--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2007-08-17  Bruno Haible  <bruno@clisp.org>
+	and Paul Eggert  <eggert@cs.ucla.edu>
+
+	* MODULES.html.sh: Add diffseq.
+	* modules/diffseq: New file.
+	* lib/diffseq.h: New file, from GNU gettext with a few minor changes,
+	extracted from GNU gettext's fstrcmp.c and GNU diff's analyze.c.
+
 2007-08-15  Paul Eggert  <eggert@cs.ucla.edu>
 
 	Import changes from coreutils for bootstrap script.

--- a/MODULES.html.sh
+++ b/MODULES.html.sh
@@ -1815,6 +1815,7 @@
   func_module base64
   func_module check-version
   func_module diacrit
+  func_module diffseq
   func_module getline
   func_module getdelim
   func_module getnline

new file mode 100644
--- /dev/null
+++ b/lib/diffseq.h
@@ -0,0 +1,460 @@
+/* Analyze differences between two vectors.
+
+   Copyright (C) 1988-1989, 1992-1995, 2001-2004, 2006, 2007 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 2, 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, write to the Free Software Foundation,
+   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+
+/* 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 Myers,
+   Algorithmica Vol. 1 No. 2, 1986, pp. 251-266;
+   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", E. Ukkonen,
+   Information and Control Vol. 64, 1985, pp. 100-118.
+
+   Unless the 'find_minimal' flag is set, this code uses the TOO_EXPENSIVE
+   heuristic, by Paul Eggert, to limit the cost to O(N**1.5 log N)
+   at the price of producing suboptimal output for large inputs with
+   many differences.  */
+
+/* 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 ssize_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].
+     USE_HEURISTIC           (Optional) Define if you want to support the
+                             heuristic for large vectors.  */
+
+/* Maximum value of type OFFSET.  */
+#define OFFSET_MAX \
+  ((((OFFSET)1 << (sizeof (OFFSET) * CHAR_BIT - 2)) - 1) * 2 + 1)
+
+/* Use this to suppress gcc's `...may be used before initialized' warnings. */
+#ifndef IF_LINT
+# ifdef lint
+#  define IF_LINT(Code) Code
+# else
+#  define IF_LINT(Code) /* empty */
+# endif
+#endif
+
+/*
+ * Context of comparison operation.
+ */
+struct context
+{
+  /* Vectors being compared. */
+  const ELEMENT *xvec;
+  const ELEMENT *yvec;
+
+  /* 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 -H flag.  With this heuristic, for
+     vectors with a constant small density of changes, the algorithm is
+     linear in the vectors size.  */
+  int heuristic;
+  #endif
+
+  /* Edit scripts longer than this are too expensive to compute.  */
+  OFFSET too_expensive;
+
+  /* Snakes bigger than this are considered `big'.  */
+  #define SNAKE_LIMIT 20
+};
+
+struct partition
+{
+  /* Midpoints of this partition.  */
+  OFFSET xmid;
+  OFFSET ymid;
+
+  /* True if low half will be analyzed minimally.  */
+  bool lo_minimal;
+
+  /* Likewise for high half.  */
+  bool hi_minimal;
+};
+
+/* 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.
+
+   If FIND_MINIMAL is true, find the minimal edit script regardless
+   of expense.  Otherwise, if the search is too expensive, use
+   heuristics to stop the search and report a suboptimal answer.
+
+   Set PART->(xmid,ymid) 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).
+
+   Set PART->lo_minimal to true iff the minimal edit script for the
+   left half of the partition is known; similarly for PART->hi_minimal.
+
+   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, bool find_minimal,
+      struct partition *part, struct context const *ctxt)
+{
+  OFFSET *const fd = ctxt->fdiag;	/* Give the compiler a chance. */
+  OFFSET *const bd = ctxt->bdiag;	/* Additional help for the compiler. */
+  const ELEMENT *const xv = ctxt->xvec;	/* Still more help for the compiler. */
+  const ELEMENT *const yv = ctxt->yvec;	/* And more and more . . . */
+  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 && EQUAL (xv[x], yv[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;
+	      part->lo_minimal = part->hi_minimal = true;
+	      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 = x - d;
+	       xoff < x && yoff < y && EQUAL (xv[x - 1], yv[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;
