bdiff: extend matches across popular lines

For very large diffs that have large numbers of identical lines (JSON
dumps) that also have large blocks of identical text, bdiff could become
confused about which block matches which because it can only match
very limited regions. The result is very large diffs for small sets of edits.

The earlier recursion rebalancing fix made this behavior more frequent because
it's now more prone to match block 1 to block 2. One frequent user of
large JSON files reported being unable to pass the resulting diffs
through their code review system.

Prior to this change, bdiff would calculate the length of a match at
(i, j) as 1 + length found at (i-1, j-1). With large number of popular
(ignored) lines, this often meant matches couldn't be extended
backwards at all and thus all matching regions were very small.
Disabling the popularity threshold is not an option because it brings
back quadratic behavior.

Instead, we extend a match backwards until we either found a previously
discovered match or we find a mismatching line. This thus successfully
bridges over any popular lines inside and before a matching region.
The larger regions then significant reduce the probability of confusion.

mercurial/bdiff.c

@@ -1,483 +1,490 b''
 /*
  bdiff.c - efficient binary diff extension for Mercurial
 
  Copyright 2005, 2006 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
 */
 
 #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;
+			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 || (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);
 	if (count < 0)
 		goto nomem;
 
 	/* calculate length of output */
 	la = lb = 0;
 	for (h = l.next; h; h = h->next) {
 		if (h->a1 != la || h->b1 != lb)
 			len += 12 + bl[h->b1].l - bl[lb].l;
 		la = h->a2;
 		lb = h->b2;
 	}
 	PyEval_RestoreThread(_save);
 	_save = NULL;
 
 	result = PyBytes_FromStringAndSize(NULL, len);
 
 	if (!result)
 		goto nomem;
 
 	/* build binary patch */
 	rb = PyBytes_AsString(result);
 	la = lb = 0;
 
 	for (h = l.next; h; h = h->next) {
 		if (h->a1 != la || h->b1 != lb) {
 			len = bl[h->b1].l - bl[lb].l;
 			putbe32((uint32_t)(al[la].l - al->l), rb);
 			putbe32((uint32_t)(al[h->a1].l - al->l), rb + 4);
 			putbe32((uint32_t)len, rb + 8);
 			memcpy(rb + 12, bl[lb].l, len);
 			rb += 12 + len;
 		}
 		la = h->a2;
 		lb = h->b2;
 	}
 
 nomem:
 	if (_save)
 		PyEval_RestoreThread(_save);
 	free(al);
 	free(bl);
 	freehunks(l.next);
 	return result ? result : PyErr_NoMemory();
 }
 
 /*
  * If allws != 0, remove all whitespace (' ', \t and \r). Otherwise,
  * reduce whitespace sequences to a single space and trim remaining whitespace
  * from end of lines.
  */
 static PyObject *fixws(PyObject *self, PyObject *args)
 {
 	PyObject *s, *result = NULL;
 	char allws, c;
 	const char *r;
 	Py_ssize_t i, rlen, wlen = 0;
 	char *w;
 
 	if (!PyArg_ParseTuple(args, "Sb:fixws", &s, &allws))
 		return NULL;
 	r = PyBytes_AsString(s);
 	rlen = PyBytes_Size(s);
 
 	w = (char *)malloc(rlen ? rlen : 1);
 	if (!w)
 		goto nomem;
 
 	for (i = 0; i != rlen; i++) {
 		c = r[i];
 		if (c == ' ' || c == '\t' || c == '\r') {
 			if (!allws && (wlen == 0 || w[wlen - 1] != ' '))
 				w[wlen++] = ' ';
 		} else if (c == '\n' && !allws
 			  && wlen > 0 && w[wlen - 1] == ' ') {
 			w[wlen - 1] = '\n';
 		} else {
 			w[wlen++] = c;
 		}
 	}
 
 	result = PyBytes_FromStringAndSize(w, wlen);
 
 nomem:
 	free(w);
 	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"},
 	{"fixws", fixws, METH_VARARGS, "normalize diff whitespaces\n"},
 	{NULL, NULL}
 };
 
 #ifdef IS_PY3K
 static struct PyModuleDef bdiff_module = {
 	PyModuleDef_HEAD_INIT,
 	"bdiff",
 	mdiff_doc,
 	-1,
 	methods
 };
 
 PyMODINIT_FUNC PyInit_bdiff(void)
 {
 	return PyModule_Create(&bdiff_module);
 }
 #else
 PyMODINIT_FUNC initbdiff(void)
 {
 	Py_InitModule3("bdiff", methods, mdiff_doc);
 }
 #endif