"""Independent PNG-only inverse arm for longest vertical step selection."""

from __future__ import annotations

import numpy as np
from PIL import Image

# Public visual palette, duplicated deliberately instead of importing renderer state.
_PALETTE: dict[str, tuple[int, int, int]] = {
    "red": (214, 54, 64),
    "blue": (40, 112, 184),
    "green": (35, 142, 82),
    "orange": (226, 126, 34),
}
_COLOR_DISTANCE = 32.0
_MIN_INK_PIXELS = 350
_MIN_VERTICAL_SPAN = 14


def _longest_vertical_extent(mask: np.ndarray) -> int:
    extents: list[int] = []
    for x in np.flatnonzero(mask.any(axis=0)):
        ys = np.flatnonzero(mask[:, x])
        if ys.size:
            extents.append(int(ys[-1] - ys[0] + 1))
    if not extents:
        raise ValueError("no colored stroke columns found")
    return max(extents)


def decision_from_image(image: Image.Image) -> str:
    arr = np.asarray(image.convert("RGB"), dtype=np.int32)
    if arr.ndim != 3 or arr.shape[2] != 3 or arr.shape[0] < 100 or arr.shape[1] < 100:
        raise ValueError("unsupported raster")

    scores: dict[str, int] = {}
    for name, rgb in _PALETTE.items():
        delta = arr - np.asarray(rgb, dtype=np.int32)
        distance = np.sqrt(np.sum(delta * delta, axis=2, dtype=np.int32))
        mask = distance <= _COLOR_DISTANCE
        if int(mask.sum()) < _MIN_INK_PIXELS:
            raise ValueError(f"missing or materially erased {name} curve")
        score = _longest_vertical_extent(mask)
        if score < _MIN_VERTICAL_SPAN:
            raise ValueError(f"no measurable vertical step for {name}")
        scores[name] = score

    ranked = sorted(((score, name) for name, score in scores.items()), reverse=True)
    if ranked[0][0] - ranked[1][0] < 2:
        raise ValueError("pixel measurement is tied or ambiguous")
    return ranked[0][1]