onix 0.1.0

// This is stripped down from the image crate.
//
// TODO: Put this implementation in a dedicated parser-only crate, which both
// image and this crate can depend on.

//! An implementation of the VP8 Video Codec
//!
//! This module contains a partial implementation of the
//! VP8 video format as defined in RFC-6386.
//!
//! It decodes Keyframes only.
//! VP8 is the underpinning of the WebP image format
//!
//! # Related Links
//! * [rfc-6386](http://tools.ietf.org/html/rfc6386) - The VP8 Data Format and Decoding Guide
//! * [VP8.pdf](http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/37073.pdf) - An overview of
//! of the VP8 format
//!

use byteorder::{LittleEndian, ReadBytesExt};
use std::default::Default;
use std::fmt;
use std::io::Read;

use crate::vp8::error::{ImageError, ImageResult};

const MAX_SEGMENTS: usize = 4;
const NUM_DCT_TOKENS: usize = 12;

type Prob = u8;

// Default probabilities for decoding the keyframe luma modes
pub(super) static KEYFRAME_YMODE_PROBS: [Prob; 4] = [145, 156, 163, 128];

// Probabilities for determining macroblock mode
pub(super) static KEYFRAME_UV_MODE_PROBS: [Prob; 3] = [142, 114, 183];

// Section 13.4
type TokenProbTables = [[[[Prob; NUM_DCT_TOKENS - 1]; 3]; 8]; 4];

// Probabilities that a token's probability will be updated
static COEFF_UPDATE_PROBS: TokenProbTables = [
    [
        [
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [176, 246, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [223, 241, 252, 255, 255, 255, 255, 255, 255, 255, 255],
            [249, 253, 253, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 244, 252, 255, 255, 255, 255, 255, 255, 255, 255],
            [234, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [253, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 246, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [239, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [254, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 248, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [251, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [251, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [254, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 254, 253, 255, 254, 255, 255, 255, 255, 255, 255],
            [250, 255, 254, 255, 254, 255, 255, 255, 255, 255, 255],
            [254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
    ],
    [
        [
            [217, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [225, 252, 241, 253, 255, 255, 254, 255, 255, 255, 255],
            [234, 250, 241, 250, 253, 255, 253, 254, 255, 255, 255],
        ],
        [
            [255, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [223, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [238, 253, 254, 254, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 248, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [249, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 253, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [247, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [252, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [253, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 254, 253, 255, 255, 255, 255, 255, 255, 255, 255],
            [250, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
    ],
    [
        [
            [186, 251, 250, 255, 255, 255, 255, 255, 255, 255, 255],
            [234, 251, 244, 254, 255, 255, 255, 255, 255, 255, 255],
            [251, 251, 243, 253, 254, 255, 254, 255, 255, 255, 255],
        ],
        [
            [255, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [236, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [251, 253, 253, 254, 254, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [254, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [254, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
    ],
    [
        [
            [248, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [250, 254, 252, 254, 255, 255, 255, 255, 255, 255, 255],
            [248, 254, 249, 253, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 253, 253, 255, 255, 255, 255, 255, 255, 255, 255],
            [246, 253, 253, 255, 255, 255, 255, 255, 255, 255, 255],
            [252, 254, 251, 254, 254, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 254, 252, 255, 255, 255, 255, 255, 255, 255, 255],
            [248, 254, 253, 255, 255, 255, 255, 255, 255, 255, 255],
            [253, 255, 254, 254, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 251, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [245, 251, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [253, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 251, 253, 255, 255, 255, 255, 255, 255, 255, 255],
            [252, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 252, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [249, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 255, 253, 255, 255, 255, 255, 255, 255, 255, 255],
            [250, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
        [
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
            [255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255],
        ],
    ],
];

