comparison ext/sord/src/zix/btree.c @ 226:c5cdc9e6a4bf

Add these external library files

+/*
+  Copyright 2011-2014 David Robillard <http://drobilla.net>
+
+  Permission to use, copy, modify, and/or distribute this software for any
+  purpose with or without fee is hereby granted, provided that the above
+  copyright notice and this permission notice appear in all copies.
+
+  THIS SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+
+#include <assert.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "zix/btree.h"
+
+// #define ZIX_BTREE_DEBUG 1
+
+#define ZIX_BTREE_PAGE_SIZE  4096
+#define ZIX_BTREE_NODE_SPACE (ZIX_BTREE_PAGE_SIZE - 2 * sizeof(uint16_t))
+#define ZIX_BTREE_LEAF_VALS  ((ZIX_BTREE_NODE_SPACE / sizeof(void*)) - 1)
+#define ZIX_BTREE_INODE_VALS (ZIX_BTREE_LEAF_VALS / 2)
+
+struct ZixBTreeImpl {
+	ZixBTreeNode*  root;
+	ZixDestroyFunc destroy;
+	ZixComparator  cmp;
+	void*          cmp_data;
+	size_t         size;
+	unsigned       height;  ///< Number of levels, i.e. root only has height 1
+};
+
+struct ZixBTreeNodeImpl {
+	uint16_t      is_leaf;
+	uint16_t      n_vals;
+	// On 64-bit we rely on some padding here to get page-sized nodes
+	void*         vals[ZIX_BTREE_INODE_VALS];  // ZIX_BTREE_LEAF_VALS for leaves
+	ZixBTreeNode* children[ZIX_BTREE_INODE_VALS + 1];  // Nonexistent for leaves
+};
+
+typedef struct {
+	ZixBTreeNode* node;
+	unsigned      index;
+} ZixBTreeIterFrame;
+
+struct ZixBTreeIterImpl {
+	unsigned          level;    ///< Current level in stack
+	ZixBTreeIterFrame stack[];  ///< Position stack
+};
+
+#ifdef ZIX_BTREE_DEBUG
+
+ZIX_PRIVATE void
+print_node(const ZixBTreeNode* n, const char* prefix)
+{
+	printf("%s[", prefix);
+	for (uint16_t v = 0; v < n->n_vals; ++v) {
+		printf(" %lu", (uintptr_t)n->vals[v]);
+	}
+	printf(" ]\n");
+}
+
+ZIX_PRIVATE void
+print_tree(const ZixBTreeNode* parent, const ZixBTreeNode* node, int level)
+{
+	if (node) {
+		if (!parent) {
+			printf("TREE {\n");
+		}
+		for (int i = 0; i < level + 1; ++i) {
+			printf("  ");
+		}
+		print_node(node, "");
+		if (!node->is_leaf) {
+			for (uint16_t i = 0; i < node->n_vals + 1; ++i) {
+				print_tree(node, node->children[i], level + 1);
+			}
+		}
+		if (!parent) {
+			printf("}\n");
+		}
+	}
+}
+
+#endif  // ZIX_BTREE_DEBUG
+
+ZIX_PRIVATE ZixBTreeNode*
+zix_btree_node_new(const bool leaf)
+{
+	assert(sizeof(ZixBTreeNode) == ZIX_BTREE_PAGE_SIZE);
+	ZixBTreeNode* node = (ZixBTreeNode*)malloc(sizeof(ZixBTreeNode));
+	if (node) {
+		node->is_leaf = leaf;
+		node->n_vals  = 0;
+	}
+	return node;
+}
+
+ZIX_API ZixBTree*
+zix_btree_new(const ZixComparator  cmp,
+              void* const          cmp_data,
+              const ZixDestroyFunc destroy)
+{
+	ZixBTree* t = (ZixBTree*)malloc(sizeof(ZixBTree));
+	if (t) {
+		t->root     = zix_btree_node_new(true);
+		t->destroy  = destroy;
+		t->cmp      = cmp;
+		t->cmp_data = cmp_data;
+		t->size     = 0;
+		t->height   = 1;
+		if (!t->root) {
+			free(t);
+			return NULL;
+		}
+	}
+	return t;
+}
+
+ZIX_PRIVATE void
+zix_btree_free_rec(ZixBTree* const t, ZixBTreeNode* const n)
+{
+	if (n) {
+		if (t->destroy) {
+			for (uint16_t i = 0; i < n->n_vals; ++i) {
+				t->destroy(n->vals[i]);
