fix(engine): populate api.otherPlayers, ensure bots can eat other b…

…ots, and fix crash that can sometimes happen when the food with the latest index is eaten (#12)

## How does this PR impact the user?

This PR fixes the following issues:
- [x] `api.otherPlayers` was always empty
- [x] bots weren't able to eat other bots
- [x] world collision computation can crash when the bot eats multiple
foods and on of them has the latest index

### Now – `api.otherPlayers` is correctly populated and the bots can eat
each other


https://github.com/move-fast-and-break-things/aibyss/assets/4187729/b7c8a438-b3a1-43ae-be11-c8df36e45797

### Before – `api.otherPlayers` is always empty, and the second bot is
stuck


https://github.com/move-fast-and-break-things/aibyss/assets/4187729/9725cc6b-46d4-4222-aeb2-c9a87aa9c0b3

## Description

- [x] move bot code preparation logic from `engine.ts` to
`prepareBotCode.ts`
- [x] fix bugs
- [x] add tests to prevent regressions

## Limitations

N/A

## Checklist

- [x] my PR is focused and contains one wholistic change
- [x] I have added screenshots or screen recordings to show the changes

server/plugins/engine.ts

@@ -1,4 +1,5 @@
 import ivm from "isolated-vm";
+import prepareBotCode from "../../utils/prepareBotCode";
 import * as botUtils from "~/utils/botstore";
 import World from "~/utils/world";
 
@@ -33,33 +34,12 @@ async function runBots({ bots, world, botApi }: RunBotArgs) {
   }
 
   for (const bot of Object.values(bots)) {
-    const { code } = bot;
-
-    const botObject = state.bots.get(bot.id);
-    if (!botObject) {
-      // TODO(yurij): add better logging
-      console.error(`Bot with id ${bot.id} not found in the world`);
-      continue;
-    }
-
-    const me = { x: botObject.x, y: botObject.y, radius: botObject.radius };
-    const otherPlayers = Object.values(state.bots)
-      .filter(b => b.id !== bot.id)
-      .map(b => ({ x: b.x, y: b.y, radius: b.radius }));
-    const food = state.food.map(f => ({ x: f.x, y: f.y, radius: f.radius }));
-
     try {
-      const preparedCode = `
-global._player = ${JSON.stringify(me)};
-global._otherPlayers = ${JSON.stringify(otherPlayers)};
-global._food = ${JSON.stringify(food)};
-global._worldWidth = ${state.width};
-global._worldHeight = ${state.height};
-
-${code}
-
-${botApi}
-`;
+      const preparedCode = prepareBotCode({ bot, state, botApi });
+      if (!preparedCode) {
+        console.error(`Failed to prepare code for bot ${bot.id}`);
+        continue;
+      }
 
       const actions = await runBot(preparedCode);
       if (actions?.[0]?.type === "move") {

utils/__snapshots__/prepareBotCode.spec.ts.snap

@@ -0,0 +1,43 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`prepares bot code correctly when there are three bots and some food in the world 1`] = `
+"
+global._player = {"x":0,"y":0,"radius":5};
+global._otherPlayers = [{"x":10,"y":10,"radius":5},{"x":20,"y":20,"radius":5}];
+global._food = [{"x":30,"y":30,"radius":5},{"x":40,"y":40,"radius":5}];
+global._worldWidth = 100;
+global._worldHeight = 100;
+
+console.log('hello world');
+
+console.log('hello bot api');
+"
+`;
+
+exports[`prepares bot code correctly when there are two bots in the world 1`] = `
+"
+global._player = {"x":0,"y":0,"radius":5};
+global._otherPlayers = [{"x":10,"y":10,"radius":5}];
+global._food = [];
+global._worldWidth = 100;
+global._worldHeight = 100;
+
+console.log('hello world');
+
+console.log('hello bot api');
+"
+`;
+
+exports[`prepares bot code correctly when there is one bot in the world 1`] = `
+"
+global._player = {"x":0,"y":0,"radius":5};
+global._otherPlayers = [];
+global._food = [];
+global._worldWidth = 100;
+global._worldHeight = 100;
+
+console.log('hello world');
+
+console.log('hello bot api');
+"
+`;

utils/botstore.ts

@@ -1,6 +1,6 @@
 import EventEmitter from "node:events";
 
