git/diff-delta.c

/*
 * diff-delta.c: generate a delta between two buffers
 *
 *  Many parts of this file have been lifted from LibXDiff version 0.10.
 *  http://www.xmailserver.org/xdiff-lib.html
 *
 *  LibXDiff was written by Davide Libenzi <davidel@xmailserver.org>
 *  Copyright (C) 2003	Davide Libenzi
 *
 *  Many mods for GIT usage by Nicolas Pitre <nico@cam.org>, (C) 2005.
 *
 *  This file is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Lesser General Public
 *  License as published by the Free Software Foundation; either
 *  version 2.1 of the License, or (at your option) any later version.
 *
 *  Use of this within git automatically means that the LGPL
 *  licensing gets turned into GPLv2 within this project.
 */

#include <stdlib.h>
#include <string.h>
#include "delta.h"


struct index {
	const unsigned char *ptr;
	struct index *next;
};

static struct index ** delta_index(const unsigned char *buf,
				   unsigned long bufsize,
				   unsigned long trg_bufsize)
{
	unsigned long hsize;
	unsigned int i, hshift, hlimit, *hash_count;
	const unsigned char *data;
	struct index *entry, **hash;
	void *mem;

	/* determine index hash size */
	hsize = bufsize / 4;
	for (i = 8; (1 << i) < hsize && i < 24; i += 2);
	hsize = 1 << i;
	hshift = (i - 8) / 2;

	/*
	 * Allocate lookup index.  Note the first hash pointer
	 * is used to store the hash shift value.
	 */
	mem = malloc((1 + hsize) * sizeof(*hash) + bufsize * sizeof(*entry));
	if (!mem)
		return NULL;
	hash = mem;
	*hash++ = (void *)hshift;
	entry = mem + (1 + hsize) * sizeof(*hash);
	memset(hash, 0, hsize * sizeof(*hash));

	/* allocate an array to count hash entries */
	hash_count = calloc(hsize, sizeof(*hash_count));
	if (!hash_count) {
		free(hash);
		return NULL;
	}

	/* then populate the index */
	data = buf + bufsize - 2;
	while (data > buf) {
		entry->ptr = --data;
		i = data[0] ^ ((data[1] ^ (data[2] << hshift)) << hshift);
		entry->next = hash[i];
		hash[i] = entry++;
		hash_count[i]++;
 	}

	/*
	 * Determine a limit on the number of entries in the same hash
	 * bucket.  This guard us against patological data sets causing
	 * really bad hash distribution with most entries in the same hash
	 * bucket that would bring us to O(m*n) computing costs (m and n
	 * corresponding to reference and target buffer sizes).
	 *
	 * The more the target buffer is large, the more it is important to
	 * have small entry lists for each hash buckets.  With such a limit
	 * the cost is bounded to something more like O(m+n).
	 */
	hlimit = (1 << 26) / trg_bufsize;
	if (hlimit < 16)
		hlimit = 16;

	/*
	 * Now make sure none of the hash buckets has more entries than
	 * we're willing to test.  Otherwise we cull the entry list to
	 * limit identical three byte prefixes to still preserve a good
	 * repartition across the reference buffer.
	 */
	for (i = 0; i < hsize; i++) {
		struct index **list, *bucket, *remaining;
		int cnt;
		if (hash_count[i] < hlimit)
			continue;

		bucket = NULL;
		list = &bucket;
		remaining = hash[i];
		cnt = 0;
		while (cnt < hlimit && remaining) {
			struct index *this = remaining, *that;
			remaining = remaining->next;
			for (that = bucket; that; that = that->next) {
				if (!memcmp(that->ptr, this->ptr, 3))
					break;
			}
			if (that)
				continue; /* discard */
			cnt++;
			*list = this;
			list = &(this->next);
			this->next = NULL;
		}
		hash[i] = bucket;
	}
	free(hash_count);

	return hash-1;
}

/* provide the size of the copy opcode given the block offset and size */
#define COPYOP_SIZE(o, s) \
    (!!(o & 0xff) + !!(o & 0xff00) + !!(o & 0xff0000) + !!(o & 0xff000000) + \
     !!(s & 0xff) + !!(s & 0xff00) + 1)

