[dragonfly.git] / sys / vfs / hammer / hammer_btree.c

/*
 * Copyright (c) 2007 The DragonFly Project.  All rights reserved.
 * 
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * 
 * $DragonFly: src/sys/vfs/hammer/hammer_btree.c,v 1.5 2007/11/20 07:16:28 dillon Exp $
 */

/*
 * HAMMER B-Tree index
 *
 * HAMMER implements a modified B+Tree.  In documentation this will
 * simply be refered to as the HAMMER B-Tree.  Basically a B-Tree
 * looks like a B+Tree (A B-Tree which stores its records only at the leafs
 * of the tree), but adds two additional boundary elements which describe
 * the left-most and right-most element a node is able to represent.  In
 * otherwords, we have boundary elements at the two ends of a B-Tree node
 * instead of sub-tree pointers.
 *
 * A B-Tree internal node looks like this:
 *
 *      B N N N N N N B   <-- boundary and internal elements
 *       S S S S S S S    <-- subtree pointers
 *
 * A B-Tree leaf node basically looks like this:
 *
 *      L L L L L L L L   <-- leaf elemenets
 *
 * The radix for an internal node is 1 less then a leaf but we get a
 * number of significant benefits for our troubles.
 *
 * The big benefit to using a B-Tree containing boundary information
 * is that it is possible to cache pointers into the middle of the tree
 * and not have to start searches, insertions, OR deletions at the root
 * node.   In particular, searches are able to progress in a definitive
 * direction from any point in the tree without revisting nodes.  This
 * greatly improves the efficiency of many operations, most especially
 * record appends.
 *
 * B-Trees also make the stacking of trees fairly straightforward.
 *
 * INTER-CLUSTER ELEMENTS:  An element of an internal node may reference
 * the root of another cluster rather then a node in the current cluster.
 * This is known as an inter-cluster references.  Only B-Tree searches
 * will cross cluster boundaries.  The rebalancing and collapse code does
 * not attempt to move children between clusters.  A major effect of this
 * is that we have to relax minimum element count requirements and allow
 * trees to become somewhat unabalanced.
 *
 * INSERTIONS AND DELETIONS:  When inserting we split full nodes on our
 * way down as an optimization.  I originally experimented with rebalancing
 * nodes on the way down for deletions but it created a huge mess due to
 * the way inter-cluster linkages work.  Instead, now I simply allow
 * the tree to become unbalanced and allow leaf nodes to become empty.
 * The delete code will try to clean things up from the bottom-up but
 * will stop if related elements are not in-core or if it cannot get a node
 * lock.
 */
#include "hammer.h"
#include <sys/buf.h>
#include <sys/buf2.h>

static int btree_search(hammer_cursor_t cursor, int flags);
static int btree_split_internal(hammer_cursor_t cursor);
static int btree_split_leaf(hammer_cursor_t cursor);
static int btree_remove(hammer_node_t node, int index);
#if 0
static int btree_rebalance(hammer_cursor_t cursor);
static int btree_collapse(hammer_cursor_t cursor);
#endif
static int btree_node_is_full(hammer_node_ondisk_t node);
static void hammer_make_separator(hammer_base_elm_t key1,
                        hammer_base_elm_t key2, hammer_base_elm_t dest);

/*
 * Iterate records after a search.  The cursor is iterated forwards past
 * the current record until a record matching the key-range requirements
 * is found.  ENOENT is returned if the iteration goes past the ending
 * key.
 *
 * key_beg/key_end is an INCLUSVE range.  i.e. if you are scanning to load
 * a 4096 byte buffer key_beg might specify an offset of 0 and key_end an
 * offset of 4095.
 *
 * cursor->key_beg may or may not be modified by this function during
 * the iteration.
 */
int
hammer_btree_iterate(hammer_cursor_t cursor)
{
        hammer_node_ondisk_t node;
        hammer_btree_elm_t elm;
        int64_t save_key;
        int error;
        int r;
        int s;

        /*
         * Skip past the current record
         */
        node = cursor->node->ondisk;
        if (node == NULL)
                return(ENOENT);
        if (cursor->index < node->count)
                ++cursor->index;

        /*
         * Loop until an element is found or we are done.
         */
        for (;;) {
                /*
                 * We iterate up the tree and then index over one element
                 * while we are at the last element in the current node.
                 *
                 * NOTE: This can pop us up to another cluster.
                 *
                 * If we are at the root of the root cluster, cursor_up
                 * returns ENOENT.
                 *
                 * NOTE: hammer_cursor_up() will adjust cursor->key_beg
                 * when told to re-search for the cluster tag.
                 *
                 * XXX this could be optimized by storing the information in
                 * the parent reference.
                 */
                if (cursor->index == node->count) {
                        error = hammer_cursor_up(cursor);
                        if (error)
                                break;
                        node = cursor->node->ondisk;
                        KKASSERT(cursor->index != node->count);
                        ++cursor->index;
                        continue;
                }

                /*
                 * Iterate down the tree while we are at an internal node.
                 * Nodes cannot be empty, assert the case because if one is
                 * we will wind up in an infinite loop.
                 *
                 * We can avoid iterating through large swaths of transaction
                 * id space if the left and right separators are the same
                 * except for their transaction spaces.  We can then skip
                 * the node if the left and right transaction spaces are the
                 * same sign.  This directly optimized accesses to files with
                 * HUGE transactional histories, such as database files,
                 * allowing us to avoid having to iterate through the entire
                 * history.
                 */
                if (node->type == HAMMER_BTREE_TYPE_INTERNAL) {
                        KKASSERT(node->count != 0);
                        elm = &node->elms[cursor->index];
                        if (elm[0].base.obj_id == elm[1].base.obj_id &&
                            elm[0].base.rec_type == elm[1].base.rec_type &&
                            elm[0].base.key == elm[1].base.key) {
                                /*
                                 * Left side transaction space
                                 */
                                save_key = cursor->key_beg.key;
                                cursor->key_beg.key = elm[0].base.key;
                                r = hammer_btree_cmp(&cursor->key_beg,
                                                     &elm[0].base);
                                cursor->key_beg.key = save_key;

                                /*
                                 * Right side transaction space
                                 */
                                save_key = cursor->key_end.key;
                                cursor->key_end.key = elm[1].base.key;
                                s = hammer_btree_cmp(&cursor->key_end,
                                                     &elm[1].base);
                                cursor->key_end.key = save_key;

                                /*
                                 * If our range is entirely on one side or
                                 * the other side we can skip the sub-tree.
                                 */
                                if ((r < 0 && s < 0) || (r > 0 && s > 0)) {
                                        ++cursor->index;
                                        continue;
                                }
                        }
                        error = hammer_cursor_down(cursor);
                        if (error)
                                break;
                        KKASSERT(cursor->index == 0);
                        node = cursor->node->ondisk;
                        continue;
                }

