/*

 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 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 <Python.h>

#include <stdlib.h>

#include <string.h>

#include <netinet/in.h>

#include <sys/types.h>

static char mpatch_doc[] = "Efficient binary patching.";

struct frag {

	int start, end, len;

	char *data;

};

struct flist {

	struct frag *base, *head, *tail;

};

static struct flist *lalloc(int size)

{

	struct flist *a;

	a = malloc(sizeof(struct flist));

	a->head = a->tail = a->base = malloc(sizeof(struct frag) * size);

	return a;

}

static void lfree(struct flist *a)

{

	free(a->base);

	free(a);

}

static int lsize(struct 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 flist *dest, struct flist *src, int cut, int offset)

{

	struct 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 flist *src, int cut, int offset)

{

	struct 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 flist *combine(struct flist *a, struct flist *b)

{

	struct flist *c;

	struct frag *bh = b->head, *ct;

	int offset = 0, post;

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

	while (bh != b->tail) {

		/* 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++;

		bh++;

		offset = post;

	}

	/* hold on to tail from a */

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

	c->tail += lsize(a);

	lfree(a);

	lfree(b);

	return c;

}

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

static struct flist *decode(char *bin, int len)

{

	struct flist *l;

	struct frag *lt;

	char *end = bin + len;

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

	l = lalloc(len / 12);

	lt = l->tail;

	while (bin < end) {

		lt->start = ntohl(*(uint32_t *)bin);

		lt->end = ntohl(*(uint32_t *)(bin + 4));

		lt->len = ntohl(*(uint32_t *)(bin + 8));

		lt->data = bin + 12;

		bin += 12 + lt->len;

		lt++;

	}

	l->tail = lt;

	return l;

}

/* calculate the size of resultant text */

static int calcsize(int len, struct flist *l)

{

	int outlen = 0, last = 0;

	struct frag *f = l->head;

	while (f != l->tail) {

		outlen += f->start - last;

		last = f->end;

		outlen += f->len;

		f++;

	}

	outlen += len - last;

	return outlen;

}

static void apply(char *buf, char *orig, int len, struct flist *l)

{

	struct frag *f = l->head;

	int last = 0;

	char *p = buf;

	while (f != l->tail) {

		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);

}

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

static struct flist *fold(PyObject *bins, int start, int end)

{

	int len;

	if (start + 1 == end) {

		/* trivial case, output a decoded list */

		PyObject *tmp = PyList_GetItem(bins, start);

		return decode(PyString_AsString(tmp), PyString_Size(tmp));

	}

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

	len = (end - start) / 2;

	return combine(fold(bins, start, start + len),

		       fold(bins, start + len, end));

}

static PyObject *

patches(PyObject *self, PyObject *args)

{

	PyObject *text, *bins, *result;

	struct flist *patch;

	char *in, *out;

	int len, outlen;

	if (!PyArg_ParseTuple(args, "OO:mpatch", &text, &bins))

		return NULL;

	len = PyList_Size(bins);

	if (!len) {

		/* nothing to do */

		Py_INCREF(text);

		return text;

	}

	patch = fold(bins, 0, len);

	outlen = calcsize(PyString_Size(text), patch);

	result = PyString_FromStringAndSize(NULL, outlen);

	in = PyString_AsString(text);

	out = PyString_AsString(result);

	apply(out, in, PyString_Size(text), patch);

	lfree(patch);

	return result;

}

static PyMethodDef methods[] = {

	{"patches", patches, METH_VARARGS, "apply a series of patches\n"},

	{NULL, NULL}

};

PyMODINIT_FUNC

initmpatch(void)

{

	Py_InitModule3("mpatch", methods, mpatch_doc);

}