/* the maximum size for any opcode */
#define MAX_OP_SIZE COPYOP_SIZE(0xffffffff, 0xffffffff)

void *diff_delta(void *from_buf, unsigned long from_size,
		 void *to_buf, unsigned long to_size,
		 unsigned long *delta_size,
		 unsigned long max_size,
		 void **from_index)
{
	unsigned int i, outpos, outsize, inscnt, hash_shift;
	const unsigned char *ref_data, *ref_top, *data, *top;
	unsigned char *out;
	struct index *entry, **hash;

	if (!from_size || !to_size)
		return NULL;
	if (from_index && *from_index) {
		hash = *from_index;
	} else {
		hash = delta_index(from_buf, from_size, to_size);
		if (!hash)
			return NULL;
		if (from_index)
			*from_index = hash;
	}
	hash_shift = (unsigned int)(*hash++);

	outpos = 0;
	outsize = 8192;
	if (max_size && outsize >= max_size)
		outsize = max_size + MAX_OP_SIZE + 1;
	out = malloc(outsize);
	if (!out) {
		if (!from_index)
			free(hash-1);
		return NULL;
	}

	ref_data = from_buf;
	ref_top = from_buf + from_size;
	data = to_buf;
	top = to_buf + to_size;

	/* store reference buffer size */
	out[outpos++] = from_size;
	from_size >>= 7;
	while (from_size) {
		out[outpos - 1] |= 0x80;
		out[outpos++] = from_size;
		from_size >>= 7;
	}

	/* store target buffer size */
	out[outpos++] = to_size;
	to_size >>= 7;
	while (to_size) {
		out[outpos - 1] |= 0x80;
		out[outpos++] = to_size;
		to_size >>= 7;
	}

	inscnt = 0;

	while (data < top) {
		unsigned int moff = 0, msize = 0;
		if (data + 3 <= top) {
			i = data[0] ^ ((data[1] ^ (data[2] << hash_shift)) << hash_shift);
			for (entry = hash[i]; entry; entry = entry->next) {
				const unsigned char *ref = entry->ptr;
				const unsigned char *src = data;
				unsigned int ref_size = ref_top - ref;
				if (ref_size > top - src)
					ref_size = top - src;
				if (ref_size > 0x10000)
					ref_size = 0x10000;
				if (ref_size <= msize)
					break;
				if (*ref != *src)
					continue;
				while (ref_size-- && *++src == *++ref);
				if (msize < ref - entry->ptr) {
					/* this is our best match so far */
					msize = ref - entry->ptr;
					moff = entry->ptr - ref_data;
				}
			}
		}

		if (!msize || msize < COPYOP_SIZE(moff, msize)) {
			if (!inscnt)
				outpos++;
			out[outpos++] = *data++;
			inscnt++;
			if (inscnt == 0x7f) {
				out[outpos - inscnt - 1] = inscnt;
				inscnt = 0;
			}
		} else {
			unsigned char *op;

			if (inscnt) {
				out[outpos - inscnt - 1] = inscnt;
				inscnt = 0;
			}

			data += msize;
			op = out + outpos++;
			i = 0x80;

			if (moff & 0xff) { out[outpos++] = moff; i |= 0x01; }
			moff >>= 8;
			if (moff & 0xff) { out[outpos++] = moff; i |= 0x02; }
			moff >>= 8;
			if (moff & 0xff) { out[outpos++] = moff; i |= 0x04; }
			moff >>= 8;
			if (moff & 0xff) { out[outpos++] = moff; i |= 0x08; }

			if (msize & 0xff) { out[outpos++] = msize; i |= 0x10; }
			msize >>= 8;
			if (msize & 0xff) { out[outpos++] = msize; i |= 0x20; }

			*op = i;
		}

		if (outpos >= outsize - MAX_OP_SIZE) {
			void *tmp = out;
			outsize = outsize * 3 / 2;
			if (max_size && outsize >= max_size)
				outsize = max_size + MAX_OP_SIZE + 1;
			if (max_size && outpos > max_size)
				out = NULL;
			else
				out = realloc(out, outsize);
			if (!out) {
				free(tmp);
				if (!from_index)
					free(hash-1);
				return NULL;
			}
		}
	}

	if (inscnt)
		out[outpos - inscnt - 1] = inscnt;

	if (!from_index)
		free(hash-1);
	*delta_size = outpos;
	return out;
}