Revert "Remove the wrong heapsort implementation"

This reverts commit aa04b6ae33.

include/ucore/heapsort.h

@@ -0,0 +1,21 @@
+#ifndef UC_HEAPSORT_H_
+#define UC_HEAPSORT_H_
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+///compare function. Needs only to return < 0 if 
+//the first element is less than the second
+typedef int (*uc_hs_cmp)(const void *, const void *);
+
+void
+uc_heapsort(void *base, size_t count, size_t width,
+     uc_hs_cmp cmp);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

src/heapsort.c

@@ -0,0 +1,41 @@
+#include <string.h>
+#include "ucore/heapsort.h"
+
+static void
+uc_sift(unsigned char *base, size_t start, size_t count, size_t width,
+     uc_hs_cmp cmp)
+{
+    size_t root = start, child;
+
+    while ((root * 2 + 1) < count) {
+        child = root * 2 + 1;
+        if (child < (count - 1)
+            && cmp(&base[child * width], &base[(child + 1) * width]) < 0)
+            child++;
+
+        if (cmp(&base[root * width], &base[child * width]) < 0) {
+            memcpy(base + root * width, base + child * width, width);
+            root = child;
+        } else
+            return;
+    }
+}
+
+void
+uc_heapsort(void *base_, size_t count, size_t width,
+     uc_hs_cmp cmp)
+{
+    int start = count / 2 - 1, end = count - 1;
+    unsigned char *base = base_;
+
+    while (start >= 0) {
+        uc_sift(base, start, count, width, cmp);
+        start--;
+    }
+
+    while (end > 0) {
+        memcpy(base + end * width, base, width);
+        uc_sift(base, 0, end, width, cmp);
+        end--;
+    }
+}