                /*
                 * We are at a leaf.
                 *
                 * Determine if the record at the cursor has gone beyond the
                 * end of our range.  Remember that our key range is inclusive.
                 *
                 * When iterating we may have to 'pick out' records matching
                 * our transaction requirements.  A comparison return of
                 * +1 or -1 indicates a transactional record that is too
                 * old or too new but does not terminate the search.
                 */
                elm = &node->elms[cursor->index];
                r = hammer_btree_cmp(&cursor->key_end, &elm->base);
                if (r == -1 || r == 1) {
                        ++cursor->index;
                        continue;
                }

                /*
                 * The search ends if the element goes beyond our key_end
                 * (after checking transactional return values above).
                 * Otherwise we have a successful match.
                 */
                error = (r < 0) ? ENOENT : 0;
                break;
        }
        return(error);
}

/*
 * Lookup cursor->key_beg.  0 is returned on success, ENOENT if the entry
 * could not be found, and a fatal error otherwise.
 * 
 * The cursor is suitably positioned for a deletion on success, and suitably
 * positioned for an insertion on ENOENT.
 *
 * The cursor may begin anywhere, the search will traverse clusters in
 * either direction to locate the requested element.
 */
int
hammer_btree_lookup(hammer_cursor_t cursor)
{
        int error;

        error = btree_search(cursor, 0);
        if (error == 0 && cursor->flags)
                error = hammer_btree_extract(cursor, cursor->flags);
        return(error);
}

/*
 * Extract the record and/or data associated with the cursor's current
 * position.  Any prior record or data stored in the cursor is replaced.
 * The cursor must be positioned at a leaf node.
 *
 * NOTE: Only records can be extracted from internal B-Tree nodes, and
 *       only for inter-cluster references.  At the moment we only support
 *       extractions from leaf nodes.
 */
int
hammer_btree_extract(hammer_cursor_t cursor, int flags)
{
        hammer_node_ondisk_t node;
        hammer_btree_elm_t elm;
        hammer_cluster_t cluster;
        int32_t cloff;
        int error;

        /*
         * A cluster record type has no data reference, the information
         * is stored directly in the record and B-Tree element.
         *
         * The case where the data reference resolves to the same buffer
         * as the record reference must be handled.
         */
        node = cursor->node->ondisk;
        KKASSERT(node->type == HAMMER_BTREE_TYPE_LEAF);
        elm = &node->elms[cursor->index];
        cluster = cursor->node->cluster;
        error = 0;

        if ((flags & HAMMER_CURSOR_GET_RECORD) && error == 0) {
                cloff = elm->leaf.rec_offset;
                cursor->record = hammer_bread(cluster, cloff,
                                              HAMMER_FSBUF_RECORDS, &error,
                                              &cursor->record_buffer);
        } else {
                cloff = 0;
        }
        if ((flags & HAMMER_CURSOR_GET_DATA) && error == 0) {
                if ((cloff ^ elm->leaf.data_offset) & ~HAMMER_BUFMASK) {
                        /*
                         * Data in different buffer than record
                         */
                        cursor->data = hammer_bread(cluster,
                                                  elm->leaf.data_offset,
                                                  HAMMER_FSBUF_DATA, &error,
                                                  &cursor->data_buffer);
                } else {
                        /*
                         * Data in same buffer as record.  Note that we
                         * leave any existing data_buffer intact, even
                         * though we don't use it in this case, in case
                         * other records extracted during an iteration
                         * go back to it.
                         */
                        cursor->data = (void *)
                                ((char *)cursor->record_buffer->ondisk +
                                 (elm->leaf.data_offset & HAMMER_BUFMASK));
                }
        }
        return(error);
}


/*
 * Insert a leaf element into the B-Tree at the current cursor position.
 * The cursor is positioned such that the element at and beyond the cursor
 * are shifted to make room for the new record.
 *
 * The caller must call hammer_btree_lookup() with the HAMMER_CURSOR_INSERT
 * flag set and that call must return ENOENT before this function can be
 * called.
 *
 * ENOSPC is returned if there is no room to insert a new record.
 */
int
hammer_btree_insert(hammer_cursor_t cursor, hammer_btree_elm_t elm)
{
        hammer_node_ondisk_t parent;
        hammer_node_ondisk_t node;
        int i;

#if 0
        /* HANDLED BY CALLER */
        /*
         * Issue a search to get our cursor at the right place.  The search
         * will get us to a leaf node.
         *
         * The search also does some setup for our insert, so there is always
         * room in the leaf.
         */
        error = btree_search(cursor, HAMMER_CURSOR_INSERT);
        if (error != ENOENT) {
                if (error == 0)
                        error = EEXIST;
                return (error);
        }
#endif

        /*
         * Insert the element at the leaf node and update the count in the
         * parent.  It is possible for parent to be NULL, indicating that
         * the root of the B-Tree in the cluster is a leaf.  It is also
         * possible for the leaf to be empty.
         *
         * Remember that the right-hand boundary is not included in the
         * count.
         */
        node = cursor->node->ondisk;
        i = cursor->index;
        KKASSERT(node->type == HAMMER_BTREE_TYPE_LEAF);
        KKASSERT(node->count < HAMMER_BTREE_LEAF_ELMS);
        if (i != node->count) {
                bcopy(&node->elms[i], &node->elms[i+1],
                      (node->count - i) * sizeof(*elm));
        }
        node->elms[i] = *elm;
        ++node->count;
        hammer_modify_node(cursor->node);

        if ((parent = cursor->parent->ondisk) != NULL) {
                i = cursor->parent_index;
                ++parent->elms[i].internal.subtree_count;
                KKASSERT(parent->elms[i].internal.subtree_count <= node->count);
                hammer_modify_node(cursor->parent);
        }
        return(0);
}

/*
 * Delete a record from the B-Tree's at the current cursor position.
 * The cursor is positioned such that the current element is the one
 * to be deleted.
 *
 * The caller must call hammer_btree_lookup() with the HAMMER_CURSOR_DELETE
 * flag set and that call must return 0 before this function can be
 * called.
 *
 * It is possible that we will be asked to delete the last element in a
 * leaf.  This case only occurs if the downward search was unable to
 * rebalance us, which in turn can occur if our parent has inter-cluster
 * elements.  So the 0-element case for a leaf is allowed.
 */
int
hammer_btree_delete(hammer_cursor_t cursor)
{
        hammer_node_ondisk_t ondisk;
        hammer_node_t node;
        hammer_node_t parent;
        hammer_btree_elm_t elm;
        int error;
        int i;

#if 0
        /* HANDLED BY CALLER */
        /*
         * Locate the leaf element to delete.  The search is also responsible
         * for doing some of the rebalancing work on its way down.
         */
        error = btree_search(cursor, HAMMER_CURSOR_DELETE);
        if (error)
                return (error);
#endif

        /*
         * Delete the element from the leaf node. 
         *
         * Remember that leaf nodes do not have boundaries.
         */
        node = cursor->node;
        ondisk = node->ondisk;
        i = cursor->index;

