aboutsummaryrefslogtreecommitdiff
path: root/editors/diff.c
blob: 8b1e92783c82f8a3cd98220b46aa2ec6fc9f5714 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
/* vi: set sw=4 ts=4: */
/*
 * Mini diff implementation for busybox, adapted from OpenBSD diff.
 *
 * Copyright (C) 2010 by Matheus Izvekov <mizvekov@gmail.com>
 * Copyright (C) 2006 by Robert Sullivan <cogito.ergo.cogito@hotmail.com>
 * Copyright (c) 2003 Todd C. Miller <Todd.Miller@courtesan.com>
 *
 * Sponsored in part by the Defense Advanced Research Projects
 * Agency (DARPA) and Air Force Research Laboratory, Air Force
 * Materiel Command, USAF, under agreement number F39502-99-1-0512.
 *
 * Licensed under GPLv2 or later, see file LICENSE in this source tree.
 */

/*
 * The following code uses an algorithm due to Harold Stone,
 * which finds a pair of longest identical subsequences in
 * the two files.
 *
 * The major goal is to generate the match vector J.
 * J[i] is the index of the line in file1 corresponding
 * to line i in file0. J[i] = 0 if there is no
 * such line in file1.
 *
 * Lines are hashed so as to work in core. All potential
 * matches are located by sorting the lines of each file
 * on the hash (called "value"). In particular, this
 * collects the equivalence classes in file1 together.
 * Subroutine equiv replaces the value of each line in
 * file0 by the index of the first element of its
 * matching equivalence in (the reordered) file1.
 * To save space equiv squeezes file1 into a single
 * array member in which the equivalence classes
 * are simply concatenated, except that their first
 * members are flagged by changing sign.
 *
 * Next the indices that point into member are unsorted into
 * array class according to the original order of file0.
 *
 * The cleverness lies in routine stone. This marches
 * through the lines of file0, developing a vector klist
 * of "k-candidates". At step i a k-candidate is a matched
 * pair of lines x,y (x in file0, y in file1) such that
 * there is a common subsequence of length k
 * between the first i lines of file0 and the first y
 * lines of file1, but there is no such subsequence for
 * any smaller y. x is the earliest possible mate to y
 * that occurs in such a subsequence.
 *
 * Whenever any of the members of the equivalence class of
 * lines in file1 matable to a line in file0 has serial number
 * less than the y of some k-candidate, that k-candidate
 * with the smallest such y is replaced. The new
 * k-candidate is chained (via pred) to the current
 * k-1 candidate so that the actual subsequence can
 * be recovered. When a member has serial number greater
 * that the y of all k-candidates, the klist is extended.
 * At the end, the longest subsequence is pulled out
 * and placed in the array J by unravel
 *
 * With J in hand, the matches there recorded are
 * checked against reality to assure that no spurious
 * matches have crept in due to hashing. If they have,
 * they are broken, and "jackpot" is recorded--a harmless
 * matter except that a true match for a spuriously
 * mated line may now be unnecessarily reported as a change.
 *
 * Much of the complexity of the program comes simply
 * from trying to minimize core utilization and
 * maximize the range of doable problems by dynamically
 * allocating what is needed and reusing what is not.
 * The core requirements for problems larger than somewhat
 * are (in words) 2*length(file0) + length(file1) +
 * 3*(number of k-candidates installed), typically about
 * 6n words for files of length n.
 */

//usage:#define diff_trivial_usage
//usage:       "[-abBdiNqrTstw] [-L LABEL] [-S FILE] [-U LINES] FILE1 FILE2"
//usage:#define diff_full_usage "\n\n"
//usage:       "Compare files line by line and output the differences between them.\n"
//usage:       "This implementation supports unified diffs only.\n"
//usage:     "\nOptions:"
//usage:     "\n	-a	Treat all files as text"
//usage:     "\n	-b	Ignore changes in the amount of whitespace"
//usage:     "\n	-B	Ignore changes whose lines are all blank"
//usage:     "\n	-d	Try hard to find a smaller set of changes"
//usage:     "\n	-i	Ignore case differences"
//usage:     "\n	-L	Use LABEL instead of the filename in the unified header"
//usage:     "\n	-N	Treat absent files as empty"
//usage:     "\n	-q	Output only whether files differ"
//usage:     "\n	-r	Recurse"
//usage:     "\n	-S	Start with FILE when comparing directories"
//usage:     "\n	-T	Make tabs line up by prefixing a tab when necessary"
//usage:     "\n	-s	Report when two files are the same"
//usage:     "\n	-t	Expand tabs to spaces in output"
//usage:     "\n	-U	Output LINES lines of context"
//usage:     "\n	-w	Ignore all whitespace"

#include "libbb.h"

