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Diffstat (limited to 'lib/diffseq.h')
| -rw-r--r-- | lib/diffseq.h | 425 |
1 files changed, 425 insertions, 0 deletions
diff --git a/lib/diffseq.h b/lib/diffseq.h new file mode 100644 index 0000000..6be7d83 --- /dev/null +++ b/lib/diffseq.h @@ -0,0 +1,425 @@ +/* Analyze differences between two vectors. + + Copyright (C) 1988-1989, 1992-1995, 2001-2004, 2006-2016 Free Software + Foundation, Inc. + + This program is free software: you can redistribute it and/or modify + it under the terms of the GNU General Public License as published by + the Free Software Foundation; either version 3 of the License, or + (at your option) any later version. + + This program 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 General Public License for more details. + + You should have received a copy of the GNU General Public License + along with this program. If not, see <http://www.gnu.org/licenses/>. */ + + +/* The basic idea is to consider two vectors as similar if, when + transforming the first vector into the second vector through a + sequence of edits (inserts and deletes of one element each), + this sequence is short - or equivalently, if the ordered list + of elements that are untouched by these edits is long. For a + good introduction to the subject, read about the "Levenshtein + distance" in Wikipedia. + + The basic algorithm is described in: + "An O(ND) Difference Algorithm and its Variations", Eugene W. Myers, + Algorithmica Vol. 1, 1986, pp. 251-266, + <http://dx.doi.org/10.1007/BF01840446>. + See especially section 4.2, which describes the variation used below. + + The basic algorithm was independently discovered as described in: + "Algorithms for Approximate String Matching", Esko Ukkonen, + Information and Control Vol. 64, 1985, pp. 100-118, + <http://dx.doi.org/10.1016/S0019-9958(85)80046-2>. */ + +/* Before including this file, you need to define: + ELEMENT The element type of the vectors being compared. + EQUAL A two-argument macro that tests two elements for + equality. + OFFSET A signed integer type sufficient to hold the + difference between two indices. Usually + something like ptrdiff_t. + EXTRA_CONTEXT_FIELDS Declarations of fields for 'struct context'. + NOTE_DELETE(ctxt, xoff) Record the removal of the object xvec[xoff]. + NOTE_INSERT(ctxt, yoff) Record the insertion of the object yvec[yoff]. + EARLY_ABORT(ctxt) (Optional) A boolean expression that triggers an + early abort of the computation. + USE_HEURISTIC (Optional) Define if you want to support the + heuristic for large vectors. + It is also possible to use this file with abstract arrays. In this case, + xvec and yvec are not represented in memory. They only exist conceptually. + In this case, the list of defines above is amended as follows: + ELEMENT Undefined. + EQUAL Undefined. + XVECREF_YVECREF_EQUAL(ctxt, xoff, yoff) + A three-argument macro: References xvec[xoff] and + yvec[yoff] and tests these elements for equality. + Before including this file, you also need to include: + #include <limits.h> + #include <stdbool.h> + #include "minmax.h" + */ + +/* Maximum value of type OFFSET. */ +#define OFFSET_MAX \ + ((((OFFSET)1 << (sizeof (OFFSET) * CHAR_BIT - 2)) - 1) * 2 + 1) + +/* Default to no early abort. */ +#ifndef EARLY_ABORT +# define EARLY_ABORT(ctxt) false +#endif + +/* Use this to suppress gcc's "...may be used before initialized" warnings. + Beware: The Code argument must not contain commas. */ +#ifndef IF_LINT +# if defined GCC_LINT || defined lint +# define IF_LINT(Code) Code +# else +# define IF_LINT(Code) /* empty */ +# endif +#endif + +/* As above, but when Code must contain one comma. */ +#ifndef IF_LINT2 +# if defined GCC_LINT || defined lint +# define IF_LINT2(Code1, Code2) Code1, Code2 +# else +# define IF_LINT2(Code1, Code2) /* empty */ +# endif +#endif + +/* + * Context of comparison operation. + */ +struct context +{ + #ifdef ELEMENT + /* Vectors being compared. */ + ELEMENT const *xvec; + ELEMENT const *yvec; + #endif + + /* Extra fields. */ + EXTRA_CONTEXT_FIELDS + + /* Vector, indexed by diagonal, containing 1 + the X coordinate of the point + furthest along the given diagonal in the forward search of the edit + matrix. */ + OFFSET *fdiag; + + /* Vector, indexed by diagonal, containing the X coordinate of the point + furthest along the given diagonal in the backward search of the edit + matrix. */ + OFFSET *bdiag; + + #ifdef USE_HEURISTIC + /* This corresponds to the diff --speed-large-files flag. With this + heuristic, for vectors with a constant small density of changes, + the algorithm is linear in the vector size. */ + bool heuristic; + #endif + + /* Snakes bigger than this are considered "big". */ + #define SNAKE_LIMIT 20 +}; + +struct partition +{ + /* Midpoints of this partition. */ + OFFSET xmid; + OFFSET ymid; +}; + + +/* Find the midpoint of the shortest edit script for a specified portion + of the two vectors. + + Scan from the beginnings of the vectors, and simultaneously from the ends, + doing a breadth-first search through the space of edit-sequence. + When the two searches meet, we have found the midpoint of the shortest + edit sequence. + + Set *PART to the midpoint (XMID,YMID). The diagonal number + XMID - YMID equals the number of inserted elements minus the number + of deleted elements (counting only elements before the midpoint). + + This function assumes that the first elements of the specified portions + of the two vectors do not match, and likewise that the last elements do not + match. The caller must trim matching elements from the beginning and end + of the portions it is going to specify. + + If we return the "wrong" partitions, the worst this can do is cause + suboptimal diff output. It cannot cause incorrect diff output. */ + +static void +diag (OFFSET xoff, OFFSET xlim, OFFSET yoff, OFFSET ylim, + struct partition *part, struct context *ctxt) +{ + OFFSET *const fd = ctxt->fdiag; /* Give the compiler a chance. */ + OFFSET *const bd = ctxt->bdiag; /* Additional help for the compiler. */ +#ifdef ELEMENT + ELEMENT const *const xv = ctxt->xvec; /* Still more help for the compiler. */ + ELEMENT const *const yv = ctxt->yvec; /* And more and more . . . */ + #define XREF_YREF_EQUAL(x,y) EQUAL (xv[x], yv[y]) +#else + #define XREF_YREF_EQUAL(x,y) XVECREF_YVECREF_EQUAL (ctxt, x, y) +#endif + const OFFSET dmin = xoff - ylim; /* Minimum valid diagonal. */ + const OFFSET dmax = xlim - yoff; /* Maximum valid diagonal. */ + const OFFSET fmid = xoff - yoff; /* Center diagonal of top-down search. */ + const OFFSET bmid = xlim - ylim; /* Center diagonal of bottom-up search. */ + OFFSET fmin = fmid; + OFFSET fmax = fmid; /* Limits of top-down search. */ + OFFSET bmin = bmid; + OFFSET bmax = bmid; /* Limits of bottom-up search. */ + OFFSET c; /* Cost. */ + bool odd = (fmid - bmid) & 1; /* True if southeast corner is on an odd + diagonal with respect to the northwest. */ + + fd[fmid] = xoff; + bd[bmid] = xlim; + + for (c = 1;; ++c) + { + OFFSET d; /* Active diagonal. */ + bool big_snake = false; + + /* Extend the top-down search by an edit step in each diagonal. */ + if (fmin > dmin) + fd[--fmin - 1] = -1; + else + ++fmin; + if (fmax < dmax) + fd[++fmax + 1] = -1; + else + --fmax; + for (d = fmax; d >= fmin; d -= 2) + { + OFFSET x; + OFFSET y; + OFFSET tlo = fd[d - 1]; + OFFSET thi = fd[d + 1]; + OFFSET x0 = tlo < thi ? thi : tlo + 1; + + for (x = x0, y = x0 - d; + x < xlim && y < ylim && XREF_YREF_EQUAL (x, y); + x++, y++) + continue; + if (x - x0 > SNAKE_LIMIT) + big_snake = true; + fd[d] = x; + if (odd && bmin <= d && d <= bmax && bd[d] <= x) + { + part->xmid = x; + part->ymid = y; + return; + } + } + + /* Similarly extend the bottom-up search. */ + if (bmin > dmin) + bd[--bmin - 1] = OFFSET_MAX; + else + ++bmin; + if (bmax < dmax) + bd[++bmax + 1] = OFFSET_MAX; + else + --bmax; + for (d = bmax; d >= bmin; d -= 2) + { + OFFSET x; + OFFSET y; + OFFSET tlo = bd[d - 1]; + OFFSET thi = bd[d + 1]; + OFFSET x0 = tlo < thi ? tlo : thi - 1; + + for (x = x0, y = x0 - d; + xoff < x && yoff < y && XREF_YREF_EQUAL (x - 1, y - 1); + x--, y--) + continue; + if (x0 - x > SNAKE_LIMIT) + big_snake = true; + bd[d] = x; + if (!odd && fmin <= d && d <= fmax && x <= fd[d]) + { + part->xmid = x; + part->ymid = y; + return; + } + } + +#ifdef USE_HEURISTIC + /* Heuristic: check occasionally for a diagonal that has made lots + of progress compared with the edit distance. If we have any + such, find the one that has made the most progress and return it + as if it had succeeded. + + With this heuristic, for vectors with a constant small density + of changes, the algorithm is linear in the vector size. */ + + if (200 < c && big_snake && ctxt->heuristic) + { + { + OFFSET best = 0; + + for (d = fmax; d >= fmin; d -= 2) + { + OFFSET dd = d - fmid; + OFFSET x = fd[d]; + OFFSET y = x - d; + OFFSET v = (x - xoff) * 2 - dd; + + if (v > 12 * (c + (dd < 0 ? -dd : dd))) + { + if (v > best + && xoff + SNAKE_LIMIT <= x && x < xlim + && yoff + SNAKE_LIMIT <= y && y < ylim) + { + /* We have a good enough best diagonal; now insist + that it end with a significant snake. */ + int k; + + for (k = 1; XREF_YREF_EQUAL (x - k, y - k); k++) + if (k == SNAKE_LIMIT) + { + best = v; + part->xmid = x; + part->ymid = y; + break; + } + } + } + } + if (best > 0) + return; + } + + { + OFFSET best = 0; + + for (d = bmax; d >= bmin; d -= 2) + { + OFFSET dd = d - bmid; + OFFSET x = bd[d]; + OFFSET y = x - d; + OFFSET v = (xlim - x) * 2 + dd; + + if (v > 12 * (c + (dd < 0 ? -dd : dd))) + { + if (v > best + && xoff < x && x <= xlim - SNAKE_LIMIT + && yoff < y && y <= ylim - SNAKE_LIMIT) + { + /* We have a good enough best diagonal; now insist + that it end with a significant snake. */ + int k; + + for (k = 0; XREF_YREF_EQUAL (x + k, y + k); k++) + if (k == SNAKE_LIMIT - 1) + { + best = v; + part->xmid = x; + part->ymid = y; + break; + } + } + } + } + if (best > 0) + return; + } + } +#endif /* USE_HEURISTIC */ + } + #undef XREF_YREF_EQUAL +} + + +/* Compare in detail contiguous subsequences of the two vectors + which are known, as a whole, to match each other. + + The subsequence of vector 0 is [XOFF, XLIM) and likewise for vector 1. + + Note that XLIM, YLIM are exclusive bounds. All indices into the vectors + are origin-0. + + The results are recorded by invoking NOTE_DELETE and NOTE_INSERT. + + Return false if terminated normally, or true if terminated through early + abort. */ + +static bool +compareseq (OFFSET xoff, OFFSET xlim, OFFSET yoff, OFFSET ylim, + struct context *ctxt) +{ +#ifdef ELEMENT + ELEMENT const *xv = ctxt->xvec; /* Help the compiler. */ + ELEMENT const *yv = ctxt->yvec; + #define XREF_YREF_EQUAL(x,y) EQUAL (xv[x], yv[y]) +#else + #define XREF_YREF_EQUAL(x,y) XVECREF_YVECREF_EQUAL (ctxt, x, y) +#endif + + /* Slide down the bottom initial diagonal. */ + while (xoff < xlim && yoff < ylim && XREF_YREF_EQUAL (xoff, yoff)) + { + xoff++; + yoff++; + } + + /* Slide up the top initial diagonal. */ + while (xoff < xlim && yoff < ylim && XREF_YREF_EQUAL (xlim - 1, ylim - 1)) + { + xlim--; + ylim--; + } + + /* Handle simple cases. */ + if (xoff == xlim) + while (yoff < ylim) + { + NOTE_INSERT (ctxt, yoff); + if (EARLY_ABORT (ctxt)) + return true; + yoff++; + } + else if (yoff == ylim) + while (xoff < xlim) + { + NOTE_DELETE (ctxt, xoff); + if (EARLY_ABORT (ctxt)) + return true; + xoff++; + } + else + { + struct partition part IF_LINT2 (= { .xmid = 0, .ymid = 0 }); + + /* Find a point of correspondence in the middle of the vectors. */ + diag (xoff, xlim, yoff, ylim, &part, ctxt); + + /* Use the partitions to split this problem into subproblems. */ + if (compareseq (xoff, part.xmid, yoff, part.ymid, ctxt)) + return true; + if (compareseq (part.xmid, xlim, part.ymid, ylim, ctxt)) + return true; + } + + return false; + #undef XREF_YREF_EQUAL +} + +#undef ELEMENT +#undef EQUAL +#undef OFFSET +#undef EXTRA_CONTEXT_FIELDS +#undef NOTE_DELETE +#undef NOTE_INSERT +#undef EARLY_ABORT +#undef USE_HEURISTIC +#undef XVECREF_YVECREF_EQUAL +#undef OFFSET_MAX |