view mercurial/thirdparty/xdiff/xprepare.c @ 36820:f33a87cf60cc

xdiff: add a preprocessing step that trims files

xdiff has a `xdl_trim_ends` step that removes common lines, unmatchable
lines. That is in theory good, but happens too late - after splitting,
hashing, and adjusting the hash values so they are unique. Those splitting,
hashing and adjusting hash values steps could have noticeable overhead.

Diffing two large files with minor (one-line-ish) changes are not uncommon.
In that case, the raw performance of those preparation steps seriously
matter. Even allocating an O(N) array and storing line offsets to it is
expensive.  Therefore my previous attempts [1] [2] cannot be good enough
since they do not remove the O(N) array assignment.

This patch adds a preprocessing step - `xdl_trim_files` that runs before
other preprocessing steps. It counts common prefix and suffix and lines in
them (needed for displaying line number), without doing anything else.

Testing with a crafted large (169MB) file, with minor change:

```
  open('a','w').write(''.join('%s\n' % (i % 100000) for i in xrange(30000000) if i != 6000000))
  open('b','w').write(''.join('%s\n' % (i % 100000) for i in xrange(30000000) if i != 6003000))
```

Running xdiff by a simple binary [3], this patch improves the xdiff perf by
more than 10x for the above case:

```
  # xdiff before this patch
  2.41s user 1.13s system 98% cpu 3.592 total
  # xdiff after this patch
  0.14s user 0.16s system 98% cpu 0.309 total
  # gnu diffutils
  0.12s user 0.15s system 98% cpu 0.272 total
  # (best of 20 runs)
```

It's still slightly slower than GNU diffutils. But it's pretty close now.

Testing with real repo data:

For the whole repo, this patch makes xdiff 25% faster:

```
  # hg perfbdiff --count 100 --alldata -c d334afc585e2 --blocks [--xdiff]
  # xdiff, after
  ! wall 0.058861 comb 0.050000 user 0.050000 sys 0.000000 (best of 100)
  # xdiff, before
  ! wall 0.077816 comb 0.080000 user 0.080000 sys 0.000000 (best of 91)
  # bdiff
  ! wall 0.117473 comb 0.120000 user 0.120000 sys 0.000000 (best of 67)
```

For files that are long (ex. commands.py), the speedup is more than 3x, very
significant:

```
  # hg perfbdiff --count 3000 --blocks commands.py.i 1 [--xdiff]
  # xdiff, after
  ! wall 0.690583 comb 0.690000 user 0.690000 sys 0.000000 (best of 12)
  # xdiff, before
  ! wall 2.240361 comb 2.210000 user 2.210000 sys 0.000000 (best of 4)
  # bdiff
  ! wall 2.469852 comb 2.440000 user 2.440000 sys 0.000000 (best of 4)
```

[1]: https://phab.mercurial-scm.org/D2631
[2]: https://phab.mercurial-scm.org/D2634
[3]:

```
// Code to run xdiff from command line. No proper error handling.
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "mercurial/thirdparty/xdiff/xdiff.h"
#define ensure(x) if (!(x)) exit(255);
mmfile_t readfile(const char *path) {
  struct stat st; int fd = open(path, O_RDONLY);
  fstat(fd, &st); mmfile_t file = { malloc(st.st_size), st.st_size };
  ensure(read(fd, file.ptr, st.st_size) == st.st_size); close(fd);
  return file;
}
int main(int argc, char const *argv[]) {
  mmfile_t a = readfile(argv[1]), b = readfile(argv[2]);
  xpparam_t xpp = {0}; xdemitconf_t xecfg = {0}; xdemitcb_t ecb = {0};
  xdl_diff(&a, &b, &xpp, &xecfg, &ecb);
  return 0;
}
```

Differential Revision: https://phab.mercurial-scm.org/D2686

/*
 *  LibXDiff by Davide Libenzi ( File Differential Library )
 *  Copyright (C) 2003  Davide Libenzi
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Lesser General Public
 *  License as published by the Free Software Foundation; either
 *  version 2.1 of the License, or (at your option) any later version.
 *
 *  This library is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *  Lesser General Public License for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public
 *  License along with this library; if not, see
 *  <http://www.gnu.org/licenses/>.
 *
 *  Davide Libenzi <[e-mail]>
 *
 */

#include "xinclude.h"


