implement stable sort for lists

also change sort_workspaces() to use it

common/list.c

@@ -59,7 +59,7 @@ void list_cat(list_t *list, list_t *source) {
 	}
 }
 
-void list_qsort(list_t* list, int compare(const void *left, const void *right)) {
+void list_qsort(list_t *list, int compare(const void *left, const void *right)) {
 	qsort(list->items, list->length, sizeof(void *), compare);
 }
 
@@ -72,3 +72,60 @@ int list_seq_find(list_t *list, int compare(const void *item, const void *data),
 	}
 	return -1;
 }
+
+static void list_swap(list_t *list, int src, int dest) {
+	void *tmp = list->items[src];
+	list->items[src] = list->items[dest];
+	list->items[dest] = tmp;
+}
+
+static void list_rotate(list_t *list, int from, int to) {
+	void *tmp = list->items[to];
+
+	while (to > from) {
+		list->items[to] = list->items[to - 1];
+		to--;
+	}
+
+	list->items[from] = tmp;
+}
+
+static void list_inplace_merge(list_t *list, int left, int last, int mid, int compare(const void *a, const void *b)) {
+	int right = mid + 1;
+
+	if (compare(&list->items[mid], &list->items[right]) <= 0) {
+		return;
+	}
+
+	while (left <= mid && right <= last) {
+		if (compare(&list->items[left], &list->items[right]) <= 0) {
+			left++;
+		} else {
+			list_rotate(list, left, right);
+			left++;
+			mid++;
+			right++;
+		}
+	}
+}
+
+static void list_inplace_sort(list_t *list, int first, int last, int compare(const void *a, const void *b)) {
+	if (first >= last) {
+		return;
+	} else if ((last - first) == 1) {
+		if (compare(&list->items[first], &list->items[last]) > 0) {
+			list_swap(list, first, last);
+		}
+	} else {
+		int mid = (int)((last + first) / 2);
+		list_inplace_sort(list, first, mid, compare);
+		list_inplace_sort(list, mid + 1, last, compare);
+		list_inplace_merge(list, first, last, mid, compare);
+	}
+}
+
+void list_stable_sort(list_t *list, int compare(const void *a, const void *b)) {
+	if (list->length > 1) {
+		list_inplace_sort(list, 0, list->length - 1, compare);
+	}
+}

include/list.h

@@ -20,5 +20,6 @@ void list_qsort(list_t *list, int compare(const void *left, const void *right));
 // Return index for first item in list that returns 0 for given compare
 // function or -1 if none matches.
 int list_seq_find(list_t *list, int compare(const void *item, const void *cmp_to), const void *cmp_to);
-
+// stable sort since qsort is not guaranteed to be stable
+void list_stable_sort(list_t *list, int compare(const void *a, const void *b));
 #endif

include/output.h

@@ -16,7 +16,7 @@ void get_absolute_position(swayc_t *container, struct wlc_point *point);
 // given wlc_point.
 void get_absolute_center_position(swayc_t *container, struct wlc_point *point);
 
-int sort_workspace_cmp_qsort(const void *a, const void *b);
+// stable sort workspaces on this output
 void sort_workspaces(swayc_t *output);
 
 #endif

sway/output.c

@@ -3,6 +3,7 @@
 #include <stdlib.h>
 #include "output.h"
 #include "log.h"
+#include "list.h"
 
 swayc_t *output_by_name(const char* name, const struct wlc_point *abs_pos) {
 	if (strcasecmp(name, "left") == 0) {
@@ -180,7 +181,7 @@ void get_absolute_center_position(swayc_t *container, struct wlc_point *point) {
 	point->y += container->height/2;
 }
 
-int sort_workspace_cmp_qsort(const void *_a, const void *_b) {
+static int sort_workspace_cmp_qsort(const void *_a, const void *_b) {
 	swayc_t *a = *(void **)_a;
 	swayc_t *b = *(void **)_b;
 	int retval = 0;
@@ -199,5 +200,5 @@ int sort_workspace_cmp_qsort(const void *_a, const void *_b) {
 }
 
 void sort_workspaces(swayc_t *output) {
-	list_qsort(output->children, sort_workspace_cmp_qsort);
+	list_stable_sort(output->children, sort_workspace_cmp_qsort);
 }