-interface Bot {
+export interface Bot {
   id: string;
   code: string;
 }

utils/prepareBotCode.spec.ts

@@ -0,0 +1,69 @@
+import { it, expect } from "vitest";
+import prepareBotCode from "./prepareBotCode";
+
+it("prepares bot code correctly when there is one bot in the world", () => {
+  const bot = {
+    id: "1",
+    code: "console.log('hello world');",
+  };
+
+  const state = {
+    bots: new Map([["1", { x: 0, y: 0, radius: 5, id: "1", color: "#00FF00" }]]),
+    food: [],
+    width: 100,
+    height: 100,
+  };
+
+  const botApi = "console.log('hello bot api');";
+
+  const preparedCode = prepareBotCode({ bot, state, botApi });
+
+  expect(preparedCode).toMatchSnapshot();
+});
+
+it("prepares bot code correctly when there are two bots in the world", () => {
+  const bot = {
+    id: "1",
+    code: "console.log('hello world');",
+  };
+
+  const state = {
+    bots: new Map([
+      ["1", { x: 0, y: 0, radius: 5, id: "1", color: "#00FF00" }],
+      ["2", { x: 10, y: 10, radius: 5, id: "2", color: "#00FF00" }],
+    ]),
+    food: [],
+    width: 100,
+    height: 100,
+  };
+
+  const botApi = "console.log('hello bot api');";
+
+  const preparedCode = prepareBotCode({ bot, state, botApi });
+
+  expect(preparedCode).toMatchSnapshot();
+});
+
+it("prepares bot code correctly when there are three bots and some food in the world", () => {
+  const bot = {
+    id: "1",
+    code: "console.log('hello world');",
+  };
+
+  const state = {
+    bots: new Map([
+      ["1", { x: 0, y: 0, radius: 5, id: "1", color: "#00FF00" }],
+      ["2", { x: 10, y: 10, radius: 5, id: "2", color: "#00FF00" }],
+      ["3", { x: 20, y: 20, radius: 5, id: "3", color: "#00FF00" }],
+    ]),
+    food: [{ x: 30, y: 30, radius: 5 }, { x: 40, y: 40, radius: 5 }],
+    width: 100,
+    height: 100,
+  };
+
+  const botApi = "console.log('hello bot api');";
+
+  const preparedCode = prepareBotCode({ bot, state, botApi });
+
+  expect(preparedCode).toMatchSnapshot();
+});

utils/prepareBotCode.ts

@@ -0,0 +1,39 @@
+import type { Bot } from "~/utils/botstore";
+import type { WorldState } from "~/utils/world";
+
+type PrepareBotCodeArgs = {
+  bot: Bot;
+  state: WorldState;
+  botApi: string;
+};
+
+export default function prepareBotCode({ bot, state, botApi }: PrepareBotCodeArgs): string | undefined {
+  const { code } = bot;
+
+  const botObject = state.bots.get(bot.id);
+  if (!botObject) {
+    // TODO(yurij): handle this better
+    console.error(`Bot with id ${bot.id} not found in the world`);
+    return;
+  }
+
+  const me = { x: botObject.x, y: botObject.y, radius: botObject.radius };
+  const otherPlayers = [...state.bots.values()]
+    .filter(b => b.id !== bot.id)
+    .map(b => ({ x: b.x, y: b.y, radius: b.radius }));
+  const food = state.food.map(f => ({ x: f.x, y: f.y, radius: f.radius }));
+
+  const preparedCode = `
+global._player = ${JSON.stringify(me)};
+global._otherPlayers = ${JSON.stringify(otherPlayers)};
+global._food = ${JSON.stringify(food)};
+global._worldWidth = ${state.width};
+global._worldHeight = ${state.height};
+
+${code}
+
+${botApi}
+`;
+
+  return preparedCode;
+}