#define XDL_KPDIS_RUN 4
#define XDL_MAX_EQLIMIT 1024
#define XDL_SIMSCAN_WINDOW 100
#define XDL_GUESS_NLINES1 256


typedef struct s_xdlclass {
	struct s_xdlclass *next;
	unsigned long ha;
	char const *line;
	long size;
	long idx;
	long len1, len2;
} xdlclass_t;

typedef struct s_xdlclassifier {
	unsigned int hbits;
	long hsize;
	xdlclass_t **rchash;
	chastore_t ncha;
	xdlclass_t **rcrecs;
	long alloc;
	long count;
	long flags;
} xdlclassifier_t;




static int xdl_init_classifier(xdlclassifier_t *cf, long size, long flags);
static void xdl_free_classifier(xdlclassifier_t *cf);
static int xdl_classify_record(unsigned int pass, xdlclassifier_t *cf, xrecord_t **rhash,
			       unsigned int hbits, xrecord_t *rec);
static int xdl_prepare_ctx(unsigned int pass, mmfile_t *mf, long narec, xpparam_t const *xpp,
			   xdlclassifier_t *cf, xdfile_t *xdf);
static void xdl_free_ctx(xdfile_t *xdf);
static int xdl_clean_mmatch(char const *dis, long i, long s, long e);
static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xdf2);
static int xdl_trim_ends(xdfile_t *xdf1, xdfile_t *xdf2);
static int xdl_optimize_ctxs(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xdf2);




static int xdl_init_classifier(xdlclassifier_t *cf, long size, long flags) {
	cf->flags = flags;

	cf->hbits = xdl_hashbits((unsigned int) size);
	cf->hsize = 1 << cf->hbits;

	if (xdl_cha_init(&cf->ncha, sizeof(xdlclass_t), size / 4 + 1) < 0) {

		return -1;
	}
	if (!(cf->rchash = (xdlclass_t **) xdl_malloc(cf->hsize * sizeof(xdlclass_t *)))) {

		xdl_cha_free(&cf->ncha);
		return -1;
	}
	memset(cf->rchash, 0, cf->hsize * sizeof(xdlclass_t *));

	cf->alloc = size;
	if (!(cf->rcrecs = (xdlclass_t **) xdl_malloc(cf->alloc * sizeof(xdlclass_t *)))) {

		xdl_free(cf->rchash);
		xdl_cha_free(&cf->ncha);
		return -1;
	}

	cf->count = 0;

	return 0;
}


static void xdl_free_classifier(xdlclassifier_t *cf) {

	xdl_free(cf->rcrecs);
	xdl_free(cf->rchash);
	xdl_cha_free(&cf->ncha);
}


static int xdl_classify_record(unsigned int pass, xdlclassifier_t *cf, xrecord_t **rhash,
			       unsigned int hbits, xrecord_t *rec) {
	long hi;
	char const *line;
	xdlclass_t *rcrec;
	xdlclass_t **rcrecs;

	line = rec->ptr;
	hi = (long) XDL_HASHLONG(rec->ha, cf->hbits);
	for (rcrec = cf->rchash[hi]; rcrec; rcrec = rcrec->next)
		if (rcrec->ha == rec->ha &&
				xdl_recmatch(rcrec->line, rcrec->size,
					rec->ptr, rec->size, cf->flags))
			break;

	if (!rcrec) {
		if (!(rcrec = xdl_cha_alloc(&cf->ncha))) {

			return -1;
		}
		rcrec->idx = cf->count++;
		if (cf->count > cf->alloc) {
			cf->alloc *= 2;
			if (!(rcrecs = (xdlclass_t **) xdl_realloc(cf->rcrecs, cf->alloc * sizeof(xdlclass_t *)))) {

				return -1;
			}
			cf->rcrecs = rcrecs;
		}
		cf->rcrecs[rcrec->idx] = rcrec;
		rcrec->line = line;
		rcrec->size = rec->size;
		rcrec->ha = rec->ha;
		rcrec->len1 = rcrec->len2 = 0;
		rcrec->next = cf->rchash[hi];
		cf->rchash[hi] = rcrec;
	}

	(pass == 1) ? rcrec->len1++ : rcrec->len2++;

	rec->ha = (unsigned long) rcrec->idx;

	hi = (long) XDL_HASHLONG(rec->ha, hbits);
	rec->next = rhash[hi];
	rhash[hi] = rec;

	return 0;
}


/*
 * Trim common prefix from files.
 *
 * Note: trimming could affect hunk shifting. But the performance benefit
 * outweighs the shift change. A diff result with suboptimal shifting is still
 * valid.
 */
static void xdl_trim_files(mmfile_t *mf1, mmfile_t *mf2, long reserved,
		xdfenv_t *xe, mmfile_t *out_mf1, mmfile_t *out_mf2) {
	mmfile_t msmall, mlarge;
	/* prefix lines, prefix bytes, suffix lines, suffix bytes */
	long plines = 0, pbytes = 0, slines = 0, sbytes = 0, i;
	/* prefix char pointer for msmall and mlarge */
	const char *pp1, *pp2;
	/* suffix char pointer for msmall and mlarge */
	const char *ps1, *ps2;

