How to convert ruby array to C array with RubyInline?

Question

I have a function which compares 2 strings char by char. I needed it to run much faster than it is in Ruby so I used RubyInline to rewrite the function in C. It did increase the speed about 100 fold. The function looks like this:

  require 'inline'

  inline do |builder|
    builder.c "
      static int distance(char *s, char *t){
        ...
      }"
  end

however I need to compare unicode strings. So I decided to use unpack("U*") and compare arrays of integers instead. I cannot figure out from a scanty documentation to RubyInline how to pass the ruby arrays into the function and how to convert them into C arrays. Any help is appreciated!

No problem, let me know how it turns out. Would love to see your implementation when done. — Corban Brook
– Corban Brook, Commented Jul 13, 2009 at 17:38
Here is the code of damerau_levenshtein distance: gist.github.com/147023 For ruby 1.8.7 works like a charm, but chokes on ruby 1.9.1 — dimus
– dimus, Commented Jul 14, 2009 at 15:51
I updated gist.github.com/147023 to work with 1.9.1 ruby as well using help from thnetos — dimus
– dimus, Commented Jul 15, 2009 at 21:30

Corban Brook · Accepted Answer · 2009-07-13 16:20:13Z

9

This has a good rundown of how to access Ruby objects from C: http://rubycentral.com/pickaxe/ext_ruby.html

inline do |builder|
  builder.c "
    static VALUE some_method(VALUE s) {
      int s_len = RARRAY(s)->len;
      int result = 0;

      VALUE *s_arr = RARRAY(s)->ptr;

      for(i = 0; i < s_len; i++) {
        result += NUM2INT(s_arr[i]); // example of reference
      }

      return INT2NUM(result); // convert C int back into ruby Numeric 
    }"
end

Then in ruby you can just pass values to it like:

object.some_method([1,2,3,4])

Hope this helps you out.

edited Jul 13, 2009 at 16:20

answered Jul 13, 2009 at 15:39

Corban Brook

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1 Comment

dimus Over a year ago

Thanks Corban, it looks exactly what I need!

thnetos · Accepted Answer · 2009-07-15 17:50:49Z

4

Given the code from the above answer, here is the code that will work for Ruby 1.8.6 and 1.9.1:

inline do |builder|
  builder.c "
    static VALUE some_method(VALUE s) {
      int s_len = RARRAY_LEN(s);
      int result = 0;
      int i = 0;

      VALUE *s_arr = RARRAY_PTR(s);

      for(i = 0; i < s_len; i++) {
        result += NUM2INT(s_arr[i]); // example of reference
      }

      return INT2NUM(result); // convert C int back into ruby Numeric 
    }"
end

Hope this helps also :)

answered Jul 15, 2009 at 17:50

community wiki

thnetos

1 Comment

dimus Over a year ago

Thanks thnetos, it did fix the problem, I updated the github gist example

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