/*

 bdiff.c - efficient binary diff extension for Mercurial

 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.

 Based roughly on Python difflib

*/

#define PY_SSIZE_T_CLEAN

#include <Python.h>

#include <stdlib.h>

#include <string.h>

#include <limits.h>

#include "util.h"

struct line {

	int hash, n, e;

	Py_ssize_t len;

	const char *l;

};

struct pos {

	int pos, len;

};

struct hunk;

struct hunk {

	int a1, a2, b1, b2;

	struct hunk *next;

};

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

{

	unsigned hash;

	int i;

	const char *p, *b = a;

	const char * const plast = a + len - 1;

	struct line *l;

	/* count the lines */

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

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

		if (*p == '\n' || p == plast)

			i++;

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

	if (!l)

		return -1;

	/* build the line array and calculate hashes */

	hash = 0;

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

		/* Leonid Yuriev's hash */

		hash = (hash * 1664525) + (unsigned char)*p + 1013904223;

		if (*p == '\n' || p == plast) {

			l->hash = hash;

			hash = 0;

			l->len = p - b + 1;

			l->l = b;

			l->n = INT_MAX;

			l++;

			b = p + 1;

		}

	}

	/* set up a sentinel */

	l->hash = 0;

	l->len = 0;

	l->l = a + len;

	return i - 1;

}

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

{

	return a->hash != b->hash || 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, scale;

	struct pos *h = NULL;

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

	while (buckets < bn + 1)

		buckets *= 2;

	/* try to allocate a large hash table to avoid collisions */

	for (scale = 4; scale; scale /= 2) {

		h = (struct pos *)malloc(scale * buckets * sizeof(struct pos));

		if (h)

			break;

	}

	if (!h)

		return 0;

	buckets = buckets * scale - 1;

	/* clear the hash table */

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

		h[i].pos = -1;

		h[i].len = 0;

	}

	/* add lines to the hash table chains */

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

		/* find the equivalence class */

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

		     j = (j + 1) & buckets)

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

				break;

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

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

		b[i].e = j;

		h[j].pos = i;

		h[j].len++; /* keep track of popularity */

	}

	/* compute popularity threshold */

	t = (bn >= 31000) ? bn / 1000 : 1000000 / (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].hash & buckets; h[j].pos != -1;

		     j = (j + 1) & buckets)

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

				break;

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

		if (h[j].len <= t)

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

		else

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

	}

	/* discard hash tables */

	free(h);

	return 1;

}

static int longest_match(struct line *a, struct line *b, struct pos *pos,

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

{

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

	/* window our search on large regions to better bound

	   worst-case performance. by choosing a window at the end, we

	   reduce skipping overhead on the b chains. */

	if (a2 - a1 > 30000)

		a1 = a2 - 30000;

	half = (a1 + a2) / 2;

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

		/* skip all lines in b after the current block */

		for (j = a[i].n; j >= b2; j = b[j].n)

			;

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

		for (; j >= b1; j = b[j].n) {

			/* does this extend an earlier match? */

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

				k = pos[j - 1].len + 1;

			else

				k = 1;

			pos[j].pos = i;

			pos[j].len = k;

			/* best match so far? we prefer matches closer

			   to the middle to balance recursion */

			if (k > mk || (k == mk && (i <= mi || i < half))) {

				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].e == b[mj - mb - 1].e)

		mb++;

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

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

		mk++;

	*omi = mi - mb;

	*omj = mj - mb;

	return mk + mb;

}

static struct hunk *recurse(struct line *a, struct line *b, struct pos *pos,

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

{

	int i, j, k;

	while (1) {

		/* find the longest match in this chunk */

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

		if (!k)

			return l;

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

		l = recurse(a, b, pos, a1, i, b1, j, l);

		if (!l)

			return NULL;

		l->next = (struct hunk *)malloc(sizeof(struct hunk));

		if (!l->next)

			return NULL;

		l = l->next;

		l->a1 = i;

		l->a2 = i + k;

		l->b1 = j;

		l->b2 = j + k;

		l->next = NULL;

		/* tail-recursion didn't happen, so do equivalent iteration */

		a1 = i + k;

		b1 = j + k;

	}

}

static int diff(struct line *a, int an, struct line *b, int bn,

		 struct hunk *base)

{

	struct hunk *curr;

	struct pos *pos;

	int t, count = 0;

	/* allocate and fill arrays */

	t = equatelines(a, an, b, bn);

	pos = (struct pos *)calloc(bn ? bn : 1, sizeof(struct pos));

	if (pos && t) {

		/* generate the matching block list */

		curr = recurse(a, b, pos, 0, an, 0, bn, base);

		if (!curr)

			return -1;

		/* sentinel end hunk */

		curr->next = (struct hunk *)malloc(sizeof(struct hunk));

		if (!curr->next)

			return -1;

		curr = curr->next;

		curr->a1 = curr->a2 = an;

		curr->b1 = curr->b2 = bn;

		curr->next = NULL;

	}

	free(pos);

	/* normalize the hunk list, try to push each hunk towards the end */

	for (curr = base->next; curr; curr = curr->next) {

		struct hunk *next = curr->next;

		if (!next)

			break;

		if (curr->a2 == next->a1 || curr->b2 == next->b1)

			while (curr->a2 < an && curr->b2 < bn

			       && next->a1 < next->a2

			       && next->b1 < next->b2

			       && !cmp(a + curr->a2, b + curr->b2)) {

				curr->a2++;

				next->a1++;

				curr->b2++;

				next->b1++;

			}

	}

	for (curr = base->next; curr; curr = curr->next)

		count++;

	return count;

}

static void freehunks(struct hunk *l)

{

	struct hunk *n;

	for (; l; l = n) {

		n = l->next;

		free(l);

	}

}

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

{

	PyObject *sa, *sb, *rl = NULL, *m;

	struct line *a, *b;

	struct hunk l, *h;

	int an, bn, count, pos = 0;

	l.next = NULL;

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

		return NULL;

	an = splitlines(PyBytes_AsString(sa), PyBytes_Size(sa), &a);

	bn = splitlines(PyBytes_AsString(sb), PyBytes_Size(sb), &b);

	if (!a || !b)

		goto nomem;

	count = diff(a, an, b, bn, &l);

	if (count < 0)

		goto nomem;

	rl = PyList_New(count);

	if (!rl)

		goto nomem;

	for (h = l.next; h; h = h->next) {

		m = Py_BuildValue("iiii", h->a1, h->a2, h->b1, h->b2);

		PyList_SetItem(rl, pos, m);

		pos++;

	}

nomem:

	free(a);

	free(b);

	freehunks(l.next);

	return rl ? rl : PyErr_NoMemory();

}

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

{

	char *sa, *sb, *rb;

	PyObject *result = NULL;

	struct line *al, *bl;

	struct hunk l, *h;

	int an, bn, count;

	Py_ssize_t len = 0, la, lb;

	PyThreadState *_save;

	l.next = NULL;

	if (!PyArg_ParseTuple(args, "s#s#:bdiff", &sa, &la, &sb, &lb))

		return NULL;

	if (la > UINT_MAX || lb > UINT_MAX) {

		PyErr_SetString(PyExc_ValueError, "bdiff inputs too large");

		return NULL;

	}

	_save = PyEval_SaveThread();

	an = splitlines(sa, la, &al);

	bn = splitlines(sb, lb, &bl);

	if (!al || !bl)

		goto nomem;

	count = diff(al, an, bl, bn, &l);