radix_sort.h

/*******************************************************************************
 * DANIEL'S ALGORITHM IMPLEMENTAIONS
 *
 *  /\  |  _   _  ._ o _|_ |_  ._ _   _ 
 * /--\ | (_| (_) |  |  |_ | | | | | _> 
 *         _|                      
 *
 * RADIX SORT 
 *
 * Features: 
 *  1. sort unsigned 32-bit array in O(n) time
 *  2. subset sorted with couting sort.
 *
 * http://en.wikipedia.org/wiki/Radix_sort
 *
 ******************************************************************************/

#ifndef ALGO_RADIX_SORT_H__
#define ALGO_RADIX_SORT_H__

#include <stdint.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <generic.h>
#include <memory>

namespace alg {
	/**
	 * couting sort
	 */
	static void radix_(int byte, const unsigned N, const uint32_t *source, uint32_t *dest) {
		unsigned count[256];
		unsigned index[256];
		memset(count, 0, sizeof (count));

		unsigned i;
		for(i=0; i<N; ++i)
			count[((source[i])>>(byte*8))&0xff]++;

		index[0]=0;
		for(i=1; i<256; ++i)
			index[i] = index[i-1] + count[i-1];

		for(i=0; i<N; ++i)
			dest[index[((source[i])>>(byte*8))&0xff]++] = source[i];
	}

	/**
	 * radix sort a given unsigned 32-bit integer array of size N
	 */
	static void radix_sort(uint32_t *source, const unsigned N) {
		uint32_t * temp = new uint32_t[N];
		radix_(0, N, source, temp);
		radix_(1, N, temp, source);
		radix_(2, N, source, temp);
		radix_(3, N, temp, source);

		delete [] temp;
	}

	/**
	 * check whether the array is in order
	 */
	static void check_order(const uint32_t *data, unsigned N) {
		for(--N ; N > 0; --N, ++data)
			assert(data[0] <= data[1]);
	}
}

#endif //