Add `git_oid_shorten` (unique OID minimzer)

Set of methods to find the minimal-length to uniquely identify every OID
in a list. Useful for GUI applications, commit logs and so on.

Includes stress test.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

1 files changed, 178 insertions, 0 deletions

diff --git a/src/oid.c b/src/oid.c
index 698d0f927..81b7d6005 100644
--- a/src/oid.c
+++ b/src/oid.c
@@ -27,6 +27,7 @@
 #include "git2/oid.h"
 #include "repository.h"
 #include <string.h>
+#include <limits.h>
 
 static signed char from_hex[] = {
 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* 00 */
@@ -166,3 +167,180 @@ int git_oid_cmp(const git_oid *a, const git_oid *b)
 {
 	return memcmp(a->id, b->id, sizeof(a->id));
 }
+
+
+typedef short node_index;
+
+typedef union {
+	const char *tail;
+	node_index children[16];
+} trie_node;
+
+struct git_oid_shorten {
+	trie_node *nodes;
+	size_t node_count, size;
+	int min_length, full;
+};
+
+static int resize_trie(git_oid_shorten *self, size_t new_size)
+{
+	self->nodes = realloc(self->nodes, new_size * sizeof(trie_node));
+	if (self->nodes == NULL)
+		return GIT_ENOMEM;
+
+	if (new_size > self->size) {
+		memset(&self->nodes[self->size], 0x0, (new_size - self->size) * sizeof(trie_node));
+	}
+
+	self->size = new_size;
+	return GIT_SUCCESS;
+}
+
+static trie_node *push_leaf(git_oid_shorten *os, node_index idx, int push_at, const char *oid)
+{
+	trie_node *node, *leaf;
+	node_index idx_leaf;
+
+	if (os->node_count >= os->size) {
+		if (resize_trie(os, os->size * 2) < GIT_SUCCESS)
+			return NULL;
+	}
+
+	idx_leaf = (node_index)os->node_count++;
+
+	if (os->node_count == SHRT_MAX)
+		os->full = 1;
+
+	node = &os->nodes[idx];
+	node->children[push_at] = -idx_leaf;
+
+	leaf = &os->nodes[idx_leaf];
+	leaf->tail = oid;
+
+	return node;
+}
+
+git_oid_shorten *git_oid_shorten_new(size_t min_length)
+{
+	git_oid_shorten *os;
+
+	os = git__malloc(sizeof(git_oid_shorten));
+	if (os == NULL)
+		return NULL;
+
+	memset(os, 0x0, sizeof(git_oid_shorten));
+
+	if (resize_trie(os, 16) < GIT_SUCCESS) {
+		free(os);
+		return NULL;
+	}
+
+	os->node_count = 1;
+	os->min_length = min_length;
+
+	return os;
+}
+
+void git_oid_shorten_free(git_oid_shorten *os)
+{
+	free(os->nodes);
+	free(os);
+}
+
+
+/*
+ * What wizardry is this?
+ *
+ * This is just a memory-optimized trie: basically a very fancy
+ * 16-ary tree, which is used to store the prefixes of the OID
+ * strings.
+ *
+ * Read more: http://en.wikipedia.org/wiki/Trie
+ *
+ * Magic that happens in this method:
+ *
+ *	- Each node in the trie is an union, so it can work both as
+ *	a normal node, or as a leaf.
+ *
+ *	- Each normal node points to 16 children (one for each possible
+ *	character in the oid). This is *not* stored in an array of
+ *	pointers, because in a 64-bit arch this would be sucking 
+ *	16*sizeof(void*) = 128 bytes of memory per node, which is fucking
+ *	insane. What we do is store Node Indexes, and use these indexes
+ *	to look up each node in the om->index array. These indexes are
+ *	signed shorts, so this limits the amount of unique OIDs that
+ *	fit in the structure to about 20000 (assuming a more or less uniform
+ *	distribution).
+ *
+ *	- All the nodes in om->index array are stored contiguously in
+ *	memory, and each of them is 32 bytes, so we fit 2x nodes per
+ *	cache line. Convenient for speed.
+ *
+ *	- To differentiate the leafs from the normal nodes, we store all
+ *	the indexes towards a leaf as a negative index (indexes to normal
+ *	nodes are positives). When we find that one of the children for
+ *	a node has a negative value, that means it's going to be a leaf.
+ *	This reduces the amount of indexes we have by two, but also reduces
+ *	the size of each node by 1-4 bytes (the amount we would need to
+ *	add a `is_leaf` field): this is good because it allows the nodes
+ *	to fit cleanly in cache lines.
+ *
+ *	- Once we reach an empty children, instead of continuing to insert
+ *	new nodes for each remaining character of the OID, we store a pointer
+ *	to the tail in the leaf; if the leaf is reached again, we turn it
+ *	into a normal node and use the tail to create a new leaf.
+ *
+ *	This is a pretty good balance between performance and memory usage.
+ */
+int git_oid_shorten_add(git_oid_shorten *os, const char *text_oid)
+{
+	int i, is_leaf;
+	node_index idx;
+
+	if (os->full)
+		return GIT_ENOMEM;
+
+	idx = 0;
+	is_leaf = 0;
+
+	for (i = 0; i < GIT_OID_HEXSZ; ++i) {
+		int c = from_hex[(int)text_oid[i]];
+		trie_node *node;
+
+		if (c == -1)
+			return GIT_ENOTOID;
+
+		node = &os->nodes[idx];
+
+		if (is_leaf) {
+			const char *tail;
+
+			tail = node->tail;
+			node->tail = NULL;
+
+			node = push_leaf(os, idx, from_hex[(int)tail[0]], &tail[1]);
+			if (node == NULL)
+				return GIT_ENOMEM;
+		}
+
+		if (node->children[c] == 0) {
+			if (push_leaf(os, idx, c, &text_oid[i + 1]) == NULL)
+				return GIT_ENOMEM;
+			break;
+		}
+
+		idx = node->children[c];
+		is_leaf = 0;
+
+		if (idx < 0) {
+			node->children[c] = idx = -idx;
+			is_leaf = 1;
+		}
+	}
+
+	if (++i > os->min_length)
+		os->min_length = i;
+
+	return os->min_length;
+}
+