Discussion on: Daily Challenge #107 - Escape the Mines

// Solution based on: https://www.redblobgames.com/pathfinding/a-star/introduction.html
// by @redblobgames

type Coords = readonly [number, number];

function solve(caveMap: readonly boolean[][], miner: Coords, exit: Coords) {
  const prevCoordsMap = calculatePrevCoordsMap(caveMap, miner);
  const path = getPath(miner, exit, prevCoordsMap);
  return getInstructionsFromPath(path);
}

function compare(c0: Coords, [x1, y1]: Coords) {
  const [x0, y0] = c0;
  return x0 === x1 && y0 === y1;
}

function getInstructionsFromPath(path: readonly Coords[]) {
  return path.reduce<string[]>((acc, cur, index, arr) => {
    if (index < arr.length - 1) {
      acc.push(getDirection(cur, arr[index + 1]));
    }
    return acc;
  }, []);
}

function calculatePrevCoordsMap(caveMap: readonly boolean[][], miner: Coords) {
  const getNeighbours = createGetNeighbours(caveMap);

  const frontier: Coords[] = [miner];
  const comeFrom: { [index: string]: Coords | null } = {
    [miner.toString()]: null,
  };

  const isInComeFrom = (coords: Coords) => comeFrom.hasOwnProperty(coords.toString());

  while (frontier.length > 0) {
    let current = frontier.shift();
    getNeighbours(current!).forEach(next => {
      if (!isInComeFrom(next)) {
        frontier.unshift(next);
        comeFrom[next.toString()] = current!;
      }
    });
  }
  return comeFrom;
}

function createGetNeighbours(caveMap: readonly boolean[][]) {
  const height = caveMap[0].length;
  const width = caveMap.length;
  const isWalkable = ([x, y]: Coords) => caveMap[x] && caveMap[x][y];
  return ([x, y]: Coords) => {
    const n: Coords | null = y > 0 ? [x, y - 1] : null;
    const s: Coords | null = y < height ? [x, y + 1] : null;
    const w: Coords | null = x > 0 ? [x - 1, y] : null;
    const e: Coords | null = x < width ? [x + 1, y] : null;
    return ([n, s, w, e].filter(c => c !== null) as readonly Coords[]).filter(
      c => isWalkable(c)
    );
  };
}

function getPath(
  start: Coords,
  goal: Coords,
  prevCoordsMap: {[index: string]: Coords | null}
) {

  const path: Coords[] = [];
  let current: Coords = goal;
  let count = 0;
  while (compare(current, start) === false) {
    if (++count > 100) break;
    path.push(current);
    const prev = prevCoordsMap[current.toString()];
    if (prev) current = prev;
  }
  path.push(start);
  return path.reverse();
}

function getDirection([x0, y0]: Coords, [x1, y1]: Coords): string {
  if (x1 > x0) return 'right';
  if (x1 < x0) return 'left';
  if (y1 > y0) return 'down';
  if (y1 < y0) return 'up';
  return '';
}

//////////////////////////////
// TESTS
//////////////////////////////

function assertEquals(name: string, a: string[], b: string[]) {
  if (a.length === b.length && a.every((step, i) => b[i] === step)) {
    console.log(`OK: ${name}`);
  } else {
    console.error(`KO: '${name}'\n${a} !== ${b}`);
  }
}

assertEquals(
  'A simple map. (2x2)',
  solve([[true, false], [true, true]], [0, 0], [1, 0]),
  ['right']
);

assertEquals(
  'A linear map. (1x4)',
  solve([[true], [true], [true], [true]], [0, 0], [3, 0]),
  ['right', 'right', 'right']
);

assertEquals(
  'Walk around an obstacle (3x3)',
  solve(
    [[true, true, true], [false, false, true], [true, true, true]],
    [0, 0],
    [2, 0]
  ),
  ['down', 'down', 'right', 'right', 'up', 'up']
);

assertEquals(
  'Change directions several times (5x5)',
  solve(
    [
      [true, true, false, false, false],
      [false, true, true, false, false],
      [false, false, true, true, false],
      [false, false, false, true, true],
      [false, false, false, false, true],
    ],
    [0, 0],
    [4, 4]
  ),
  ['down', 'right', 'down', 'right', 'down', 'right', 'down', 'right']
);