87 lines
2.5 KiB
C
87 lines
2.5 KiB
C
#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 substitution, 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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