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use crate::bit_io::{BitReader, BitReaderReversed};
use crate::decoding::errors::{FSEDecoderError, FSETableError};
use alloc::vec::Vec;
pub struct FSEDecoder<'table> {
/// An FSE state value represents an index in the FSE table.
pub state: Entry,
/// A reference to the table used for decoding.
table: &'table FSETable,
}
impl<'t> FSEDecoder<'t> {
/// Initialize a new Finite State Entropy decoder.
pub fn new(table: &'t FSETable) -> FSEDecoder<'t> {
FSEDecoder {
state: table.decode.first().copied().unwrap_or(Entry {
base_line: 0,
num_bits: 0,
symbol: 0,
}),
table,
}
}
/// Returns the byte associated with the symbol the internal cursor is pointing at.
pub fn decode_symbol(&self) -> u8 {
self.state.symbol
}
/// Initialize internal state and prepare for decoding. After this, `decode_symbol` can be called
/// to read the first symbol and `update_state` can be called to prepare to read the next symbol.
pub fn init_state(&mut self, bits: &mut BitReaderReversed<'_>) -> Result<(), FSEDecoderError> {
if self.table.accuracy_log == 0 {
return Err(FSEDecoderError::TableIsUninitialized);
}
let new_state = bits.get_bits(self.table.accuracy_log);
self.state = self.table.decode[new_state as usize];
Ok(())
}
/// Advance the internal state to decode the next symbol in the bitstream.
pub fn update_state(&mut self, bits: &mut BitReaderReversed<'_>) {
let num_bits = self.state.num_bits;
let add = bits.get_bits(num_bits);
let base_line = self.state.base_line;
let new_state = base_line + add as u32;
self.state = self.table.decode[new_state as usize];
//println!("Update: {}, {} -> {}", base_line, add, self.state);
}
}
/// FSE decoding involves a decoding table that describes the probabilities of
/// all literals from 0 to the highest present one
///
/// <https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#fse-table-description>
#[derive(Debug, Clone)]
pub struct FSETable {
/// The maximum symbol in the table (inclusive). Limits the probabilities length to max_symbol + 1.
max_symbol: u8,
/// The actual table containing the decoded symbol and the compression data
/// connected to that symbol.
pub decode: Vec<Entry>, //used to decode symbols, and calculate the next state
/// The size of the table is stored in logarithm base 2 format,
/// with the **size of the table** being equal to `(1 << accuracy_log)`.
/// This value is used so that the decoder knows how many bits to read from the bitstream.
pub accuracy_log: u8,
/// In this context, probability refers to the likelihood that a symbol occurs in the given data.
/// Given this info, the encoder can assign shorter codes to symbols that appear more often,
/// and longer codes that appear less often, then the decoder can use the probability
/// to determine what code was assigned to what symbol.
///
/// The probability of a single symbol is a value representing the proportion of times the symbol
/// would fall within the data.
///
/// If a symbol probability is set to `-1`, it means that the probability of a symbol
/// occurring in the data is less than one.
pub symbol_probabilities: Vec<i32>, //used while building the decode Vector
/// The number of times each symbol occurs (The first entry being 0x0, the second being 0x1) and so on
/// up until the highest possible symbol (255).
symbol_counter: Vec<u32>,
}
impl FSETable {
/// Initialize a new empty Finite State Entropy decoding table.
pub fn new(max_symbol: u8) -> FSETable {
FSETable {
max_symbol,
symbol_probabilities: Vec::with_capacity(256), //will never be more than 256 symbols because u8
symbol_counter: Vec::with_capacity(256), //will never be more than 256 symbols because u8
decode: Vec::new(), //depending on acc_log.
accuracy_log: 0,
}
}
/// Reset `self` and update `self`'s state to mirror the provided table.
pub fn reinit_from(&mut self, other: &Self) {
self.reset();
self.symbol_counter.extend_from_slice(&other.symbol_counter);
self.symbol_probabilities
.extend_from_slice(&other.symbol_probabilities);
self.decode.extend_from_slice(&other.decode);
self.accuracy_log = other.accuracy_log;
}
/// Empty the table and clear all internal state.
