From e260e75ec5fcd7e46d1218445be4fa1094af5c99 Mon Sep 17 00:00:00 2001
From: Alistair Buxton <a.j.buxton@gmail.com>
Date: Sun, 12 Sep 2021 13:46:26 +0100
Subject: [PATCH] Prevent the snake from going back on itself

There are two ways that could happen:

If you are going up and press left and down on the same frame, pressing
left switches your movement to left, and then the check for vertical
movement passes and you start going down in the same update.

The other way is if two updates happen in one game tick.

Fix both of these by remembering the last move. Switch the direction
variable to an enum to make it easier to test the current and previous
direction, and use a switch statement to calculate the actual changes
to the position.
---
 snake.cpp | 60 ++++++++++++++++++++++++++++++-------------------------
 1 file changed, 33 insertions(+), 27 deletions(-)

diff --git a/snake.cpp b/snake.cpp
index 0f2d19f..e0c3813 100644
--- a/snake.cpp
+++ b/snake.cpp
@@ -13,11 +13,9 @@ using namespace blit;
 const uint32_t UPDATE_RATE = 200;
 
 // Some handy constants for movement directions and "stopped" state
-const Vec2 MOVE_UP(0, -1);
-const Vec2 MOVE_DOWN(0, 1);
-const Vec2 MOVE_LEFT(-1, 0);
-const Vec2 MOVE_RIGHT(1, 0);
-const Vec2 STOP(0, 0);
+enum Direction {
+    Up, Down, Left, Right, Stop
+};
 
 // These points index into the spritesheet, using x/y values
 // makes it easy to identify a specific sprite.
@@ -30,7 +28,8 @@ const uint32_t NUM_SPRITES = 14; // First 14 sprites on the top row
 Rect game_bounds;
 
 // Current snake direction
-Vec2 direction(0, 0);
+Direction direction = Stop;
+Direction last_move = Stop;
 
 // The snake itself is a vector of Points
 std::vector<Point> snake;
@@ -53,7 +52,7 @@ void restart_game() {
     // Remove all snake 
     snake.clear();
     snake.emplace_back(game_bounds.center());
-    direction = STOP;
+    direction = Stop;
     score = 0;
     random_reset();
     apple = get_random_point(game_bounds.size());
@@ -64,11 +63,27 @@ void restart_game() {
 // this allows us to run the game at a speed (more or less) of our choosing.
 // TASK you could try making Snake harder over time by speeding it up!
 void move(Timer &t) {
-    // No movement means nothing to do here
-    // exit early so the snake doesn't continually collide with itself
-    if (direction == Vec2(0.0f, 0.0f)) return;
 
-    Point head = snake.back() + direction;
+    Point head = snake.back();
+    switch(direction) {
+        case Up:
+            head.y -= 1;
+            break;
+        case Down:
+            head.y += 1;
+            break;
+        case Left:
+            head.x -= 1;
+            break;
+        case Right:
+            head.x += 1;
+            break;
+        default:
+            // No movement means nothing to do here
+            // exit early so the snake doesn't continually collide with itself
+            return;
+    }
+
     for(auto segment : snake) {
         // If the head x/y coordinates match any body/segment
         // coordinates then we've collided with ourselves. BAD LUCK!
@@ -99,6 +114,7 @@ void move(Timer &t) {
         // which is A BAD THING, but for the sake of snake... it's fiiiinnnee!
         snake.erase(snake.begin());
     }
+    last_move = direction;
 }
 
 void init() {
@@ -150,20 +166,10 @@ void render(uint32_t time_ms) {
 
 void update(uint32_t time) {
     // Movement is easy. You can't go back on yourself.
-    // These checks ensure you can't be moving left,
-    // ie: directon.x == -1
-    // and change direction to go back over yourself,
-    // ie: direction.x == 1
-    if(direction.x == 0) {
-        // If we're not already moving along the X axis,
-        // allow a change in direction along X
-        if(buttons.pressed & DPAD_RIGHT) direction = MOVE_RIGHT;
-        if(buttons.pressed & DPAD_LEFT) direction = MOVE_LEFT;
-    }
-    if(direction.y == 0) {
-        // If we're not already moving along the Y axis,
-        // allow a change in direction along Y
-        if(buttons.pressed & DPAD_DOWN) direction = MOVE_DOWN;
-        if(buttons.pressed & DPAD_UP) direction = MOVE_UP;
-    }
+    // The last actual move made is remembered in last_move.
+    // You can go in any direction except the opposite of it.
+    if(buttons.pressed & DPAD_RIGHT && last_move != Left) direction = Right;
+    if(buttons.pressed & DPAD_LEFT && last_move != Right) direction = Left;
+    if(buttons.pressed & DPAD_DOWN && last_move != Up) direction = Down;
+    if(buttons.pressed & DPAD_UP && last_move != Down) direction = Up;
 }