/*
From: Andre Reinald
To: submissive

I though you might be intersted by some modifications I brought to your
code in order to improve legibility and slightly speed.

Well, my source is commented and I often wrote 4 lines where you had
only one, but the generated code is smaller and it gives more
opportunity to check intermediate steps while debugging.

By the way, I did so on a PowerMac with CodeWarrior compiler, so your
source is really portable.
*/

#include <stdlib.h>
#include <assert.h>

// complicated expression better fits as macro (or inline in C++)
#define ByteOf(x) (((x) >> bitsOffset) & 0xff)

typedef unsigned long ulong;

// replaced byte with bitsOffset to avoid *8 operation in loop
static void radix (short bitsOffset, ulong N, ulong *source, ulong *dest)
{
// suppressed the need for index as it is reported in count
        ulong count[256];
// added temp variables to simplify writing, understanding and compiler optimization job
// most of them will be allocated as registers
        ulong *cp, *sp, s, c, i;

// faster than MemSet
        cp = count;
        for (i = 256; i > 0; --i, ++cp)
                *cp = 0;

// count occurences of every byte value
        sp = source;
        for (i = N; i > 0; --i, ++sp) {
                cp = count + ByteOf (*sp);
                ++(*cp);
        }

// transform count into index by summing elements and storing into same array
        s = 0;
        cp = count;
        for (i = 256; i > 0; --i, ++cp) {
                c = *cp;
                *cp = s;
                s += c;
        }

// fill dest with the right values in the right place
        sp = source;
        for (i = N; i > 0; --i, ++sp) {
                s = *sp;
                cp = count + ByteOf (s);
                dest[*cp] = s;
                ++(*cp);
        }
}

static void radix_sort (ulong *source, ulong N)
{
// allocate heap memory to avoid the need of additional parameter
        ulong *temp = malloc (N * sizeof (ulong));
        assert (temp != NULL);

        radix (0, N, source, temp);
        radix (8, N, temp, source);
        radix (16, N, source, temp);
        radix (24, N, temp, source);

        free (temp);
}

static void make_random (ulong *data, ulong N)
{
        for ( ; N > 0; --N, ++data)
                *data = rand () | (rand () << 16);
}

static void check_order (ulong *data, ulong N)
{
// only signal errors if any (should not be)
        --N;
        for ( ; N > 0; --N, ++data)
                assert (data[0] <= data[1]);
}

// test for big number of elements
static void test_radix (ulong N)
{
        ulong *data = malloc (N * sizeof (ulong));
        assert (data != NULL);

        make_random (data, N);
        radix_sort (data, N);
        check_order (data, N);

        free (data);
}

void main (void)
{
        test_radix (500000);
}