	/* reserved must >= 0 for the line boundary adjustment to work */
	if (reserved < 0)
		reserved = 0;

	if (mf1->size < mf2->size) {
		memcpy(&msmall, mf1, sizeof(mmfile_t));
		memcpy(&mlarge, mf2, sizeof(mmfile_t));
	} else {
		memcpy(&msmall, mf2, sizeof(mmfile_t));
		memcpy(&mlarge, mf1, sizeof(mmfile_t));
	}

	pp1 = msmall.ptr, pp2 = mlarge.ptr;
	for (i = 0; i < msmall.size && *pp1 == *pp2; ++i) {
		plines += (*pp1 == '\n');
		pp1++, pp2++;
	}

	ps1 = msmall.ptr + msmall.size - 1, ps2 = mlarge.ptr + mlarge.size - 1;
	while (ps1 > pp1 && *ps1 == *ps2) {
		slines += (*ps1 == '\n');
		ps1--, ps2--;
	}

	/* Retract common prefix and suffix boundaries for reserved lines */
	if (plines <= reserved + 1) {
		plines = 0;
	} else {
		i = 0;
		while (i <= reserved) {
			pp1--;
			i += (*pp1 == '\n');
		}
		/* The new mmfile starts at the next char just after '\n' */
		pbytes = pp1 - msmall.ptr + 1;
		plines -= reserved;
	}

	if (slines <= reserved + 1) {
		slines = 0;
	} else {
		/* Note: with compiler SIMD support (ex. -O3 -mavx2), this
		 * might perform better than memchr. */
		i = 0;
		while (i <= reserved) {
			ps1++;
			i += (*ps1 == '\n');
		}
		/* The new mmfile includes this '\n' */
		sbytes = msmall.ptr + msmall.size - ps1 - 1;
		slines -= reserved;
		if (msmall.ptr[msmall.size - 1] == '\n')
			slines -= 1;
	}

	xe->nprefix = plines;
	xe->nsuffix = slines;
	out_mf1->ptr = mf1->ptr + pbytes;
	out_mf1->size = mf1->size - pbytes - sbytes;
	out_mf2->ptr = mf2->ptr + pbytes;
	out_mf2->size = mf2->size - pbytes - sbytes;
}


static int xdl_prepare_ctx(unsigned int pass, mmfile_t *mf, long narec, xpparam_t const *xpp,
			   xdlclassifier_t *cf, xdfile_t *xdf) {
	unsigned int hbits;
	long nrec, hsize, bsize;
	unsigned long hav;
	char const *blk, *cur, *top, *prev;
	xrecord_t *crec;
	xrecord_t **recs, **rrecs;
	xrecord_t **rhash;
	unsigned long *ha;
	char *rchg;
	long *rindex;

	ha = NULL;
	rindex = NULL;
	rchg = NULL;
	rhash = NULL;
	recs = NULL;

	if (xdl_cha_init(&xdf->rcha, sizeof(xrecord_t), narec / 4 + 1) < 0)
		goto abort;
	if (!(recs = (xrecord_t **) xdl_malloc(narec * sizeof(xrecord_t *))))
		goto abort;

	{
		hbits = xdl_hashbits((unsigned int) narec);
		hsize = 1 << hbits;
		if (!(rhash = (xrecord_t **) xdl_malloc(hsize * sizeof(xrecord_t *))))
			goto abort;
		memset(rhash, 0, hsize * sizeof(xrecord_t *));
	}

	nrec = 0;
	if ((cur = blk = xdl_mmfile_first(mf, &bsize)) != NULL) {
		for (top = blk + bsize; cur < top; ) {
			prev = cur;
			hav = xdl_hash_record(&cur, top, xpp->flags);
			if (nrec >= narec) {
				narec *= 2;
				if (!(rrecs = (xrecord_t **) xdl_realloc(recs, narec * sizeof(xrecord_t *))))
					goto abort;
				recs = rrecs;
			}
			if (!(crec = xdl_cha_alloc(&xdf->rcha)))
				goto abort;
			crec->ptr = prev;
			crec->size = (long) (cur - prev);
			crec->ha = hav;
			recs[nrec++] = crec;

			if (xdl_classify_record(pass, cf, rhash, hbits, crec) < 0)
				goto abort;
		}
	}