// Section 13.5
// Default Probabilities for tokens
pub(super) static COEFF_PROBS: TokenProbTables = [
    [
        [
            [128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128],
            [128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128],
            [128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128],
        ],
        [
            [253, 136, 254, 255, 228, 219, 128, 128, 128, 128, 128],
            [189, 129, 242, 255, 227, 213, 255, 219, 128, 128, 128],
            [106, 126, 227, 252, 214, 209, 255, 255, 128, 128, 128],
        ],
        [
            [1, 98, 248, 255, 236, 226, 255, 255, 128, 128, 128],
            [181, 133, 238, 254, 221, 234, 255, 154, 128, 128, 128],
            [78, 134, 202, 247, 198, 180, 255, 219, 128, 128, 128],
        ],
        [
            [1, 185, 249, 255, 243, 255, 128, 128, 128, 128, 128],
            [184, 150, 247, 255, 236, 224, 128, 128, 128, 128, 128],
            [77, 110, 216, 255, 236, 230, 128, 128, 128, 128, 128],
        ],
        [
            [1, 101, 251, 255, 241, 255, 128, 128, 128, 128, 128],
            [170, 139, 241, 252, 236, 209, 255, 255, 128, 128, 128],
            [37, 116, 196, 243, 228, 255, 255, 255, 128, 128, 128],
        ],
        [
            [1, 204, 254, 255, 245, 255, 128, 128, 128, 128, 128],
            [207, 160, 250, 255, 238, 128, 128, 128, 128, 128, 128],
            [102, 103, 231, 255, 211, 171, 128, 128, 128, 128, 128],
        ],
        [
            [1, 152, 252, 255, 240, 255, 128, 128, 128, 128, 128],
            [177, 135, 243, 255, 234, 225, 128, 128, 128, 128, 128],
            [80, 129, 211, 255, 194, 224, 128, 128, 128, 128, 128],
        ],
        [
            [1, 1, 255, 128, 128, 128, 128, 128, 128, 128, 128],
            [246, 1, 255, 128, 128, 128, 128, 128, 128, 128, 128],
            [255, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128],
        ],
    ],
    [
        [
            [198, 35, 237, 223, 193, 187, 162, 160, 145, 155, 62],
            [131, 45, 198, 221, 172, 176, 220, 157, 252, 221, 1],
            [68, 47, 146, 208, 149, 167, 221, 162, 255, 223, 128],
        ],
        [
            [1, 149, 241, 255, 221, 224, 255, 255, 128, 128, 128],
            [184, 141, 234, 253, 222, 220, 255, 199, 128, 128, 128],
            [81, 99, 181, 242, 176, 190, 249, 202, 255, 255, 128],
        ],
        [
            [1, 129, 232, 253, 214, 197, 242, 196, 255, 255, 128],
            [99, 121, 210, 250, 201, 198, 255, 202, 128, 128, 128],
            [23, 91, 163, 242, 170, 187, 247, 210, 255, 255, 128],
        ],
        [
            [1, 200, 246, 255, 234, 255, 128, 128, 128, 128, 128],
            [109, 178, 241, 255, 231, 245, 255, 255, 128, 128, 128],
            [44, 130, 201, 253, 205, 192, 255, 255, 128, 128, 128],
        ],
        [
            [1, 132, 239, 251, 219, 209, 255, 165, 128, 128, 128],
            [94, 136, 225, 251, 218, 190, 255, 255, 128, 128, 128],
            [22, 100, 174, 245, 186, 161, 255, 199, 128, 128, 128],
        ],
        [
            [1, 182, 249, 255, 232, 235, 128, 128, 128, 128, 128],
            [124, 143, 241, 255, 227, 234, 128, 128, 128, 128, 128],
            [35, 77, 181, 251, 193, 211, 255, 205, 128, 128, 128],
        ],
        [
            [1, 157, 247, 255, 236, 231, 255, 255, 128, 128, 128],
            [121, 141, 235, 255, 225, 227, 255, 255, 128, 128, 128],
            [45, 99, 188, 251, 195, 217, 255, 224, 128, 128, 128],
        ],
        [
            [1, 1, 251, 255, 213, 255, 128, 128, 128, 128, 128],
            [203, 1, 248, 255, 255, 128, 128, 128, 128, 128, 128],
