class Containers::SuffixArray

  1. lib/containers/suffix_array.rb
Parent: Containers

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.

Methods

Public Class

  1. new

Public Instance

  1. has_substring?

Public Instance Aliases

[] -> has_substring?

Public Class methods

new (string)

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("abracadabra")
s_array["abra"] #=> true

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

Public Instance methods

has_substring? (substring)

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
[show source] 
# 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