[ruby-talk:4858] Re: FEATURE REQUEST: Fixnum bitfields

    >I just realized that the '[]=' is not implimented for Fixnum and Bignum.
    >Add that to my feature request. :)
    >
    >There is no reason we just be able to write bitfields as well.  I will
    >try to write a patch if people (and Matz) like the feature.  I have been
    >wanting to learn Ruby internals.

I wrote the following code to create a BitVector type as an extension
some time ago.  It was a quick hack, and I make no guarantees
that it even works with the latest verions...

Enjoy or dispose of as you see fit :-)

/\ndy

-------------------------------------------

#include "ruby.h"
#include <memory.h>
#include <stdio.h>

static ID numChars;
static ID numBits;
static ID storage;

static VALUE 
bv_init(VALUE obj, VALUE size)
{
  unsigned int size_in_chars;
  unsigned char *arr;
  unsigned int length;

  size_in_chars = (NUM2INT(size) / sizeof(char)) +1;
  rb_ivar_set(obj, numChars, size_in_chars);
  rb_ivar_set(obj, numBits, size);
  arr = malloc(size_in_chars);
  memset(arr,0,size_in_chars);
  rb_ivar_set(obj, storage, rb_str_new(arr, size_in_chars));
  return obj;
}

static VALUE
bv_set_bit(VALUE obj, VALUE aNum, VALUE aVal)
{
  unsigned int byte; 
  unsigned int bitnum; 
  VALUE st;

  st = rb_ivar_get(obj, storage);
  bitnum = NUM2INT(aNum);
  byte = bitnum >> 3;

  if (NUM2INT(aVal) != 0) {
    RSTRING(st)->ptr[byte] |= 1 << (bitnum & 0x7);
  } else {
    RSTRING(st)->ptr[byte] &= ~(1 << (bitnum & 0x7));
  }
  return obj;
}

static VALUE
bv_get_size(VALUE obj)
{
  return rb_ivar_get(obj, numChars);
}


static VALUE
bv_get_bit(VALUE obj, VALUE aNum)
{
  unsigned int byte; 
  unsigned int bitnum; 
  VALUE st;

  st = rb_ivar_get(obj, storage);
  bitnum = NUM2INT(aNum);
  byte = bitnum >> 3;

  if (RSTRING(st)->ptr[byte] & (1 << (bitnum & 0x7)))
    return INT2FIX(1);
  return INT2FIX(0);
}

static VALUE cBitVector;

void Init_BitVector() {
  numChars = rb_intern("numChars");
  numBits = rb_intern("numBits");
  storage = rb_intern("storage");

  cBitVector = rb_define_class("BitVector", rb_cObject);
  rb_define_method(cBitVector, "initialize", bv_init, 1);
  rb_define_method(cBitVector, "[]", bv_get_bit, 1);
  rb_define_method(cBitVector, "[]=", bv_set_bit, 2);
  rb_define_method(cBitVector, "size", bv_get_size, 0);
}