Sliding Tiles Left

# lesson4_compress_row_start.py
import random
# ===== Existing working functions =====
def add_random_tile(board: list[list[int]]) -> None:
    empty_positions = [
        (r, c)
        for r in range(4)
        for c in range(4)
        if board[r][c] == 0
    ]
    if not empty_positions:
        return
    r, c = random.choice(empty_positions)
    board[r][c] = 4 if random.random() < 0.1 else 2
def create_initial_board() -> list[list[int]]:
    board = [[0] * 4 for _ in range(4)]
    add_random_tile(board)
    add_random_tile(board)
    return board
def render_board(board: list[list[int]]) -> str:
    size = len(board)
    cell_width = 5
    line = "+" + ("-" * cell_width + "+") * size
    lines = [line]
    for r in range(size):
        row_text = "|"
        for c in range(size):
            val = board[r][c]
            text = "" if val == 0 else str(val)
            row_text += text.rjust(cell_width) + "|"
        lines.append(row_text)
        lines.append(line)
    return "\n".join(lines)
# ===== Lesson 4 target function =====
def compress_row(row: list[int]) -> list[int]:
    """
    Slide non-zero tiles in the row to the LEFT, keeping their order,
    and move zeros to the right. Do NOT merge tiles here.
    Examples:
    [2, 0, 2, 4] -> [2, 2, 4, 0]
    [0, 0, 2, 0] -> [2, 0, 0, 0]
    [0, 4, 0, 4] -> [4, 4, 0, 0]
    """
    # TODO: implement in this lesson
    raise NotImplementedError
# ===== Still stubs for future lessons =====
def merge_row(row: list[int]) -> tuple[list[int], int]:
    raise NotImplementedError
def move_board(board: list[list[int]], direction: str) -> tuple[list[list[int]], int, bool]:
    raise NotImplementedError
def check_game_state(board: list[list[int]], target: int = 2048) -> str:
    raise NotImplementedError
def main():
    board = create_initial_board()
    score = 0
    print("Initial board:")
    print("Score:", score)
    print(render_board(board))
    # We'll add compress_row demos here later in the lesson.
if __name__ == "__main__":
    main()