#if 0
# define dbg_error_msg(...) bb_error_msg(__VA_ARGS__)
#else
# define dbg_error_msg(...) ((void)0)
#endif

enum {                  /* print_status() and diffreg() return values */
	STATUS_SAME,    /* files are the same */
	STATUS_DIFFER,  /* files differ */
	STATUS_BINARY,  /* binary files differ */
};

enum {                  /* Commandline flags */
	FLAG_a,
	FLAG_b,
	FLAG_d,
	FLAG_i,
	FLAG_L,         /* never used, handled by getopt32 */
	FLAG_N,
	FLAG_q,
	FLAG_r,
	FLAG_s,
	FLAG_S,         /* never used, handled by getopt32 */
	FLAG_t,
	FLAG_T,
	FLAG_U,         /* never used, handled by getopt32 */
	FLAG_w,
	FLAG_u,         /* ignored, this is the default */
	FLAG_p,         /* not implemented */
	FLAG_B,
	FLAG_E,         /* not implemented */
};
#define FLAG(x) (1 << FLAG_##x)

/* We cache file position to avoid excessive seeking */
typedef struct FILE_and_pos_t {
	FILE *ft_fp;
	off_t ft_pos;
} FILE_and_pos_t;

struct globals {
	smallint exit_status;
	int opt_U_context;
	const char *other_dir;
	char *label[2];
	struct stat stb[2];
};
#define G (*ptr_to_globals)
#define exit_status        (G.exit_status       )
#define opt_U_context      (G.opt_U_context     )
#define label              (G.label             )
#define stb                (G.stb               )
#define INIT_G() do { \
	SET_PTR_TO_GLOBALS(xzalloc(sizeof(G))); \
	opt_U_context = 3; \
} while (0)

typedef int token_t;

enum {
	/* Public */
	TOK_EMPTY = 1 << 9,  /* Line fully processed, you can proceed to the next */
	TOK_EOF   = 1 << 10, /* File ended */
	/* Private (Only to be used by read_token() */
	TOK_EOL   = 1 << 11, /* we saw EOL (sticky) */
	TOK_SPACE = 1 << 12, /* used -b code, means we are skipping spaces */
	SHIFT_EOF = (sizeof(token_t)*8 - 8) - 1,
	CHAR_MASK = 0x1ff,   /* 8th bit is used to distinguish EOF from 0xff */
};

/* Restores full EOF from one 8th bit: */
//#define TOK2CHAR(t) (((t) << SHIFT_EOF) >> SHIFT_EOF)
/* We don't really need the above, we only need to have EOF != any_real_char: */
#define TOK2CHAR(t) ((t) & CHAR_MASK)

static void seek_ft(FILE_and_pos_t *ft, off_t pos)
{
	if (ft->ft_pos != pos) {
		ft->ft_pos = pos;
		fseeko(ft->ft_fp, pos, SEEK_SET);
	}
}

/* Reads tokens from given fp, handling -b and -w flags
 * The user must reset tok every line start
 */
static int read_token(FILE_and_pos_t *ft, token_t tok)
{
	tok |= TOK_EMPTY;
	while (!(tok & TOK_EOL)) {
		bool is_space;
		int t;

		t = fgetc(ft->ft_fp);
		if (t != EOF)
			ft->ft_pos++;
		is_space = (t == EOF || isspace(t));

		/* If t == EOF (-1), set both TOK_EOF and TOK_EOL */
		tok |= (t & (TOK_EOF + TOK_EOL));
		/* Only EOL? */
		if (t == '\n')
			tok |= TOK_EOL;

		if (option_mask32 & FLAG(i)) /* Handcoded tolower() */
			t = (t >= 'A' && t <= 'Z') ? t - ('A' - 'a') : t;

		if ((option_mask32 & FLAG(w)) && is_space)
			continue;

		/* Trim char value to low 9 bits */
		t &= CHAR_MASK;

		if (option_mask32 & FLAG(b)) {
			/* Was prev char whitespace? */
			if (tok & TOK_SPACE) { /* yes */
				if (is_space) /* this one too, ignore it */
					continue;
				tok &= ~TOK_SPACE;
			} else if (is_space) {
				/* 1st whitespace char.
				 * Set TOK_SPACE and replace char by ' ' */
				t = TOK_SPACE + ' ';
			}
		}
		/* Clear EMPTY */
		tok &= ~(TOK_EMPTY + CHAR_MASK);
		/* Assign char value (low 9 bits) and maybe set TOK_SPACE */
		tok |= t;
		break;
	}
#if 0
	bb_error_msg("fp:%p tok:%x '%c'%s%s%s%s", fp, tok, tok & 0xff
		, tok & TOK_EOF ? " EOF" : ""
		, tok & TOK_EOL ? " EOL" : ""
		, tok & TOK_EMPTY ? " EMPTY" : ""
		, tok & TOK_SPACE ? " SPACE" : ""
	);
#endif
	return tok;
}