+			}
+		}
+		if (!n->is_leaf) {
+			for (uint16_t i = 0; i < n->n_vals + 1; ++i) {
+				zix_btree_free_rec(t, n->children[i]);
+			}
+		}
+		free(n);
+	}
+}
+
+ZIX_API void
+zix_btree_free(ZixBTree* const t)
+{
+	if (t) {
+		zix_btree_free_rec(t, t->root);
+		free(t);
+	}
+}
+
+ZIX_API size_t
+zix_btree_size(const ZixBTree* const t)
+{
+	return t->size;
+}
+
+ZIX_PRIVATE uint16_t
+zix_btree_max_vals(const ZixBTreeNode* const node)
+{
+	return node->is_leaf ? ZIX_BTREE_LEAF_VALS : ZIX_BTREE_INODE_VALS;
+}
+
+ZIX_PRIVATE uint16_t
+zix_btree_min_vals(const ZixBTreeNode* const node)
+{
+	return ((zix_btree_max_vals(node) + 1) / 2) - 1;
+}
+
+/** Shift pointers in `array` of length `n` right starting at `i`. */
+ZIX_PRIVATE void
+zix_btree_ainsert(void** const   array,
+                  const uint16_t n,
+                  const uint16_t i,
+                  void* const    e)
+{
+	memmove(array + i + 1, array + i, (n - i) * sizeof(e));
+	array[i] = e;
+}
+
+/** Erase element `i` in `array` of length `n` and return erased element. */
+ZIX_PRIVATE void*
+zix_btree_aerase(void** const array, const uint16_t n, const uint16_t i)
+{
+	void* const ret = array[i];
+	memmove(array + i, array + i + 1, (n - i) * sizeof(ret));
+	return ret;
+}
+
+/** Split lhs, the i'th child of `n`, into two nodes. */
+ZIX_PRIVATE ZixBTreeNode*
+zix_btree_split_child(ZixBTreeNode* const n,
+                      const uint16_t      i,
+                      ZixBTreeNode* const lhs)
+{
+	assert(lhs->n_vals == zix_btree_max_vals(lhs));
+	assert(n->n_vals < ZIX_BTREE_INODE_VALS);
+	assert(i < n->n_vals + 1);
+	assert(n->children[i] == lhs);
+
+	const uint16_t max_n_vals = zix_btree_max_vals(lhs);
+	ZixBTreeNode*  rhs        = zix_btree_node_new(lhs->is_leaf);
+	if (!rhs) {
+		return NULL;
+	}
+
+	// LHS and RHS get roughly half, less the middle value which moves up
+	lhs->n_vals = max_n_vals / 2;
+	rhs->n_vals = max_n_vals - lhs->n_vals - 1;
+
+	// Copy large half of values from LHS to new RHS node
+	memcpy(rhs->vals,
+	       lhs->vals + lhs->n_vals + 1,
+	       rhs->n_vals * sizeof(void*));
+
+	// Copy large half of children from LHS to new RHS node
+	if (!lhs->is_leaf) {
+		memcpy(rhs->children,
+		       lhs->children + lhs->n_vals + 1,
+		       (rhs->n_vals + 1) * sizeof(ZixBTreeNode*));
+	}
+
+	// Move middle value up to parent
+	zix_btree_ainsert(n->vals, n->n_vals, i, lhs->vals[lhs->n_vals]);
+
+	// Insert new RHS node in parent at position i
+	zix_btree_ainsert((void**)n->children, ++n->n_vals, i + 1, rhs);
+
+	return rhs;
+}
+
+/** Find the first value in `n` that is not less than `e` (lower bound). */
+ZIX_PRIVATE uint16_t
+zix_btree_node_find(const ZixBTree* const     t,
+                    const ZixBTreeNode* const n,
+                    const void* const         e,
+                    bool* const               equal)
+{
+	uint16_t first = 0;
+	uint16_t len   = n->n_vals;
+	while (len > 0) {
+		const uint16_t half = len >> 1;
+		const uint16_t i    = first + half;
+		const int      cmp  = t->cmp(n->vals[i], e, t->cmp_data);
+		if (cmp == 0) {
+			*equal = true;
+			len    = half;  // Keep searching for wildcard matches
+		} else if (cmp < 0) {
