/*

 mpatch.c - efficient binary patching for Mercurial

 This implements a patch algorithm that's O(m + nlog n) where m is the

 size of the output and n is the number of patches.

 Given a list of binary patches, it unpacks each into a hunk list,

 then combines the hunk lists with a treewise recursion to form a

 single hunk list. This hunk list is then applied to the original

 text.

 The text (or binary) fragments are copied directly from their source

 Python objects into a preallocated output string to avoid the

 allocation of intermediate Python objects. Working memory is about 2x

 the total number of hunks.

 Copyright 2005, 2006 Matt Mackall <mpm@selenic.com>

 This software may be used and distributed according to the terms

 of the GNU General Public License, incorporated herein by reference.

*/

#include <stdlib.h>

#include <string.h>

#include "bitmanipulation.h"

#include "compat.h"

#include "mpatch.h"

static struct mpatch_flist *lalloc(ssize_t size)

{

	struct mpatch_flist *a = NULL;

	if (size < 1)

		size = 1;

	a = (struct mpatch_flist *)malloc(sizeof(struct mpatch_flist));

	if (a) {

		a->base = (struct mpatch_frag *)malloc(

		    sizeof(struct mpatch_frag) * size);

		if (a->base) {

			a->head = a->tail = a->base;

			return a;

		}

		free(a);

	}

	return NULL;

}

void mpatch_lfree(struct mpatch_flist *a)

{

	if (a) {

		free(a->base);

		free(a);

	}

}

static ssize_t lsize(struct mpatch_flist *a)

{

	return a->tail - a->head;

}

/* move hunks in source that are less cut to dest, compensating

   for changes in offset. the last hunk may be split if necessary.

*/

static int gather(struct mpatch_flist *dest, struct mpatch_flist *src, int cut,

                  int offset)

{

	struct mpatch_frag *d = dest->tail, *s = src->head;

	int postend, c, l;

	while (s != src->tail) {

		if (s->start + offset >= cut)

			break; /* we've gone far enough */

		postend = offset + s->start + s->len;

		if (postend <= cut) {

			/* save this hunk */

			offset += s->start + s->len - s->end;

			*d++ = *s++;

		} else {

			/* break up this hunk */

			c = cut - offset;

			if (s->end < c)

				c = s->end;

			l = cut - offset - s->start;

			if (s->len < l)

				l = s->len;

			offset += s->start + l - c;

			d->start = s->start;

			d->end = c;

			d->len = l;

			d->data = s->data;

			d++;

			s->start = c;

			s->len = s->len - l;

			s->data = s->data + l;

			break;

		}

	}

	dest->tail = d;

	src->head = s;

	return offset;

}

/* like gather, but with no output list */

static int discard(struct mpatch_flist *src, int cut, int offset)

{

	struct mpatch_frag *s = src->head;

	int postend, c, l;

	while (s != src->tail) {

		if (s->start + offset >= cut)

			break;

		postend = offset + s->start + s->len;

		if (postend <= cut) {

			offset += s->start + s->len - s->end;

			s++;

		} else {

			c = cut - offset;

			if (s->end < c)

				c = s->end;

			l = cut - offset - s->start;

			if (s->len < l)

				l = s->len;

			offset += s->start + l - c;

			s->start = c;

			s->len = s->len - l;

			s->data = s->data + l;

			break;

		}

	}

	src->head = s;

	return offset;

}

/* combine hunk lists a and b, while adjusting b for offset changes in a/

   this deletes a and b and returns the resultant list. */

static struct mpatch_flist *combine(struct mpatch_flist *a,

                                    struct mpatch_flist *b)

{

	struct mpatch_flist *c = NULL;

	struct mpatch_frag *bh, *ct;

	int offset = 0, post;

	if (a && b)

		c = lalloc((lsize(a) + lsize(b)) * 2);

	if (c) {

		for (bh = b->head; bh != b->tail; bh++) {

			/* save old hunks */

			offset = gather(c, a, bh->start, offset);

			/* discard replaced hunks */

			post = discard(a, bh->end, offset);

			/* insert new hunk */

			ct = c->tail;

			ct->start = bh->start - offset;

			ct->end = bh->end - post;

			ct->len = bh->len;

			ct->data = bh->data;

			c->tail++;

			offset = post;

		}

		/* hold on to tail from a */

		memcpy(c->tail, a->head, sizeof(struct mpatch_frag) * lsize(a));

		c->tail += lsize(a);

	}

	mpatch_lfree(a);

	mpatch_lfree(b);

	return c;

}

/* decode a binary patch into a hunk list */

int mpatch_decode(const char *bin, ssize_t len, struct mpatch_flist **res)

{

	struct mpatch_flist *l;

	struct mpatch_frag *lt;

	int pos = 0;

	/* assume worst case size, we won't have many of these lists */

	l = lalloc(len / 12 + 1);

	if (!l)

		return MPATCH_ERR_NO_MEM;

	lt = l->tail;

	while (pos >= 0 && pos < len) {

		lt->start = getbe32(bin + pos);

		lt->end = getbe32(bin + pos + 4);

		lt->len = getbe32(bin + pos + 8);

		lt->data = bin + pos + 12;

		pos += 12 + lt->len;

		if (lt->start > lt->end || lt->len < 0)

			break; /* sanity check */

		lt++;

	}

	if (pos != len) {

		mpatch_lfree(l);

		return MPATCH_ERR_CANNOT_BE_DECODED;

	}

	l->tail = lt;

	*res = l;

	return 0;

}

/* calculate the size of resultant text */

ssize_t mpatch_calcsize(ssize_t len, struct mpatch_flist *l)

{

	ssize_t outlen = 0, last = 0;

	struct mpatch_frag *f = l->head;

	while (f != l->tail) {

		if (f->start < last || f->end > len) {

			return MPATCH_ERR_INVALID_PATCH;

		}

		outlen += f->start - last;

		last = f->end;

		outlen += f->len;

		f++;

	}

	outlen += len - last;

	return outlen;

}

int mpatch_apply(char *buf, const char *orig, ssize_t len,

                 struct mpatch_flist *l)

{

	struct mpatch_frag *f = l->head;

	int last = 0;

	char *p = buf;

	while (f != l->tail) {

		if (f->start < last || f->end > len) {

			return MPATCH_ERR_INVALID_PATCH;

		}

		memcpy(p, orig + last, f->start - last);

		p += f->start - last;

		memcpy(p, f->data, f->len);

		last = f->end;

		p += f->len;

		f++;

	}

	memcpy(p, orig + last, len - last);

	return 0;

}

/* recursively generate a patch of all bins between start and end */

struct mpatch_flist *

mpatch_fold(void *bins, struct mpatch_flist *(*get_next_item)(void *, ssize_t),

            ssize_t start, ssize_t end)

{

	ssize_t len;

	if (start + 1 == end) {

		/* trivial case, output a decoded list */

		return get_next_item(bins, start);

	}

	/* divide and conquer, memory management is elsewhere */

	len = (end - start) / 2;

	return combine(mpatch_fold(bins, get_next_item, start, start + len),

	               mpatch_fold(bins, get_next_item, start + len, end));

}