struct cand {
	int x;
	int y;
	int pred;
};

static int search(const int *c, int k, int y, const struct cand *list)
{
	int i, j;

	if (list[c[k]].y < y)  /* quick look for typical case */
		return k + 1;

	for (i = 0, j = k + 1;;) {
		const int l = (i + j) >> 1;
		if (l > i) {
			const int t = list[c[l]].y;
			if (t > y)
				j = l;
			else if (t < y)
				i = l;
			else
				return l;
		} else
			return l + 1;
	}
}

static unsigned isqrt(unsigned n)
{
	unsigned x = 1;
	while (1) {
		const unsigned y = x;
		x = ((n / x) + x) >> 1;
		if (x <= (y + 1) && x >= (y - 1))
			return x;
	}
}

static void stone(const int *a, int n, const int *b, int *J, int pref)
{
	const unsigned isq = isqrt(n);
	const unsigned bound =
		(option_mask32 & FLAG(d)) ? UINT_MAX : MAX(256, isq);
	int clen = 1;
	int clistlen = 100;
	int k = 0;
	struct cand *clist = xzalloc(clistlen * sizeof(clist[0]));
	struct cand cand;
	struct cand *q;
	int *klist = xzalloc((n + 2) * sizeof(klist[0]));
	/*clist[0] = (struct cand){0}; - xzalloc did it */
	/*klist[0] = 0; */

	for (cand.x = 1; cand.x <= n; cand.x++) {
		int j = a[cand.x], oldl = 0;
		unsigned numtries = 0;
		if (j == 0)
			continue;
		cand.y = -b[j];
		cand.pred = klist[0];
		do {
			int l, tc;
			if (cand.y <= clist[cand.pred].y)
				continue;
			l = search(klist, k, cand.y, clist);
			if (l != oldl + 1)
				cand.pred = klist[l - 1];
			if (l <= k && clist[klist[l]].y <= cand.y)
				continue;
			if (clen == clistlen) {
				clistlen = clistlen * 11 / 10;
				clist = xrealloc(clist, clistlen * sizeof(clist[0]));
			}
			clist[clen] = cand;
			tc = klist[l];
			klist[l] = clen++;
			if (l <= k) {
				cand.pred = tc;
				oldl = l;
				numtries++;
			} else {
				k++;
				break;
			}
		} while ((cand.y = b[++j]) > 0 && numtries < bound);
	}
	/* Unravel */
	for (q = clist + klist[k]; q->y; q = clist + q->pred)
		J[q->x + pref] = q->y + pref;
	free(klist);
	free(clist);
}

struct line {
	/* 'serial' is not used in the begining, so we reuse it
	 * to store line offsets, thus reducing memory pressure
	 */
	union {
		unsigned serial;
		off_t offset;
	};
	unsigned value;
};

static void equiv(struct line *a, int n, struct line *b, int m, int *c)
{
	int i = 1, j = 1;

	while (i <= n && j <= m) {
		if (a[i].value < b[j].value)
			a[i++].value = 0;
		else if (a[i].value == b[j].value)
			a[i++].value = j;
		else
			j++;
	}
	while (i <= n)
		a[i++].value = 0;
	b[m + 1].value = 0;
	j = 0;
	while (++j <= m) {
		c[j] = -b[j].serial;
		while (b[j + 1].value == b[j].value) {
			j++;
			c[j] = b[j].serial;
		}
	}
	c[j] = -1;
}

static void unsort(const struct line *f, int l, int *b)
{
	int i;
	int *a = xmalloc((l + 1) * sizeof(a[0]));
	for (i = 1; i <= l; i++)
		a[f[i].serial] = f[i].value;
	for (i = 1; i <= l; i++)
		b[i] = a[i];
	free(a);
}

static int line_compar(const void *a, const void *b)
{
#define l0 ((const struct line*)a)
#define l1 ((const struct line*)b)
	int r = l0->value - l1->value;
	if (r)
		return r;
	return l0->serial - l1->serial;
#undef l0
#undef l1
}

static void fetch(FILE_and_pos_t *ft, const off_t *ix, int a, int b, int ch)
{
	int i, j, col;
	for (i = a; i <= b; i++) {
		seek_ft(ft, ix[i - 1]);
		putchar(ch);
		if (option_mask32 & FLAG(T))
			putchar('\t');
		for (j = 0, col = 0; j < ix[i] - ix[i - 1]; j++) {
			int c = fgetc(ft->ft_fp);
			if (c == EOF) {
				printf("\n\\ No newline at end of file\n");
				return;
			}
			ft->ft_pos++;
			if (c == '\t' && (option_mask32 & FLAG(t)))
				do putchar(' '); while (++col & 7);
			else {
				putchar(c);
				col++;
			}
		}
	}
}

