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/// The arithmetic decoder, as described in Annex E.
pub(crate) struct ArithmeticDecoder<'a> {
/// The underlying encoded data.
data: &'a [u8],
/// `C` - The `c_high` and `c_low` registers.
code_register: u32,
/// `A` - The probability interval register.
interval: u32,
/// `BP` - A pointer to the compressed data.
byte_offset: u32,
/// `CT` - The count of available bits in the current byte.
bits_available: u32,
}
impl<'a> ArithmeticDecoder<'a> {
#[inline(always)]
pub(crate) fn new(data: &'a [u8]) -> Self {
let mut decoder = ArithmeticDecoder {
data,
code_register: 0,
interval: 0,
byte_offset: 0,
bits_available: 0,
};
decoder.initialize();
decoder
}
/// Read the next bit using the given context.
#[inline(always)]
pub(crate) fn decode(&mut self, context: &mut Context) -> u32 {
self.decode_internal(context)
}
/// The DECODE procedure (E.3.2, Figure G.2).
#[inline(always)]
fn decode_internal(&mut self, context: &mut Context) -> u32 {
let qe_entry = &QE_TABLE[context.state_index as usize];
// Figure G.2: "A = A - Qe(I(CX))"
self.interval -= qe_entry.probability;
// `D` - The decoded binary decision.
let decoded_bit;
// Figure G.2: "Chigh < A?"
if (self.code_register >> 16) < self.interval {
// Figure G.2: "A AND 0x8000 = 0?"
if self.interval & 0x8000 == 0 {
// Figure G.2: "D = MPS_EXCHANGE; RENORMD"
decoded_bit = self.exchange_mps(context, qe_entry);
self.renormalize();
} else {
// Figure G.2: "D = MPS(CX)"
decoded_bit = context.mps;
}
} else {
// Figure G.2: "Chigh = Chigh - A; D = LPS_EXCHANGE; RENORMD"
self.code_register -= self.interval << 16;
decoded_bit = self.exchange_lps(context, qe_entry);
self.renormalize();
}
decoded_bit
}
/// The INITDEC procedure (E.3.5, Figure G.1).
#[inline(always)]
fn initialize(&mut self) {
// Figure G.1: "C = (B XOR 0xFF) << 16"
self.code_register = ((self.current_byte() as u32) ^ 0xff) << 16;
// Figure G.1: "BYTEIN"
self.read_byte();
// Figure G.1: "C = C << 7; CT = CT - 7; A = 0x8000"
self.code_register <<= 7;
self.bits_available -= 7;
self.interval = 0x8000;
}
/// The BYTEIN procedure (E.3.4, Figure G.3).
#[inline(always)]
fn read_byte(&mut self) {
// Figure G.3: "B = 0xFF?"
if self.current_byte() == 0xff {
// `B1` - The next byte.
let next_byte = self.next_byte();
// Figure G.3: "B1 > 0x8F?"
if next_byte > 0x8f {
// Figure G.3: "CT = 8"
self.bits_available = 8;
} else {
// Figure G.3: "BP = BP + 1; C = C + 0xFE00 - (B << 9); CT = 7"
self.byte_offset += 1;
self.code_register = self
.code_register
.wrapping_add(0xfe00)
.wrapping_sub((self.current_byte() as u32) << 9);
self.bits_available = 7;
}
} else {
// Figure G.3: "BP = BP + 1; C = C + 0xFF00 - (B << 8); CT = 8"
self.byte_offset += 1;
self.code_register = self
.code_register
.wrapping_add(0xff00)
.wrapping_sub((self.current_byte() as u32) << 8);
self.bits_available = 8;
}
}
/// The RENORMD procedure (E.3.3, Figure E.18).
#[inline(always)]
fn renormalize(&mut self) {
loop {
// Figure E.18: "CT = 0?"
if self.bits_available == 0 {
// Figure E.18: "BYTEIN"
self.read_byte();
}
// Figure E.18: "A = A << 1; C = C << 1; CT = CT - 1"
self.interval <<= 1;
self.code_register <<= 1;
self.bits_available -= 1;
// Figure E.18: "A AND 0x8000 = 0?"
if self.interval & 0x8000 != 0 {
break;
}
}
}
/// The `LPS_EXCHANGE` procedure (E.3.2, Figure E.17).
#[inline(always)]
fn exchange_lps(&mut self, context: &mut Context, qe_entry: &QeData) -> u32 {
// `D` - The decoded binary decision.
let decoded_bit;
// Figure E.17: "A < Qe(I(CX))?"
if self.interval < qe_entry.probability {
// Figure E.17: "A = Qe(I(CX)); D = MPS(CX); I(CX) = NMPS(I(CX))"
self.interval = qe_entry.probability;
decoded_bit = context.mps;
context.state_index = qe_entry.next_index_on_mps;
} else {
// Figure E.17: "A = Qe(I(CX)); D = 1 - MPS(CX)"
self.interval = qe_entry.probability;
decoded_bit = 1 - context.mps;
// Figure E.17: "SWITCH(I(CX)) = 1?"
if qe_entry.switch_mps_sense {
// Figure E.17: "MPS(CX) = 1 - MPS(CX)"
context.mps = 1 - context.mps;
}
// Figure E.17: "I(CX) = NLPS(I(CX))"
context.state_index = qe_entry.next_index_on_lps;
}
decoded_bit
}
/// The `MPS_EXCHANGE` procedure (E.3.2, Figure E.16).
