// ============================================================================
// Snake — one movement tick with growth, wall and self collision
// ============================================================================
//
// The snake is a list of {y, x} segments, head first. tick() moves the head one
// cell in the current direction; if it lands on food the tail is kept (grow),
// otherwise the tail is dropped. A move into a wall or into the snake's own
// body is reported as a crash.
//
// Run:  soma run snake.cell --signal demo
// Test: soma test snake.cell

cell Snake {
    face {
        signal tick(snake: List, dir: String, food: Map, w: Int, h: Int) -> Map
        signal hits_body(snake: List, y: Int, x: Int) -> Bool
        signal demo() -> Map
    }

    on hits_body(snake: List, y: Int, x: Int) {
        return any(snake, seg => seg.y == y && seg.x == x)
    }

    on tick(snake: List, dir: String, food: Map, w: Int, h: Int) {
        let head = snake[0]
        let ny = head.y
        let nx = head.x
        if dir == "N" { ny = head.y - 1 }
        if dir == "S" { ny = head.y + 1 }
        if dir == "W" { nx = head.x - 1 }
        if dir == "E" { nx = head.x + 1 }

        // wall collision
        if ny < 0 || ny >= h || nx < 0 || nx >= w {
            return map("snake", snake, "crashed", true, "ate", false)
        }

        let ate = ny == food.y && nx == food.x
        // build the new body: new head, then old segments (drop tail if not eating)
        let body = [map("y", ny, "x", nx)]
        let keep = len(snake) - 1
        if ate { keep = len(snake) }
        for i in range(0, keep) {
            body = push(body, snake[i])
        }
        // self collision: new head lands on a surviving segment
        let crashed = hits_body(snake, ny, nx) && ate == false
        return map("snake", body, "crashed", crashed, "ate", ate)
    }

    on demo() {
        // 3-segment snake moving east into food at (0,3)
        let s = [map("y",0,"x",2), map("y",0,"x",1), map("y",0,"x",0)]
        return tick(s, "E", map("y",0,"x",3), 5, 5)
    }
}

cell test SnakeTests {
    rules {
        // moving east onto food grows the snake from 3 to 4
        assert len(tick([map("y",0,"x",2), map("y",0,"x",1), map("y",0,"x",0)],
            "E", map("y",0,"x",3), 5, 5).snake) == 4
        assert tick([map("y",0,"x",2), map("y",0,"x",1), map("y",0,"x",0)],
            "E", map("y",0,"x",3), 5, 5).ate == true
        // moving without eating keeps the length at 3
        assert len(tick([map("y",0,"x",2), map("y",0,"x",1), map("y",0,"x",0)],
            "E", map("y",9,"x",9), 5, 5).snake) == 3
        // running into the west wall crashes
        assert tick([map("y",0,"x",0)], "W", map("y",9,"x",9), 5, 5).crashed == true
        // body collision helper
        assert hits_body([map("y",1,"x",1), map("y",1,"x",2)], 1, 2) == true
        assert hits_body([map("y",1,"x",1)], 3, 3) == false
    }
}