/* Creates the match vector J, where J[i] is the index
 * of the line in the new file corresponding to the line i
 * in the old file. Lines start at 1 instead of 0, that value
 * being used instead to denote no corresponding line.
 * This vector is dynamically allocated and must be freed by the caller.
 *
 * * fp is an input parameter, where fp[0] and fp[1] are the open
 *   old file and new file respectively.
 * * nlen is an output variable, where nlen[0] and nlen[1]
 *   gets the number of lines in the old and new file respectively.
 * * ix is an output variable, where ix[0] and ix[1] gets
 *   assigned dynamically allocated vectors of the offsets of the lines
 *   of the old and new file respectively. These must be freed by the caller.
 */
static NOINLINE int *create_J(FILE_and_pos_t ft[2], int nlen[2], off_t *ix[2])
{
	int *J, slen[2], *class, *member;
	struct line *nfile[2], *sfile[2];
	int pref = 0, suff = 0, i, j, delta;

	/* Lines of both files are hashed, and in the process
	 * their offsets are stored in the array ix[fileno]
	 * where fileno == 0 points to the old file, and
	 * fileno == 1 points to the new one.
	 */
	for (i = 0; i < 2; i++) {
		unsigned hash;
		token_t tok;
		size_t sz = 100;
		nfile[i] = xmalloc((sz + 3) * sizeof(nfile[i][0]));
		/* ft gets here without the correct position, cant use seek_ft */
		ft[i].ft_pos = 0;
		fseeko(ft[i].ft_fp, 0, SEEK_SET);

		nlen[i] = 0;
		/* We could zalloc nfile, but then zalloc starts showing in gprof at ~1% */
		nfile[i][0].offset = 0;
		goto start; /* saves code */
		while (1) {
			tok = read_token(&ft[i], tok);
			if (!(tok & TOK_EMPTY)) {
				/* Hash algorithm taken from Robert Sedgewick, Algorithms in C, 3d ed., p 578. */
				/*hash = hash * 128 - hash + TOK2CHAR(tok);
				 * gcc insists on optimizing above to "hash * 127 + ...", thus... */
				unsigned o = hash - TOK2CHAR(tok);
				hash = hash * 128 - o; /* we want SPEED here */
				continue;
			}
			if (nlen[i]++ == sz) {
				sz = sz * 3 / 2;
				nfile[i] = xrealloc(nfile[i], (sz + 3) * sizeof(nfile[i][0]));
			}
			/* line_compar needs hashes fit into positive int */
			nfile[i][nlen[i]].value = hash & INT_MAX;
			/* like ftello(ft[i].ft_fp) but faster (avoids lseek syscall) */
			nfile[i][nlen[i]].offset = ft[i].ft_pos;
			if (tok & TOK_EOF) {
				/* EOF counts as a token, so we have to adjust it here */
				nfile[i][nlen[i]].offset++;
				break;
			}
start:
			hash = tok = 0;
		}
		/* Exclude lone EOF line from the end of the file, to make fetch()'s job easier */
		if (nfile[i][nlen[i]].offset - nfile[i][nlen[i] - 1].offset == 1)
			nlen[i]--;
		/* Now we copy the line offsets into ix */
		ix[i] = xmalloc((nlen[i] + 2) * sizeof(ix[i][0]));
		for (j = 0; j < nlen[i] + 1; j++)
			ix[i][j] = nfile[i][j].offset;
	}

	/* length of prefix and suffix is calculated */
	for (; pref < nlen[0] && pref < nlen[1] &&
	       nfile[0][pref + 1].value == nfile[1][pref + 1].value;
	       pref++);
	for (; suff < nlen[0] - pref && suff < nlen[1] - pref &&
	       nfile[0][nlen[0] - suff].value == nfile[1][nlen[1] - suff].value;
	       suff++);
	/* Arrays are pruned by the suffix and prefix length,
	 * the result being sorted and stored in sfile[fileno],
	 * and their sizes are stored in slen[fileno]
	 */
	for (j = 0; j < 2; j++) {
		sfile[j] = nfile[j] + pref;
		slen[j] = nlen[j] - pref - suff;
		for (i = 0; i <= slen[j]; i++)
			sfile[j][i].serial = i;
		qsort(sfile[j] + 1, slen[j], sizeof(*sfile[j]), line_compar);
	}
	/* nfile arrays are reused to reduce memory pressure
	 * The #if zeroed out section performs the same task as the
	 * one in the #else section.
	 * Peak memory usage is higher, but one array copy is avoided
	 * by not using unsort()
	 */
#if 0
	member = xmalloc((slen[1] + 2) * sizeof(member[0]));
	equiv(sfile[0], slen[0], sfile[1], slen[1], member);
	free(nfile[1]);

