regehr_sort_simple.c

// Copyright 2015, Mike Giroux
// Licensed under Apache License 2.0 : http://www.apache.org/licenses/LICENSE-2.0
// Permission is explicitly granted to aggregate this solution with any other
// solutions to the Regehr Nibble-sort Challenge (http://blog.regehr.org/archives/1213) 
// for benchmarking or pedagogical purposes, or any other purposes allowed by AL2.0.

unsigned long nibble_sort_word(unsigned long word) {
    char          nibble_counts[16] = {0};
    unsigned long result = 0;
    unsigned char nibble_index = 15;

    for(int i = 0; i < sizeof(word)*2; ++i) {
        nibble_counts[word&0x0F]++;
        word>>=4;
    }

    for(unsigned long int i = 15; i > 0; --i) {
        while (nibble_counts[i]--) {
            result|=(i << nibble_index*4);
            nibble_index--;
        }
    }

    return result;
}