            [137, 1, 177, 255, 224, 255, 128, 128, 128, 128, 128],
        ],
    ],
    [
        [
            [253, 9, 248, 251, 207, 208, 255, 192, 128, 128, 128],
            [175, 13, 224, 243, 193, 185, 249, 198, 255, 255, 128],
            [73, 17, 171, 221, 161, 179, 236, 167, 255, 234, 128],
        ],
        [
            [1, 95, 247, 253, 212, 183, 255, 255, 128, 128, 128],
            [239, 90, 244, 250, 211, 209, 255, 255, 128, 128, 128],
            [155, 77, 195, 248, 188, 195, 255, 255, 128, 128, 128],
        ],
        [
            [1, 24, 239, 251, 218, 219, 255, 205, 128, 128, 128],
            [201, 51, 219, 255, 196, 186, 128, 128, 128, 128, 128],
            [69, 46, 190, 239, 201, 218, 255, 228, 128, 128, 128],
        ],
        [
            [1, 191, 251, 255, 255, 128, 128, 128, 128, 128, 128],
            [223, 165, 249, 255, 213, 255, 128, 128, 128, 128, 128],
            [141, 124, 248, 255, 255, 128, 128, 128, 128, 128, 128],
        ],
        [
            [1, 16, 248, 255, 255, 128, 128, 128, 128, 128, 128],
            [190, 36, 230, 255, 236, 255, 128, 128, 128, 128, 128],
            [149, 1, 255, 128, 128, 128, 128, 128, 128, 128, 128],
        ],
        [
            [1, 226, 255, 128, 128, 128, 128, 128, 128, 128, 128],
            [247, 192, 255, 128, 128, 128, 128, 128, 128, 128, 128],
            [240, 128, 255, 128, 128, 128, 128, 128, 128, 128, 128],
        ],
        [
            [1, 134, 252, 255, 255, 128, 128, 128, 128, 128, 128],
            [213, 62, 250, 255, 255, 128, 128, 128, 128, 128, 128],
            [55, 93, 255, 128, 128, 128, 128, 128, 128, 128, 128],
        ],
        [
            [128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128],
            [128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128],
            [128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128],
        ],
    ],
    [
        [
            [202, 24, 213, 235, 186, 191, 220, 160, 240, 175, 255],
            [126, 38, 182, 232, 169, 184, 228, 174, 255, 187, 128],
            [61, 46, 138, 219, 151, 178, 240, 170, 255, 216, 128],
        ],
        [
            [1, 112, 230, 250, 199, 191, 247, 159, 255, 255, 128],
            [166, 109, 228, 252, 211, 215, 255, 174, 128, 128, 128],
            [39, 77, 162, 232, 172, 180, 245, 178, 255, 255, 128],
        ],
        [
            [1, 52, 220, 246, 198, 199, 249, 220, 255, 255, 128],
            [124, 74, 191, 243, 183, 193, 250, 221, 255, 255, 128],
            [24, 71, 130, 219, 154, 170, 243, 182, 255, 255, 128],
        ],
        [
            [1, 182, 225, 249, 219, 240, 255, 224, 128, 128, 128],
            [149, 150, 226, 252, 216, 205, 255, 171, 128, 128, 128],
            [28, 108, 170, 242, 183, 194, 254, 223, 255, 255, 128],
        ],
        [
            [1, 81, 230, 252, 204, 203, 255, 192, 128, 128, 128],
            [123, 102, 209, 247, 188, 196, 255, 233, 128, 128, 128],
            [20, 95, 153, 243, 164, 173, 255, 203, 128, 128, 128],
        ],
        [
            [1, 222, 248, 255, 216, 213, 128, 128, 128, 128, 128],
            [168, 175, 246, 252, 235, 205, 255, 255, 128, 128, 128],
            [47, 116, 215, 255, 211, 212, 255, 255, 128, 128, 128],
        ],
        [
            [1, 121, 236, 253, 212, 214, 255, 255, 128, 128, 128],
            [141, 84, 213, 252, 201, 202, 255, 219, 128, 128, 128],
            [42, 80, 160, 240, 162, 185, 255, 205, 128, 128, 128],
        ],
        [
            [1, 1, 255, 128, 128, 128, 128, 128, 128, 128, 128],
            [244, 1, 255, 128, 128, 128, 128, 128, 128, 128, 128],
            [238, 1, 255, 128, 128, 128, 128, 128, 128, 128, 128],
        ],
    ],
];

