oid: optimize git_oid_fromstrn by using memset

[libgit2.git] / src / oid.c

/*
 * Copyright (C) 2009-2011 the libgit2 contributors
 *
 * This file is part of libgit2, distributed under the GNU GPL v2 with
 * a Linking Exception. For full terms see the included COPYING file.
 */

#include "common.h"
#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 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* 10 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* 20 */
 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, -1, -1, -1, -1, -1, -1, /* 30 */
-1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* 40 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* 50 */
-1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* 60 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* 70 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* 80 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* 90 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* a0 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* b0 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* c0 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* d0 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* e0 */
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, /* f0 */
};
static char to_hex[] = "0123456789abcdef";

int git_oid_fromstrn(git_oid *out, const char *str, size_t length)
{
        size_t p;
        int v;

        if (length > GIT_OID_HEXSZ)
                length = GIT_OID_HEXSZ;

        for (p = 0; p < length - 1; p += 2) {
                v = (from_hex[(unsigned char)str[p + 0]] << 4)
                                | from_hex[(unsigned char)str[p + 1]];

                if (v < 0)
                        return git__throw(GIT_ENOTOID, "Failed to generate sha1. Given string is not a valid sha1 hash");

                out->id[p / 2] = (unsigned char)v;
        }

        if (length % 2) {
                v = (from_hex[(unsigned char)str[p + 0]] << 4);
                out->id[p / 2] = (unsigned char)v;
                p += 2;
        }

        memset(out->id + p / 2, 0, (GIT_OID_HEXSZ - p) / 2);

        return GIT_SUCCESS;
}

int git_oid_fromstr(git_oid *out, const char *str)
{
        return git_oid_fromstrn(out, str, GIT_OID_HEXSZ);
}

GIT_INLINE(char) *fmt_one(char *str, unsigned int val)
{
        *str++ = to_hex[val >> 4];
        *str++ = to_hex[val & 0xf];
        return str;
}

void git_oid_fmt(char *str, const git_oid *oid)
{
        size_t i;

        for (i = 0; i < sizeof(oid->id); i++)
                str = fmt_one(str, oid->id[i]);
}

void git_oid_pathfmt(char *str, const git_oid *oid)
{
        size_t i;

        str = fmt_one(str, oid->id[0]);
        *str++ = '/';
        for (i = 1; i < sizeof(oid->id); i++)
                str = fmt_one(str, oid->id[i]);
}

char *git_oid_allocfmt(const git_oid *oid)
{
        char *str = git__malloc(GIT_OID_HEXSZ + 1);
        if (!str)
                return NULL;
        git_oid_fmt(str, oid);
        str[GIT_OID_HEXSZ] = '\0';
        return str;
}

char *git_oid_to_string(char *out, size_t n, const git_oid *oid)
{
        char str[GIT_OID_HEXSZ];

        if (!out || n == 0 || !oid)
                return "";

        n--; /* allow room for terminating NUL */

        if (n > 0) {
                git_oid_fmt(str, oid);
                if (n > GIT_OID_HEXSZ)
                        n = GIT_OID_HEXSZ;
                memcpy(out, str, n);
        }

        out[n] = '\0';

        return out;
}

int git_oid__parse(git_oid *oid, const char **buffer_out,
                const char *buffer_end, const char *header)
{
        const size_t sha_len = GIT_OID_HEXSZ;
        const size_t header_len = strlen(header);

        const char *buffer = *buffer_out;

        if (buffer + (header_len + sha_len + 1) > buffer_end)
                return git__throw(GIT_EOBJCORRUPTED, "Failed to parse OID. Buffer too small");

        if (memcmp(buffer, header, header_len) != 0)
                return git__throw(GIT_EOBJCORRUPTED, "Failed to parse OID. Buffer and header do not match");

        if (buffer[header_len + sha_len] != '\n')
                return git__throw(GIT_EOBJCORRUPTED, "Failed to parse OID. Buffer not terminated correctly");

        if (git_oid_fromstr(oid, buffer + header_len) < GIT_SUCCESS)
                return git__throw(GIT_EOBJCORRUPTED, "Failed to parse OID. Failed to generate sha1");

        *buffer_out = buffer + (header_len + sha_len + 1);

        return GIT_SUCCESS;
}

void git_oid__writebuf(git_buf *buf, const char *header, const git_oid *oid)
{
        char hex_oid[GIT_OID_HEXSZ];

        git_oid_fmt(hex_oid, oid);
        git_buf_puts(buf, header);
        git_buf_put(buf, hex_oid, GIT_OID_HEXSZ);
        git_buf_putc(buf, '\n');
}

void git_oid_fromraw(git_oid *out, const unsigned char *raw)
{
        memcpy(out->id, raw, sizeof(out->id));
}

void git_oid_cpy(git_oid *out, const git_oid *src)
{
        memcpy(out->id, src->id, sizeof(out->id));
}

int git_oid_cmp(const git_oid *a, const git_oid *b)
{
        return memcmp(a->id, b->id, sizeof(a->id));
}

int git_oid_ncmp(const git_oid *oid_a, const git_oid *oid_b, unsigned int len)
{
        const unsigned char *a = oid_a->id;
        const unsigned char *b = oid_b->id;

        do {
                if (*a != *b)
                        return 1;
                a++;
                b++;
                len -= 2;
        } while (len > 1);

        if (len)
                if ((*a ^ *b) & 0xf0)
                        return 1;

        return 0;
}

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;

        if (text_oid == NULL)
                return os->min_length;

        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__throw(GIT_ENOTOID, "Failed to shorten OID. Invalid hex value");

                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;
}