#[inline(always)]
fn exchange_mps(&mut self, context: &mut Context, qe_entry: &QeData) -> u32 {
// `D` - The decoded binary decision.
let decoded_bit;
// Figure E.16: "A < Qe(I(CX))?"
if self.interval < qe_entry.probability {
// Figure E.16: "D = 1 - MPS(CX)"
decoded_bit = 1 - context.mps;
// Figure E.16: "SWITCH(I(CX)) = 1?"
if qe_entry.switch_mps_sense {
// "MPS(CX) = 1 - MPS(CX)"
context.mps = 1 - context.mps;
}
// Figure E.16: "I(CX) = NLPS(I(CX))"
context.state_index = qe_entry.next_index_on_lps;
} else {
// Figure E.16: "D = MPS(CX); I(CX) = NMPS(I(CX))"
decoded_bit = context.mps;
context.state_index = qe_entry.next_index_on_mps;
}
decoded_bit
}
#[inline(always)]
fn current_byte(&self) -> u8 {
self.data
.get(self.byte_offset as usize)
.copied()
.unwrap_or(0xFF)
}
#[inline(always)]
fn next_byte(&self) -> u8 {
self.data
.get((self.byte_offset + 1) as usize)
.copied()
.unwrap_or(0xFF)
}
}
/// Context for the arithmetic decoder.
#[derive(Copy, Clone, Debug, Default)]
pub(crate) struct Context {
/// `I(CX)` - Index into the probability estimation state machine.
pub(crate) state_index: u32,
/// `MPS(CX)` - The sense of MPS for context CX.
pub(crate) mps: u32,
}
/// Qe value table entry (Table E.1).
#[derive(Debug, Clone, Copy)]
struct QeData {
/// `Qe` - The probability estimate value.
probability: u32,
/// `NMPS` - Next state index if MPS is coded.
next_index_on_mps: u32,
/// `NLPS` - Next state index if LPS is coded.
next_index_on_lps: u32,
/// `SWITCH` - Whether to flip the MPS sense.
switch_mps_sense: bool,
}
macro_rules! qe {
($($qe:expr, $nmps:expr, $nlps:expr, $switch:expr),+ $(,)?) => {
[
$(
QeData {
probability: $qe,
next_index_on_mps: $nmps,
next_index_on_lps: $nlps,
switch_mps_sense: $switch,
}
),+
]
};
}
/// "Table E.1 - Qe values and probability estimation process"
#[rustfmt::skip]
static QE_TABLE: [QeData; 47] = qe!(
// Index Qe_Value NMPS NLPS SWITCH
/* 0 */ 0x5601, 1, 1, true,
/* 1 */ 0x3401, 2, 6, false,
/* 2 */ 0x1801, 3, 9, false,
/* 3 */ 0x0AC1, 4, 12, false,
/* 4 */ 0x0521, 5, 29, false,
/* 5 */ 0x0221, 38, 33, false,
/* 6 */ 0x5601, 7, 6, true,
/* 7 */ 0x5401, 8, 14, false,
/* 8 */ 0x4801, 9, 14, false,
/* 9 */ 0x3801, 10, 14, false,
/* 10 */ 0x3001, 11, 17, false,
/* 11 */ 0x2401, 12, 18, false,
/* 12 */ 0x1C01, 13, 20, false,
/* 13 */ 0x1601, 29, 21, false,
/* 14 */ 0x5601, 15, 14, true,
/* 15 */ 0x5401, 16, 14, false,
/* 16 */ 0x5101, 17, 15, false,
/* 17 */ 0x4801, 18, 16, false,
/* 18 */ 0x3801, 19, 17, false,
/* 19 */ 0x3401, 20, 18, false,
/* 20 */ 0x3001, 21, 19, false,
/* 21 */ 0x2801, 22, 19, false,
/* 22 */ 0x2401, 23, 20, false,
/* 23 */ 0x2201, 24, 21, false,
/* 24 */ 0x1C01, 25, 22, false,
/* 25 */ 0x1801, 26, 23, false,
/* 26 */ 0x1601, 27, 24, false,
/* 27 */ 0x1401, 28, 25, false,
/* 28 */ 0x1201, 29, 26, false,
/* 29 */ 0x1101, 30, 27, false,
/* 30 */ 0x0AC1, 31, 28, false,
/* 31 */ 0x09C1, 32, 29, false,
/* 32 */ 0x08A1, 33, 30, false,
/* 33 */ 0x0521, 34, 31, false,
/* 34 */ 0x0441, 35, 32, false,
/* 35 */ 0x02A1, 36, 33, false,
/* 36 */ 0x0221, 37, 34, false,
/* 37 */ 0x0141, 38, 35, false,
/* 38 */ 0x0111, 39, 36, false,
/* 39 */ 0x0085, 40, 37, false,
/* 40 */ 0x0049, 41, 38, false,
/* 41 */ 0x0025, 42, 39, false,
/* 42 */ 0x0015, 43, 40, false,
/* 43 */ 0x0009, 44, 41, false,
/* 44 */ 0x0005, 45, 42, false,
/* 45 */ 0x0001, 45, 43, false,
/* 46 */ 0x5601, 46, 46, false,
);