/// m03 rules: the pure game rules. Each function takes a state and returns a new one.
module Snake.Rules

open Snake.Model

let move (p: Point) (dir: Direction) : Point =
    let d = delta dir
    { X = p.X + d.X; Y = p.Y + d.Y }

let inBounds (state: GameState) (p: Point) : bool =
    p.X >= 0 && p.X < state.Width && p.Y >= 0 && p.Y < state.Height

// Validate against `Dir` (last moved), not `NextDir`: Up then Left within one tick must not fold the snake back on itself.
let turn (state: GameState) (dir: Direction) : GameState =
    if state.Status = Playing && dir <> opposite state.Dir then { state with NextDir = dir } else state

let advance (state: GameState) : GameState =
    if state.Status <> Playing then
        state
    else
        let d = state.NextDir
        let head = move (List.head state.Snake) d
        let eating = state.Apple = Some head
        // When not eating, the tail leaves its cell this step, so the head may move into it.
        let body = if eating then state.Snake else Util.dropLast state.Snake

        if not (inBounds state head) || Util.contains body head (=) then
            { state with Dir = d; Status = GameOver }
        else
            { state with
                Snake = head :: body
                Dir = d
                Apple = if eating then None else state.Apple
                Score = state.Score + (if eating then 1 else 0) }