feat: implement shellsort

1 files changed, 64 insertions, 0 deletions

diff --git a/shellsort.c b/shellsort.c
new file mode 100644
index 0000000..b19f547
--- /dev/null
+++ b/shellsort.c
@@ -0,0 +1,64 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+#include "util.h"
+
+typedef int T;
+
+void
+shellsort(T a[], size_t n)
+{
+        //static const size_t gaps[] = { 701, 301, 132, 57, 23, 10, 4, 1 };
+
+        // Wikipedia:
+        //static const size_t gaps[] = { 2147483647, 1131376761, 410151271, 157840433, 58548857, 21521774, 8810089, 3501671, 1355339, 543749, 213331, 84801, 27901, 11969, 4711, 1968, 815, 271, 111, 41, 13, 4, 1 };
+
+        // https://www.cs.princeton.edu/~rs/shell/paperF.pdf
+        static const size_t gaps[] = { 1391376, 463792, 198768, 86961, 33936, 13776, 4592, 1968, 861, 336, 112, 48, 21, 7, 3, 1 };
+
+        for ( size_t k = 0; k < NELEM(gaps); ++k ) {
+                const size_t gap = gaps[k];
+
+                for ( size_t i = gap; i < n; i++ ) {
+                        size_t j    = i;
+                        T      temp = a[i];
+
+                        for ( ; j >= gap && a[j - gap] > temp; j -= gap )
+                                a[j] = a[j - gap];
+
+                        a[j] = temp;
+                }
+        }
+}
+
+static void
+test(T a[], size_t n)
+{
+        for ( size_t i = 1; i < n; ++i ) {
+                if ( a[i - 1] > a[i] ) {
+                        fprintf(stderr, "Error\n");
+                        exit(EXIT_FAILURE);
+                }
+        }
+}
+
+int
+main(void)
+{
+        T a[1000000];
+
+        srand(time(NULL));
+        for ( size_t i = 0; i != NELEM(a); ++i )
+                a[i] = rand() % 1000;
+
+        clock_t start = clock();
+        shellsort(a, NELEM(a));
+        clock_t end = clock();
+
+        test(a, NELEM(a));
+
+        printf("duration: %.3f sec\n", ((double) (end - start)) / CLOCKS_PER_SEC);
+
+        return EXIT_SUCCESS;
+}