+	      part->lo_minimal = part->hi_minimal = true;
+	      return;
+	    }
+	}
+
+      if (find_minimal)
+	continue;
+
+#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; EQUAL (xv[x - k], yv[y - k]); k++)
+			if (k == SNAKE_LIMIT)
+			  {
+			    best = v;
+			    part->xmid = x;
+			    part->ymid = y;
+			    break;
+			  }
+		    }
+		}
+	    }
+	  if (best > 0)
+	    {
+	      part->lo_minimal = true;
+	      part->hi_minimal = false;
+	      return;
+	    }
+
+	  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; EQUAL (xv[x + k], yv[y + k]); k++)
+			if (k == SNAKE_LIMIT - 1)
+			  {
+			    best = v;
+			    part->xmid = x;
+			    part->ymid = y;
+			    break;
+			  }
+		    }
+		}
+	    }
+	  if (best > 0)
+	    {
+	      part->lo_minimal = false;
+	      part->hi_minimal = true;
+	      return;
+	    }
+	}
+#endif /* USE_HEURISTIC */
+
+      /* Heuristic: if we've gone well beyond the call of duty, give up
+	 and report halfway between our best results so far.  */
+      if (c >= ctxt->too_expensive)
+	{
+	  OFFSET fxybest;
+	  OFFSET fxbest IF_LINT (= 0);
+	  OFFSET bxybest;
+	  OFFSET bxbest IF_LINT (= 0);
+
+	  /* Find forward diagonal that maximizes X + Y.  */
+	  fxybest = -1;
+	  for (d = fmax; d >= fmin; d -= 2)
+	    {
+	      OFFSET x = MIN (fd[d], xlim);
+	      OFFSET y = x - d;
+	      if (ylim < y)
+		{
+		  x = ylim + d;
+		  y = ylim;
+		}
+	      if (fxybest < x + y)
+		{
+		  fxybest = x + y;
+		  fxbest = x;
+		}
+	    }
+
+	  /* Find backward diagonal that minimizes X + Y.  */
+	  bxybest = OFFSET_MAX;
+	  for (d = bmax; d >= bmin; d -= 2)
+	    {
+	      OFFSET x = MAX (xoff, bd[d]);
+	      OFFSET y = x - d;
+	      if (y < yoff)
+		{
+		  x = yoff + d;
+		  y = yoff;
+		}
+	      if (x + y < bxybest)
+		{
+		  bxybest = x + y;
+		  bxbest = x;
+		}
+	    }
+
+	  /* Use the better of the two diagonals.  */
+	  if ((xlim + ylim) - bxybest < fxybest - (xoff + yoff))
+	    {
+	      part->xmid = fxbest;
+	      part->ymid = fxybest - fxbest;
+	      part->lo_minimal = true;
+	      part->hi_minimal = false;
+	    }
+	  else
+	    {
+	      part->xmid = bxbest;
+	      part->ymid = bxybest - bxbest;
+	      part->lo_minimal = false;
+	      part->hi_minimal = true;
+	    }
+	  return;
+	}
+    }
+}
+
+/* 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.
+
+   If FIND_MINIMAL, find a minimal difference no matter how
+   expensive it is.
+
+   The results are recorded in the vectors files[N].changed, by storing 1
+   in the element for each line that is an insertion or deletion.  */
+
+static void
+compareseq (OFFSET xoff, OFFSET xlim, OFFSET yoff, OFFSET ylim,
+	    bool find_minimal, struct context const *ctxt)
+{
+  ELEMENT const *xv = ctxt->xvec; /* Help the compiler.  */
+  ELEMENT const *yv = ctxt->yvec;
+
+  /* Slide down the bottom initial diagonal.  */
+  while (xoff < xlim && yoff < ylim && EQUAL (xv[xoff], yv[yoff]))
+    {
+      xoff++;
+      yoff++;
+    }
+
+  /* Slide up the top initial diagonal. */
+  while (xoff < xlim && yoff < ylim && EQUAL (xv[xlim - 1], yv[ylim - 1]))
+    {
+      xlim--;
+      ylim--;
+    }
+
+  /* Handle simple cases. */
+  if (xoff == xlim)
+    while (yoff < ylim)
+      {
+	NOTE_INSERT (ctxt, yoff);
+	yoff++;
+      }
+  else if (yoff == ylim)
+    while (xoff < xlim)
+      {
+	NOTE_DELETE (ctxt, xoff);
+	xoff++;
+      }
+  else
+    {
+      struct partition part;
+
+      /* Find a point of correspondence in the middle of the vectors.  */
+      diag (xoff, xlim, yoff, ylim, find_minimal, &part, ctxt);
+
+      /* Use the partitions to split this problem into subproblems.  */
+      compareseq (xoff, part.xmid, yoff, part.ymid, part.lo_minimal, ctxt);
+      compareseq (part.xmid, xlim, part.ymid, ylim, part.hi_minimal, ctxt);
+    }
+}
+
+#undef ELEMENT
+#undef EQUAL
+#undef OFFSET
+#undef OFFSET_MAX
+#undef EXTRA_CONTEXT_FIELDS
+#undef NOTE_DELETE
+#undef NOTE_INSERT

new file mode 100644
--- /dev/null
+++ b/modules/diffseq
@@ -0,0 +1,21 @@
+Description:
+Compute the difference between two sequences.
+
+Files:
+lib/diffseq.h
+
+Depends-on:
+
+configure.ac:
+
+Makefile.am:
+lib_SOURCES += diffseq.h
+
+Include:
+"diffseq.h"
+
+License:
+GPL
+
+Maintainer:
+Paul Eggert, Bruno Haible