"""Crop geometry for the zoom. Pure PIL, no model code, so it stays cheap to test."""
from PIL import Image

PROCESS_SIZE = 512


def resize_and_center_crop(img, size=PROCESS_SIZE):
    """CoZ's entry crop: shortest side to `size`, then the centre square."""
    w, h = img.size
    scale = size / min(w, h)
    nw, nh = int(w * scale), int(h * scale)
    img = img.resize((nw, nh), Image.LANCZOS)
    left, top = (nw - size) // 2, (nh - size) // 2
    return img.crop((left, top, left + size, top + size))


def window_rect(size, upscale, center=(0.5, 0.5)):
    """The next level's field of view: a 1/upscale square around `center`, clamped inside."""
    w, h = size
    nw, nh = w // upscale, h // upscale
    left = min(max(int(center[0] * w - nw / 2), 0), w - nw)
    top = min(max(int(center[1] * h - nh / 2), 0), h - nh)
    return left, top, left + nw, top + nh


def zoom_window(img, upscale, center=(0.5, 0.5)):
    return img.crop(window_rect(img.size, upscale, center))


def nested_rects(size, levels, upscale=4, center=(0.5, 0.5)):
    """Where each zoom step lands, in the coordinates of the first canvas."""
    rects, rect = [], (0, 0, size[0], size[1])
    for _ in range(levels):
        side = rect[2] - rect[0]
        l, t, r, b = window_rect((side, side), upscale, center)
        rect = (rect[0] + l, rect[1] + t, rect[0] + r, rect[1] + b)
        rects.append(rect)
    return rects