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/* Find a best match between two vectors.
Copyright (C) 2009-2011 Free Software Foundation, Inc.
Written by Andreas Gruenbacher <agruen@gnu.org>, 2009.
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/>. */
/* Before including this file, you need to define:
EQUAL_IDX(x, y) A two-argument macro that tests elements
at index x and y for equality.
OFFSET A signed integer type sufficient to hold the
difference between two indices. Usually
something like ssize_t. */
/*
* Shortest Edit Sequence
*
* Based on the Greedy LCS/SES Algorithm (Figure 2) in:
*
* Eugene W. Myers, "An O(ND) Difference Algorithm and Its Variations",
* Algorithmica, Vol. 1, No. 1, pp. 251-266, March 1986.
* Available: http://dx.doi.org/10.1007/BF01840446
* http://xmailserver.org/diff2.pdf
*
* Returns the number of changes (insertions and deletions) required to get
* from a[] to b[]. Returns MAX + 1 if a[] cannot be turned into b[] with
* MAX or fewer changes.
*
* MIN specifies the minimum number of elements in which a[] and b[] must
* match. This allows to prevent trivial matches in which a sequence is
* completely discarded, or completely made up.
*
* If PY is not NULL, matches a[] against a prefix of b[], and returns the
* number of elements in b[] that were matched in *PY. Otherwise, matches
* all elements of b[].
*
* Note that the divide-and-conquer strategy discussed in section 4b of the
* paper is more efficient, but does not allow an open-ended prefix string
* search.
*/
OFFSET
bestmatch(OFFSET xoff, OFFSET xlim, OFFSET yoff, OFFSET ylim,
OFFSET min, OFFSET max, OFFSET *py)
{
const OFFSET dmin = xoff - ylim; /* Minimum valid diagonal. */
const OFFSET dmax = xlim - yoff; /* Maximum valid diagonal. */
const OFFSET fmid = xoff - yoff; /* Center diagonal. */
OFFSET fmin = fmid;
OFFSET fmax = fmid;
OFFSET *V, *fd;
OFFSET fmid_plus_2_min, ymax = -1;
OFFSET c;
V = malloc ((2 * max + 3) * sizeof (OFFSET));
fd = V + max + 1 - fmid;
/*
The number of elements that were matched in x and in y can be
computed as either (x - x_skipped) or (y - y_skipped), with:
delta = (x - xoff) - (y - yoff)
x_skipped = (c + delta) / 2
y_skipped = (c - delta) / 2
For searching for a minimum number of matching elements, we end up
with this check:
(x - x_skipped) >= min
...
x + y - c >= (xoff - yoff) + 2 * min
x + y - c >= fmid + 2 * min
*/
if (min)
{
fmid_plus_2_min = fmid + 2 * min;
min += yoff;
if (min > ylim)
return max + 1;
}
else
fmid_plus_2_min = 0; /* disable this check */
if (!py)
min = ylim;
/* Handle the exact-match case. */
while (xoff < xlim && yoff < ylim && EQUAL_IDX (xoff, yoff))
{
xoff++;
yoff++;
}
if (xoff == xlim && yoff >= min
&& xoff + yoff >= fmid_plus_2_min)
{
ymax = yoff;
c = 0;
}
else
{
fd[fmid] = xoff;
for (c = 1; c <= max; c++)
{
OFFSET d;
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, y;
if (fd[d - 1] < fd[d + 1])
x = fd[d + 1];
else
x = fd[d - 1] + 1;
for (y = x - d;
x < xlim && y < ylim && EQUAL_IDX (x, y);
x++, y++)
/* do nothing */ ;
fd[d] = x;
if (x == xlim && y >= min
&& x + y - c >= fmid_plus_2_min)
{
if (ymax < y)
ymax = y;
if (y == ylim)
goto done;
}
}
if (ymax != -1)
goto done;
}
}
done:
if (py)
*py = ymax;
free (V);
return c;
}
#undef OFFSET
#undef EQUAL_IDX
|
