"""Book cover generation: stdlib-only PNG with gradient + title text.

Covers are a vertical gradient between two random light colors with the
book title rendered on top in white with a black outline and a drop
shadow, using an embedded 5x7 bitmap font. No third-party image libraries
are required: PNG scanlines are packed and compressed with zlib/struct
directly.
"""

import logging
import random
import struct
import zlib
from colorsys import hsv_to_rgb
from pathlib import Path
from typing import List, Optional, Tuple

logger = logging.getLogger(__name__)

COVER_WIDTH = 600
COVER_HEIGHT = 900

# 5x7 bitmap font for ASCII 32..126. Each glyph is 7 rows of 5 bits
# (MSB left), encoded as ints for compactness.
_FONT = {
    " ": [0, 0, 0, 0, 0, 0, 0],
    "!": [0x04, 0x04, 0x04, 0x04, 0x04, 0x00, 0x04],
    '"': [0x0A, 0x0A, 0x00, 0x00, 0x00, 0x00, 0x00],
    "#": [0x0A, 0x0A, 0x1F, 0x0A, 0x1F, 0x0A, 0x0A],
    "$": [0x04, 0x0F, 0x14, 0x0E, 0x05, 0x1E, 0x04],
    "%": [0x18, 0x19, 0x02, 0x04, 0x08, 0x13, 0x03],
    "&": [0x08, 0x14, 0x14, 0x08, 0x15, 0x12, 0x0D],
    "'": [0x04, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00],
    "(": [0x02, 0x04, 0x08, 0x08, 0x08, 0x04, 0x02],
    ")": [0x08, 0x04, 0x02, 0x02, 0x02, 0x04, 0x08],
    "*": [0x00, 0x04, 0x15, 0x0E, 0x15, 0x04, 0x00],
    "+": [0x00, 0x04, 0x04, 0x1F, 0x04, 0x04, 0x00],
    ",": [0x00, 0x00, 0x00, 0x00, 0x04, 0x04, 0x08],
    "-": [0x00, 0x00, 0x00, 0x1F, 0x00, 0x00, 0x00],
    ".": [0x00, 0x00, 0x00, 0x00, 0x00, 0x0C, 0x0C],
    "/": [0x00, 0x01, 0x02, 0x04, 0x08, 0x10, 0x00],
    "0": [0x0E, 0x11, 0x13, 0x15, 0x19, 0x11, 0x0E],
    "1": [0x04, 0x0C, 0x04, 0x04, 0x04, 0x04, 0x0E],
    "2": [0x0E, 0x11, 0x01, 0x02, 0x04, 0x08, 0x1F],
    "3": [0x1F, 0x02, 0x04, 0x02, 0x01, 0x11, 0x0E],
    "4": [0x02, 0x06, 0x0A, 0x12, 0x1F, 0x02, 0x02],
    "5": [0x1F, 0x10, 0x1E, 0x01, 0x01, 0x11, 0x0E],
    "6": [0x06, 0x08, 0x10, 0x1E, 0x11, 0x11, 0x0E],
    "7": [0x1F, 0x01, 0x02, 0x04, 0x08, 0x08, 0x08],
    "8": [0x0E, 0x11, 0x11, 0x0E, 0x11, 0x11, 0x0E],
    "9": [0x0E, 0x11, 0x11, 0x0F, 0x01, 0x02, 0x0C],
    ":": [0x00, 0x0C, 0x0C, 0x00, 0x0C, 0x0C, 0x00],
    ";": [0x00, 0x0C, 0x0C, 0x00, 0x0C, 0x04, 0x08],
    "<": [0x02, 0x04, 0x08, 0x10, 0x08, 0x04, 0x02],
    "=": [0x00, 0x00, 0x1F, 0x00, 0x1F, 0x00, 0x00],
    ">": [0x08, 0x04, 0x02, 0x01, 0x02, 0x04, 0x08],
    "?": [0x0E, 0x11, 0x01, 0x02, 0x04, 0x00, 0x04],
    "@": [0x0E, 0x11, 0x17, 0x15, 0x17, 0x10, 0x0E],
    "A": [0x0E, 0x11, 0x11, 0x1F, 0x11, 0x11, 0x11],
    "B": [0x1E, 0x11, 0x11, 0x1E, 0x11, 0x11, 0x1E],
    "C": [0x0E, 0x11, 0x10, 0x10, 0x10, 0x11, 0x0E],
    "D": [0x1C, 0x12, 0x11, 0x11, 0x11, 0x12, 0x1C],
    "E": [0x1F, 0x10, 0x10, 0x1E, 0x10, 0x10, 0x1F],
    "F": [0x1F, 0x10, 0x10, 0x1E, 0x10, 0x10, 0x10],
    "G": [0x0E, 0x11, 0x10, 0x17, 0x11, 0x11, 0x0F],
    "H": [0x11, 0x11, 0x11, 0x1F, 0x11, 0x11, 0x11],
    "I": [0x0E, 0x04, 0x04, 0x04, 0x04, 0x04, 0x0E],