	class = xmalloc((slen[0] + 1) * sizeof(class[0]));
	for (i = 1; i <= slen[0]; i++) /* Unsorting */
		class[sfile[0][i].serial] = sfile[0][i].value;
	free(nfile[0]);
#else
	member = (int *)nfile[1];
	equiv(sfile[0], slen[0], sfile[1], slen[1], member);
	member = xrealloc(member, (slen[1] + 2) * sizeof(member[0]));

	class = (int *)nfile[0];
	unsort(sfile[0], slen[0], (int *)nfile[0]);
	class = xrealloc(class, (slen[0] + 2) * sizeof(class[0]));
#endif
	J = xmalloc((nlen[0] + 2) * sizeof(J[0]));
	/* The elements of J which fall inside the prefix and suffix regions
	 * are marked as unchanged, while the ones which fall outside
	 * are initialized with 0 (no matches), so that function stone can
	 * then assign them their right values
	 */
	for (i = 0, delta = nlen[1] - nlen[0]; i <= nlen[0]; i++)
		J[i] = i <= pref            ?  i :
		       i > (nlen[0] - suff) ? (i + delta) : 0;
	/* Here the magic is performed */
	stone(class, slen[0], member, J, pref);
	J[nlen[0] + 1] = nlen[1] + 1;

	free(class);
	free(member);

	/* Both files are rescanned, in an effort to find any lines
	 * which, due to limitations intrinsic to any hashing algorithm,
	 * are different but ended up confounded as the same
	 */
	for (i = 1; i <= nlen[0]; i++) {
		if (!J[i])
			continue;

		seek_ft(&ft[0], ix[0][i - 1]);
		seek_ft(&ft[1], ix[1][J[i] - 1]);

		for (j = J[i]; i <= nlen[0] && J[i] == j; i++, j++) {
			token_t tok0 = 0, tok1 = 0;
			do {
				tok0 = read_token(&ft[0], tok0);
				tok1 = read_token(&ft[1], tok1);

				if (((tok0 ^ tok1) & TOK_EMPTY) != 0 /* one is empty (not both) */
				 || (!(tok0 & TOK_EMPTY) && TOK2CHAR(tok0) != TOK2CHAR(tok1))
				) {
					J[i] = 0; /* Break the correspondence */
				}
			} while (!(tok0 & tok1 & TOK_EMPTY));
		}
	}

	return J;
}

static bool diff(FILE* fp[2], char *file[2])
{
	int nlen[2];
	off_t *ix[2];
	FILE_and_pos_t ft[2];
	typedef struct { int a, b; } vec_t[2];
	vec_t *vec = NULL;
	int i = 1, j, k, idx = -1;
	bool anychange = false;
	int *J;

	ft[0].ft_fp = fp[0];
	ft[1].ft_fp = fp[1];
	/* note that ft[i].ft_pos is unintitalized, create_J()
	 * must not assume otherwise */
	J = create_J(ft, nlen, ix);

	do {
		bool nonempty = false;

		while (1) {
			vec_t v;

			for (v[0].a = i; v[0].a <= nlen[0] && J[v[0].a] == J[v[0].a - 1] + 1; v[0].a++)
				continue;
			v[1].a = J[v[0].a - 1] + 1;

			for (v[0].b = v[0].a - 1; v[0].b < nlen[0] && !J[v[0].b + 1]; v[0].b++)
				continue;
			v[1].b = J[v[0].b + 1] - 1;
			/*
			 * Indicate that there is a difference between lines a and b of the 'from' file
			 * to get to lines c to d of the 'to' file. If a is greater than b then there
			 * are no lines in the 'from' file involved and this means that there were
			 * lines appended (beginning at b).  If c is greater than d then there are
			 * lines missing from the 'to' file.
			 */
			if (v[0].a <= v[0].b || v[1].a <= v[1].b) {
				/*
				 * If this change is more than 'context' lines from the
				 * previous change, dump the record and reset it.
				 */
				int ct = (2 * opt_U_context) + 1;
				if (idx >= 0
				 && v[0].a > vec[idx][0].b + ct
				 && v[1].a > vec[idx][1].b + ct
				) {
					break;
				}

				for (j = 0; j < 2; j++)
					for (k = v[j].a; k < v[j].b; k++)
						nonempty |= (ix[j][k+1] - ix[j][k] != 1);

