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 <zlib.h>
#include "delta.h"


/* block size: min = 16, max = 64k, power of 2 */
#define BLK_SIZE 16

#define MIN(a, b) ((a) < (b) ? (a) : (b))

#define GR_PRIME 0x9e370001
#define HASH(v, shift) (((unsigned int)(v) * GR_PRIME) >> (shift))

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

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

	/* determine index hash size */
	entries = (bufsize + BLK_SIZE - 1) / BLK_SIZE;
	hsize = entries / 4;
	for (i = 4; (1 << i) < hsize && i < 16; i++);
	hsize = 1 << i;
	hshift = 32 - i;
	*hash_shift = hshift;

	/* allocate lookup index */
	mem = malloc(hsize * sizeof(*hash) + entries * sizeof(*entry));
	if (!mem)
		return NULL;
	hash = mem;
	entry = mem + hsize * sizeof(*hash);
	memset(hash, 0, hsize * sizeof(*hash));

	/* then populate it */
	data = buf + entries * BLK_SIZE - BLK_SIZE;
	blksize = bufsize - (data - buf);
	while (data >= buf) {
		unsigned int val = adler32(0, data, blksize);
		i = HASH(val, hshift);
		entry->ptr = data;
		entry->val = val;
		entry->next = hash[i];
		hash[i] = entry++;
		blksize = BLK_SIZE;
		data -= BLK_SIZE;
 	}

	return hash;
}

/* 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)
{
	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;
	hash = delta_index(from_buf, from_size, &hash_shift);
	if (!hash)
		return NULL;

	outpos = 0;
	outsize = 8192;
	if (max_size && outsize >= max_size)
		outsize = max_size + MAX_OP_SIZE + 1;
	out = malloc(outsize);
	if (!out) {
		free(hash);
		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;
		unsigned int blksize = MIN(top - data, BLK_SIZE);
		unsigned int val = adler32(0, data, blksize);
		i = HASH(val, 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 (entry->val != val)
				continue;
			if (ref_size > top - src)
				ref_size = top - src;
			while (ref_size && *src++ == *ref) {
				ref++;
				ref_size--;
			}
			ref_size = ref - entry->ptr;
			if (ref_size > msize) {
				/* this is our best match so far */
				moff = entry->ptr - ref_data;
				msize = ref_size;
				if (msize >= 0x10000) {
					msize = 0x10000;
					break;
				}
			}
		}

		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);
				free(hash);
				return NULL;
			}
		}
	}

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

	free(hash);
	*delta_size = outpos;
	return out;
}