    "J": [0x01, 0x01, 0x01, 0x01, 0x01, 0x11, 0x0E],
    "K": [0x11, 0x12, 0x14, 0x18, 0x14, 0x12, 0x11],
    "L": [0x10, 0x10, 0x10, 0x10, 0x10, 0x10, 0x1F],
    "M": [0x11, 0x1B, 0x15, 0x15, 0x11, 0x11, 0x11],
    "N": [0x11, 0x19, 0x15, 0x13, 0x11, 0x11, 0x11],
    "O": [0x0E, 0x11, 0x11, 0x11, 0x11, 0x11, 0x0E],
    "P": [0x1E, 0x11, 0x11, 0x1E, 0x10, 0x10, 0x10],
    "Q": [0x0E, 0x11, 0x11, 0x11, 0x15, 0x12, 0x0D],
    "R": [0x1E, 0x11, 0x11, 0x1E, 0x14, 0x12, 0x11],
    "S": [0x0F, 0x10, 0x10, 0x0E, 0x01, 0x01, 0x1E],
    "T": [0x1F, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04],
    "U": [0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x0E],
    "V": [0x11, 0x11, 0x11, 0x11, 0x11, 0x0A, 0x04],
    "W": [0x11, 0x11, 0x11, 0x15, 0x15, 0x15, 0x0A],
    "X": [0x11, 0x11, 0x0A, 0x04, 0x0A, 0x11, 0x11],
    "Y": [0x11, 0x11, 0x0A, 0x04, 0x04, 0x04, 0x04],
    "Z": [0x1F, 0x01, 0x02, 0x04, 0x08, 0x10, 0x1F],
    "[": [0x0E, 0x08, 0x08, 0x08, 0x08, 0x08, 0x0E],
    "\\": [0x00, 0x10, 0x08, 0x04, 0x02, 0x01, 0x00],
    "]": [0x0E, 0x02, 0x02, 0x02, 0x02, 0x02, 0x0E],
    "^": [0x04, 0x0A, 0x11, 0x00, 0x00, 0x00, 0x00],
    "_": [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x1F],
    "`": [0x08, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00],
    "a": [0x00, 0x00, 0x0E, 0x01, 0x0F, 0x11, 0x0F],
    "b": [0x10, 0x10, 0x1E, 0x11, 0x11, 0x11, 0x1E],
    "c": [0x00, 0x00, 0x0F, 0x10, 0x10, 0x10, 0x0F],
    "d": [0x01, 0x01, 0x0F, 0x11, 0x11, 0x11, 0x0F],
    "e": [0x00, 0x00, 0x0E, 0x11, 0x1F, 0x10, 0x0E],
    "f": [0x06, 0x08, 0x1E, 0x08, 0x08, 0x08, 0x08],
    "g": [0x00, 0x0F, 0x11, 0x11, 0x0F, 0x01, 0x1E],
    "h": [0x10, 0x10, 0x1E, 0x11, 0x11, 0x11, 0x11],
    "i": [0x04, 0x00, 0x0C, 0x04, 0x04, 0x04, 0x0E],
    "j": [0x02, 0x00, 0x06, 0x02, 0x02, 0x12, 0x0C],
    "k": [0x10, 0x10, 0x12, 0x14, 0x18, 0x14, 0x12],
    "l": [0x0C, 0x04, 0x04, 0x04, 0x04, 0x04, 0x0E],
    "m": [0x00, 0x00, 0x1A, 0x15, 0x15, 0x15, 0x15],
    "n": [0x00, 0x00, 0x1E, 0x11, 0x11, 0x11, 0x11],
    "o": [0x00, 0x00, 0x0E, 0x11, 0x11, 0x11, 0x0E],
    "p": [0x00, 0x00, 0x1E, 0x11, 0x11, 0x1E, 0x10],
    "q": [0x00, 0x00, 0x0F, 0x11, 0x11, 0x0F, 0x01],
    "r": [0x00, 0x00, 0x16, 0x09, 0x08, 0x08, 0x08],
    "s": [0x00, 0x00, 0x0F, 0x10, 0x0E, 0x01, 0x1E],
    "t": [0x08, 0x08, 0x1E, 0x08, 0x08, 0x08, 0x06],
    "u": [0x00, 0x00, 0x11, 0x11, 0x11, 0x13, 0x0D],
    "v": [0x00, 0x00, 0x11, 0x11, 0x11, 0x0A, 0x04],
    "w": [0x00, 0x00, 0x11, 0x15, 0x15, 0x15, 0x0A],
    "x": [0x00, 0x00, 0x11, 0x0A, 0x04, 0x0A, 0x11],
    "y": [0x00, 0x00, 0x11, 0x11, 0x0F, 0x01, 0x1E],
    "z": [0x00, 0x00, 0x1F, 0x02, 0x04, 0x08, 0x1F],
    "{": [0x06, 0x08, 0x08, 0x04, 0x08, 0x08, 0x06],
    "|": [0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04],
    "}": [0x0C, 0x02, 0x02, 0x04, 0x02, 0x02, 0x0C],
    "~": [0x00, 0x00, 0x08, 0x15, 0x02, 0x00, 0x00],
}

