Algorithm used in Ruby for "String#include?"

Question

Is anyone able to pinpoint which algorithm is used for the include? method in Ruby? For example

"helloworld".include?("hello")

Answer 1

As emboss states in his answer, String#include calls rb_str_index. This function in turn calls rb_memsearch, which implements the Rabin-Karp string search algorithm, according to this post on ruby-forum.com.

Answer 2

The Ruby Language Specification doesn't prescribe any particular algorithm. Every implementation can use whatever algorithm they want.

For example, in Rubinius, String#include? calls String#find_string:

def include?(needle)
  if needle.kind_of? Fixnum
    needle = needle % 256
    str_needle = needle.chr
  else
    str_needle = StringValue(needle)
  end

  !!find_string(str_needle, 0)
end

String#find_string in turn is implemented via the string_index primitive:

def find_string(pattern, start)
  Rubinius.primitive :string_index
  raise PrimitiveFailure, "String#find_string failed"
end

The string_index primitive is implemented by the rubinius::String::index function:

// Rubinius.primitive :string_index
Fixnum* index(STATE, String* pattern, Fixnum* start);

rubinius::String::index:

Fixnum* String::index(STATE, String* pattern, Fixnum* start) {
  native_int total = size();
  native_int match_size = pattern->size();

  if(start->to_native() < 0) {
    Exception::argument_error(state, "negative start given");
  }

  switch(match_size) {
  case 0:
    return start;
  case 1:
    {
      uint8_t* buf = byte_address();
      uint8_t matcher = pattern->byte_address()[0];

      for(native_int pos = start->to_native(); pos < total; pos++) {
        if(buf[pos] == matcher) return Fixnum::from(pos);
      }
    }
    return nil<Fixnum>();
  default:
    {
      uint8_t* buf = byte_address();
      uint8_t* matcher = pattern->byte_address();

      uint8_t* last = buf + (total - match_size);
      uint8_t* pos = buf + start->to_native();

      while(pos <= last) {
        // Checking *pos directly then also checking memcmp is an
        // optimization. It's about 10x faster than just calling memcmp
        // everytime.
        if(*pos == *matcher &&
            memcmp(pos, matcher, match_size) == 0) {
          return Fixnum::from(pos - buf);
        }
        pos++;
      }
    }
    return nil<Fixnum>();
  }
}