        KKASSERT(ondisk->type == HAMMER_BTREE_TYPE_LEAF);
        if (i + 1 != ondisk->count) {
                bcopy(&ondisk->elms[i+1], &ondisk->elms[i],
                      (ondisk->count - i - 1) * sizeof(ondisk->elms[0]));
        }
        --ondisk->count;
        if (cursor->parent != NULL) {
                /*
                 * Adjust parent's notion of the leaf's count.  subtree_count
                 * is only approximate, it is allowed to be too small but
                 * never allowed to be too large.  Make sure we don't drop
                 * the count below 0.
                 */
                parent = cursor->parent;
                elm = &parent->ondisk->elms[cursor->parent_index];
                if (elm->internal.subtree_count)
                        --elm->internal.subtree_count;
                KKASSERT(elm->internal.subtree_count <= ondisk->count);
                hammer_modify_node(parent);
        }

        /*
         * If the leaf is empty try to remove the subtree reference
         * in at (parent, parent_index).  This will unbalance the
         * tree.
         *
         * Note that internal nodes must have at least one element
         * so their boundary information is properly laid out.  If
         * we would cause our parent to become empty we try to
         * recurse up the tree, but if that doesn't work we just
         * leave the tree with an empty leaf.
         */
        if (ondisk->count == 0) {
                error = btree_remove(cursor->parent, cursor->parent_index);
                if (error == 0) {
                        hammer_free_btree(node->cluster, node->node_offset);
                } else if (error == EAGAIN) {
                        hammer_modify_node(node);
                        error = 0;
                } /* else a real error occured XXX */
        } else {
                hammer_modify_node(node);
                error = 0;
        }
        return(error);
}

/*
 * PRIMAY B-TREE SEARCH SUPPORT PROCEDURE
 *
 * Search a cluster's B-Tree for cursor->key_beg, return the matching node.
 *
 * The search begins at the current node and will instantiate a NULL
 * parent if necessary and if not at the root of the cluster.  On return
 * parent will be non-NULL unless the cursor is sitting at a root-leaf.
 *
 * The search code may be forced to iterate up the tree if the conditions
 * required for an insertion or deletion are not met.  This does not occur
 * very often.
 *
 * INSERTIONS: The search will split full nodes and leaves on its way down
 * and guarentee that the leaf it ends up on is not full.
 *
 * DELETIONS: The search will rebalance the tree on its way down.
 */
static 
int
btree_search(hammer_cursor_t cursor, int flags)
{
        hammer_node_ondisk_t node;
        hammer_cluster_t cluster;
        int error;
        int i;
        int r;

        flags |= cursor->flags;

        /*
         * Move our cursor up the tree until we find a node whos range covers
         * the key we are trying to locate.  This may move us between
         * clusters.
         *
         * The left bound is inclusive, the right bound is non-inclusive.
         * It is ok to cursor up too far so when cursoring across a cluster
         * boundary.
         *
         * First see if we can skip the whole cluster.  hammer_cursor_up()
         * handles both cases but this way we don't check the cluster
         * bounds when going up the tree within a cluster.
         */
        cluster = cursor->node->cluster;
        while (
            hammer_btree_cmp(&cursor->key_beg, &cluster->clu_btree_beg) < 0 ||
            hammer_btree_cmp(&cursor->key_beg, &cluster->clu_btree_end) >= 0) {
                error = hammer_cursor_toroot(cursor);
                if (error)
                        goto done;
                error = hammer_cursor_up(cursor);
                if (error)
                        goto done;
                cluster = cursor->node->cluster;
        }

        /*
         * Deal with normal cursoring within a cluster.  The right bound
         * is non-inclusive.  That is, the bounds form a separator.
         */
        while (hammer_btree_cmp(&cursor->key_beg, cursor->left_bound) < 0 ||
               hammer_btree_cmp(&cursor->key_beg, cursor->right_bound) >= 0) {
                error = hammer_cursor_up(cursor);
                if (error)
                        goto done;
        }

        /*
         * We better have ended up with a node somewhere, and our second
         * while loop had better not have traversed up a cluster.
         */
        KKASSERT(cursor->node != NULL && cursor->node->cluster == cluster);

        /*
         * If we are inserting we can't start at a full node if the parent
         * is also full (because there is no way to split the node),
         * continue running up the tree until we hit the root of the
         * root cluster or until the requirement is satisfied.
         *
         * NOTE: These cursor-up's CAN continue to cross cluster boundaries.
         *
         * XXX as an optimization it should be possible to unbalance the tree
         * and stop at the root of the current cluster.
         */
        while (flags & HAMMER_CURSOR_INSERT) {
                if (btree_node_is_full(cursor->node->ondisk) == 0)
                        break;
                if (cursor->parent == NULL)
                        break;
                if (cursor->parent->ondisk->count != HAMMER_BTREE_INT_ELMS)
                        break;
                error = hammer_cursor_up(cursor);
                /* cluster and node are now may become stale */
                if (error)
                        goto done;
        }
        /* cluster = cursor->node->cluster; not needed until next cluster = */

#if 0
        /*
         * If we are deleting we can't start at an internal node with only
         * one element unless it is root, because all of our code assumes
         * that internal nodes will never be empty.  Just do this generally
         * for both leaf and internal nodes to get better balance.
         *
         * This handles the case where the cursor is sitting at a leaf and
         * either the leaf or parent contain an insufficient number of
         * elements.
         *
         * NOTE: These cursor-up's CAN continue to cross cluster boundaries.
         *
         * XXX NOTE: Iterations may not set this flag anyway.
         */
        while (flags & HAMMER_CURSOR_DELETE) {
                if (cursor->node->ondisk->count > 1)
                        break;
                if (cursor->parent == NULL)
                        break;
                KKASSERT(cursor->node->ondisk->count != 0);
                error = hammer_cursor_up(cursor);
                /* cluster and node are now may become stale */
                if (error)
                        goto done;
        }
#endif

/*new_cluster:*/
        /*
         * Push down through internal nodes to locate the requested key.
         */
        cluster = cursor->node->cluster;
        node = cursor->node->ondisk;
        while (node->type == HAMMER_BTREE_TYPE_INTERNAL) {
#if 0
                /*
                 * If we are a the root node and deleting, try to collapse
                 * all of the root's children into the root.  This is the
                 * only point where tree depth is reduced.
                 *
                 * XXX NOTE: Iterations may not set this flag anyway.
                 */
                if ((flags & HAMMER_CURSOR_DELETE) && cursor->parent == NULL) {
                        error = btree_collapse(cursor);
                        /* node becomes stale after call */
                        if (error)
                                goto done;
                }
                node = cursor->node->ondisk;
#endif

                /*
                 * Scan the node to find the subtree index to push down into.
                 * We go one-past, then back-up.  The key should never be
                 * less then the left-hand boundary so I should never wind
                 * up 0.
                 */
                for (i = 0; i < node->count; ++i) {
                        r = hammer_btree_cmp(&cursor->key_beg,
                                             &node->elms[i].base);
                        if (r < 0)
                                break;
                }
                KKASSERT(i != 0);