+			const uint16_t chop = half + 1;
+			first += chop;
+			len   -= chop;
+		} else {
+			len = half;
+		}
+	}
+	assert(!*equal || t->cmp(n->vals[first], e, t->cmp_data) == 0);
+	return first;
+}
+
+ZIX_API ZixStatus
+zix_btree_insert(ZixBTree* const t, void* const e)
+{
+	ZixBTreeNode* parent = NULL;     // Parent of n
+	ZixBTreeNode* n      = t->root;  // Current node
+	uint16_t      i      = 0;        // Index of n in parent
+	while (n) {
+		if (n->n_vals == zix_btree_max_vals(n)) {
+			// Node is full, split to ensure there is space for a leaf split
+			if (!parent) {
+				// Root is full, grow tree upwards
+				if (!(parent = zix_btree_node_new(false))) {
+					return ZIX_STATUS_NO_MEM;
+				}
+				t->root             = parent;
+				parent->children[0] = n;
+				++t->height;
+			}
+
+			ZixBTreeNode* const rhs = zix_btree_split_child(parent, i, n);
+			if (!rhs) {
+				return ZIX_STATUS_NO_MEM;
+			}
+
+			const int cmp = t->cmp(parent->vals[i], e, t->cmp_data);
+			if (cmp == 0) {
+				return ZIX_STATUS_EXISTS;
+			} else if (cmp < 0) {
+				// Move to new RHS
+				n = rhs;
+				++i;
+			}
+		}
+
+		assert(!parent || parent->children[i] == n);
+
+		bool equal = false;
+		i          = zix_btree_node_find(t, n, e, &equal);
+		if (equal) {
+			return ZIX_STATUS_EXISTS;
+		} else if (!n->is_leaf) {
+			// Descend to child node left of value
+			parent = n;
+			n      = n->children[i];
+		} else {
+			// Insert into internal node
+			zix_btree_ainsert(n->vals, n->n_vals++, i, e);
+			break;
+		}
+	}
+
+	++t->size;
+
+	return ZIX_STATUS_SUCCESS;
+}
+
+ZIX_PRIVATE ZixBTreeIter*
+zix_btree_iter_new(const ZixBTree* const t)
+{
+	const size_t        s = t->height * sizeof(ZixBTreeIterFrame);
+	ZixBTreeIter* const i = (ZixBTreeIter*)malloc(sizeof(ZixBTreeIter) + s);
+	if (i) {
+		i->level = 0;
+	}
+	return i;
+}
+
+ZIX_PRIVATE void
+zix_btree_iter_set_frame(ZixBTreeIter* const ti,
+                         ZixBTreeNode* const n,
+                         const uint16_t      i)
+{
+	if (ti) {
+		ti->stack[ti->level].node  = n;
+		ti->stack[ti->level].index = i;
+	}
+}
+
+ZIX_PRIVATE bool
+zix_btree_node_is_minimal(ZixBTreeNode* const n)
+{
+	assert(n->n_vals >= zix_btree_min_vals(n));
+	return n->n_vals == zix_btree_min_vals(n);
+}
+
+/** Enlarge left child by stealing a value from its right sibling. */
+ZIX_PRIVATE ZixBTreeNode*
+zix_btree_rotate_left(ZixBTreeNode* const parent, const uint16_t i)
+{
+	ZixBTreeNode* const lhs = parent->children[i];
+	ZixBTreeNode* const rhs = parent->children[i + 1];
+
+	// Move parent value to end of LHS
+	lhs->vals[lhs->n_vals++] = parent->vals[i];
+
+	// Move first child pointer from RHS to end of LHS
+	if (!lhs->is_leaf) {
+		lhs->children[lhs->n_vals] = (ZixBTreeNode*)zix_btree_aerase(
+			(void**)rhs->children, rhs->n_vals, 0);
+	}
+
+	// Move first value in RHS to parent
+	parent->vals[i] = zix_btree_aerase(rhs->vals, --rhs->n_vals, 0);
+
+	return lhs;
+}
+
+/** Enlarge right child by stealing a value from its left sibling. */
+ZIX_PRIVATE ZixBTreeNode*
+zix_btree_rotate_right(ZixBTreeNode* const parent, const uint16_t i)
+{
+	ZixBTreeNode* const lhs = parent->children[i - 1];