utils/world.spec.ts

@@ -0,0 +1,78 @@
+import { it, expect } from "vitest";
+import World, { type BotSprite, type Sprite } from "./world";
+
+it("correctly removes the smaller bot when the bigger bot eats it", () => {
+  const world = new World({ width: 100, height: 100 });
+  world.addBot("1");
+  world.addBot("2");
+
+  // @ts-expect-error - world.bots is private
+  const bot1 = world.bots.get("1") as BotSprite;
+  bot1.radius = 10;
+  bot1.x = 10;
+  bot1.y = 10;
+
+  // @ts-expect-error - world.bots is private
+  const bot2 = world.bots.get("2") as BotSprite;
+  bot2.radius = 5;
+  bot2.x = 10;
+  bot2.y = 10;
+
+  world.checkCollisions("1");
+
+  // @ts-expect-error - world.bots is private
+  expect(world.bots.has("1")).toBe(true);
+  // @ts-expect-error - world.bots is private
+  expect(world.bots.has("2")).toBe(false);
+});
+
+it("correctly removes food when the bot eats it", () => {
+  const world = new World({ width: 100, height: 100 });
+  world.addBot("1");
+
+  // @ts-expect-error - world.bots is private
+  const bot = world.bots.get("1") as BotSprite;
+  bot.x = 10;
+  bot.y = 10;
+
+  // @ts-expect-error - world.food is private
+  expect(world.food.length).toBe(100);
+
+  // @ts-expect-error - world.food is private
+  const food = world.food[0] as Sprite;
+  food.x = 10;
+  food.y = 10;
+
+  world.checkCollisions("1");
+
+  // @ts-expect-error - world.food is private
+  expect(world.food.length).toBeLessThan(100);
+});
+
+it("shouldn't crash when bot eats two food items and one of them with the last index", () => {
+  const world = new World({ width: 100, height: 100 });
+  world.addBot("1");
+
+  // @ts-expect-error - world.bots is private
+  const bot = world.bots.get("1") as BotSprite;
+  bot.x = 10;
+  bot.y = 10;
+
+  // @ts-expect-error - world.food is private
+  expect(world.food.length).toBe(100);
+
+  // @ts-expect-error - world.food is private
+  const food1 = world.food[0] as Sprite;
+  food1.x = 10;
+  food1.y = 10;
+
+  // @ts-expect-error - world.food is private
+  const food2 = world.food[99] as Sprite;
+  food2.x = 10;
+  food2.y = 10;
+
+  world.checkCollisions("1");
+
+  // @ts-expect-error - world.food is private
+  expect(world.food.length).toBeLessThan(100);
+});

utils/world.ts

@@ -1,6 +1,6 @@
 import getRandomElement from "./getRandomElement";
 
-interface Sprite {
+export interface Sprite {
   x: number;
   y: number;
   radius: number;
@@ -16,6 +16,13 @@ export interface BotSprite extends Sprite {
   color: string;
 }
 
+export interface WorldState {
+  bots: BotSprites;
+  food: Sprite[];
+  width: number;
+  height: number;
+}
+
 type BotSprites = Map<string, BotSprite>;
 
 type WorldArgs = {
@@ -117,7 +124,7 @@ export default class World {
     return newBot;
   }
 
-  getState() {
+  getState(): WorldState {
     return {
       bots: this.bots,
       food: this.food,
@@ -171,7 +178,7 @@ export default class World {
     // check if bot eats other bots
     const bots = this.bots;
     const botIdsToRemove: string[] = [];
-    for (const [otherBotId, otherBot] of Object.entries(bots)) {
+    for (const [otherBotId, otherBot] of bots.entries()) {
       if (otherBot.id === botId) {
         continue;
       }
@@ -203,8 +210,10 @@ export default class World {
       bots.delete(botIdToRemove);
     }
 
+    // sort in descending order to avoid index shifting when removing elements
+    foodIdxToRemove.sort((a, b) => b - a);
     for (const foodIdx of foodIdxToRemove) {
-      // it's safe to cast to Sprite because know that `foodIdx` is within the bounds of the array
+      // it's safe to cast to Sprite because we know that `foodIdx` is within the bounds of the array
       bot.radius += (food[foodIdx] as Sprite).radius;
       food.splice(foodIdx, 1);
     }