	if (!(rchg = (char *) xdl_malloc((nrec + 2) * sizeof(char))))
		goto abort;
	memset(rchg, 0, (nrec + 2) * sizeof(char));

	if (!(rindex = (long *) xdl_malloc((nrec + 1) * sizeof(long))))
		goto abort;
	if (!(ha = (unsigned long *) xdl_malloc((nrec + 1) * sizeof(unsigned long))))
		goto abort;

	xdf->nrec = nrec;
	xdf->recs = recs;
	xdf->hbits = hbits;
	xdf->rhash = rhash;
	xdf->rchg = rchg + 1;
	xdf->rindex = rindex;
	xdf->nreff = 0;
	xdf->ha = ha;
	xdf->dstart = 0;
	xdf->dend = nrec - 1;

	return 0;

abort:
	xdl_free(ha);
	xdl_free(rindex);
	xdl_free(rchg);
	xdl_free(rhash);
	xdl_free(recs);
	xdl_cha_free(&xdf->rcha);
	return -1;
}


static void xdl_free_ctx(xdfile_t *xdf) {

	xdl_free(xdf->rhash);
	xdl_free(xdf->rindex);
	xdl_free(xdf->rchg - 1);
	xdl_free(xdf->ha);
	xdl_free(xdf->recs);
	xdl_cha_free(&xdf->rcha);
}

/* Reserved lines for trimming, to leave room for shifting */
#define TRIM_RESERVED_LINES 100

int xdl_prepare_env(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
		    xdfenv_t *xe) {
	long enl1, enl2, sample;
	mmfile_t tmf1, tmf2;
	xdlclassifier_t cf;

	memset(&cf, 0, sizeof(cf));

	sample = XDL_GUESS_NLINES1;

	enl1 = xdl_guess_lines(mf1, sample) + 1;
	enl2 = xdl_guess_lines(mf2, sample) + 1;

	if (xdl_init_classifier(&cf, enl1 + enl2 + 1, xpp->flags) < 0)
		return -1;

	xdl_trim_files(mf1, mf2, TRIM_RESERVED_LINES, xe, &tmf1, &tmf2);

	if (xdl_prepare_ctx(1, &tmf1, enl1, xpp, &cf, &xe->xdf1) < 0) {

		xdl_free_classifier(&cf);
		return -1;
	}
	if (xdl_prepare_ctx(2, &tmf2, enl2, xpp, &cf, &xe->xdf2) < 0) {

		xdl_free_ctx(&xe->xdf1);
		xdl_free_classifier(&cf);
		return -1;
	}

	if (xdl_optimize_ctxs(&cf, &xe->xdf1, &xe->xdf2) < 0) {
		xdl_free_ctx(&xe->xdf2);
		xdl_free_ctx(&xe->xdf1);
		xdl_free_classifier(&cf);
		return -1;
	}

	xdl_free_classifier(&cf);

	return 0;
}


void xdl_free_env(xdfenv_t *xe) {

	xdl_free_ctx(&xe->xdf2);
	xdl_free_ctx(&xe->xdf1);
}


static int xdl_clean_mmatch(char const *dis, long i, long s, long e) {
	long r, rdis0, rpdis0, rdis1, rpdis1;

	/*
	 * Limits the window the is examined during the similar-lines
	 * scan. The loops below stops when dis[i - r] == 1 (line that
	 * has no match), but there are corner cases where the loop
	 * proceed all the way to the extremities by causing huge
	 * performance penalties in case of big files.
	 */
	if (i - s > XDL_SIMSCAN_WINDOW)
		s = i - XDL_SIMSCAN_WINDOW;
	if (e - i > XDL_SIMSCAN_WINDOW)
		e = i + XDL_SIMSCAN_WINDOW;

	/*
	 * Scans the lines before 'i' to find a run of lines that either
	 * have no match (dis[j] == 0) or have multiple matches (dis[j] > 1).
	 * Note that we always call this function with dis[i] > 1, so the
	 * current line (i) is already a multimatch line.
	 */
	for (r = 1, rdis0 = 0, rpdis0 = 1; (i - r) >= s; r++) {
		if (!dis[i - r])
			rdis0++;
		else if (dis[i - r] == 2)
			rpdis0++;
		else
			break;
	}
	/*
	 * If the run before the line 'i' found only multimatch lines, we
	 * return 0 and hence we don't make the current line (i) discarded.
	 * We want to discard multimatch lines only when they appear in the
	 * middle of runs with nomatch lines (dis[j] == 0).
	 */
	if (rdis0 == 0)
		return 0;
	for (r = 1, rdis1 = 0, rpdis1 = 1; (i + r) <= e; r++) {
		if (!dis[i + r])
			rdis1++;
		else if (dis[i + r] == 2)
			rpdis1++;
		else
			break;
	}
	/*
	 * If the run after the line 'i' found only multimatch lines, we
	 * return 0 and hence we don't make the current line (i) discarded.
	 */
	if (rdis1 == 0)
		return 0;
	rdis1 += rdis0;
	rpdis1 += rpdis0;