+	ZixBTreeNode* const rhs = parent->children[i];
+
+	// Prepend parent value to RHS
+	zix_btree_ainsert(rhs->vals, rhs->n_vals++, 0, parent->vals[i - 1]);
+
+	// Move last child pointer from LHS and prepend to RHS
+	if (!lhs->is_leaf) {
+		zix_btree_ainsert((void**)rhs->children,
+		                  rhs->n_vals,
+		                  0,
+		                  lhs->children[lhs->n_vals]);
+	}
+
+	// Move last value from LHS to parent
+	parent->vals[i - 1] = lhs->vals[--lhs->n_vals];
+
+	return rhs;
+}
+
+/** Move n[i] down, merge the left and right child, return the merged node. */
+ZIX_PRIVATE ZixBTreeNode*
+zix_btree_merge(ZixBTree* const t, ZixBTreeNode* const n, const uint16_t i)
+{
+	ZixBTreeNode* const lhs = n->children[i];
+	ZixBTreeNode* const rhs = n->children[i + 1];
+
+	assert(zix_btree_node_is_minimal(n->children[i]));
+	assert(lhs->n_vals + rhs->n_vals < zix_btree_max_vals(lhs));
+
+	// Move parent value to end of LHS
+	lhs->vals[lhs->n_vals++] = zix_btree_aerase(n->vals, n->n_vals, i);
+
+	// Erase corresponding child pointer (to RHS) in parent
+	zix_btree_aerase((void**)n->children, n->n_vals, i + 1);
+
+	// Add everything from RHS to end of LHS
+	memcpy(lhs->vals + lhs->n_vals, rhs->vals, rhs->n_vals * sizeof(void*));
+	if (!lhs->is_leaf) {
+		memcpy(lhs->children + lhs->n_vals,
+		       rhs->children,
+		       (rhs->n_vals + 1) * sizeof(void*));
+	}
+	lhs->n_vals += rhs->n_vals;
+
+	if (--n->n_vals == 0) {
+		// Root is now empty, replace it with its only child
+		assert(n == t->root);
+		t->root = lhs;
+		free(n);
+	}
+
+	free(rhs);
+	return lhs;
+}
+
+/** Remove and return the min value from the subtree rooted at `n`. */
+ZIX_PRIVATE void*
+zix_btree_remove_min(ZixBTree* const t, ZixBTreeNode* n)
+{
+	while (!n->is_leaf) {
+		if (zix_btree_node_is_minimal(n->children[0])) {
+			// Leftmost child is minimal, must expand
+			if (!zix_btree_node_is_minimal(n->children[1])) {
+				// Child's right sibling has at least one key to steal
+				n = zix_btree_rotate_left(n, 0);
+			} else {
+				// Both child and right sibling are minimal, merge
+				n = zix_btree_merge(t, n, 0);
+			}
+		} else {
+			n = n->children[0];
+		}
+	}
+
+	return zix_btree_aerase(n->vals, --n->n_vals, 0);
+}
+
+/** Remove and return the max value from the subtree rooted at `n`. */
+ZIX_PRIVATE void*
+zix_btree_remove_max(ZixBTree* const t, ZixBTreeNode* n)
+{
+	while (!n->is_leaf) {
+		if (zix_btree_node_is_minimal(n->children[n->n_vals])) {
+			// Leftmost child is minimal, must expand
+			if (!zix_btree_node_is_minimal(n->children[n->n_vals - 1])) {
+				// Child's left sibling has at least one key to steal
+				n = zix_btree_rotate_right(n, n->n_vals);
+			} else {
+				// Both child and left sibling are minimal, merge
+				n = zix_btree_merge(t, n, n->n_vals - 1);
+			}
+		} else {
+			n = n->children[n->n_vals];
+		}
+	}
+
+	return n->vals[--n->n_vals];
+}
+
+ZIX_API ZixStatus
+zix_btree_remove(ZixBTree* const      t,
+                 const void* const    e,
+                 void** const         out,
+                 ZixBTreeIter** const next)
+{
+	ZixBTreeNode* n         = t->root;
+	ZixBTreeIter* ti        = NULL;
+	const bool    user_iter = next && *next;