				vec = xrealloc_vector(vec, 6, ++idx);
				memcpy(vec[idx], v, sizeof(v));
			}

			i = v[0].b + 1;
			if (i > nlen[0])
				break;
			J[v[0].b] = v[1].b;
		}
		if (idx < 0 || ((option_mask32 & FLAG(B)) && !nonempty))
			goto cont;
		if (!(option_mask32 & FLAG(q))) {
			int lowa;
			vec_t span, *cvp = vec;

			if (!anychange) {
				/* Print the context/unidiff header first time through */
				printf("--- %s\n", label[0] ? label[0] : file[0]);
				printf("+++ %s\n", label[1] ? label[1] : file[1]);
			}

			printf("@@");
			for (j = 0; j < 2; j++) {
				int a = span[j].a = MAX(1, (*cvp)[j].a - opt_U_context);
				int b = span[j].b = MIN(nlen[j], vec[idx][j].b + opt_U_context);

				printf(" %c%d", j ? '+' : '-', MIN(a, b));
				if (a == b)
					continue;
				printf(",%d", (a < b) ? b - a + 1 : 0);
			}
			printf(" @@\n");
			/*
			 * Output changes in "unified" diff format--the old and new lines
			 * are printed together.
			 */
			for (lowa = span[0].a; ; lowa = (*cvp++)[0].b + 1) {
				bool end = cvp > &vec[idx];
				fetch(&ft[0], ix[0], lowa, end ? span[0].b : (*cvp)[0].a - 1, ' ');
				if (end)
					break;
				for (j = 0; j < 2; j++)
					fetch(&ft[j], ix[j], (*cvp)[j].a, (*cvp)[j].b, j ? '+' : '-');
			}
		}
		anychange = true;
 cont:
		idx = -1;
	} while (i <= nlen[0]);

	free(vec);
	free(ix[0]);
	free(ix[1]);
	free(J);
	return anychange;
}

static int diffreg(char *file[2])
{
	FILE *fp[2] = { stdin, stdin };
	bool binary = false, differ = false;
	int status = STATUS_SAME, i;

	for (i = 0; i < 2; i++) {
		int fd = open_or_warn_stdin(file[i]);
		if (fd == -1)
			goto out;
		/* Our diff implementation is using seek.
		 * When we meet non-seekable file, we must make a temp copy.
		 */
		if (lseek(fd, 0, SEEK_SET) == -1 && errno == ESPIPE) {
			char name[] = "/tmp/difXXXXXX";
			int fd_tmp = xmkstemp(name);

			unlink(name);
			if (bb_copyfd_eof(fd, fd_tmp) < 0)
				xfunc_die();
			if (fd) /* Prevents closing of stdin */
				close(fd);
			fd = fd_tmp;
		}
		fp[i] = fdopen(fd, "r");
	}

	while (1) {
		const size_t sz = COMMON_BUFSIZE / 2;
		char *const buf0 = bb_common_bufsiz1;
		char *const buf1 = buf0 + sz;
		int j, k;
		i = fread(buf0, 1, sz, fp[0]);
		j = fread(buf1, 1, sz, fp[1]);
		if (i != j) {
			differ = true;
			i = MIN(i, j);
		}
		if (i == 0)
			break;
		for (k = 0; k < i; k++) {
			if (!buf0[k] || !buf1[k])
				binary = true;
			if (buf0[k] != buf1[k])
				differ = true;
		}
	}
	if (differ) {
		if (binary && !(option_mask32 & FLAG(a)))
			status = STATUS_BINARY;
		else if (diff(fp, file))
			status = STATUS_DIFFER;
	}
	if (status != STATUS_SAME)
		exit_status |= 1;
out:
	fclose_if_not_stdin(fp[0]);
	fclose_if_not_stdin(fp[1]);

	return status;
}

static void print_status(int status, char *path[2])
{
	switch (status) {
	case STATUS_BINARY:
	case STATUS_DIFFER:
		if ((option_mask32 & FLAG(q)) || status == STATUS_BINARY)
			printf("Files %s and %s differ\n", path[0], path[1]);
		break;
	case STATUS_SAME:
		if (option_mask32 & FLAG(s))
			printf("Files %s and %s are identical\n", path[0], path[1]);
		break;
	}
}

#if ENABLE_FEATURE_DIFF_DIR
struct dlist {
	size_t len;
	int s, e;
	char **dl;
};

/* This function adds a filename to dl, the directory listing. */
static int FAST_FUNC add_to_dirlist(const char *filename,
		struct stat *sb UNUSED_PARAM,
		void *userdata, int depth UNUSED_PARAM)
{
	struct dlist *const l = userdata;
	const char *file = filename + l->len;
	while (*file == '/')
		file++;
	l->dl = xrealloc_vector(l->dl, 6, l->e);
	l->dl[l->e] = xstrdup(file);
	l->e++;
	return TRUE;
}

