package q1970

var dirs = [4][2]int{{0, 1}, {0, -1}, {1, 0}, {-1, 0}}

func isPossibleOnDay(row, col int, cells [][]bool, day int, floods [][]int) bool {
	for i := range col {
		cells[0][i] = true
	}
	for i := range row - 1 {
		copy(cells[i+1], cells[0])
	}
	for i := range day {
		cells[floods[i][0]-1][floods[i][1]-1] = false
	}

	// Use bfs to find a path from top to bottom
	queue := make([][2]int, 0, col) // {row, col}
	for x, ok := range cells[0] {
		if ok {
			queue = append(queue, [2]int{0, x})
			cells[0][x] = false
		}
	}
	for ; len(queue) > 0; queue = queue[1:] {
		r, c := queue[0][0], queue[0][1]
		for _, d := range dirs {
			tr, tc := r+d[0], c+d[1]
			if tr < 0 || tc < 0 || tr >= row || tc >= col {
				continue
			}
			if !cells[tr][tc] {
				continue
			}
			if tr == row-1 {
				return true // reached bottom
			}
			cells[tr][tc] = false
			queue = append(queue, [2]int{tr, tc})
		}
	}
	return false
}

func latestDayToCross(row int, col int, cells [][]int) int {
	walkableBuf := make([]bool, col*row)
	walkable := make([][]bool, row)
	for i := range row {
		walkable[i] = walkableBuf[i*col : (i+1)*col]
	}

	// Use binary search to find the last day
	l, r := 1, len(cells)+1
	for l < r {
		if l+1 == r {
			return l
		}
		m := (l + r) / 2
		if isPossibleOnDay(row, col, walkable, m, cells) {
			l = m
		} else {
			r = m
		}
	}
	return 0 // impossible
}

var _ = latestDayToCross