ff.ts

type Point = readonly [x: number, y: number]

class Board {
  static of(cells: string) {
    return new Board(cells.split("\n").map((x) => x.split("")))
  }

  constructor(readonly cells: string[][]) {}

  *search(endpoints: string, as: string) {
    let a, b

    find: for (let i = 0; i < this.cells.length; i++) {
      for (let j = 0; j < this.cells[i]!.length; j++) {
        if (this.cells[i]![j]! == endpoints) {
          if (a) {
            b = [i, j] satisfies Point as Point
            break find
          } else {
            a = [i, j] satisfies Point as Point
          }
        }
      }
    }

    if (!a || !b) {
      console.warn("no endpoints")
      return
    }

    yield* this.explore(a, b, as)
  }

  *explore([sx, sy]: Point, [dx, dy]: Point, as: string): Generator<string> {
    for (const [ex, ey] of [
      [0, 1],
      [0, -1],
      [1, 0],
      [-1, 0],
    ] as Point[]) {
      const x = sx + ex
      const y = sy + ey
      if (x == dx && y == dy) {
        yield this.join()
      } else if (this.cells[y]?.[x] == ".") {
        this.cells[y]![x] = as
        yield* this.explore([x, y], [dx, dy], as)
        this.cells[y]![x] = "."
      }
    }
  }

  join() {
    return this.cells.map((x) => x.join("")).join("\n")
  }
}

const src = new Board(
  `A....
.....
..B..
CBD.A
D...C`
    .split("\n")
    .map((x) => x.split("")),
)

for (const a of src.search("A", "a")) {
  console.log()
  console.log(a)
  // for (const b of Board.of(a).search("B", "b")) {
  // for (const c of Board.of(b).search("C", "c")) {
  // for (const d of c.search("D", "d")) {
  // }
  // }
  // }
}