/* If recursion is not set, this function adds the directory
 * to the list and prevents recursive_action from recursing into it.
 */
static int FAST_FUNC skip_dir(const char *filename,
		struct stat *sb, void *userdata,
		int depth)
{
	if (!(option_mask32 & FLAG(r)) && depth) {
		add_to_dirlist(filename, sb, userdata, depth);
		return SKIP;
	}
	if (!(option_mask32 & FLAG(N))) {
		/* -r without -N: no need to recurse into dirs
		 * which do not exist on the "other side".
		 * Testcase: diff -r /tmp /
		 * (it would recurse deep into /proc without this code) */
		struct dlist *const l = userdata;
		filename += l->len;
		if (filename[0]) {
			struct stat osb;
			char *othername = concat_path_file(G.other_dir, filename);
			int r = stat(othername, &osb);
			free(othername);
			if (r != 0 || !S_ISDIR(osb.st_mode)) {
				/* other dir doesn't have similarly named
				 * directory, don't recurse; return 1 upon
				 * exit, just like diffutils' diff */
				exit_status |= 1;
				return SKIP;
			}
		}
	}
	return TRUE;
}

static void diffdir(char *p[2], const char *s_start)
{
	struct dlist list[2];
	int i;

	memset(&list, 0, sizeof(list));
	for (i = 0; i < 2; i++) {
		/*list[i].s = list[i].e = 0; - memset did it */
		/*list[i].dl = NULL; */

		G.other_dir = p[1 - i];
		/* We need to trim root directory prefix.
		 * Using list.len to specify its length,
		 * add_to_dirlist will remove it. */
		list[i].len = strlen(p[i]);
		recursive_action(p[i], ACTION_RECURSE | ACTION_FOLLOWLINKS,
		                 add_to_dirlist, skip_dir, &list[i], 0);
		/* Sort dl alphabetically.
		 * GNU diff does this ignoring any number of trailing dots.
		 * We don't, so for us dotted files almost always are
		 * first on the list.
		 */
		qsort_string_vector(list[i].dl, list[i].e);
		/* If -S was set, find the starting point. */
		if (!s_start)
			continue;
		while (list[i].s < list[i].e && strcmp(list[i].dl[list[i].s], s_start) < 0)
			list[i].s++;
	}
	/* Now that both dirlist1 and dirlist2 contain sorted directory
	 * listings, we can start to go through dirlist1. If both listings
	 * contain the same file, then do a normal diff. Otherwise, behaviour
	 * is determined by whether the -N flag is set. */
	while (1) {
		char *dp[2];
		int pos;
		int k;

		dp[0] = list[0].s < list[0].e ? list[0].dl[list[0].s] : NULL;
		dp[1] = list[1].s < list[1].e ? list[1].dl[list[1].s] : NULL;
		if (!dp[0] && !dp[1])
			break;
		pos = !dp[0] ? 1 : (!dp[1] ? -1 : strcmp(dp[0], dp[1]));
		k = pos > 0;
		if (pos && !(option_mask32 & FLAG(N))) {
			printf("Only in %s: %s\n", p[k], dp[k]);
			exit_status |= 1;
		} else {
			char *fullpath[2], *path[2]; /* if -N */

			for (i = 0; i < 2; i++) {
				if (pos == 0 || i == k) {
					path[i] = fullpath[i] = concat_path_file(p[i], dp[i]);
					stat(fullpath[i], &stb[i]);
				} else {
					fullpath[i] = concat_path_file(p[i], dp[1 - i]);
					path[i] = (char *)bb_dev_null;
				}
			}
			if (pos)
				stat(fullpath[k], &stb[1 - k]);

			if (S_ISDIR(stb[0].st_mode) && S_ISDIR(stb[1].st_mode))
				printf("Common subdirectories: %s and %s\n", fullpath[0], fullpath[1]);
			else if (!S_ISREG(stb[0].st_mode) && !S_ISDIR(stb[0].st_mode))
				printf("File %s is not a regular file or directory and was skipped\n", fullpath[0]);
			else if (!S_ISREG(stb[1].st_mode) && !S_ISDIR(stb[1].st_mode))
				printf("File %s is not a regular file or directory and was skipped\n", fullpath[1]);
			else if (S_ISDIR(stb[0].st_mode) != S_ISDIR(stb[1].st_mode)) {
				if (S_ISDIR(stb[0].st_mode))
					printf("File %s is a %s while file %s is a %s\n", fullpath[0], "directory", fullpath[1], "regular file");
				else
					printf("File %s is a %s while file %s is a %s\n", fullpath[0], "regular file", fullpath[1], "directory");
			} else
				print_status(diffreg(path), fullpath);

			free(fullpath[0]);
			free(fullpath[1]);
		}
		free(dp[k]);
		list[k].s++;
		if (pos == 0) {
			free(dp[1 - k]);
			list[1 - k].s++;
		}
	}
	if (ENABLE_FEATURE_CLEAN_UP) {
		free(list[0].dl);
		free(list[1].dl);
	}
}
#endif

