/*

 bdiff.c - efficient binary diff extension for Mercurial

 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.

 Based roughly on Python difflib

*/

#include <Python.h>

#include <stdlib.h>

#include <string.h>

#include <netinet/in.h>

#include <sys/types.h>

struct line {

	int h, len, n;

	const char *l;

};

struct hunk {

	int a1, a2, b1, b2;

};

struct hunklist {

	struct hunk *base, *head;

};

static inline uint32_t rol32(uint32_t word, unsigned int shift)

{

        return (word << shift) | (word >> (32 - shift));

}

int splitlines(const char *a, int len, struct line **lr)

{

	int h, i;

	const char *p, *b = a;

	struct line *l;

	/* count the lines */

	i = 1; /* extra line for sentinel */

	for (p = a; p < a + len; p++)

		if (*p == '\n' || p == a + len - 1)

			i++;

	*lr = l = malloc(sizeof(struct line) * i);

	if (!l)

		return -1;

	/* build the line array and calculate hashes */

	h = 0;

	for (p = a; p < a + len; p++) {

		h = *p + rol32(h, 7); /* a simple hash from GNU diff */

		if (*p == '\n' || p == a + len - 1) {

			l->len = p - b + 1;

			l->h = h;

			l->l = b;

			l->n = -1;

			l++;

			b = p + 1;

			h = 0;

		}

	}

	/* set up a sentinel */

	l->h = l->len = 0;

	l->l = a + len;

	return i - 1;

}

int inline cmp(struct line *a, struct line *b)

{

	return a->len != b->len || memcmp(a->l, b->l, a->len);

}

static int equatelines(struct line *a, int an, struct line *b, int bn)

{

	int i, j, buckets = 1, t, *h, *l;

	/* build a hash table of the next highest power of 2 */

	while (buckets < bn + 1)

		buckets *= 2;

	h = malloc(buckets * sizeof(int));

	l = calloc(buckets, sizeof(int));

	buckets = buckets - 1;

	if (!h || !l) {

		free(h);

		return 0;

	}

	/* clear the hash table */

	for (i = 0; i <= buckets; i++)

		h[i] = -1;

	/* add lines to the hash table chains */

	for (i = bn - 1; i >= 0; i--) {

		/* find the equivalence class */

		for (j = b[i].h & buckets; h[j] != -1; j = (j + 1) & buckets)

			if (!cmp(b + i, b + h[j]))

				break;

		/* add to the head of the equivalence class */

		b[i].n = h[j];

		b[i].h = j;

		h[j] = i;

		l[j]++; /* keep track of popularity */

	}

	/* compute popularity threshold */

	t = (bn >= 200) ? bn / 100 : bn + 1;

	/* match items in a to their equivalence class in b */

	for (i = 0; i < an; i++) {

		/* find the equivalence class */

		for (j = a[i].h & buckets; h[j] != -1; j = (j + 1) & buckets)

			if (!cmp(a + i, b + h[j]))

				break;

		a[i].h = j; /* use equivalence class for quick compare */

		if(l[j] <= t)

			a[i].n = h[j]; /* point to head of match list */

		else

			a[i].n = -1; /* too popular */

	}

	/* discard hash tables */

	free(h);

	free(l);

	return 1;

}

static int longest_match(struct line *a, struct line *b, int *jpos, int *jlen,

			 int a1, int a2, int b1, int b2, int *omi, int *omj)

{

	int mi = a1, mj = b1, mk = 0, mb = 0, i, j, k;

	for (i = a1; i < a2; i++) {

		/* skip things before the current block */

		for (j = a[i].n; j != -1 && j < b1; j = b[j].n)

			;

		/* loop through all lines match a[i] in b */

		for (; j != -1 && j < b2; j = b[j].n) {

			/* does this extend an earlier match? */

			if (i > a1 && j > b1 && jpos[j - 1] == i)

				k = jlen[j - 1] + 1;

			else

				k = 1;

			jpos[j] = i + 1;

			jlen[j] = k;

