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/**
* Histogram sort
*
* This sort is interesting in that it is a "comparison-free" sort. That is to
* say that it doesn't ever directly compare two items against each other to
* determine the larger one, much like conventional sorting algorithms does.
*
* It basically counts the frequency of each input value into a histogram.
* So it'd take an input set
*
* {5,4,0,5,0,1,4,2,1,1,3}
*
* and compress it into the histogram
*
* {2,3,1,1,2,2}
*
* by counting the number of occurences of 0, 1, … 5. Then it just builds the
* sorted set by walking the histogram from left to right.
*
* {0,0,1,1,1,2,3,4,4,5,5}
*
* Essentially, a histogram is built by counting the frequency of each value in
* the input set. Then, a sorted output set is constructed from this histogram
* since it is implicitly in order.
*
* This has the disadvantage of becoming heavy on memory usage when the input
* values increase in range. Theoretically, if the range was high, but sparse,
* some simple compression could be applied. There's an area for
* experimentation.
*/
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int
is_sorted(unsigned int *data, size_t length) {
size_t i = 0;
for (i = 0; i < length-1; i++)
if (data[i] > data[i+1])
return 0;
return 1;
}
void
fill_random(unsigned int *data, size_t length, unsigned int max) {
size_t i = 0;
for (i = 0; i < length; i++)
data[i] = rand()%max+1;
}
void
sort(unsigned int *data, size_t length, unsigned int max) {
size_t *hist = calloc(max+1, sizeof(size_t));
size_t i = 0;
size_t j = 0;
if (!hist) {
perror("calloc");
return;
}
/* build the histogram */
for (i = 0; i < length; i++)
hist[data[i]]++;
/* expand histogram to form sorted set */
j = 0;
for (i = 0; i < max+1; i++)
for (; hist[i]; hist[i]--, j++)
data[j] = i;
}
void
show_usage(const char *argv0) {
fprintf(stderr, "usage: %s <item_count> <max_value>\n", argv0);
}
int
main(int argc, char **argv) {
clock_t start, end;
unsigned int *data = NULL;
int range = 0;
int count = 0;
if (argc != 3) {
show_usage(argv[0]);
return 1;
}
count = atoi(argv[1]);
range = atoi(argv[2]);
if (count <= 0 || range <= 0) {
show_usage(argv[0]);
return 1;
}
data = calloc(count, sizeof(unsigned int));
if (!data) {
perror("calloc");
return 1;
}
srand(time(NULL));
fill_random(data, count, range);
start = clock();
sort(data, count, range);
end = clock();
printf("Time taken to sort: %.4f\n",
((double)(end-start))/CLOCKS_PER_SEC);
if (!is_sorted(data, count)) {
fprintf(stderr, "Failed: out of order\n");
} else {
printf("Success.\n");
}
}
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