pub(super) struct BoolReader {
    buf: Vec<u8>,
    index: usize,

    pub(super) range: u32,
    pub(super) value: u32,
    pub(super) bit_count: u8,
}

impl Debug for BoolReader {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt.debug_struct("BoolReader")
            .field("index", &self.index)
            .field("range", &self.range)
            .field("value", &self.value)
            .field("bit_count", &self.bit_count)
            .finish()
    }
}

impl BoolReader {
    pub(crate) fn new() -> BoolReader {
        BoolReader {
            buf: Vec::new(),
            range: 0,
            value: 0,
            bit_count: 0,
            index: 0,
        }
    }

    pub(crate) fn init(&mut self, buf: Vec<u8>) -> ImageResult<()> {
        if buf.len() < 2 {
            return Err(ImageError);
        }

        self.buf = buf;
        // Direct access safe, since length has just been validated.
        self.value = (u32::from(self.buf[0]) << 8) | u32::from(self.buf[1]);
        self.index = 2;
        self.range = 255;
        self.bit_count = 0;

        Ok(())
    }

    pub(crate) fn read_bool(&mut self, probability: u8) -> bool {
        let split = 1 + (((self.range - 1) * u32::from(probability)) >> 8);
        let bigsplit = split << 8;

        let retval = if self.value >= bigsplit {
            self.range -= split;
            self.value -= bigsplit;
            true
        } else {
            self.range = split;
            false
        };

        while self.range < 128 {
            self.value <<= 1;
            self.range <<= 1;
            self.bit_count += 1;

            if self.bit_count == 8 {
                self.bit_count = 0;

                // If no more bits are available, just don't do anything.
                // This strategy is suggested in the reference implementation of RFC6386 (p.135)
                if self.index < self.buf.len() {
                    self.value |= u32::from(self.buf[self.index]);
                    self.index += 1;
                }
            }
        }

        retval
    }

    pub(crate) fn read_literal(&mut self, n: u8) -> u8 {
        let mut v = 0u8;
        let mut n = n;

        while n != 0 {
            v = (v << 1) + self.read_bool(128u8) as u8;
            n -= 1;
        }

        v
    }

    pub(crate) fn read_magnitude_and_sign(&mut self, n: u8) -> i32 {
        let magnitude = self.read_literal(n);
        let sign = self.read_literal(1);

        if sign == 1 {
            -i32::from(magnitude)
        } else {
            i32::from(magnitude)
        }
    }

    pub(crate) fn read_flag(&mut self) -> bool {
        0 != self.read_literal(1)
    }
}

/// A Representation of the last decoded video frame
#[derive(Default, Clone)]
pub struct Frame {
    /// The width of the luma plane
    pub width: u16,

    /// The height of the luma plane
    pub height: u16,

    pub(super) horizontal_scale: u8,
    pub(super) vertical_scale: u8,

    /// Indicates whether this frame is a keyframe
    pub keyframe: bool,

    pub(super) version: u8,

    /// Indicates whether this frame is intended for display
    pub for_display: bool,

    // Section 9.2
    /// The pixel type of the frame as defined by Section 9.2
    /// of the VP8 Specification
    pub pixel_type: u8,

