Snakes-And-Ladders/src/game/board.ts

import { readFile } from "fs/promises";
import { BoardLoadError } from "./errors/board_load_error.js";

interface ISnakeOrLadder {
  landing: number;
  destination: number;
}

export interface IBoardData {
  size: number;
  ladders: ISnakeOrLadder[];
  snakes: ISnakeOrLadder[];
}

export class Board {
  public size: number;
  private snakeMap: Map<number, number> = new Map();
  private ladderMap: Map<number, number> = new Map();

  static async loadFromFile(
    filename: string,
    encoding: BufferEncoding = "utf-8"
  ) {
    let rawData;
    try {
      rawData = await readFile(filename, { encoding });
    } catch (err) {
      throw new BoardLoadError(`Unable to read file: ${filename}`);
    }

    let jsonData;
    try {
      jsonData = JSON.parse(rawData);
    } catch (err) {
      throw new BoardLoadError("Invalid JSON");
    }

    if (typeof jsonData !== "object") {
      throw new BoardLoadError("JSON does not contain board object");
    }

    if (
      !("ladders" in jsonData && "snakes" in jsonData && "size" in jsonData)
    ) {
      throw new BoardLoadError("Missing properties in board object");
    }

    if (!Number.isInteger(jsonData.size)) {
      throw new BoardLoadError("Invalid board size");
    }

    const boardData: IBoardData = {
      size: jsonData.size,
      snakes: [],
      ladders: [],
    };

    const props: Array<"snakes" | "ladders"> = ["snakes", "ladders"];
    for (const prop of props) {
      const transportData = jsonData[prop];

      if (!Array.isArray(transportData)) {
        throw new BoardLoadError(`Invalid "${prop}" property in board object`);
      }

      for (const transport of transportData) {
        if (
          typeof transport !== "object" ||
          !("landing" in transport) ||
          !Number.isInteger(transport.landing) ||
          !("destination" in transport) ||
          !Number.isInteger(transport.destination)
        ) {
          const repr = JSON.stringify(transport);
          // Remove plural
          const transportType = prop.replace(/s$/, prop);
          throw new BoardLoadError(`Invalid ${transportType} entry: ${repr}`);
        }

        boardData[prop].push({
          landing: transport.landing,
          destination: transport.destination,
        });
      }
    }

    return new Board(boardData);
  }

  constructor(boardData: IBoardData) {
    this.size = boardData.size;
    for (const transport of boardData.snakes) {
      if (transport.landing <= transport.destination) {
        throw new BoardLoadError(
          "Invalid snake, landing not greater than destination"
        );
      }
      this.snakeMap.set(transport.landing, transport.destination);
    }
    for (const transport of boardData.ladders) {
      if (transport.landing >= transport.destination) {
        throw new BoardLoadError(
          "Invalid ladder, landing not less than destination"
        );
      }
      this.ladderMap.set(transport.landing, transport.destination);
    }
  }

  tileHasSnake(pos: number) {
    return this.snakeMap.has(pos);
  }

  tileHasLadder(pos: number) {
    return this.ladderMap.has(pos);
  }

  calculateLandingPosition(pos: number) {
    return Math.min(
      this.ladderMap.get(pos) ?? this.snakeMap.get(pos) ?? pos,
      this.size
    );
  }
}