			/* best match so far? */

			if (k > mk) {

				mi = i;

				mj = j;

				mk = k;

			}

		}

	}

	if (mk) {

		mi = mi - mk + 1;

		mj = mj - mk + 1;

	}

	/* expand match to include neighboring popular lines */

	while (mi - mb > a1 && mj - mb > b1 &&

	       a[mi - mb - 1].h == b[mj - mb - 1].h)

		mb++;

	while (mi + mk < a2 && mj + mk < b2 &&

	       a[mi + mk].h == b[mj + mk].h)

		mk++;

	*omi = mi - mb;

	*omj = mj - mb;

	return mk + mb;

}

static void recurse(struct line *a, struct line *b, int *jpos, int *jlen,

		    int a1, int a2, int b1, int b2, struct hunklist *l)

{

	int i, j, k;

	/* find the longest match in this chunk */

	k = longest_match(a, b, jpos, jlen, a1, a2, b1, b2, &i, &j);

	if (!k)

		return;

	/* and recurse on the remaining chunks on either side */

	recurse(a, b, jpos, jlen, a1, i, b1, j, l);

	l->head->a1 = i;

	l->head->a2 = i + k;

	l->head->b1 = j;

	l->head->b2 = j + k;

	l->head++;

	recurse(a, b, jpos, jlen, i + k, a2, j + k, b2, l);

}

static PyObject *bdiff(PyObject *self, PyObject *args)

{

	PyObject *sa, *sb, *result = NULL;

	struct hunklist l;

	struct hunk *h;

	struct line *al, *bl;

	char encode[12], *rb;

	int an, bn, len = 0, t, la = 0, lb = 0, *jpos, *jlen;

	if (!PyArg_ParseTuple(args, "SS:bdiff", &sa, &sb))

		return NULL;

	/* allocate and fill arrays */

	an = splitlines(PyString_AsString(sa), PyString_Size(sa), &al);

	bn = splitlines(PyString_AsString(sb), PyString_Size(sb), &bl);

	t = equatelines(al, an, bl, bn);

	jpos = calloc(bn, sizeof(int));

	jlen = calloc(bn, sizeof(int));

	l.head = l.base = malloc(sizeof(struct hunk) * ((an + bn) / 4 + 2));

	if (!al || !bl || !jpos || !jlen || !l.base || !t)

		goto nomem;

	/* generate the matching block list */

	recurse(al, bl, jpos, jlen, 0, an, 0, bn, &l);

	l.head->a1 = an;

	l.head->b1 = bn;

	l.head++;

	/* calculate length of output */

	for(h = l.base; h != l.head; h++) {

		if (h->a1 != la || h->b1 != lb)

			len += 12 + bl[h->b1].l - bl[lb].l;

		la = h->a2;

		lb = h->b2;

	}

	result = PyString_FromStringAndSize(NULL, len);

	if (!result)

		goto nomem;

	/* build binary patch */

	rb = PyString_AsString(result);

	la = lb = 0;

	for(h = l.base; h != l.head; h++) {

		if (h->a1 != la || h->b1 != lb) {

			len = bl[h->b1].l - bl[lb].l;

			*(uint32_t *)(encode)     = htonl(al[la].l - al->l);

			*(uint32_t *)(encode + 4) = htonl(al[h->a1].l - al->l);

			*(uint32_t *)(encode + 8) = htonl(len);

			memcpy(rb, encode, 12);

			memcpy(rb + 12, bl[lb].l, len);

			rb += 12 + len;

		}

		la = h->a2;

		lb = h->b2;

	}

nomem:

	free(al);

	free(bl);

	free(jpos);

	free(jlen);

	free(l.base);

	return result ? result : PyErr_NoMemory();

}

static char mdiff_doc[] = "Efficient binary diff.";

static PyMethodDef methods[] = {

	{"bdiff", bdiff, METH_VARARGS, "calculate a binary diff\n"},

	{NULL, NULL}

};

PyMODINIT_FUNC initbdiff(void)

{

	Py_InitModule3("bdiff", methods, mdiff_doc);

}