Some hash functions to generate pseudo-random values per-coordinate, in C, with OCaml FFI.

hashes.c

/* Some hash functions to generate pseudo-random values per-coordinate.
 * Useful for spatial synthesis, such as procedural textures.
 */

/*** Bob Jenkins' mix and final ***/

#define rot(x,k) (((x)<<(k)) | ((x)>>(32-(k))))

#define mix(a,b,c) \
{ \
  a -= c;  a ^= rot(c, 4);  c += b; \
  b -= a;  b ^= rot(a, 6);  a += c; \
  c -= b;  c ^= rot(b, 8);  b += a; \
  a -= c;  a ^= rot(c,16);  c += b; \
  b -= a;  b ^= rot(a,19);  a += c; \
  c -= b;  c ^= rot(b, 4);  b += a; \
}

#define final(a,b,c) \
{ \
  c ^= b; c -= rot(b,14); \
  a ^= c; a -= rot(c,11); \
  b ^= a; b -= rot(a,25); \
  c ^= b; c -= rot(b,16); \
  a ^= c; a -= rot(c,4);  \
  b ^= a; b -= rot(a,14); \
  c ^= b; c -= rot(b,24); \
}

/* ------------------------------------ */

/* A portion of Paul Hsieh's hash */
/* use this to generate a PRN sequence without state */
uint32_t HsiehHashNext( uint32_t hash ){
  hash ^= hash << 3;
  hash += hash >> 5;
  hash ^= hash << 4;
  hash += hash >> 17;
  hash ^= hash << 25;
  hash += hash >> 6;
  return hash;
}

/* ------------------------------------ */

/*** OCaml FFI ***/

#define Uint32_val(v) (*((uint32_t *) Data_custom_val(v)))

CAMLprim value ocaml_float_of_u32( value i ){
  return caml_copy_double((double)Uint32_val(i));
}

/* it's fine if we return int32 type to OCaml... so it can understand it...
 * since we'll be using a C-call for float_of_u32 anyway */
CAMLprim value ocaml_init_rnd( value data, value i ){
  CAMLparam2( data, i );
  uint32_t *p = (uint32_t*)data;
  uint32_t a = p[0];
  uint32_t b = p[1];
  uint32_t c = p[2];

  mix(a,b,c);
  a+= p[3]; b+= p[4]; c+= p[5];
  mix(a,b,c);
  a+= Long_val(i);
  final(a,b,c);

  CAMLreturn( caml_copy_int32(c) );
}

CAMLprim value ocaml_next_rnd( value i ){
   return caml_copy_int32( HsiehHashNext( Uint32_val(i) ) );
}