From 56dbf9dbb423dc70839c353f63355df56b27a32d Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cdo@wip.sh>
Date: Sat, 4 Jan 2025 09:26:42 +0000
Subject: [PATCH] Construct drawlist at end of game_init()

---
 game/main.c | 77 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)

diff --git a/game/main.c b/game/main.c
index a033d6e..5cdf643 100644
--- a/game/main.c
+++ b/game/main.c
@@ -51,6 +51,9 @@
 
 #define MAXOBJTYPES 8
 #define INITOBJCOLCAP 16
+#define MAXENTITIES 32
+
+#define QSORT_STACKSIZE 64
 
 #define NELEMS(a) (sizeof(a) / sizeof(a[0]))
 
@@ -128,8 +131,14 @@ typedef struct {
 	objtype_t objtypes[MAXOBJTYPES];
 	objcol_t objcol;
 	dynentity_t p;
+	entity_t *drawlist;
 } gamestate_t;
 
+typedef struct {
+	int y;
+	entity_t *e;
+} y_entity_ref_t;
+
 const engineconf_t game_conf = {
 	.win = {
 		.title = "2D Game",
@@ -339,6 +348,72 @@ load_map(gamestate_t *state, const char *path, SDL_Renderer *renderer)
 	xmlFreeDoc(doc);
 }
 
+static void y_entity_ref_swap(y_entity_ref_t *a, y_entity_ref_t *b)
+{
+	y_entity_ref_t tmp = *a;
+	*a = *b;
+	*b = tmp;
+}
+
+void init_drawlist(gamestate_t *state)
+{
+	y_entity_ref_t buf[MAXENTITIES];
+	int n = 0;
+
+	// Populate buffer with player and objects
+	buf[n].y = (int)rint(state->p.pos.y) + state->p.ext.y / 2;
+	buf[n++].e = (entity_t *)&state->p;
+	for (unsigned i = 0; i < state->objcol.n; ++i) {
+		assert(n < MAXENTITIES);
+		objentity_t *obj = state->objcol.buf + i;
+		buf[n].y = obj->pos.y;
+		buf[n++].e = (entity_t *)obj;
+	}
+
+	// Quicksort on y value
+	int stack[QSORT_STACKSIZE], *sp = stack;
+	int l = 0, h = n;
+	assert(n != 0);
+	while (1) {
+		int pivot = buf[l].y;
+		int i = l, j = h;
+		do {
+			for (++i; buf[i].y <= pivot; ++i)
+				;
+			for (--j; buf[j].y > pivot; --j)
+				;
+			if (i < j)
+				y_entity_ref_swap(buf + i, buf + j);
+		} while (i < j);
+		y_entity_ref_swap(buf + l, buf + j);
+		assert(sp + 2 <= stack + QSORT_STACKSIZE);
+		if (j + 2 < h) {
+			*sp++ = j + 1;
+			*sp++ = h;
+		}
+		if (l + 1 < j) {
+			h = j;
+		} else if (sp != stack) {
+			h = *(--sp);
+			l = *(--sp);
+		} else {
+			break;
+		}
+	}
+
+	// Set up drawlist from buffer
+	state->drawlist = buf[0].e;
+	int i = 0;
+	buf[i].e->prev = NULL;
+	buf[i].e->next = buf[i + 1].e;
+	for (++i; i < n - 1; ++i) {
+		buf[i].e->prev = buf[i - 1].e;
+		buf[i].e->next = buf[i + 1].e;
+	}
+	buf[i].e->prev = buf[i - 1].e;
+	buf[i].e->next = NULL;
+}
+
 void game_init(int argc, char *argv[], void *mem, SDL_Renderer *renderer)
 {
 	char path[MAX_PATH_LEN];
@@ -389,6 +464,8 @@ void game_init(int argc, char *argv[], void *mem, SDL_Renderer *renderer)
 	state->p.ext.x = PWIDTH;
 	state->p.ext.y = PHEIGHT;
 	state->p.tex = state->pidle;
+
+	init_drawlist(state);
 }
 
 void game_teardown(void *mem)