Method: Algorithms::Search.kmp_search

Defined in:
lib/algorithms/search.rb

.kmp_search(string, substring) ⇒ Object

Knuth-Morris-Pratt Algorithm substring search algorithm: Efficiently finds the starting position of a substring in a string. The algorithm calculates the best position to resume searching from if a failure occurs.

The method returns the index of the starting position in the string where the substring is found. If there is no match, nil is returned.

Complexity: O(n + k), where n is the length of the string and k is the length of the substring.

Algorithms::Search.kmp_search("ABC ABCDAB ABCDABCDABDE", "ABCDABD") #=> 15
Algorithms::Search.kmp_search("ABC ABCDAB ABCDABCDABDE", "ABCDEF") #=> nil



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# File 'lib/algorithms/search.rb', line 43

def self.kmp_search(string, substring)
  return nil if string.nil? or substring.nil?
  
  # create failure function table
  pos = 2
  cnd = 0
  failure_table = [-1, 0]
  while pos < substring.length
    if substring[pos - 1] == substring[cnd]
      failure_table[pos] = cnd + 1
      pos += 1
      cnd += 1
    elsif cnd > 0
      cnd = failure_table[cnd]
    else
      failure_table[pos] = 0
      pos += 1
    end
  end

  m = i = 0
  while m + i < string.length
    if substring[i] == string[m + i]
      i += 1
      return m if i == substring.length
    else
      m = m + i - failure_table[i]
      i = failure_table[i] if i > 0
    end
  end
  return nil
end