--!name Snake
--!icon game

local N = 17                      -- board is N x N cells
local CELL_EMPTY = "."            -- ground, drawn as a dot so an empty board
local CELL_BODY  = "#"            -- still reads as a field rather than a void
local CELL_HEAD  = "@"            -- a block with the centre punched out
local CELL_FOOD  = "*"            -- a disc: food has its own silhouette
local START_LEN  = 3
local FOOD_SCORE = 10

local PACE_MAX  = 600
local PACE_MIN  = 450

local ALPHA = "0123456789abcdefg"

local DX = {0, 0, -1, 1}
local DY = {-1, 1, 0, 0}
local OPPOSITE = {2, 1, 4, 3}
local DIR_OF_KEY = {up = 1, down = 2, left = 3, right = 4}
local DIR_NAME = {"NORTH", "SOUTH", "WEST", "EAST"}

local body, dir, food = {}, 4, 0

local q, qn = {}, 0

local scratch = {}

local function spec_index(cell)
  return (cell // N) * (N + 1) + (cell % N) + 1
end
local score, best, alive, note = 0, 0, true, ""
local hi = false
local running = false

local function place_food()
  local occ = scratch
  for i = 1, N * N do occ[i] = false end
  for i = 1, #body do occ[body[i] + 1] = true end
  local free = 0
  for i = 1, N * N do if not occ[i] then free = free + 1 end end
  if free == 0 then
    alive, note = false, "PERFECT BOARD"
    return
  end
  local pick, seen = math.random(free), 0
  for c = 0, N * N - 1 do
    if not occ[c + 1] then
      seen = seen + 1
      if seen == pick then food = c return end
    end
  end
end

local function new_game()
  body = {}
  local cy = N // 2
  for i = 1, START_LEN do
    body[i] = cy * N + (N // 2) - i + 1
  end
  dir, qn, score, alive, note = 4, 0, 0, true, "GO"
  hi = false
  running = false
  place_food()
end

local function encode(from, to)
  local buf, n = scratch, 0
  for i = from, to do
    local x, y = body[i] % N, body[i] // N
    n = n + 1 buf[n] = ALPHA:sub(x + 1, x + 1)
    n = n + 1 buf[n] = ALPHA:sub(y + 1, y + 1)
  end
  return table.concat(buf, "", 1, n)
end

local function save()
  local half = #body // 2
  snail.save(string.format("%d %d %d %d ;%d %d %d %s",
    best, score, #body, alive and 1 or 0,
    dir, food % N, food // N, encode(1, half) .. encode(half + 1, #body)))
end

local function load()
  local s = snail.load()
  if not s then return end
  local b, sc, ln, al, d, fx, fy, segs =
    s:match("^(%d+) (%d+) (%d+) (%d+) ;(%d+) (%d+) (%d+) (%w*)$")
  if not b then return end
  best = tonumber(b)
  ln = tonumber(ln)
  if #segs ~= ln * 2 or ln < 1 then return end
  local rebuilt, occ = {}, scratch
  for i = 1, N * N do occ[i] = false end
  local px, py
  for i = 1, ln do
    local x = ALPHA:find(segs:sub(i * 2 - 1, i * 2 - 1), 1, true)
    local y = ALPHA:find(segs:sub(i * 2, i * 2), 1, true)
    if not x or not y then return end
    local c = (y - 1) * N + (x - 1)
    if occ[c + 1] then return end
    occ[c + 1] = true
    if i > 1 and math.abs(x - px) + math.abs(y - py) ~= 1 then return end
    px, py = x, y
    rebuilt[i] = c
  end
  body, score, alive = rebuilt, tonumber(sc), tonumber(al) == 1
  if score > best then best = score end
  dir = tonumber(d)
  if dir < 1 or dir > 4 then dir = 4 end
  local fx2, fy2 = tonumber(fx), tonumber(fy)
  if fx2 >= N or fy2 >= N then
    place_food()
  else
    food = fy2 * N + fx2
    if occ[food + 1] then place_food() end
  end
  qn = 0
  if #body > 1 then
    local gx = body[1] % N - body[2] % N
    local gy = body[1] // N - body[2] // N
    dir = (gy < 0 and 1) or (gy > 0 and 2) or (gx < 0 and 3) or 4
  end
  note = alive and "RESUMED" or "GAME OVER"
end