                /*
                 * The push-down index is now i - 1.
                 */
                --i;
                cursor->index = i;

                /*
                 * Handle insertion and deletion requirements.
                 *
                 * If inserting split full nodes.  The split code will
                 * adjust cursor->node and cursor->index if the current
                 * index winds up in the new node.
                 */
                if (flags & HAMMER_CURSOR_INSERT) {
                        if (node->count == HAMMER_BTREE_INT_ELMS) {
                                error = btree_split_internal(cursor);
                                if (error)
                                        goto done;
                                /*
                                 * reload stale pointers
                                 */
                                i = cursor->index;
                                node = cursor->node->ondisk;
                        }
                }

#if 0
                /*
                 * If deleting rebalance - do not allow the child to have
                 * just one element or we will not be able to delete it.
                 *
                 * Neither internal or leaf nodes (except a root-leaf) are
                 * allowed to drop to 0 elements.  (XXX - well, leaf nodes
                 * can at the moment).
                 *
                 * Our separators may have been reorganized after rebalancing,
                 * so we have to pop back up and rescan.
                 *
                 * XXX test for subtree_count < maxelms / 2, minus 1 or 2
                 * for hysteresis?
                 *
                 * XXX NOTE: Iterations may not set this flag anyway.
                 */
                if (flags & HAMMER_CURSOR_DELETE) {
                        if (node->elms[i].internal.subtree_count <= 1) {
                                error = btree_rebalance(cursor);
                                if (error)
                                        goto done;
                                /* cursor->index is invalid after call */
                                goto new_cluster;
                        }
                }
#endif

                /*
                 * Push down (push into new node, existing node becomes
                 * the parent).
                 */
                error = hammer_cursor_down(cursor);
                /* node and cluster become stale */
                if (error)
                        goto done;
                node = cursor->node->ondisk;
                cluster = cursor->node->cluster;
        }

        /*
         * We are at a leaf, do a linear search of the key array.
         * (XXX do a binary search).  On success the index is set to the
         * matching element, on failure the index is set to the insertion
         * point.
         *
         * Boundaries are not stored in leaf nodes, so the index can wind
         * up to the left of element 0 (index == 0) or past the end of
         * the array (index == node->count).
         */
        KKASSERT(node->count <= HAMMER_BTREE_LEAF_ELMS);

        for (i = 0; i < node->count; ++i) {
                r = hammer_btree_cmp(&cursor->key_beg, &node->elms[i].base);

                /*
                 * Stop if we've flipped past key_beg
                 */
                if (r < 0)
                        break;

                /*
                 * Return an exact match
                 */
                if (r == 0) {
                        cursor->index = i;
                        error = 0;
                        goto done;
                }
        }

        /*
         * No exact match was found, i is now at the insertion point.
         *
         * If inserting split a full leaf before returning.  This
         * may have the side effect of adjusting cursor->node and
         * cursor->index.
         */
        cursor->index = i;
        if ((flags & HAMMER_CURSOR_INSERT) &&
            node->count == HAMMER_BTREE_LEAF_ELMS) {
                error = btree_split_leaf(cursor);
                /* NOT USED
                i = cursor->index;
                node = &cursor->node->internal;
                */
                if (error)
                        goto done;
        }
        error = ENOENT;
done:
        return(error);
}


/************************************************************************
 *                         SPLITTING AND MERGING                        *
 ************************************************************************
 *
 * These routines do all the dirty work required to split and merge nodes.
 */

/*
 * Split an internal node into two nodes and move the separator at the split
 * point to the parent.  Note that the parent's parent's element pointing
 * to our parent will have an incorrect subtree_count (we don't update it).
 * It will be low, which is ok.
 *
 * (cursor->node, cursor->index) indicates the element the caller intends
 * to push into.  We will adjust node and index if that element winds
 * up in the split node.
 *
 * If we are at the root of a cluster a new root must be created with two
 * elements, one pointing to the original root and one pointing to the
 * newly allocated split node.
 *
 * NOTE! Being at the root of a cluster is different from being at the
 * root of the root cluster.  cursor->parent will not be NULL and
 * cursor->node->ondisk.parent must be tested against 0.  Theoretically
 * we could propogate the algorithm into the parent and deal with multiple
 * 'roots' in the cluster header, but it's easier not to.
 */
static
int
btree_split_internal(hammer_cursor_t cursor)
{
        hammer_node_ondisk_t ondisk;
        hammer_node_t node;
        hammer_node_t parent;
        hammer_node_t new_node;
        hammer_btree_elm_t elm;
        hammer_btree_elm_t parent_elm;
        int parent_index;
        int made_root;
        int split;
        int error;
        const int esize = sizeof(*elm);

        /* 
         * We are splitting but elms[split] will be promoted to the parent,
         * leaving the right hand node with one less element.  If the
         * insertion point will be on the left-hand side adjust the split
         * point to give the right hand side one additional node.
         */
        node = cursor->node;
        ondisk = node->ondisk;
        split = (ondisk->count + 1) / 2;
        if (cursor->index <= split)
                --split;
        error = 0;

        /*
         * If we are at the root of the cluster, create a new root node with
         * 1 element and split normally.  Avoid making major modifications
         * until we know the whole operation will work.
         *
         * The root of the cluster is different from the root of the root
         * cluster.  Use the node's on-disk structure's parent offset to
         * detect the case.
         */
        if (ondisk->parent == 0) {
                parent = hammer_alloc_btree(node->cluster, &error);
                if (parent == NULL)
                        return(error);
                hammer_lock_ex(&parent->lock);
                ondisk = parent->ondisk;
                ondisk->count = 1;
                ondisk->parent = 0;
                ondisk->type = HAMMER_BTREE_TYPE_INTERNAL;
                ondisk->elms[0].base = node->cluster->clu_btree_beg;
                ondisk->elms[0].internal.subtree_type = node->ondisk->type;
                ondisk->elms[0].internal.subtree_offset = node->node_offset;
                ondisk->elms[1].base = node->cluster->clu_btree_end;
                made_root = 1;
                parent_index = 0;       /* index of current node in parent */
        } else {
                made_root = 0;
                parent = cursor->parent;
                parent_index = cursor->parent_index;
        }

        /*
         * Split node into new_node at the split point.
         *
         *  B O O O P N N B     <-- P = node->elms[split]
         *   0 1 2 3 4 5 6      <-- subtree indices
         *
         *       x x P x x
         *        s S S s  
         *         /   \
         *  B O O O B    B N N B        <--- inner boundary points are 'P'
         *   0 1 2 3      4 5 6  
         *
         */
        new_node = hammer_alloc_btree(node->cluster, &error);
        if (new_node == NULL) {
                if (made_root) {
                        hammer_unlock(&parent->lock);
                        hammer_free_btree(node->cluster, parent->node_offset);
                        hammer_rel_node(parent);
                }
                return(error);
        }
        hammer_lock_ex(&new_node->lock);

