/*
 * 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 int *hash_shift)
{
        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;
        *hash_shift = hshift;

        /* allocate lookup index */
        mem = malloc(hsize * sizeof(*hash) + bufsize * sizeof(*entry));
        if (!mem)
                return NULL;
        hash = mem;
        entry = mem + 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 short-circuit the entry
         * list uniformly to still preserve a good repartition across
         * the reference buffer.
         */
        for (i = 0; i < hsize; i++) {
                if (hash_count[i] < hlimit)
                        continue;
                entry = hash[i];
                do {
                        struct index *keep = entry;
                        int skip = hash_count[i] / hlimit / 2;
                        do {
                                entry = entry->next;
                        } while(--skip && entry);
                        keep->next = entry;
                } while(entry);
        }
        free(hash_count);

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

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

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