About Array#<=> (a.k.a. Spacecraft Operator)

The documentations about Array#<=> are:

• http://www.ruby-doc.org/core/classes/Array.html#M002227 (in English)
• http://doc.okkez.net/191/view/method/Array/i/=3c=3d=3e (in Japanese)

For example:

[1, 2, 3, 4] <=> [1, 2, 3, 3] #=> 1
[1, 2, 3, 4] <=> [1, 2, 3, 4] #=> 0
[1, 2, 3, 4] <=> [1, 2, 3, 5] #=> -1
[1, 2, 3, 4] <=> [1, 2, 3] #=> 1

The implementation in MRI 1.9 is:

VALUE
rb_ary_cmp(VALUE ary1, VALUE ary2)
{
    long len;
    VALUE v;

    ary2 = to_ary(ary2);
    if (ary1 == ary2) return INT2FIX(0);
    v = rb_exec_recursive(recursive_cmp, ary1, ary2);
    if (v != Qundef) return v;
    len = RARRAY_LEN(ary1) - RARRAY_LEN(ary2);
    if (len == 0) return INT2FIX(0);
    if (len > 0) return INT2FIX(1);
    return INT2FIX(-1);
}

static VALUE
recursive_cmp(VALUE ary1, VALUE ary2, int recur)
{
    long i, len;

    if (recur) return Qnil;
    len = RARRAY_LEN(ary1);
    if (len > RARRAY_LEN(ary2)) {
        len = RARRAY_LEN(ary2);
    }
    for (i=0; i<len; i++) {
        VALUE v = rb_funcall(rb_ary_elt(ary1, i), id_cmp, 1, rb_ary_elt(ary2, i));
        if (v != INT2FIX(0)) {
            return v;
        }
    }
    return Qundef;
}

I translated it from C to Ruby literally:

class Array
  def yet_another_cmp(you)
    you = you.to_ary
    return 0 if self == you
    v = nil
    (0...[self.length, you.length].min).each do |i|
      v = self[i] <=> you[i]
      return v if v != 0
    end
    len = self.length - you.length
    return 0 if len == 0
    return 1 if len > 0
    -1
  end
end

It works exactly the same.

You may think it is easy to write a simpler equivalent implement with Array#zip, but unfortunately I found that it was not so simple. The following is the simplest code I can write. Of course it works exactly the same as original <=>.

class Array
  def simple_cmp(you)
    self.zip(you) {|x, y|
      break if (x && y).nil?
      (v = x <=> y) == 0 or return v
    }
    self.length <=> you.length
  end
end

In conclusion, Array#<=> itself is complicated one. Enjoy your space travel!