        /*
         * Create the new node.  P becomes the left-hand boundary in the
         * new node.  Copy the right-hand boundary as well.
         *
         * elm is the new separator.
         */
        ondisk = node->ondisk;
        elm = &ondisk->elms[split];
        bcopy(elm, &new_node->ondisk->elms[0],
              (ondisk->count - split + 1) * esize);
        new_node->ondisk->count = ondisk->count - split;
        new_node->ondisk->parent = parent->node_offset;
        new_node->ondisk->type = HAMMER_BTREE_TYPE_INTERNAL;
        KKASSERT(ondisk->type == new_node->ondisk->type);

        /*
         * Cleanup the original node.  P becomes the new boundary, its
         * subtree_offset was moved to the new node.  If we had created
         * a new root its parent pointer may have changed.
         */
        elm->internal.subtree_offset = 0;

        /*
         * Insert the separator into the parent, fixup the parent's
         * reference to the original node, and reference the new node.
         * The separator is P.
         *
         * Remember that base.count does not include the right-hand boundary.
         */
        ondisk = parent->ondisk;
        ondisk->elms[parent_index].internal.subtree_count = split;
        parent_elm = &ondisk->elms[parent_index+1];
        bcopy(parent_elm, parent_elm + 1,
              (ondisk->count - parent_index) * esize);
        parent_elm->internal.base = elm->base;  /* separator P */
        parent_elm->internal.subtree_offset = new_node->node_offset;
        parent_elm->internal.subtree_count = new_node->ondisk->count;

        /*
         * The cluster's root pointer may have to be updated.
         */
        if (made_root) {
                node->cluster->ondisk->clu_btree_root = parent->node_offset;
                hammer_modify_cluster(node->cluster);
                node->ondisk->parent = parent->node_offset;
                if (cursor->parent) {
                        hammer_unlock(&cursor->parent->lock);
                        hammer_rel_node(cursor->parent);
                }
                cursor->parent = parent;        /* lock'd and ref'd */
        }

        hammer_modify_node(node);
        hammer_modify_node(new_node);
        hammer_modify_node(parent);

        /*
         * Ok, now adjust the cursor depending on which element the original
         * index was pointing at.  If we are >= the split point the push node
         * is now in the new node.
         *
         * NOTE: If we are at the split point itself we cannot stay with the
         * original node because the push index will point at the right-hand
         * boundary, which is illegal.
         *
         * NOTE: The cursor's parent or parent_index must be adjusted for
         * the case where a new parent (new root) was created, and the case
         * where the cursor is now pointing at the split node.
         */
        if (cursor->index >= split) {
                cursor->parent_index = parent_index + 1;
                cursor->index -= split;
                hammer_unlock(&cursor->node->lock);
                hammer_rel_node(cursor->node);
                cursor->node = new_node;        /* locked and ref'd */
        } else {
                cursor->parent_index = parent_index;
                hammer_unlock(&new_node->lock);
                hammer_rel_node(new_node);
        }
        return (0);
}

/*
 * Same as the above, but splits a full leaf node.
 */
static
int
btree_split_leaf(hammer_cursor_t cursor)
{
        hammer_node_ondisk_t ondisk;
        hammer_node_t parent;
        hammer_node_t leaf;
        hammer_node_t new_leaf;
        hammer_btree_elm_t elm;
        hammer_btree_elm_t parent_elm;
        int parent_index;
        int made_root;
        int split;
        int error;
        const size_t esize = sizeof(*elm);

        /* 
         * Calculate the split point.  If the insertion point will be on
         * the left-hand side adjust the split point to give the right
         * hand side one additional node.
         */
        leaf = cursor->node;
        ondisk = leaf->ondisk;
        split = (ondisk->count + 1) / 2;
        if (cursor->index <= split)
                --split;
        error = 0;

        /*
         * If we are at the root of the tree, create a new root node with
         * 1 element and split normally.  Avoid making major modifications
         * until we know the whole operation will work.
         */
        if (ondisk->parent == 0) {
                parent = hammer_alloc_btree(leaf->cluster, &error);
                if (parent == NULL)
                        return(error);
                hammer_lock_ex(&parent->lock);
                ondisk = parent->ondisk;
                ondisk->count = 1;
                ondisk->parent = 0;
                ondisk->type = HAMMER_BTREE_TYPE_INTERNAL;
                ondisk->elms[0].base = leaf->cluster->clu_btree_beg;
                ondisk->elms[0].internal.subtree_type = leaf->ondisk->type;
                ondisk->elms[0].internal.subtree_offset = leaf->node_offset;
                ondisk->elms[1].base = leaf->cluster->clu_btree_end;
                made_root = 1;
                parent_index = 0;       /* insertion point in parent */
        } else {
                made_root = 0;
                parent = cursor->parent;
                parent_index = cursor->parent_index;
        }

        /*
         * Split leaf into new_leaf at the split point.  Select a separator
         * value in-between the two leafs but with a bent towards the right
         * leaf since comparisons use an 'elm >= separator' inequality.
         *
         *  L L L L L L L L
         *
         *       x x P x x
         *        s S S s  
         *         /   \
         *  L L L L     L L L L
         */
        new_leaf = hammer_alloc_btree(leaf->cluster, &error);
        if (new_leaf == NULL) {
                if (made_root) {
                        hammer_unlock(&parent->lock);
                        hammer_free_btree(leaf->cluster, parent->node_offset);
                        hammer_rel_node(parent);
                }
                return(error);
        }
        hammer_lock_ex(&new_leaf->lock);

        /*
         * Create the new node.  P become the left-hand boundary in the
         * new node.  Copy the right-hand boundary as well.
         */
        ondisk = leaf->ondisk;
        elm = &ondisk->elms[split];
        bcopy(elm, &new_leaf->ondisk->elms[0], (ondisk->count - split) * esize);
        new_leaf->ondisk->count = ondisk->count - split;
        new_leaf->ondisk->parent = parent->node_offset;
        new_leaf->ondisk->type = HAMMER_BTREE_TYPE_LEAF;
        KKASSERT(ondisk->type == new_leaf->ondisk->type);

        /*
         * Cleanup the original node.  Because this is a leaf node and
         * leaf nodes do not have a right-hand boundary, there
         * aren't any special edge cases to clean up.
         */
        /* nothing to do */

        /*
         * Insert the separator into the parent, fixup the parent's
         * reference to the original node, and reference the new node.
         * The separator is P.
         *
         * Remember that base.count does not include the right-hand boundary.
         * We are copying parent_index+1 to parent_index+2, not +0 to +1.
         */
        ondisk = parent->ondisk;
        ondisk->elms[parent_index].internal.subtree_count = split;
        parent_elm = &ondisk->elms[parent_index+1];
        if (parent_index + 1 != ondisk->count) {
                bcopy(parent_elm, parent_elm + 1,
                      (ondisk->count - parent_index - 1) * esize);
        }
        hammer_make_separator(&elm[-1].base, &elm[0].base, &parent_elm->base);
        parent_elm->internal.subtree_offset = new_leaf->node_offset;
        parent_elm->internal.subtree_count = new_leaf->ondisk->count;