#if ENABLE_FEATURE_DIFF_LONG_OPTIONS
static const char diff_longopts[] ALIGN1 =
	"ignore-case\0"              No_argument       "i"
	"ignore-tab-expansion\0"     No_argument       "E"
	"ignore-space-change\0"      No_argument       "b"
	"ignore-all-space\0"         No_argument       "w"
	"ignore-blank-lines\0"       No_argument       "B"
	"text\0"                     No_argument       "a"
	"unified\0"                  Required_argument "U"
	"label\0"                    Required_argument "L"
	"show-c-function\0"          No_argument       "p"
	"brief\0"                    No_argument       "q"
	"expand-tabs\0"              No_argument       "t"
	"initial-tab\0"              No_argument       "T"
	"recursive\0"                No_argument       "r"
	"new-file\0"                 No_argument       "N"
	"report-identical-files\0"   No_argument       "s"
	"starting-file\0"            Required_argument "S"
	"minimal\0"                  No_argument       "d"
	;
#endif

int diff_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int diff_main(int argc UNUSED_PARAM, char **argv)
{
	int gotstdin = 0, i;
	char *file[2], *s_start = NULL;
	llist_t *L_arg = NULL;

	INIT_G();

	/* exactly 2 params; collect multiple -L <label>; -U N */
	opt_complementary = "=2:L::U+";
#if ENABLE_FEATURE_DIFF_LONG_OPTIONS
	applet_long_options = diff_longopts;
#endif
	getopt32(argv, "abdiL:NqrsS:tTU:wupBE",
			&L_arg, &s_start, &opt_U_context);
	argv += optind;
	while (L_arg)
		label[!!label[0]] = llist_pop(&L_arg);
	xfunc_error_retval = 2;
	for (i = 0; i < 2; i++) {
		file[i] = argv[i];
		/* Compat: "diff file name_which_doesnt_exist" exits with 2 */
		if (LONE_DASH(file[i])) {
			fstat(STDIN_FILENO, &stb[i]);
			gotstdin++;
		} else
			xstat(file[i], &stb[i]);
	}
	xfunc_error_retval = 1;
	if (gotstdin && (S_ISDIR(stb[0].st_mode) || S_ISDIR(stb[1].st_mode)))
		bb_error_msg_and_die("can't compare stdin to a directory");

	/* Compare metadata to check if the files are the same physical file.
	 *
	 * Comment from diffutils source says:
	 * POSIX says that two files are identical if st_ino and st_dev are
	 * the same, but many file systems incorrectly assign the same (device,
	 * inode) pair to two distinct files, including:
	 * GNU/Linux NFS servers that export all local file systems as a
	 * single NFS file system, if a local device number (st_dev) exceeds
	 * 255, or if a local inode number (st_ino) exceeds 16777215.
	 */
	if (ENABLE_DESKTOP
	 && stb[0].st_ino == stb[1].st_ino
	 && stb[0].st_dev == stb[1].st_dev
	 && stb[0].st_size == stb[1].st_size
	 && stb[0].st_mtime == stb[1].st_mtime
	 && stb[0].st_ctime == stb[1].st_ctime
	 && stb[0].st_mode == stb[1].st_mode
	 && stb[0].st_nlink == stb[1].st_nlink
	 && stb[0].st_uid == stb[1].st_uid
	 && stb[0].st_gid == stb[1].st_gid
	) {
		/* files are physically the same; no need to compare them */
		return STATUS_SAME;
	}

	if (S_ISDIR(stb[0].st_mode) && S_ISDIR(stb[1].st_mode)) {
#if ENABLE_FEATURE_DIFF_DIR
		diffdir(file, s_start);
#else
		bb_error_msg_and_die("no support for directory comparison");
#endif
	} else {
		bool dirfile = S_ISDIR(stb[0].st_mode) || S_ISDIR(stb[1].st_mode);
		bool dir = S_ISDIR(stb[1].st_mode);
		if (dirfile) {
			const char *slash = strrchr(file[!dir], '/');
			file[dir] = concat_path_file(file[dir], slash ? slash + 1 : file[!dir]);
			xstat(file[dir], &stb[dir]);
		}
		/* diffreg can get non-regular files here */
		print_status(gotstdin > 1 ? STATUS_SAME : diffreg(file), file);

		if (dirfile)
			free(file[dir]);
	}

	return exit_status;
}