+	if (next) {
+		if (!*next && !(*next = zix_btree_iter_new(t))) {
+			return ZIX_STATUS_NO_MEM;
+		}
+		ti        = *next;
+		ti->level = 0;
+	}
+
+	while (true) {
+		/* To remove in a single walk down, the tree is adjusted along the way
+		   so that the current node always has at least one more value than the
+		   minimum required in general. Thus, there is always room to remove
+		   without adjusting on the way back up. */
+		assert(n == t->root || !zix_btree_node_is_minimal(n));
+
+		bool           equal = false;
+		const uint16_t i     = zix_btree_node_find(t, n, e, &equal);
+		zix_btree_iter_set_frame(ti, n, i);
+		if (n->is_leaf) {
+			if (equal) {
+				// Found in leaf node
+				*out = zix_btree_aerase(n->vals, --n->n_vals, i);
+				if (ti && i == n->n_vals) {
+					if (i == 0) {
+						ti->stack[ti->level = 0].node = NULL;
+					} else {
+						--ti->stack[ti->level].index;
+						zix_btree_iter_increment(ti);
+					}
+				}
+				--t->size;
+				return ZIX_STATUS_SUCCESS;
+			} else {
+				// Not found in leaf node, or tree
+				if (ti && !user_iter) {
+					zix_btree_iter_free(ti);
+					*next = NULL;
+				}
+				return ZIX_STATUS_NOT_FOUND;
+			}
+		} else if (equal) {
+			// Found in internal node
+			if (!zix_btree_node_is_minimal(n->children[i])) {
+				// Left child can remove without merge
+				*out = n->vals[i];
+				n->vals[i] = zix_btree_remove_max(t, n->children[i]);
+				--t->size;
+				return ZIX_STATUS_SUCCESS;
+			} else if (!zix_btree_node_is_minimal(n->children[i + 1])) {
+				// Right child can remove without merge
+				*out = n->vals[i];
+				n->vals[i] = zix_btree_remove_min(t, n->children[i + 1]);
+				--t->size;
+				return ZIX_STATUS_SUCCESS;
+			} else {
+				// Both preceding and succeeding child are minimal
+				n = zix_btree_merge(t, n, i);
+			}
+		} else {
+			// Not found in internal node, key is in/under children[i]
+			if (zix_btree_node_is_minimal(n->children[i])) {
+				if (i > 0 && !zix_btree_node_is_minimal(n->children[i - 1])) {
+					// Steal a key from child's left sibling
+					n = zix_btree_rotate_right(n, i);
+				} else if (i < n->n_vals &&
+				           !zix_btree_node_is_minimal(n->children[i + 1])) {
+					// Steal a key from child's right sibling
+					n = zix_btree_rotate_left(n, i);
+				} else {
+					// Both child's siblings are minimal, merge them
+					if (i < n->n_vals) {
+						n = zix_btree_merge(t, n, i);
+					} else {
+						n = zix_btree_merge(t, n, i - 1);
+						if (ti) {
+							--ti->stack[ti->level].index;
+						}
+					}
+				}
+			} else {
+				n = n->children[i];
+			}
+		}
+		if (ti) {
+			++ti->level;
+		}
+	}
+
+	assert(false);  // Not reached
+	return ZIX_STATUS_ERROR;
+}
+
+ZIX_API ZixStatus
+zix_btree_find(const ZixBTree* const t,
+               const void* const     e,
+               ZixBTreeIter** const  ti)
+{
+	ZixBTreeNode* n = t->root;
+	if (!(*ti = zix_btree_iter_new(t))) {
+		return ZIX_STATUS_NO_MEM;
+	}
+
+	while (n) {
+		bool           equal = false;
+		const uint16_t i     = zix_btree_node_find(t, n, e, &equal);
+
+		zix_btree_iter_set_frame(*ti, n, i);
+
+		if (equal) {
+			return ZIX_STATUS_SUCCESS;
