Class: Containers::SuffixArray

Inherits:
Object
  • Object
show all
Defined in:
lib/containers/suffix_array.rb

Overview

A suffix array enables fast substring search of a given string. An array of all possible substrings

is constructed and stored, and a binary search is then done to find a desired substring among those
stored. While more storage (and thus memory) is needed to create the SuffixArray, the advantage is
that substrings can be found in O(m log n) time, where m is the length of the substring to search for
and n is the total number of substrings.

Instance Method Summary collapse

Constructor Details

#initialize(string) ⇒ SuffixArray

Creates a new SuffixArray with a given string. Object of any class implementing a #to_s method can be passed in, such as integers.

Complexity: O(n^2 log n)

s_array = Containers::SuffixArray.new("abracadabra")
s_array["abra"] #=> true

number = Containers::SuffixArray.new(1234567)
number[1] #=> true
number[13] #=> false

Raises:

  • (ArgumentError) 


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# File 'lib/containers/suffix_array.rb', line 21

def initialize(string)
  string = string.to_s
  raise ArgumentError, "SuffixArray needs to be initialized with a non-empty string" if string.empty?
  @original_string = string
  @suffixes = []
  string.length.times do |i|
    @suffixes << string[i..-1]
  end

  # Sort the suffixes in ascending order
  @suffixes.sort! { |x, y| x <=> y }
end

Instance Method Details

#has_substring?(substring) ⇒ Boolean Also known as: []

Returns true if the substring occurs in the string, false otherwise.

Complexity: O(m + log n)

s_array = Containers::SuffixArray.new("abracadabra")
s_array.has_substring?("a") #=> true
s_array.has_substring?("abra") #=> true
s_array.has_substring?("abracadabra") #=> true
s_array.has_substring?("acadabra") #=> true
s_array.has_substring?("adabra") #=> true
s_array.has_substring?("bra") #=> true
s_array.has_substring?("bracadabra") #=> true
s_array.has_substring?("cadabra") #=> true
s_array.has_substring?("dabra") #=> true
s_array.has_substring?("ra") #=> true
s_array.has_substring?("racadabra") #=> true
s_array.has_substring?("nope") #=> false

Returns:

  • (Boolean) 


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# File 'lib/containers/suffix_array.rb', line 51

def has_substring?(substring)
  substring = substring.to_s
  return false if substring.empty?
  substring_length = substring.length-1
  l, r = 0, @suffixes.size-1
  while(l <= r)
    mid = (l + r) / 2
    suffix = @suffixes[mid][0..substring_length]
    case substring <=> suffix
    when 0 then return true
    when 1 then l = mid + 1
    when -1 then r = mid - 1
    end
  end
  return false
end