_GLYPH_WIDTH = 5
_GLYPH_HEIGHT = 7
_TEXT_SCALE = 6  # render each font pixel as a 6x6 block
_TEXT_MARGIN = 60  # horizontal padding when wrapping
_TEXT_COLOR = (255, 255, 255)
_STROKE_COLOR = (0, 0, 0)
_STROKE_WIDTH = 3  # outline thickness in pixels
_SHADOW_OFFSET = 4  # drop shadow shift in pixels (down-right)
_SHADOW_DARKEN = 0.55  # shadow keeps 55% of the background color


def _random_light_color(rng: random.Random) -> Tuple[int, int, int]:
    """A random pastel: any hue, low saturation, high value."""
    hue = rng.random()
    saturation = rng.uniform(0.25, 0.55)
    value = rng.uniform(0.82, 0.95)
    r, g, b = hsv_to_rgb(hue, saturation, value)
    return int(r * 255), int(g * 255), int(b * 255)


def _lerp(a: int, b: int, t: float) -> int:
    return int(round(a + (b - a) * t))


def _text_width(text: str) -> int:
    """Pixel width of ``text`` at the rendered scale (spaces count too)."""
    if not text:
        return 0
    return (len(text) * (_GLYPH_WIDTH + 1) - 1) * _TEXT_SCALE


def _wrap_title(title: str, max_width: int) -> List[str]:
    """Word-wrap ``title`` into lines that fit ``max_width`` pixels."""
    words = title.split()
    if not words:
        return []
    lines: List[str] = []
    current = ""
    for word in words:
        candidate = f"{current} {word}".strip()
        if _text_width(candidate) <= max_width or not current:
            current = candidate
        else:
            lines.append(current)
            current = word
    if current:
        lines.append(current)
    return lines


def _set_pixel(pixels: List[List[Tuple[int, int, int]]], x: int, y: int,
               color: Tuple[int, int, int], darken: Optional[float] = None) -> None:
    """Set one pixel: solid ``color``, or darken the existing pixel by ``darken``."""
    if not 0 <= y < len(pixels):
        return
    if not 0 <= x < len(pixels[y]):
        return
    if darken is None:
        pixels[y][x] = color
    else:
        r, g, b = pixels[y][x]
        pixels[y][x] = (int(r * darken), int(g * darken), int(b * darken))


def _render_line(pixels: List[List[Tuple[int, int, int]]], text: str, x0: int, y0: int,
                 color: Tuple[int, int, int] = (0, 0, 0),
                 darken: Optional[float] = None) -> None:
    """Blit one line of bitmap text onto the pixel grid in place.

