46 lines
1.2 KiB
Ruby
46 lines
1.2 KiB
Ruby
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# Knuth-Morris-Pratt Algorithm substring search algorithm: Efficiently finds the starting position of a
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# substring in a string. The algorithm calculates the best position to resume searching from if a failure
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# occurs.
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#
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# The method returns the index of the starting position in the string where the substring is found. If there
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# is no match, nil is returned.
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#
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# Complexity: O(n + k), where n is the length of the string and k is the length of the substring.
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#
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# "ABC ABCDAB ABCDABCDABDE".kmp_search("ABCDABD") #=> 15
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# "ABC ABCDAB ABCDABCDABDE".kmp_search("ABCDEF") #=> nil
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class Array
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def self.kmp_search(substring)
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return nil if self.nil? or substring.nil?
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pos = 2
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cnd = 0
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failure_table = [-1, 0]
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while pos < substring.length
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if substring[pos - 1] == substring[cnd]
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failure_table[pos] = cnd + 1
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pos += 1
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cnd += 1
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elsif cnd > 0
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cnd = failure_table[cnd]
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else
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failure_table[pos] = 0
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pos += 1
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end
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end
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m = i = 0
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while m + i < self.length
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if substring[i] == self[m + i]
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i += 1
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return m if i == substring.length
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else
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m = m + i - failure_table[i]
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i = failure_table[i] if i > 0
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end
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end
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return nil
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end
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end
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