import Foundation

struct Coord : Hashable, CustomStringConvertible {
    let (x, y): (Int, Int)
    var description: String { return "(\(x), \(y))" }
}

struct Maze : CustomStringConvertible {
    let (w, h): (Int, Int)
    let walls: Set<Coord>
    let (start, end): (Coord, Coord)
    var description: String {
        var s = Array(repeating: Array(repeating: " ", count: w), count: (h+1)/2)
        walls.forEach { wall in
            if wall.y.isMultiple(of: 2) {
                s[wall.y/2][wall.x] = (s[wall.y/2][wall.x] == " ") ? "▀" : "█"
            } else {
                s[wall.y/2][wall.x] = (s[wall.y/2][wall.x] == " ") ? "▄" : "█"
            }
        }
        return s.map { $0.joined() + "\n" }.joined() + "\(start), \(end)"
    }

    func valid(_ cell: Coord) -> Bool {
        return cell.x >= 0 && cell.x < w && cell.y >= 0 && cell.y < h &&
            !walls.contains(cell)
    }

    func neighbors(of cell: Coord) -> [Coord] {
        return [(-1, 0), (1, 0), (0, -1), (0, 1)]
            .map { dx, dy in Coord(x: cell.x + dx, y: cell.y + dy) }
    }

    func cheats(of cell: Coord) -> [Coord] {
        return [(-1, 0), (1, 0), (0, -1), (0, 1)]
            .map { dx, dy in Coord(x: cell.x + dx*2, y: cell.y + dy*2) }
            .filter(valid)
    }

    func cheats() -> [Int] {
        var cost: [Coord: Int] = [start: 0]
        var jumps: [(Coord, Coord)] = []
        var cell = start
        var prev = start
        var currentCost = 0
        while cell != end {
            // list potential cheats from current cell
            let newCheats = cheats(of: cell).filter { !cost.keys.contains($0) }
            jumps.append(contentsOf: newCheats.map { dest in (cell, dest)} )
            // add cost to table
            let next = neighbors(of: cell).filter(valid).filter { $0 != prev }[0]
            currentCost += 1
            cost[next] = currentCost
            prev = cell
            cell = next
        }
        return jumps.map { from, to in cost[to]! - cost[from]! - 2 }
    }
}

func readInput(_ filePath: String) throws -> Maze {
    let content = try String(contentsOfFile: filePath, encoding: .ascii)
    let lines = content.split(separator: "\n")
    let (w, h) = (lines[0].count, lines.count)
    var walls: Set<Coord> = []
    var (start, end) = (Coord(x: 0, y: 0), Coord(x: w-1, y: h-1))
    lines.enumerated().forEach { i, line in
        line.enumerated().forEach { j, cell in
            let coord = Coord(x: j, y: i)
            if cell == "#" {
                walls.insert(coord)
            } else if cell == "S" {
                start = coord
            } else if cell == "E" {
                end = coord
            }
        }
    }
    return Maze(w: w, h: h, walls: walls, start: start, end: end)
}

let maze = try readInput(CommandLine.arguments[1])
print(maze)
let cheats = maze.cheats()
let hist = cheats.reduce(into: [:]) { counts, t in counts[t, default: 0] += 1 }
hist.keys.sorted().forEach { print("\($0) -> \(hist[$0]!)") }
print(cheats.filter { $0 >= 100 }.count)