local function die(why)
  alive, note = false, why
  if score > best then best, hi = score, true end
end

local function queue_turn(turn)
  local ref = qn > 0 and q[qn] or dir
  if turn == ref or turn == OPPOSITE[ref] then return end
  if qn >= 2 then return end
  qn = qn + 1
  q[qn] = turn
end

local function step()
  if not alive then return end
  if qn > 0 then                      -- pop one queued turn per step
    dir = q[1]
    q[1] = q[2]
    qn = qn - 1
  end

  local head = body[1]
  local nx, ny = head % N + DX[dir], head // N + DY[dir]
  if nx < 0 or nx >= N or ny < 0 or ny >= N then
    die("HIT THE WALL")
    return
  end
  local cell = ny * N + nx

  local grow = (cell == food)
  local last = grow and #body or #body - 1
  for i = 1, last do
    if body[i] == cell then
      die("ATE ITSELF")
      return
    end
  end

  table.insert(body, 1, cell)
  if grow then
    score = score + FOOD_SCORE
    if score > best then best, hi = score, true end
    place_food()
  else
    body[#body] = nil
  end
end

local function set_hint()
  if not alive then
    snail.hint("OK plays again   BACK menu")
  elseif running then
    snail.hint("ARROWS steer   OK pause   2x SIDE new   BACK menu")
  else
    snail.hint("ARROWS start and steer   OK starts   BACK menu")
  end
end

local function set_pace()
  if alive and running then
    local pace = PACE_MAX - 10 * ((#body - START_LEN))
    if pace < PACE_MIN then pace = PACE_MIN end
    snail.tick(pace)
  else
    snail.tick(0)
  end
end

function start()
  snail.ink("fast")   -- solid tiles, so the short waveform is honest here
  new_game()
  load()
  set_hint()
  set_pace()
end

function key(k)
  if k == "top" then
    new_game()
    save()
  elseif not alive then
    if k == "ok" then
      new_game()
      save()
    end
  elseif k == "ok" then
    running = not running
    qn = 0
  else
    local turn = DIR_OF_KEY[k]
    if turn then queue_turn(turn) end
    running = true
  end
  set_hint()
  set_pace()
end

function tick()
  local before = score
  step()
  if not alive then running = false end
  set_hint()
  set_pace()
  if not alive or score ~= before then save() end
end

local function board_spec()
  local buf, n = scratch, 0
  for y = 0, N - 1 do
    if y > 0 then n = n + 1 buf[n] = "/" end
    for x = 0, N - 1 do n = n + 1 buf[n] = CELL_EMPTY end
  end
  buf[spec_index(food)] = CELL_FOOD
  for i = 1, #body do buf[spec_index(body[i])] = CELL_BODY end
  buf[spec_index(body[1])] = CELL_HEAD
  return table.concat(buf, "", 1, n)
end

function draw()
  snail.center(true)
  if not alive then
    snail.title("GAME OVER")
    snail.title(string.format("%d", score))
    if hi then
      snail.small("NEW BEST")
    else
      snail.small(string.format("BEST %d", best))
    end
    snail.gap()
    snail.board(board_spec(), nil, note)
    return
  end
  snail.title("SNAKE")
  snail.small(string.format("SCORE %d   BEST %d   LEN %d", score, best, #body))
  if running then
    snail.small("HEADING " .. DIR_NAME[qn > 0 and q[qn] or dir])
  else
    snail.small("STOPPED  --  FACING " .. DIR_NAME[dir])
  end
  snail.gap()
  snail.board(board_spec())
end