#!/usr/bin/env julia

using Test

struct Move
  direction :: Char
  count :: Int
end

mutable struct KnotPair
  head_x :: Int
  head_y :: Int
  tail_x :: Int
  tail_y :: Int
end

function parse_move(line)
  parts = split(line, limit=2)
  return Move(parts[1][1], parse(Int, parts[2]))
end


function state_changed(old_state, new_state)
  return (new_state.head_x != old_state.head_x
          || new_state.head_y != old_state.head_y
          || new_state.tail_x != old_state.tail_x
          || new_state.tail_y != old_state.tail_y)
end

function read_moves(io)
  return [parse_move(line) for line in readlines(io)]
end

function apply_move(state, direction)
  if direction == 'U'
    state.head_y += 1
  elseif direction == 'D'
    state.head_y -= 1
  elseif direction == 'L'
    state.head_x -= 1
  elseif direction == 'R'
    state.head_x += 1
  elseif direction != 'O'
    throw(DomainError("Unknown direction"))
  end
  x_diff = state.head_x - state.tail_x
  y_diff = state.head_y - state.tail_y
  if abs(x_diff) <= 1 && abs(y_diff) <= 1
    return state
  end
  if abs(x_diff) <= 1
    state.tail_x += x_diff
    state.tail_y += sign(y_diff) * (abs(y_diff) - 1)
  elseif abs(y_diff) <= 1
    state.tail_y += y_diff
    state.tail_x += sign(x_diff) * (abs(x_diff) - 1)
  elseif abs(x_diff) == 2 && abs(y_diff) == 2
    # possible with more than 2 knots
    state.tail_x += sign(x_diff) * (abs(x_diff) - 1)
    state.tail_y += sign(y_diff) * (abs(y_diff) - 1)
  else
    throw(DomainError("invalid state " * string(state)))
  end 
  return state
end

function get_visited(moves; npairs::Int=1)
  visited = Set{NTuple{2, Int}}()
  states = [KnotPair(0, 0, 0, 0) for _ in 1:npairs]
  push!(visited, (states[npairs].tail_x, states[npairs].tail_y))
  for move in moves
    for m in 1:move.count
      # println(move, " ", m)
      # print(" 1 ", states[1], " -> ")
      states[1] = apply_move(states[1], move.direction)
      # println(states[1])
      for i in 2:npairs
        states[i].head_x = states[i-1].tail_x
        states[i].head_y = states[i-1].tail_y
        # old_state = deepcopy(states[i])
        states[i] = apply_move(states[i], 'O')
        # if state_changed(states[i], old_state)
        #   println(" ", i, " ", old_state, " -> ", states[i])
        # end
      end
      push!(visited, (states[npairs].tail_x, states[npairs].tail_y))
    end
  end
  return visited
end

function visited_map(visited)
  (min_x, max_x) = extrema([t[1] for t in visited])
  (min_y, max_y) = extrema([t[2] for t in visited])
  nrows = max_y - min_y + 1
  ncols = max_x - min_x + 1
  vmap = zeros(Int, nrows, ncols)
  for (x, y) in visited
    vmap[nrows - (y - min_y), (x - min_x + 1)] = 1
  end
  return vmap
end

function test()
  @testset "visited count 2 knot" verbose=true begin
    @test length(get_visited(read_moves("example.txt"))) == 13
    @test length(get_visited(read_moves("input.txt"))) == 6037
  end
  @testset "visited count 10 knot" verbose=true begin
    @test length(get_visited(read_moves("example.txt"), npairs=9)) == 1
    @test length(get_visited(read_moves("input.txt"), npairs=9)) == 2485
  end
end

function main()
  if size(ARGS, 1) == 0
    test()
  else
    infile = ARGS[1]
    println("infile = ", infile)
    moves = read_moves(infile)
    println("moves: ", moves)
    visited = get_visited(moves)
    println("visited: ", visited)
    println("count  : ", length(visited))
    display(visited_map(visited))
    println()

    visited10 = get_visited(moves, npairs=9)
    println("visited10: ", visited10)
    println("count10  : ", length(visited10))
    display(visited_map(visited10))
    println()

  end
end

main()