        /*
         * The cluster's root pointer may have to be updated.
         */
        if (made_root) {
                leaf->cluster->ondisk->clu_btree_root = parent->node_offset;
                hammer_modify_cluster(leaf->cluster);
                leaf->ondisk->parent = parent->node_offset;
                if (cursor->parent) {
                        hammer_unlock(&cursor->parent->lock);
                        hammer_rel_node(cursor->parent);
                }
                cursor->parent = parent;        /* lock'd and ref'd */
        }

        hammer_modify_node(leaf);
        hammer_modify_node(new_leaf);
        hammer_modify_node(parent);

        /*
         * Ok, now adjust the cursor depending on which element the original
         * index was pointing at.  If we are >= the split point the push node
         * is now in the new node.
         *
         * NOTE: If we are at the split point itself we cannot stay with the
         * original node because the push index will point at the right-hand
         * boundary, which is illegal.
         */
        if (cursor->index >= split) {
                cursor->parent_index = parent_index + 1;
                cursor->index -= split;
                cursor->node = new_leaf;
                hammer_unlock(&cursor->node->lock);
                hammer_rel_node(cursor->node);
                cursor->node = new_leaf;
        } else {
                cursor->parent_index = parent_index;
                hammer_unlock(&new_leaf->lock);
                hammer_rel_node(new_leaf);
        }
        return (0);
}

/*
 * Remove the element at (node, index).  If the internal node would become
 * empty passively recurse up the tree.
 *
 * A locked internal node is passed to this function, the node remains
 * locked on return.  Leaf nodes cannot be passed to this function.
 *
 * Returns EAGAIN if we were unable to acquire the needed locks.  The caller
 * does not deal with the empty leaf until determines whether this recursion
 * has succeeded or not.
 */
int
btree_remove(hammer_node_t node, int index)
{
        hammer_node_ondisk_t ondisk;
        hammer_node_t parent;
        int error;

        ondisk = node->ondisk;
        KKASSERT(ondisk->count > 0);

        /*
         * Remove the element, shifting remaining elements left one.
         * Note that our move must include the right-boundary element.
         */
        if (ondisk->count != 1) {
                bcopy(&ondisk->elms[index+1], &ondisk->elms[index],
                      (ondisk->count - index) * sizeof(ondisk->elms[0]));
                --ondisk->count;
                hammer_modify_node(node);
                return(0);
        }

        /*
         * Internal nodes cannot drop to 0 elements, so remove the node
         * from ITS parent.  If the node is the root node, convert it to
         * an empty leaf node (which can drop to 0 elements).
         */
        if (ondisk->parent == 0) {
                ondisk->count = 0;
                ondisk->type = HAMMER_BTREE_TYPE_LEAF;
                hammer_modify_node(node);
                return(0);
        }

        /*
         * Try to remove the node from its parent.  Return EAGAIN if we
         * cannot.
         */
        parent = hammer_get_node(node->cluster, ondisk->parent, &error);
        if (hammer_lock_ex_try(&parent->lock)) {
                hammer_rel_node(parent);
                return(EAGAIN);
        }
        ondisk = parent->ondisk;
        for (index = 0; index < ondisk->count; ++index) {
                if (ondisk->elms[index].internal.subtree_offset ==
                    node->node_offset) {
                        break;
                }
        }
        if (index == ondisk->count) {
                kprintf("btree_remove: lost parent linkage to node\n");
                error = EIO;
        } else {
                error = btree_remove(parent, index);
                if (error == 0) {
                        hammer_free_btree(node->cluster, node->node_offset);
                        /* NOTE: node can be reallocated at any time now */
                }
        }
        hammer_unlock(&parent->lock);
        hammer_rel_node(parent);
        return (error);
}

#if 0

/*
 * This routine is called on the internal node (node) prior to recursing down
 * through (node, index) when the node referenced by (node, index) MIGHT
 * have too few elements for the caller to perform a deletion.
 *
 * cursor->index is invalid on return because the separators may have gotten
 * adjusted, the caller must rescan the node's elements.  The caller may set
 * cursor->index to -1 if it wants us to do a general rebalancing.
 *
 * This routine rebalances the children of the (node), collapsing children
 * together if possible.  On return each child will have at least L/2-1
 * elements unless the node only has one child.
 * 
 * NOTE: Because we do not update the parent's parent in the split code,
 * the subtree_count used by the caller may be incorrect.  We correct it
 * here.  Also note that we cannot change the depth of the tree's leaf
 * nodes here (see btree_collapse()).
 *
 * NOTE: We make no attempt to rebalance inter-cluster elements.
 */
static
int
btree_rebalance(hammer_cursor_t cursor)
{
        hammer_node_ondisk_t ondisk;
        hammer_node_t node;
        hammer_node_t children[HAMMER_BTREE_INT_ELMS];
        hammer_node_t child;
        hammer_btree_elm_t elm;
        hammer_btree_elm_t elms;
        int i, j, n, nelms, goal;
        int maxelms, halfelms;
        int error;

        /*
         * If the elm being recursed through is an inter-cluster reference,
         * don't worry about it.
         */
        ondisk = cursor->node->ondisk;
        elm = &ondisk->elms[cursor->index];
        if (elm->internal.subtree_type == HAMMER_BTREE_TYPE_CLUSTER)
                return(0);

        KKASSERT(elm->internal.subtree_offset != 0);
        error = 0;

        /*
         * Load the children of node and do any necessary corrections
         * to subtree_count.  subtree_count may be too low due to the
         * way insertions split nodes.  Get a count of the total number
         * of actual elements held by our children.
         */
        error = 0;

        for (i = n = 0; i < node->base.count; ++i) {
                struct hammer_btree_internal_elm *elm;

                elm = &node->elms[i];
                children[i] = NULL;
                child_buffer[i] = NULL; /* must be preinitialized for bread */
                if (elm->subtree_offset == 0)
                        continue;
                child = hammer_bread(cursor->cluster, elm->subtree_offset,
                                     HAMMER_FSBUF_BTREE, &error,
                                     &child_buffer[i], XXX);
                children[i] = child;
                if (child == NULL)
                        continue;
                XXX
                KKASSERT(node->base.subtype == child->base.type);

                /*
                 * Accumulate n for a good child, update the node's count
                 * if it was wrong.
                 */
                if (node->elms[i].subtree_count != child->base.count) {
                        node->elms[i].subtree_count = child->base.count;
                }
                n += node->elms[i].subtree_count;
        }
        if (error)
                goto failed;

        /*
         * Collect all the children's elements together
         */
        nelms = n;
        elms = kmalloc(sizeof(*elms) * (nelms + 1), M_HAMMER, M_WAITOK|M_ZERO);
        for (i = n = 0; i < node->base.count; ++i) {
                child = children[i];
                for (j = 0; j < child->base.count; ++j) {
                        elms[n].owner = child;
                        if (node->base.subtype == HAMMER_BTREE_TYPE_LEAF)
                                elms[n].u.leaf = child->leaf.elms[j];
                        else
                                elms[n].u.internal = child->internal.elms[j];
                        ++n;
                }
        }
        KKASSERT(n == nelms);