    With ``darken``, each glyph pixel darkens whatever is underneath it
    instead of painting a solid color (used for the drop shadow, which
    stays tinted by the gradient behind it).
    """
    for char_index, char in enumerate(text):
        glyph = _FONT.get(char)
        if glyph is None:
            continue
        x_off = x0 + char_index * (_GLYPH_WIDTH + 1) * _TEXT_SCALE
        for gy, bits in enumerate(glyph):
            for gx in range(_GLYPH_WIDTH):
                if not bits & (1 << (_GLYPH_WIDTH - 1 - gx)):
                    continue
                for dy in range(_TEXT_SCALE):
                    for dx in range(_TEXT_SCALE):
                        _set_pixel(pixels,
                                   x_off + gx * _TEXT_SCALE + dx,
                                   y0 + gy * _TEXT_SCALE + dy,
                                   color, darken)


def _encode_png(width: int, height: int, pixels: List[List[Tuple[int, int, int]]]) -> bytes:
    """Encode an RGB pixel grid as a PNG using only the stdlib."""
    def chunk(chunk_type: bytes, data: bytes) -> bytes:
        payload = chunk_type + data
        return (struct.pack(">I", len(data)) + payload
                + struct.pack(">I", zlib.crc32(payload) & 0xFFFFFFFF))

    raw = b"".join(
        b"\x00" + bytes(channel for pixel in scanline for channel in pixel)
        for scanline in pixels
    )
    ihdr = struct.pack(">IIBBBBB", width, height, 8, 2, 0, 0, 0)  # 8-bit RGB
    return (b"\x89PNG\r\n\x1a\n"
            + chunk(b"IHDR", ihdr)
            + chunk(b"IDAT", zlib.compress(raw, 9))
            + chunk(b"IEND", b""))


def generate_cover(title: str, path: Path, width: int = COVER_WIDTH,
                   height: int = COVER_HEIGHT, seed: Optional[int] = None) -> Optional[Path]:
    """Write a gradient cover PNG with ``title`` centered on it.

    ``seed`` makes the gradient reproducible (used by tests). Returns the
    path on success, or None when the PNG cannot be written (the audiobook
    still gets tags, just without cover art).
    """
    rng = random.Random(seed)
    top_color = _random_light_color(rng)
    bottom_color = _random_light_color(rng)

    pixels = []
    for y in range(height):
        t = y / max(1, height - 1)
        color = (_lerp(top_color[0], bottom_color[0], t),
                 _lerp(top_color[1], bottom_color[1], t),
                 _lerp(top_color[2], bottom_color[2], t))
        pixels.append([color] * width)

    lines = _wrap_title((title or "").strip(), width - 2 * _TEXT_MARGIN)
    if lines:
        line_height = _GLYPH_HEIGHT * _TEXT_SCALE + _TEXT_SCALE * 3
        total_height = len(lines) * line_height
        y_start = max(0, (height - total_height) // 2)
        for line_index, line in enumerate(lines):
            x0 = (width - _text_width(line)) // 2
            y0 = y_start + line_index * line_height
            # Paint order: drop shadow (the outlined glyph's full silhouette
            # — glyph dilated by the stroke width — shifted down-right and
            # darkened, tinted by the gradient underneath), then the black
            # outline traced around the glyph, then the solid white text.
            for dx in range(-_STROKE_WIDTH, _STROKE_WIDTH + 1):
                for dy in range(-_STROKE_WIDTH, _STROKE_WIDTH + 1):
                    _render_line(pixels, line, x0 + dx + _SHADOW_OFFSET,
                                 y0 + dy + _SHADOW_OFFSET, darken=_SHADOW_DARKEN)
            for dx in range(-_STROKE_WIDTH, _STROKE_WIDTH + 1):
                for dy in range(-_STROKE_WIDTH, _STROKE_WIDTH + 1):
                    _render_line(pixels, line, x0 + dx, y0 + dy,
                                 color=_STROKE_COLOR)
            _render_line(pixels, line, x0, y0, color=_TEXT_COLOR)

    try:
        path.write_bytes(_encode_png(width, height, pixels))
    except OSError as exc:
        logger.warning("Could not write cover image %s: %s", path, exc)
        return None
    logger.info("Generated cover: %s (gradient %s -> %s)", path, top_color, bottom_color)
    return path