annotate mercurial/bdiff.c @ 990:5007e0bdeed2

Fix long-standing excessive file merges

Since switching to the multihead approach, we've been creating
excessive file-level merges where files are marked as merged with
their ancestors.

This explicitly checks at commit time whether the two parent versions
are linearly related, and if so, reduces the file check-in to a
non-merge. Then the file is compared against the remaining parent,
and, if equal, skips check-in of that file (as it's not changed).

Since we're not checking in all files that were different between
versions, we no longer need to mark so many files for merge. This
removes most of the 'm' state marking as well.

Finally, it is possible to do a tree-level merge with no file-level
changes. This will happen if one user changes file A and another
changes file B. Thus, if we have have two parents, we allow commit to
proceed even if there are no file-level changes.

/*
 bdiff.c - efficient binary diff extension for Mercurial

 Copyright 2005 Matt Mackall <[e-mail]>

 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>

#ifdef __hpux
#define inline
#endif

#ifdef _WIN32
#ifdef _MSC_VER
#define inline __inline
typedef unsigned long uint32_t;
#else
#include <stdint.h>
#endif
static uint32_t htonl(uint32_t x)
{
	return ((x & 0x000000ffUL) << 24) |
		((x & 0x0000ff00UL) <<  8) |
		((x & 0x00ff0000UL) >>  8) |
		((x & 0xff000000UL) >> 24);
}
#else
#include <sys/types.h>
#include <arpa/inet.h>
#endif

struct line {
	int h, len, n, e;
	const char *l;
};

struct pos {
	int pos, len;
};

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->len;
			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->h != b->h || 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;
	struct pos *h;

	/* build a hash table of the next highest power of 2 */
	while (buckets < bn + 1)
		buckets *= 2;

	h = malloc(buckets * sizeof(struct pos));
	buckets = buckets - 1;
	if (!h)
		return 0;

	/* 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 = bn - 1; i >= 0; i--) {
		/* find the equivalence class */
		for (j = b[i].h & 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 >= 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].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;

	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 && 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? */
			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].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 void recurse(struct line *a, struct line *b, struct pos *pos,
		    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, pos, a1, a2, b1, b2, &i, &j);
	if (!k)
		return;

	/* and recurse on the remaining chunks on either side */
	recurse(a, b, pos, 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, pos, i + k, a2, j + k, b2, l);
}

static struct hunklist diff(struct line *a, int an, struct line *b, int bn)
{
	struct hunklist l;
	struct pos *pos;
	int t;

	/* allocate and fill arrays */
	t = equatelines(a, an, b, bn);
	pos = calloc(bn, sizeof(struct pos));
	/* we can't have more matches than lines in the shorter file */
	l.head = l.base = malloc(sizeof(struct hunk) * ((an<bn ? an:bn) + 1));

	if (pos && l.base && t) {
		/* generate the matching block list */
		recurse(a, b, pos, 0, an, 0, bn, &l);
		l.head->a1 = an;
		l.head->b1 = bn;
		l.head++;
	}

	free(pos);
	return l;
}

static PyObject *blocks(PyObject *self, PyObject *args)
{
	PyObject *sa, *sb, *rl = NULL, *m;
	struct line *a, *b;
	struct hunklist l = {NULL, NULL};
	struct hunk *h;
	int an, bn, pos = 0;

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

	an = splitlines(PyString_AsString(sa), PyString_Size(sa), &a);
	bn = splitlines(PyString_AsString(sb), PyString_Size(sb), &b);
	if (!a || !b)
		goto nomem;

	l = diff(a, an, b, bn);
	rl = PyList_New(l.head - l.base);
	if (!l.head || !rl)
		goto nomem;

	for(h = l.base; h != l.head; h++) {
		m = Py_BuildValue("iiii", h->a1, h->a2, h->b1, h->b2);
		PyList_SetItem(rl, pos, m);
		pos++;
	}

nomem:
	free(a);
	free(b);
	free(l.base);
	return rl ? rl : PyErr_NoMemory();
}

static PyObject *bdiff(PyObject *self, PyObject *args)
{
	PyObject *sa, *sb, *result = NULL;
	struct line *al, *bl;
	struct hunklist l = {NULL, NULL};
	struct hunk *h;
	char encode[12], *rb;
	int an, bn, len = 0, la = 0, lb = 0;

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

	an = splitlines(PyString_AsString(sa), PyString_Size(sa), &al);
	bn = splitlines(PyString_AsString(sb), PyString_Size(sb), &bl);
	if (!al || !bl)
		goto nomem;

	l = diff(al, an, bl, bn);
	if (!l.head)
		goto nomem;

	/* 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(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"},
	{"blocks", blocks, METH_VARARGS, "find a list of matching lines\n"},
	{NULL, NULL}
};

PyMODINIT_FUNC initbdiff(void)
{
	Py_InitModule3("bdiff", methods, mdiff_doc);
}