	return rpdis1 * XDL_KPDIS_RUN < (rpdis1 + rdis1);
}


/*
 * Try to reduce the problem complexity, discard records that have no
 * matches on the other file. Also, lines that have multiple matches
 * might be potentially discarded if they happear in a run of discardable.
 */
static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xdf2) {
	long i, nm, nreff, mlim;
	xrecord_t **recs;
	xdlclass_t *rcrec;
	char *dis, *dis1, *dis2;

	if (!(dis = (char *) xdl_malloc(xdf1->nrec + xdf2->nrec + 2))) {

		return -1;
	}
	memset(dis, 0, xdf1->nrec + xdf2->nrec + 2);
	dis1 = dis;
	dis2 = dis1 + xdf1->nrec + 1;

	if ((mlim = xdl_bogosqrt(xdf1->nrec)) > XDL_MAX_EQLIMIT)
		mlim = XDL_MAX_EQLIMIT;
	for (i = xdf1->dstart, recs = &xdf1->recs[xdf1->dstart]; i <= xdf1->dend; i++, recs++) {
		rcrec = cf->rcrecs[(*recs)->ha];
		nm = rcrec ? rcrec->len2 : 0;
		dis1[i] = (nm == 0) ? 0: (nm >= mlim) ? 2: 1;
	}

	if ((mlim = xdl_bogosqrt(xdf2->nrec)) > XDL_MAX_EQLIMIT)
		mlim = XDL_MAX_EQLIMIT;
	for (i = xdf2->dstart, recs = &xdf2->recs[xdf2->dstart]; i <= xdf2->dend; i++, recs++) {
		rcrec = cf->rcrecs[(*recs)->ha];
		nm = rcrec ? rcrec->len1 : 0;
		dis2[i] = (nm == 0) ? 0: (nm >= mlim) ? 2: 1;
	}

	for (nreff = 0, i = xdf1->dstart, recs = &xdf1->recs[xdf1->dstart];
	     i <= xdf1->dend; i++, recs++) {
		if (dis1[i] == 1 ||
		    (dis1[i] == 2 && !xdl_clean_mmatch(dis1, i, xdf1->dstart, xdf1->dend))) {
			xdf1->rindex[nreff] = i;
			xdf1->ha[nreff] = (*recs)->ha;
			nreff++;
		} else
			xdf1->rchg[i] = 1;
	}
	xdf1->nreff = nreff;

	for (nreff = 0, i = xdf2->dstart, recs = &xdf2->recs[xdf2->dstart];
	     i <= xdf2->dend; i++, recs++) {
		if (dis2[i] == 1 ||
		    (dis2[i] == 2 && !xdl_clean_mmatch(dis2, i, xdf2->dstart, xdf2->dend))) {
			xdf2->rindex[nreff] = i;
			xdf2->ha[nreff] = (*recs)->ha;
			nreff++;
		} else
			xdf2->rchg[i] = 1;
	}
	xdf2->nreff = nreff;

	xdl_free(dis);

	return 0;
}


/*
 * Early trim initial and terminal matching records.
 */
static int xdl_trim_ends(xdfile_t *xdf1, xdfile_t *xdf2) {
	long i, lim;
	xrecord_t **recs1, **recs2;

	recs1 = xdf1->recs;
	recs2 = xdf2->recs;
	for (i = 0, lim = XDL_MIN(xdf1->nrec, xdf2->nrec); i < lim;
	     i++, recs1++, recs2++)
		if ((*recs1)->ha != (*recs2)->ha)
			break;

	xdf1->dstart = xdf2->dstart = i;

	recs1 = xdf1->recs + xdf1->nrec - 1;
	recs2 = xdf2->recs + xdf2->nrec - 1;
	for (lim -= i, i = 0; i < lim; i++, recs1--, recs2--)
		if ((*recs1)->ha != (*recs2)->ha)
			break;

	xdf1->dend = xdf1->nrec - i - 1;
	xdf2->dend = xdf2->nrec - i - 1;

	return 0;
}


static int xdl_optimize_ctxs(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xdf2) {

	if (xdl_trim_ends(xdf1, xdf2) < 0 ||
	    xdl_cleanup_records(cf, xdf1, xdf2) < 0) {

		return -1;
	}

	return 0;
}