+		} else if (n->is_leaf) {
+			break;
+		} else {
+			++(*ti)->level;
+			n = n->children[i];
+		}
+	}
+
+	zix_btree_iter_free(*ti);
+	*ti = NULL;
+	return ZIX_STATUS_NOT_FOUND;
+}
+
+ZIX_API ZixStatus
+zix_btree_lower_bound(const ZixBTree* const t,
+                      const void* const     e,
+                      ZixBTreeIter** const  ti)
+{
+	if (!t) {
+		*ti = NULL;
+		return ZIX_STATUS_BAD_ARG;
+	}
+
+	ZixBTreeNode* n           = t->root;
+	bool          found       = false;
+	unsigned      found_level = 0;
+	if (!(*ti = zix_btree_iter_new(t))) {
+		return ZIX_STATUS_NO_MEM;
+	}
+
+	while (n) {
+		bool           equal = false;
+		const uint16_t i     = zix_btree_node_find(t, n, e, &equal);
+
+		zix_btree_iter_set_frame(*ti, n, i);
+
+		if (equal) {
+			found_level = (*ti)->level;
+			found       = true;
+		}
+
+		if (n->is_leaf) {
+			break;
+		} else {
+			++(*ti)->level;
+			n = n->children[i];
+			assert(n);
+		}
+	}
+
+	const ZixBTreeIterFrame* const frame = &(*ti)->stack[(*ti)->level];
+	if (frame->index == frame->node->n_vals) {
+		if (found) {
+			// Found on a previous level but went too far
+			(*ti)->level = found_level;
+		} else {
+			// Reached end (key is greater than everything in tree)
+			(*ti)->stack[0].node = NULL;
+		}
+	}
+
+	return ZIX_STATUS_SUCCESS;
+}
+
+ZIX_API void*
+zix_btree_get(const ZixBTreeIter* const ti)
+{
+	const ZixBTreeIterFrame* const frame = &ti->stack[ti->level];
+	assert(frame->index < frame->node->n_vals);
+	return frame->node->vals[frame->index];
+}
+
+ZIX_API ZixBTreeIter*
+zix_btree_begin(const ZixBTree* const t)
+{
+	ZixBTreeIter* const i = zix_btree_iter_new(t);
+	if (!i) {
+		return NULL;
+	} else if (t->size == 0) {
+		i->stack[0].node = NULL;
+	} else {
+		ZixBTreeNode* n = t->root;
+		i->stack[0].node  = n;
+		i->stack[0].index = 0;
+		while (!n->is_leaf) {
+			n = n->children[0];
+			++i->level;
+			i->stack[i->level].node  = n;
+			i->stack[i->level].index = 0;
+		}
+	}
+	return i;
+}
+
+ZIX_API bool
+zix_btree_iter_is_end(const ZixBTreeIter* const i)
+{
+	return !i || i->stack[0].node == NULL;
+}
+
+ZIX_API void
+zix_btree_iter_increment(ZixBTreeIter* const i)
+{
+	ZixBTreeIterFrame* f = &i->stack[i->level];
+	if (f->node->is_leaf) {
+		// Leaf, move right
+		assert(f->index < f->node->n_vals);
+		if (++f->index == f->node->n_vals) {
+			// Reached end of leaf, move up
+			f = &i->stack[i->level];
+			while (i->level > 0 && f->index == f->node->n_vals) {
+				f = &i->stack[--i->level];
+				assert(f->index <= f->node->n_vals);
+			}
+
+			if (f->index == f->node->n_vals) {
+				// Reached end of tree
+				assert(i->level == 0);
+				f->node  = NULL;
+				f->index = 0;
+			}
+		}
+	} else {
+		// Internal node, move down to next child
+		assert(f->index < f->node->n_vals);
+		ZixBTreeNode* child = f->node->children[++f->index];
+
+		f        = &i->stack[++i->level];
+		f->node  = child;
+		f->index = 0;
+
+		// Move down and left until we hit a leaf
+		while (!f->node->is_leaf) {
+			child    = f->node->children[0];
+			f        = &i->stack[++i->level];
+			f->node  = child;
+			f->index = 0;
+		}
+	}
+}
+
+ZIX_API void
+zix_btree_iter_free(ZixBTreeIter* const i)
+{
+	free(i);
+}