add stable_sort

include/pocketpy/common/algorithm.h

@@ -25,6 +25,18 @@ extern "C" {
         *(out_index) = __first - (T*)(ptr);                                                        \
     } while(0)
 
+/**
+ * @brief Sorts an array of elements of the same type, using the given comparison function.
+ * @param ptr Pointer to the first element of the array.
+ * @param count Number of elements in the array.
+ * @param elem_size Size of each element in the array.
+ * @param cmp Comparison function that takes two elements and returns an integer similar to `strcmp`.
+ */
+void c11__stable_sort(void* ptr,
+                      int count,
+                      int elem_size,
+                      int (*cmp)(const void* a, const void* b));
+
 #ifdef __cplusplus
 }
 

src/common/algorithm.c

@@ -1,2 +1,47 @@
 #include "pocketpy/common/algorithm.h"
+#include <string.h>
+#include <stdlib.h>
 
+static void merge(void* a_begin,
+                  void* a_end,
+                  void* b_begin,
+                  void* b_end,
+                  void* res,
+                  int elem_size,
+                  int (*cmp)(const void* a, const void* b)) {
+    void *a = a_begin, *b = b_begin, *r = res;
+    while(a < a_end && b < b_end) {
+        if(cmp(a, b) <= 0) {
+            memcpy(r, a, elem_size);
+            a += elem_size;
+        } else {
+            memcpy(r, b, elem_size);
+            b += elem_size;
+        }
+        r += elem_size;
+    }
+
+    // one of the arrays is empty
+    for(; a < a_end; a += elem_size, r += elem_size)
+        memcpy(r, a, elem_size);
+    for(; b < b_end; b += elem_size, r += elem_size)
+        memcpy(r, b, elem_size);
+}
+
+void c11__stable_sort(void* ptr,
+                      int count,
+                      int elem_size,
+                      int (*cmp)(const void* a, const void* b)) {
+    // merge sort
+    void* tmp = malloc(count * elem_size);
+    for(int seg = 1; seg < count; seg *= 2) {
+        for(void* a = ptr; a < ptr + (count - seg) * elem_size; a += 2 * seg * elem_size) {
+            void* b = a + seg * elem_size, *a_end = b, *b_end = b + seg * elem_size;
+			if (b_end > ptr + count * elem_size)
+				b_end = ptr + count * elem_size;
+			merge(a, a_end, b, b_end, tmp, elem_size, cmp);
+			memcpy(a, tmp, (b_end - a) * elem_size);
+        }
+    }
+	free(tmp);
+}