Document levenshtein.c
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Johannes Schindelin committed
Nov 20, 2008 at 14:27 UTC
850fb6ff81a151887043b7edd10681640b0e91c1
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levenshtein.c
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#include "cache.h"
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#include "levenshtein.h"
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+/*
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+ * This function implements the Damerau-Levenshtein algorithm to
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+ * calculate a distance between strings.
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+ *
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+ * Basically, it says how many letters need to be swapped, substituted,
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+ * deleted from, or added to string1, at least, to get string2.
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+ *
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+ * The idea is to build a distance matrix for the substrings of both
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+ * strings. To avoid a large space complexity, only the last three rows
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+ * are kept in memory (if swaps had the same or higher cost as one deletion
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+ * plus one insertion, only two rows would be needed).
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+ *
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+ * At any stage, "i + 1" denotes the length of the current substring of
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+ * string1 that the distance is calculated for.
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+ *
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+ * row2 holds the current row, row1 the previous row (i.e. for the substring
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+ * of string1 of length "i"), and row0 the row before that.
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+ *
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+ * In other words, at the start of the big loop, row2[j + 1] contains the
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+ * Damerau-Levenshtein distance between the substring of string1 of length
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+ * "i" and the substring of string2 of length "j + 1".
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+ *
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+ * All the big loop does is determine the partial minimum-cost paths.
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+ *
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+ * It does so by calculating the costs of the path ending in characters
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+ * i (in string1) and j (in string2), respectively, given that the last
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+ * operation is a substition, a swap, a deletion, or an insertion.
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+ *
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+ * This implementation allows the costs to be weighted:
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+ *
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+ * - w (as in "sWap")
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+ * - s (as in "Substitution")
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+ * - a (for insertion, AKA "Add")
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+ * - d (as in "Deletion")
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+ *
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+ * Note that this algorithm calculates a distance _iff_ d == a.
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+ */
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int levenshtein(const char *string1, const char *string2,
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int w, int s, int a, int d)
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{