    // Section 9.4 and 15
    pub(super) filter_type: bool, //if true uses simple filter // if false uses normal filter
    pub(super) filter_level: u8,
    pub(super) sharpness_level: u8,

    pub(super) first_partition_size: u32,
}

impl Debug for Frame {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt.debug_struct("Frame")
            .field("width", &self.width)
            .field("height", &self.height)
            .field("keyframe", &self.keyframe)
            .field("version", &self.version)
            .field("for_display", &self.for_display)
            .field("pixel_type", &self.pixel_type)
            .field("filter_type", &self.filter_type)
            .field("filter_level", &self.filter_level)
            .field("sharpness_level", &self.sharpness_level)
            .finish()
    }
}

#[derive(Clone, Copy, Debug, Default)]
pub(super) struct Segment {
    delta_values: bool,

    pub(super) quantizer_level: i8,
    pub(super) loopfilter_level: i8,
}

#[derive(Clone, Copy, Debug, Default)]
pub(super) struct Quant {
    pub(super) y_ac_qi: u8,
    pub(super) y_dc_delta: i8,

    pub(super) y2_dc_delta: i8,
    pub(super) y2_ac_delta: i8,

    pub(super) uv_dc_delta: i8,
    pub(super) uv_ac_delta: i8,
}

use core::fmt::Debug;

/// VP8 Decoder
///
/// Only decodes keyframes
#[derive(Debug)]
pub struct Vp8Decoder<R: Debug> {
    r: R,
    pub(super) b: BoolReader,

    mbwidth: u16,
    mbheight: u16,

    pub(super) frame: Frame,

    pub(super) quant: Quant,
    pub(super) segments_enabled: bool,
    pub(super) segments_update_map: bool,
    pub(super) update_segment_feature_data: bool,
    pub(super) segment_feature_mode: bool,
    pub(super) lf_adjust_enable: bool,
    pub(super) delta_update: bool,
    pub(super) segment: [Segment; MAX_SEGMENTS],

    pub(super) ref_delta: [i8; 4],
    pub(super) mode_delta: [i8; 4],

    partitions: [BoolReader; 8],
    pub(super) num_partitions: u8,

    pub(super) segment_tree_probs: [Prob; 3],
    pub(super) token_probs: Box<TokenProbTables>,

    // Section 9.10
    prob_intra: Prob,

    // Section 9.11
    pub(super) prob_skip_false: Option<Prob>,
}

impl<R: Read + Debug> Vp8Decoder<R> {
    /// Create a new decoder.
    /// The reader must present a raw vp8 bitstream to the decoder
    pub fn new(r: R) -> Vp8Decoder<R> {
        let f = Frame::default();
        let s = Segment::default();

        Vp8Decoder {
            r,
            b: BoolReader::new(),

            mbwidth: 0,
            mbheight: 0,

            frame: f,
            quant: Quant::default(),
            segments_enabled: false,
            segments_update_map: false,
            update_segment_feature_data: false,
            segment_feature_mode: false,
            lf_adjust_enable: false,
            delta_update: false,
            segment: [s; MAX_SEGMENTS],

            ref_delta: [0; 4],
            mode_delta: [0; 4],

            partitions: [
                BoolReader::new(),
                BoolReader::new(),
                BoolReader::new(),
                BoolReader::new(),
                BoolReader::new(),
                BoolReader::new(),
                BoolReader::new(),
                BoolReader::new(),
            ],

            num_partitions: 1,

            segment_tree_probs: [255u8; 3],
            token_probs: Box::new(COEFF_PROBS),

            // Section 9.10
            prob_intra: 0u8,

            // Section 9.11
            prob_skip_false: None,
        }
    }

    fn update_token_probabilities(&mut self) {
        for (i, is) in COEFF_UPDATE_PROBS.iter().enumerate() {
            for (j, js) in is.iter().enumerate() {
                for (k, ks) in js.iter().enumerate() {
                    for (t, prob) in ks.iter().enumerate().take(NUM_DCT_TOKENS - 1) {
                        if self.b.read_bool(*prob) {
                            let v = self.b.read_literal(8);
                            self.token_probs[i][j][k][t] = v;
                        }
                    }
                }
            }
        }
    }

