Is anyone able to pinpoint which algorithm is used for the include? method in Ruby? For exa开发者_开发知识库mple
"helloworld".include?("hello")
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
.
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>();
}
}
This is the actual implementation of String#include?
:
static VALUE
rb_str_include(VALUE str, VALUE arg)
{
long i;
StringValue(arg);
i = rb_str_index(str, arg, 0);
if (i == -1) return Qfalse;
return Qtrue;
}
So the actual algorithm used can be found in rb_str_index.
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