Snake game

The problem

A snake moves on a grid. The player steers it. Eating food makes it longer and scores a point. Hitting a wall or itself ends the game.

Entities

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Figure SDA-LLD-13-1 · Snake classes · © forcsguy.com

One step of the game

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Figure SDA-LLD-13-2 · One game step · © forcsguy.com

type P = { x: number; y: number };
const delta = { UP: { x: 0, y: -1 }, DOWN: { x: 0, y: 1 }, LEFT: { x: -1, y: 0 }, RIGHT: { x: 1, y: 0 } };
const opposite = { UP: "DOWN", DOWN: "UP", LEFT: "RIGHT", RIGHT: "LEFT" } as const;

class Game {
  body: P[] = [{ x: 5, y: 5 }];      // index 0 is the head
  dir: keyof typeof delta = "RIGHT"; food: P = { x: 8, y: 5 }; score = 0; over = false;
  constructor(readonly w = 20, readonly h = 20) {}

  turn(d: keyof typeof delta) { if (d !== opposite[this.dir]) this.dir = d; }

  step() {
    if (this.over) return;
    const head = { x: this.body[0].x + delta[this.dir].x, y: this.body[0].y + delta[this.dir].y };
    const outside = head.x < 0 || head.y < 0 || head.x >= this.w || head.y >= this.h;
    const ate = head.x === this.food.x && head.y === this.food.y;
    const hitsSelf = this.body.slice(0, ate ? undefined : -1).some(c => c.x === head.x && c.y === head.y);
    if (outside || hitsSelf) { this.over = true; return; }
    this.body.unshift(head);
    if (ate) { this.score++; this.food = this.freeCell(); } else this.body.pop();
  }
  freeCell(): P { /* pick a random cell not in body */ return { x: 0, y: 0 }; }
}

The subtle detail: when not eating, the tail moves away this step, so the head may legally enter the cell the tail is leaving. That is why the self-collision check excludes the tail unless the snake grows.

Design notes

• Separate model from loop and view: step() is pure game logic. An outer timer calls it; a renderer draws the state. Tests call step() directly.
• Input buffering: store the next direction and apply it at the next step, so quick double taps do not skip.
• Free-cell search: for a nearly full board, keep a set of free cells rather than retrying random picks.
• Extensions: levels and speed, walls, two players, power-ups, saved high scores.

Real-world reading