#!/usr/bin/env python3
"""Build a four-direction exploration sprite sheet from a three-view turnaround.

The input is expected to contain transparent front, right, and back views in
three equal horizontal columns. The output matches the runtime exploration
contract: 192px frames, four rows (south/east/north/west), and sixteen frames
per row (eight idle followed by eight walk frames).
"""

from __future__ import annotations

import argparse
from pathlib import Path

from PIL import Image


FRAME_SIZE = 192
IDLE_FRAME_COUNT = 8
WALK_FRAME_COUNT = 8
FRAMES_PER_DIRECTION = IDLE_FRAME_COUNT + WALK_FRAME_COUNT
DIRECTIONS = ("south", "east", "north", "west")

IDLE_OFFSETS = (
    (0, 0),
    (0, -1),
    (0, -2),
    (0, -1),
    (0, 0),
    (0, 1),
    (0, 0),
    (0, -1),
)
WALK_OFFSETS = (
    (-2, 0),
    (-1, -2),
    (0, -3),
    (1, -1),
    (2, 0),
    (1, -2),
    (0, -3),
    (-1, -1),
)


def parse_args() -> argparse.Namespace:
    parser = argparse.ArgumentParser(
        description="Build a 3072x768 exploration SD character sheet."
    )
    parser.add_argument("--input", required=True, type=Path)
    parser.add_argument("--output", required=True, type=Path)
    parser.add_argument(
        "--max-height",
        type=int,
        default=184,
        help="Maximum character height inside each 192px frame.",
    )
    parser.add_argument(
        "--max-width",
        type=int,
        default=172,
        help="Maximum character width inside each 192px frame.",
    )
    return parser.parse_args()


def split_turnaround(source: Image.Image) -> tuple[Image.Image, Image.Image, Image.Image]:
    views: list[Image.Image] = []
    for index in range(3):
        left = round(source.width * index / 3)
        right = round(source.width * (index + 1) / 3)
        view = source.crop((left, 0, right, source.height))
        alpha = view.getchannel("A")
        bbox = alpha.getbbox()
        if not bbox:
            raise ValueError(f"Turnaround column {index + 1} contains no visible pixels.")
        views.append(view.crop(bbox))
    return views[0], views[1], views[2]


def resize_views(
    views: tuple[Image.Image, Image.Image, Image.Image],
    max_width: int,
    max_height: int,
) -> tuple[Image.Image, Image.Image, Image.Image]:
    widest = max(view.width for view in views)
    tallest = max(view.height for view in views)
    scale = min(max_width / widest, max_height / tallest)
    if scale <= 0:
        raise ValueError("The requested sprite bounds must be positive.")
    return tuple(
        view.resize(
            (max(1, round(view.width * scale)), max(1, round(view.height * scale))),
            Image.Resampling.LANCZOS,
        )
        for view in views
    )


def place_frame(view: Image.Image, offset: tuple[int, int]) -> Image.Image:
    frame = Image.new("RGBA", (FRAME_SIZE, FRAME_SIZE), (0, 0, 0, 0))
    x = (FRAME_SIZE - view.width) // 2 + offset[0]
    y = FRAME_SIZE - 4 - view.height + offset[1]
    frame.alpha_composite(view, (x, y))
    return frame


def build_sheet(source: Image.Image, max_width: int, max_height: int) -> Image.Image:
    front, right, back = resize_views(
        split_turnaround(source),
        max_width=max_width,
        max_height=max_height,
    )
    directional_views = {
        "south": front,
        "east": right,
        "north": back,
        "west": right.transpose(Image.Transpose.FLIP_LEFT_RIGHT),
    }
    sheet = Image.new(
        "RGBA",
        (FRAME_SIZE * FRAMES_PER_DIRECTION, FRAME_SIZE * len(DIRECTIONS)),
        (0, 0, 0, 0),
    )
    for row, direction in enumerate(DIRECTIONS):
        view = directional_views[direction]
        offsets = (*IDLE_OFFSETS, *WALK_OFFSETS)
        for column, offset in enumerate(offsets):
            sheet.alpha_composite(
                place_frame(view, offset),
                (column * FRAME_SIZE, row * FRAME_SIZE),
            )
    return sheet


def main() -> None:
    args = parse_args()
    source = Image.open(args.input).convert("RGBA")
    if source.getchannel("A").getextrema()[0] != 0:
        raise ValueError("The turnaround must contain transparent background pixels.")
    sheet = build_sheet(source, max_width=args.max_width, max_height=args.max_height)
    args.output.parent.mkdir(parents=True, exist_ok=True)
    sheet.save(args.output, "WEBP", lossless=True, method=6)
    print(
        f"Wrote {args.output} "
        f"({sheet.width}x{sheet.height}, {FRAMES_PER_DIRECTION} frames x {len(DIRECTIONS)} rows)."
    )


if __name__ == "__main__":
    main()