"""Independent pixel-only inverse arm for the guitar string count."""

from __future__ import annotations

import numpy as np
from PIL import Image


def _runs(values: np.ndarray) -> list[tuple[int, int]]:
    padded = np.pad(values.astype(np.int8), (1, 1))
    changes = np.diff(padded)
    starts = np.flatnonzero(changes == 1)
    stops = np.flatnonzero(changes == -1)
    return list(zip(starts.tolist(), stops.tolist()))


def decision_from_image(image: Image.Image) -> str:
    rgb = np.asarray(image.convert("RGB"), dtype=np.int16)
    if rgb.shape[0] < 64 or rgb.shape[1] < 64:
        raise ValueError("image is too small")

    # Estimate the paper background only from corner patches, then locate a
    # broad, bright non-background horizontal band (the visibly drawn bridge).
    patches = np.concatenate((rgb[:12, :12].reshape(-1, 3), rgb[:12, -12:].reshape(-1, 3), rgb[-12:, :12].reshape(-1, 3), rgb[-12:, -12:].reshape(-1, 3)))
    bg = np.median(patches, axis=0)
    lum = rgb.mean(axis=2)
    distance = np.sqrt(((rgb - bg) ** 2).sum(axis=2))
    chroma = rgb.max(axis=2) - rgb.min(axis=2)
    bridge_like = (lum > 178) & (distance > 30) & (chroma > 20)
    row_scores = bridge_like.sum(axis=1)
    candidate_rows = np.flatnonzero(row_scores >= 50)
    if candidate_rows.size < 8:
        raise ValueError("visible bridge band not found")
    # The bridge forms the lowest coherent run of qualifying rows; tuning pegs
    # are much narrower and cannot satisfy the row-width criterion.
    groups = _runs(np.isin(np.arange(rgb.shape[0]), candidate_rows))
    groups = [(a, b) for a, b in groups if b - a >= 8]
    if not groups:
        raise ValueError("bridge band is not coherent")
    y0 = (groups[-1][0] + groups[-1][1] - 1) // 2
    bright_x = np.flatnonzero(bridge_like[y0])
    if bright_x.size < 60:
        raise ValueError("bridge row is too narrow")
    xlo, xhi = int(bright_x.min()), int(bright_x.max())

    # Take a three-row vote. Each qualifying string is a narrow dark run wholly
    # inside the recovered bridge extent; body edges and sound hole lie outside.
    counts = []
    for y in range(y0 - 1, y0 + 2):
        strip = rgb[y, xlo:xhi+1]
        dark = strip.mean(axis=1) < 105
        runs = [(a, b) for a, b in _runs(dark) if 1 <= b-a <= 7 and a > 2 and b < len(dark)-2]
        counts.append(len(runs))
    count = int(np.median(counts))
    if count not in {4, 5, 6, 7, 8} or max(counts) != min(counts):
        raise ValueError(f"ambiguous string runs: {counts}")
    return str(count)