view mercurial/bdiff.c @ 47072:4c041c71ec01

revlog: introduce an explicit tracking of what the revlog is about

Since the dawn of time, people have been forced to rely to lossy introspection
of the index filename to determine what the purpose and role of the revlog they
encounter is. This is hacky, error prone, inflexible, abstraction-leaky,
<insert-your-own-complaints-here>.

In f63299ee7e4d Raphaël introduced a new attribute to track this information:
`revlog_kind`. However it is initialized in an odd place and various instances
end up not having it set. In addition is only tracking some of the information
we end up having to introspect in various pieces of code.

So we add a new attribute that holds more data and is more strictly enforced.
This work is done in collaboration with Raphaël.

The `revlog_kind` one will be removed/adapted in the next changeset. We expect
to be able to clean up various existing piece of code and to simplify coming
work around the newer revlog format.

Differential Revision: https://phab.mercurial-scm.org/D10352

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

 Copyright 2005, 2006 Olivia 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 <limits.h>
#include <stdlib.h>
#include <string.h>

#include "bdiff.h"
#include "bitmanipulation.h"
#include "compat.h"

/* Hash implementation from diffutils */
#define ROL(v, n) ((v) << (n) | (v) >> (sizeof(v) * CHAR_BIT - (n)))
#define HASH(h, c) ((c) + ROL(h, 7))

struct pos {
	int pos, len;
};

int bdiff_splitlines(const char *a, ssize_t len, struct bdiff_line **lr)
{
	unsigned hash;
	int i;
	const char *p, *b = a;
	const char *const plast = a + len - 1;
	struct bdiff_line *l;

	/* count the lines */
	i = 1; /* extra line for sentinel */
	for (p = a; p < plast; p++) {
		if (*p == '\n') {
			i++;
		}
	}
	if (p == plast) {
		i++;
	}

	*lr = l = (struct bdiff_line *)calloc(i, sizeof(struct bdiff_line));
	if (!l) {
		return -1;
	}

	/* build the line array and calculate hashes */
	hash = 0;
	for (p = a; p < plast; p++) {
		hash = HASH(hash, *p);

		if (*p == '\n') {
			l->hash = hash;
			hash = 0;
			l->len = p - b + 1;
			l->l = b;
			l->n = INT_MAX;
			l++;
			b = p + 1;
		}
	}

	if (p == plast) {
		hash = HASH(hash, *p);
		l->hash = hash;
		l->len = p - b + 1;
		l->l = b;
		l->n = INT_MAX;
		l++;
	}

	/* set up a sentinel */
	l->hash = 0;
	l->len = 0;
	l->l = a + len;
	return i - 1;
}

static inline int cmp(struct bdiff_line *a, struct bdiff_line *b)
{
	return a->hash != b->hash || a->len != b->len ||
	       memcmp(a->l, b->l, a->len);
}

static int equatelines(struct bdiff_line *a, int an, struct bdiff_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 *)calloc(buckets, scale * 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 bdiff_line *a, struct bdiff_line *b,
                         struct pos *pos, int a1, int a2, int b1, int b2,
                         int *omi, int *omj)
{
	int mi = a1, mj = b1, mk = 0, i, j, k, half, bhalf;

	/* 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 - 1) / 2;
	bhalf = (b1 + b2 - 1) / 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? */
			for (k = 1; j - k >= b1 && i - k >= a1; k++) {
				/* reached an earlier match? */
				if (pos[j - k].pos == i - k) {
					k += pos[j - k].len;
					break;
				}
				/* previous line mismatch? */
				if (a[i - k].e != b[j - k].e) {
					break;
				}
			}

			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) {
				/* a longer match */
				mi = i;
				mj = j;
				mk = k;
			} else if (k == mk) {
				if (i > mi && i <= half && j > b1) {
					/* same match but closer to half */
					mi = i;
					mj = j;
				} else if (i == mi && (mj > bhalf || i == a1)) {
					/* same i but best earlier j */
					mj = j;
				}
			}
		}
	}

	if (mk) {
		mi = mi - mk + 1;
		mj = mj - mk + 1;
	}

	/* expand match to include subsequent popular lines */
	while (mi + mk < a2 && mj + mk < b2 && a[mi + mk].e == b[mj + mk].e) {
		mk++;
	}

	*omi = mi;
	*omj = mj;

	return mk;
}

static struct bdiff_hunk *recurse(struct bdiff_line *a, struct bdiff_line *b,
                                  struct pos *pos, int a1, int a2, int b1,
                                  int b2, struct bdiff_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 bdiff_hunk *)malloc(sizeof(struct bdiff_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;
	}
}

int bdiff_diff(struct bdiff_line *a, int an, struct bdiff_line *b, int bn,
               struct bdiff_hunk *base)
{
	struct bdiff_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 bdiff_hunk *)malloc(sizeof(struct bdiff_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 bdiff_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;
}

/* deallocate list of hunks; l may be NULL */
void bdiff_freehunks(struct bdiff_hunk *l)
{
	struct bdiff_hunk *n;
	for (; l; l = n) {
		n = l->next;
		free(l);
	}
}