From 864e053487967faa7dd51709df1869d87544b64f Mon Sep 17 00:00:00 2001
From: P. J. McDermott <pj@pehjota.net>
Date: Sat, 27 Mar 2021 18:14:52 -0400
Subject: ball: Avoid colliding with multiple coaxial walls

---
diff --git a/src/ball.c b/src/ball.c
index 14ee8de..d4c27a4 100644
--- a/src/ball.c
+++ b/src/ball.c
@@ -118,11 +118,13 @@ db_balls_collisions(struct db_ball *ball)
 	int             x_max;
 	int             y_min;
 	int             y_max;
+	int             prev_col_x[2];
+	int             prev_col_y[2];
 	int             x;
 	int             y;
-	struct db_ball *other;
 	double          col_x;
 	double          col_y;
+	struct db_ball *other;
 
 	wall.w = map_tw = db_map_get_tilewidth (ball->map);
 	wall.h = map_th = db_map_get_tileheight(ball->map);
@@ -130,6 +132,10 @@ db_balls_collisions(struct db_ball *ball)
 	x_max = ceil ((ball->x + ball->r) / map_tw);
 	y_min = floor((ball->y - ball->r) / map_th);
 	y_max = ceil ((ball->y + ball->r) / map_th);
+	prev_col_x[0] = -1;
+	prev_col_x[1] = -1;
+	prev_col_y[0] = -1;
+	prev_col_y[1] = -1;
 	for (y = y_min; y < y_max; ++y) {
 		for (x = x_min; x < x_max; ++x) {
 			if (db_map_tile_ball_collides(ball->map, x, y)) {
@@ -137,7 +143,26 @@ db_balls_collisions(struct db_ball *ball)
 				wall.y = y * map_th;
 				if (db_col_cir_rect(ball->x, ball->y, ball->r,
 							&wall, &col_x, &col_y)){
+					if (x == prev_col_x[0]) {
+						continue;
+					} else if (x == prev_col_x[1]) {
+						continue;
+					} else if (y == prev_col_y[0]) {
+						continue;
+					} else if (y == prev_col_y[1]) {
+						continue;
+					}
 					_db_ball_bounce(ball, col_x, col_y);
+					if (prev_col_x[0] >= 0) {
+						prev_col_x[0] = x;
+					} else {
+						prev_col_x[1] = x;
+					}
+					if (prev_col_y[0] >= 0) {
+						prev_col_y[0] = y;
+					} else {
+						prev_col_y[1] = y;
+					}
 				}
 			}
 		}
--
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