Module: Algorithms::Search

### Home Classes Methods

### In Files

- lib/algorithms/search.rb

### Methods

- ::binary_search
- ::kmp_search
- #kmp_search

### Files

- Makefile
- Makefile
- Makefile
- Makefile
- Makefile

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# Algorithms::Search

This module implements search algorithms. Documentation is provided for each algorithm.

### Public Class Methods

binary_search(container, item) click to toggle source

Binary Search: This search finds an item in log(n) time provided that the container is already sorted. The method returns the item if it is found, or nil if it is not. If there are duplicates, the first one found is returned, and this is not guaranteed to be the smallest or largest item.

Complexity: O(lg N)

    Algorithms::Search.binary_search([1, 2, 3], 1) #=> 1
    Algorithms::Search.binary_search([1, 2, 3], 4) #=> nil

    # File lib/algorithms/search.rb, line 14
    def self.binary_search(container, item)
      return nil if item.nil?
      low = 0
      high = container.size - 1
      while low <= high
        mid = (low + high) / 2
        val = container[mid]
        if val > item
          high = mid - 1
        elsif val < item
          low = mid + 1
        else
          return val
        end
      end
      nil
    end

kmp_search(string, substring) click to toggle source

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

    # 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

### Public Instance Methods

kmp_search(substring) click to toggle source

Allows kmp_search to be called as an instance method in classes that include the Search module.

    class String; include Algorithms::Search; end
    "ABC ABCDAB ABCDABCDABDE".kmp_search("ABCDABD") #=> 15

    # File lib/algorithms/search.rb, line 80
    def kmp_search(substring)
      Algorithms::Search.kmp_search(self, substring)
    end

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