        /*
         * Store a boundary in the elms array to ease the code below.  This
         * is only used if the children are internal nodes.
         */
        elms[n].u.internal = node->elms[i];

        /*
         * Calculate the number of elements each child should have (goal) by
         * reducing the number of elements until we achieve at least
         * halfelms - 1 per child, unless we are a degenerate case.
         */
        maxelms = btree_max_elements(node->base.subtype);
        halfelms = maxelms / 2;

        goal = halfelms - 1;
        while (i && n / i < goal)
                --i;

        /*
         * Now rebalance using the specified goal
         */
        for (i = n = 0; i < node->base.count; ++i) {
                struct hammer_buffer *subchild_buffer = NULL;
                struct hammer_btree_internal_node *subchild;

                child = children[i];
                for (j = 0; j < goal && n < nelms; ++j) {
                        if (node->base.subtype == HAMMER_BTREE_TYPE_LEAF) {
                                child->leaf.elms[j] = elms[n].u.leaf;
                        } else {
                                child->internal.elms[j] = elms[n].u.internal;
                        }

                        /*
                         * If the element's parent has changed we have to
                         * update the parent pointer.  This is somewhat
                         * expensive.
                         */
                        if (elms[n].owner != child &&
                            node->base.subtype == HAMMER_BTREE_TYPE_INTERNAL) {
                                subchild = hammer_bread(cursor->cluster,
                                                        elms[n].u.internal.subtree_offset,
                                                        HAMMER_FSBUF_BTREE,
                                                        &error,
                                                        &subchild_buffer, XXX);
                                if (subchild) {
                                        subchild->base.parent =
                                            hammer_bclu_offset(child_buffer[i],
                                                                child);
                                        hammer_modify_buffer(subchild_buffer);
                                }
                                /* XXX error */
                        }
                        ++n;
                }
                /* 
                 * Set right boundary if the children are internal nodes.
                 */
                if (node->base.subtype == HAMMER_BTREE_TYPE_INTERNAL)
                        child->internal.elms[j] = elms[n].u.internal;
                child->base.count = j;
                hammer_modify_buffer(child_buffer[i]);
                if (subchild_buffer)
                        hammer_put_buffer(subchild_buffer, 0);

                /*
                 * If we have run out of elements, break out
                 */
                if (n == nelms)
                        break;
        }

        /*
         * Physically destroy any left-over children.  These children's
         * elements have been packed into prior children.  The node's
         * right hand boundary and count gets shifted to index i.
         *
         * The subtree count in the node's parent MUST be updated because
         * we are removing elements.  The subtree_count field is allowed to
         * be too small, but not too large!
         */
        if (i != node->base.count) {
                n = i;
                node->elms[n] = node->elms[node->base.count];
                while (i < node->base.count) {
                        hammer_free_btree_ptr(child_buffer[i], children[i]);
                        hammer_put_buffer(child_buffer[i], 0);
                        ++i;
                }
                node->base.count = n;
                if (cursor->parent) {
                        cursor->parent->elms[cursor->parent_index].subtree_count = n;
                        hammer_modify_buffer(cursor->parent_buffer);
                }
        }

        kfree(elms, M_HAMMER);
failed:
        hammer_modify_buffer(cursor->node_buffer);
        for (i = 0; i < node->base.count; ++i) {
                if (child_buffer[i])
                        hammer_put_buffer(child_buffer[i], 0);
        }
        return (error);
}

/*
 * This routine is only called if the cursor is at the root node and the
 * root node is an internal node.  We attempt to collapse the root node
 * by replacing it with all of its children, reducing tree depth by one.
 *
 * This is the only way to reduce tree depth in a HAMMER filesystem.
 * Note that all leaf nodes are at the same depth.
 *
 * This is a fairly expensive operation because we not only have to load
 * the root's children, we also have to scan each child and adjust the
 * parent offset for each element in each child.  Nasty all around.
 */
static
int
btree_collapse(hammer_cursor_t cursor)
{
        hammer_btree_node_ondisk_t root, child;
        hammer_btree_node_ondisk_t children[HAMMER_BTREE_INT_ELMS];
        struct hammer_buffer *child_buffer[HAMMER_BTREE_INT_ELMS];
        int count;
        int i, j, n;
        int root_modified;
        int error;
        int32_t root_offset;
        u_int8_t subsubtype;

        root = cursor->node;
        count = root->base.count;
        root_offset = hammer_bclu_offset(cursor->node_buffer, root);

        /*
         * Sum up the number of children each element has.  This value is
         * only approximate due to the way the insertion node works.  It
         * may be too small but it will never be too large.
         *
         * Quickly terminate the collapse if the elements have too many
         * children.
         */
        KKASSERT(root->base.parent == 0);       /* must be root node */
        KKASSERT(root->base.type == HAMMER_BTREE_TYPE_INTERNAL);
        KKASSERT(count <= HAMMER_BTREE_INT_ELMS);

        for (i = n = 0; i < count; ++i) {
                n += root->internal.elms[i].subtree_count;
        }
        if (n > btree_max_elements(root->base.subtype))
                return(0);

        /*
         * Iterate through the elements again and correct the subtree_count.
         * Terminate the collapse if we wind up with too many.
         */
        error = 0;
        root_modified = 0;

        for (i = n = 0; i < count; ++i) {
                struct hammer_btree_internal_elm *elm;

                elm = &root->internal.elms[i];
                child_buffer[i] = NULL;
                children[i] = NULL;
                if (elm->subtree_offset == 0)
                        continue;
                child = hammer_bread(cursor->cluster, elm->subtree_offset,
                                     HAMMER_FSBUF_BTREE, &error,
                                     &child_buffer[i], XXX);
                children[i] = child;
                if (child == NULL)
                        continue;
                KKASSERT(root->base.subtype == child->base.type);

                /*
                 * Accumulate n for a good child, update the root's count
                 * if it was wrong.
                 */
                if (root->internal.elms[i].subtree_count != child->base.count) {
                        root->internal.elms[i].subtree_count = child->base.count;
                        root_modified = 1;
                }
                n += root->internal.elms[i].subtree_count;
        }
        if (error || n > btree_max_elements(root->base.subtype))
                goto done;