    fn init_partitions(&mut self, n: usize) -> ImageResult<()> {
        if n > 1 {
            let mut sizes = vec![0; 3 * n - 3];
            self.r.read_exact(sizes.as_mut_slice())?;

            for (i, s) in sizes.chunks(3).enumerate() {
                let size = { s }
                    .read_u24::<LittleEndian>()
                    .expect("Reading from &[u8] can't fail and the chunk is complete");

                let mut buf = vec![0; size as usize];
                self.r.read_exact(buf.as_mut_slice())?;

                self.partitions[i].init(buf)?;
            }
        }

        let mut buf = Vec::new();
        self.r.read_to_end(&mut buf)?;
        self.partitions[n - 1].init(buf)?;

        Ok(())
    }

    fn read_quantization_indices(&mut self) {
        let yac_abs = i16::from(self.b.read_literal(7));
        let ydc_delta = if self.b.read_flag() {
            self.b.read_magnitude_and_sign(4)
        } else {
            0
        };

        let y2dc_delta = if self.b.read_flag() {
            self.b.read_magnitude_and_sign(4)
        } else {
            0
        };

        let y2ac_delta = if self.b.read_flag() {
            self.b.read_magnitude_and_sign(4)
        } else {
            0
        };

        let uvdc_delta = if self.b.read_flag() {
            self.b.read_magnitude_and_sign(4)
        } else {
            0
        };

        let uvac_delta = if self.b.read_flag() {
            self.b.read_magnitude_and_sign(4)
        } else {
            0
        };

        let n = if self.segments_enabled {
            MAX_SEGMENTS
        } else {
            1
        };

        self.quant.y_ac_qi = yac_abs as u8;
        self.quant.y_dc_delta = ydc_delta as i8;
        self.quant.y2_dc_delta = y2dc_delta as i8;
        self.quant.y2_ac_delta = y2ac_delta as i8;
        self.quant.uv_dc_delta = uvdc_delta as i8;
        self.quant.uv_ac_delta = uvac_delta as i8;

        /*
            for i in 0usize..n {
                let base = i32::from(if !self.segment[i].delta_values {
                    i16::from(self.segment[i].quantizer_level)
                } else {
                    i16::from(self.segment[i].quantizer_level) + yac_abs
                });

                self.segment[i].ydc = dc_quant(base + ydc_delta);
                self.segment[i].yac = ac_quant(base);

                self.segment[i].y2dc = dc_quant(base + y2dc_delta) * 2;
                // The intermediate result (max`284*155`) can be larger than the `i16` range.
                self.segment[i].y2ac = (i32::from(ac_quant(base + y2ac_delta)) * 155 / 100) as i16;

                self.segment[i].uvdc = dc_quant(base + uvdc_delta);
                self.segment[i].uvac = ac_quant(base + uvac_delta);

                if self.segment[i].y2ac < 8 {
                    self.segment[i].y2ac = 8;
                }

                if self.segment[i].uvdc > 132 {
                    self.segment[i].uvdc = 132;
                }
            }
        */
    }

    fn read_loop_filter_adjustments(&mut self) {
        self.delta_update = self.b.read_flag();
        if self.delta_update {
            for i in 0usize..4 {
                let ref_frame_delta_update_flag = self.b.read_flag();

                self.ref_delta[i] = if ref_frame_delta_update_flag {
                    self.b.read_magnitude_and_sign(6) as i8
                } else {
                    0i8
                };
            }

            for i in 0usize..4 {
                let mb_mode_delta_update_flag = self.b.read_flag();

                self.mode_delta[i] = if mb_mode_delta_update_flag {
                    self.b.read_magnitude_and_sign(6) as i8
                } else {
                    0i8
                };
            }
        }
    }