        /*
         * Ok, we can collapse the root.  If the root's children are leafs
         * the collapse is really simple.  If they are internal nodes the
         * collapse is not so simple because we have to fixup the parent
         * pointers for the root's children's children.
         *
         * When collapsing an internal node the far left and far right
         * element's boundaries should match the root's left and right
         * boundaries.
         */
        if (root->base.subtype == HAMMER_BTREE_TYPE_LEAF) {
                for (i = n = 0; i < count; ++i) {
                        child = children[i];
                        for (j = 0; j < child->base.count; ++j) {
                                root->leaf.elms[n] = child->leaf.elms[j];
                                ++n;
                        }
                }
                root->base.type = root->base.subtype;
                root->base.subtype = 0;
                root->base.count = n;
                root->leaf.link_left = 0;
                root->leaf.link_right = 0;
        } else {
                struct hammer_btree_internal_elm *elm;
                struct hammer_btree_internal_node *subchild;
                struct hammer_buffer *subchild_buffer = NULL;

                if (count) {
                        child = children[0];
                        subsubtype = child->base.subtype;
                        KKASSERT(child->base.count > 0);
                        KKASSERT(root->internal.elms[0].base.key ==
                                 child->internal.elms[0].base.key);
                        child = children[count-1];
                        KKASSERT(child->base.count > 0);
                        KKASSERT(root->internal.elms[count].base.key ==
                             child->internal.elms[child->base.count].base.key);
                } else {
                        subsubtype = 0;
                }
                for (i = n = 0; i < count; ++i) {
                        child = children[i];
                        KKASSERT(child->base.subtype == subsubtype);
                        for (j = 0; j < child->base.count; ++j) {
                                elm = &child->internal.elms[j];

                                root->internal.elms[n] = *elm;
                                subchild = hammer_bread(cursor->cluster,
                                                        elm->subtree_offset,
                                                        HAMMER_FSBUF_BTREE,
                                                        &error,
                                                        &subchild_buffer,
                                                        XXX);
                                if (subchild) {
                                        subchild->base.parent = root_offset;
                                        hammer_modify_buffer(subchild_buffer);
                                }
                                ++n;
                        }
                        /* make sure the right boundary is correct */
                        /* (this gets overwritten when the loop continues) */
                        /* XXX generate a new separator? */
                        root->internal.elms[n] = child->internal.elms[j];
                }
                root->base.type = HAMMER_BTREE_TYPE_INTERNAL;
                root->base.subtype = subsubtype;
                if (subchild_buffer)
                        hammer_put_buffer(subchild_buffer, 0);
        }
        root_modified = 1;

        /*
         * Cleanup
         */
done:
        if (root_modified)
                hammer_modify_buffer(cursor->node_buffer);
        for (i = 0; i < count; ++i) {
                if (child_buffer[i])
                        hammer_put_buffer(child_buffer[i], 0);
        }
        return(error);
}

#endif

/************************************************************************
 *                         MISCELLANIOUS SUPPORT                        *
 ************************************************************************/

/*
 * Compare two B-Tree elements, return -1, 0, or +1 (e.g. similar to strcmp).
 *
 * See also hammer_rec_rb_compare() and hammer_rec_cmp() in hammer_object.c.
 *
 * Note that key1 and key2 are treated differently.  key1 is allowed to
 * wildcard some of its fields by setting them to 0, while key2 is expected
 * to be in an on-disk form (no wildcards).
 */
int
hammer_btree_cmp(hammer_base_elm_t key1, hammer_base_elm_t key2)
{
#if 0
        kprintf("compare obj_id %016llx %016llx\n",
                key1->obj_id, key2->obj_id);
        kprintf("compare rec_type %04x %04x\n",
                key1->rec_type, key2->rec_type);
        kprintf("compare key %016llx %016llx\n",
                key1->key, key2->key);
#endif

        /*
         * A key1->obj_id of 0 matches any object id
         */
        if (key1->obj_id) {
                if (key1->obj_id < key2->obj_id)
                        return(-4);
                if (key1->obj_id > key2->obj_id)
                        return(4);
        }

        /*
         * A key1->rec_type of 0 matches any record type.
         */
        if (key1->rec_type) {
                if (key1->rec_type < key2->rec_type)
                        return(-3);
                if (key1->rec_type > key2->rec_type)
                        return(3);
        }

        /*
         * There is no special case for key.  0 means 0.
         */
        if (key1->key < key2->key)
                return(-2);
        if (key1->key > key2->key)
                return(2);

        /*
         * This test has a number of special cases.  create_tid in key1 is
         * the as-of transction id, and delete_tid in key1 is NOT USED.
         *
         * A key1->create_tid of 0 matches any record regardles of when
         * it was created or destroyed.  0xFFFFFFFFFFFFFFFFULL should be
         * used to search for the most current state of the object.
         *
         * key2->create_tid is a HAMMER record and will never be
         * 0.   key2->delete_tid is the deletion transaction id or 0 if 
         * the record has not yet been deleted.
         */
        if (key1->create_tid) {
                if (key1->create_tid < key2->create_tid)
                        return(-1);
                if (key2->delete_tid && key1->create_tid >= key2->delete_tid)
                        return(1);
        }

        return(0);
}

/*
 * Create a separator half way inbetween key1 and key2.  For fields just
 * one unit apart, the separator will match key2.
 *
 * The handling of delete_tid is a little confusing.  It is only possible
 * to have one record in the B-Tree where all fields match except delete_tid.
 * This means, worse case, two adjacent elements may have a create_tid that
 * is one-apart and cause the separator to choose the right-hand element's
 * create_tid.  e.g.  (create,delete):  (1,x)(2,x) -> separator is (2,x).
 *
 * So all we have to do is set delete_tid to the right-hand element to
 * guarentee that the separator is properly between the two elements.
 */
#define MAKE_SEPARATOR(key1, key2, dest, field) \
        dest->field = key1->field + ((key2->field - key1->field + 1) >> 1);

static void
hammer_make_separator(hammer_base_elm_t key1, hammer_base_elm_t key2,
                      hammer_base_elm_t dest)
{
        bzero(dest, sizeof(*dest));
        MAKE_SEPARATOR(key1, key2, dest, obj_id);
        MAKE_SEPARATOR(key1, key2, dest, rec_type);
        MAKE_SEPARATOR(key1, key2, dest, key);
        MAKE_SEPARATOR(key1, key2, dest, create_tid);
        dest->delete_tid = key2->delete_tid;
}

#undef MAKE_SEPARATOR

/*
 * Return whether a generic internal or leaf node is full
 */
static int
btree_node_is_full(hammer_node_ondisk_t node)
{
        switch(node->type) {
        case HAMMER_BTREE_TYPE_INTERNAL:
                if (node->count == HAMMER_BTREE_INT_ELMS)
                        return(1);
                break;
        case HAMMER_BTREE_TYPE_LEAF:
                if (node->count == HAMMER_BTREE_LEAF_ELMS)
                        return(1);
                break;
        default:
                panic("illegal btree subtype");
        }
        return(0);
}

#if 0
static int
btree_max_elements(u_int8_t type)
{
        if (type == HAMMER_BTREE_TYPE_LEAF)
                return(HAMMER_BTREE_LEAF_ELMS);
        if (type == HAMMER_BTREE_TYPE_INTERNAL)
                return(HAMMER_BTREE_INT_ELMS);
        panic("btree_max_elements: bad type %d\n", type);
}
#endif