    fn read_segment_updates(&mut self) {
        // Section 9.3
        self.segments_update_map = self.b.read_flag();
        self.update_segment_feature_data = self.b.read_flag();

        if self.update_segment_feature_data {
            self.segment_feature_mode = self.b.read_flag();

            for i in 0usize..MAX_SEGMENTS {
                self.segment[i].delta_values = !self.segment_feature_mode;
            }

            for i in 0usize..MAX_SEGMENTS {
                let update = self.b.read_flag();

                self.segment[i].quantizer_level = if update {
                    self.b.read_magnitude_and_sign(7)
                } else {
                    0i32
                } as i8;
            }

            for i in 0usize..MAX_SEGMENTS {
                let update = self.b.read_flag();

                self.segment[i].loopfilter_level = if update {
                    self.b.read_magnitude_and_sign(6)
                } else {
                    0i32
                } as i8;
            }
        }

        if self.segments_update_map {
            for i in 0usize..3 {
                let update = self.b.read_flag();

                self.segment_tree_probs[i] = if update { self.b.read_literal(8) } else { 255 };
            }
        }
    }

    pub(super) fn read_frame_header(&mut self) -> ImageResult<()> {
        let tag = self.r.read_u24::<LittleEndian>()?;

        self.frame.keyframe = tag & 1 == 0;
        self.frame.version = ((tag >> 1) & 7) as u8;
        self.frame.for_display = (tag >> 4) & 1 != 0;
        self.frame.first_partition_size = tag >> 5;

        if self.frame.keyframe {
            let mut tag = [0u8; 3];
            self.r.read_exact(&mut tag)?;

            if tag != [0x9d, 0x01, 0x2a] {
                return Err(ImageError);
            }

            let w = self.r.read_u16::<LittleEndian>()?;
            let h = self.r.read_u16::<LittleEndian>()?;

            self.frame.width = w & 0x3FFF;
            self.frame.height = h & 0x3FFF;

            self.frame.horizontal_scale = (w >> 14) as u8;
            self.frame.vertical_scale = (h >> 14) as u8;

            self.mbwidth = (self.frame.width + 15) / 16;
            self.mbheight = (self.frame.height + 15) / 16;
        }

        let mut buf = vec![0; self.frame.first_partition_size as usize];
        self.r.read_exact(&mut buf)?;

        // initialise binary decoder
        self.b.init(buf)?;

        if self.frame.keyframe {
            let color_space = self.b.read_literal(1);
            self.frame.pixel_type = self.b.read_literal(1);

            if color_space != 0 {
                return Err(ImageError);
            }
        }

        self.segments_enabled = self.b.read_flag();
        if self.segments_enabled {
            self.read_segment_updates();
        }

        self.frame.filter_type = self.b.read_flag();
        self.frame.filter_level = self.b.read_literal(6);
        self.frame.sharpness_level = self.b.read_literal(3);

        self.lf_adjust_enable = self.b.read_flag();
        if self.lf_adjust_enable {
            self.read_loop_filter_adjustments();
        }

        self.num_partitions = (1usize << self.b.read_literal(2) as usize) as u8;
        let num_partitions = self.num_partitions as usize;
        self.init_partitions(num_partitions)?;

        self.read_quantization_indices();

        if !self.frame.keyframe {
            // 9.7 refresh golden frame and altref frame
            // FIXME: support this?
            return Err(ImageError);
        } else {
            // Refresh entropy probs ?????
            let _ = self.b.read_literal(1);
        }

        self.update_token_probabilities();

        let mb_no_skip_coeff = self.b.read_literal(1);
        self.prob_skip_false = if mb_no_skip_coeff == 1 {
            Some(self.b.read_literal(8))
        } else {
            None
        };

        if !self.frame.keyframe {
            // 9.10 remaining frame data
            self.prob_intra = 0;

            // FIXME: support this?
            return Err(ImageError);
        } else {
            // Reset motion vectors
        }

        Ok(())
    }
}