pub fn reset(&mut self) {
self.symbol_counter.clear();
self.symbol_probabilities.clear();
self.decode.clear();
self.accuracy_log = 0;
}
/// returns how many BYTEs (not bits) were read while building the decoder
pub fn build_decoder(&mut self, source: &[u8], max_log: u8) -> Result<usize, FSETableError> {
self.accuracy_log = 0;
let bytes_read = self.read_probabilities(source, max_log)?;
self.build_decoding_table()?;
Ok(bytes_read)
}
/// Given the provided accuracy log, build a decoding table from that log.
pub fn build_from_probabilities(
&mut self,
acc_log: u8,
probs: &[i32],
) -> Result<(), FSETableError> {
if acc_log == 0 {
return Err(FSETableError::AccLogIsZero);
}
self.symbol_probabilities = probs.to_vec();
self.accuracy_log = acc_log;
self.build_decoding_table()
}
/// Build the actual decoding table after probabilities have been read into the table.
/// After this function is called, the decoding process can begin.
fn build_decoding_table(&mut self) -> Result<(), FSETableError> {
if self.symbol_probabilities.len() > self.max_symbol as usize + 1 {
return Err(FSETableError::TooManySymbols {
got: self.symbol_probabilities.len(),
});
}
self.decode.clear();
let table_size = 1 << self.accuracy_log;
if self.decode.len() < table_size {
self.decode.reserve(table_size - self.decode.len());
}
//fill with dummy entries
self.decode.resize(
table_size,
Entry {
base_line: 0,
num_bits: 0,
symbol: 0,
},
);
let mut negative_idx = table_size; //will point to the highest index with is already occupied by a negative-probability-symbol
//first scan for all -1 probabilities and place them at the top of the table
for symbol in 0..self.symbol_probabilities.len() {
if self.symbol_probabilities[symbol] == -1 {
negative_idx -= 1;
let entry = &mut self.decode[negative_idx];
entry.symbol = symbol as u8;
entry.base_line = 0;
entry.num_bits = self.accuracy_log;
}
}
//then place in a semi-random order all of the other symbols
let mut position = 0;
for idx in 0..self.symbol_probabilities.len() {
let symbol = idx as u8;
if self.symbol_probabilities[idx] <= 0 {
continue;
}
//for each probability point the symbol gets on slot
let prob = self.symbol_probabilities[idx];
for _ in 0..prob {
let entry = &mut self.decode[position];
entry.symbol = symbol;
position = next_position(position, table_size);
while position >= negative_idx {
position = next_position(position, table_size);
//everything above negative_idx is already taken
}
}
}
// baselines and num_bits can only be calculated when all symbols have been spread
self.symbol_counter.clear();
self.symbol_counter
.resize(self.symbol_probabilities.len(), 0);
for idx in 0..negative_idx {
let entry = &mut self.decode[idx];
let symbol = entry.symbol;
let prob = self.symbol_probabilities[symbol as usize];
let symbol_count = self.symbol_counter[symbol as usize];
let (bl, nb) = calc_baseline_and_numbits(table_size as u32, prob as u32, symbol_count);
//println!("symbol: {:2}, table: {}, prob: {:3}, count: {:3}, bl: {:3}, nb: {:2}", symbol, table_size, prob, symbol_count, bl, nb);
assert!(nb <= self.accuracy_log);
self.symbol_counter[symbol as usize] += 1;
entry.base_line = bl;
entry.num_bits = nb;
}
Ok(())
}
/// Read the accuracy log and the probability table from the source and return the number of bytes
/// read. If the size of the table is larger than the provided `max_log`, return an error.
fn read_probabilities(&mut self, source: &[u8], max_log: u8) -> Result<usize, FSETableError> {
self.symbol_probabilities.clear(); //just clear, we will fill a probability for each entry anyways. No need to force new allocs here
let mut br = BitReader::new(source);
self.accuracy_log = ACC_LOG_OFFSET + (br.get_bits(4)? as u8);
if self.accuracy_log > max_log {
return Err(FSETableError::AccLogTooBig {
got: self.accuracy_log,
max: max_log,
});
}
if self.accuracy_log == 0 {
return Err(FSETableError::AccLogIsZero);
}
let probability_sum = 1 << self.accuracy_log;
let mut probability_counter = 0;
while probability_counter < probability_sum {
let max_remaining_value = probability_sum - probability_counter + 1;
let bits_to_read = highest_bit_set(max_remaining_value);
let unchecked_value = br.get_bits(bits_to_read as usize)? as u32;
let low_threshold = ((1 << bits_to_read) - 1) - (max_remaining_value);
let mask = (1 << (bits_to_read - 1)) - 1;
let small_value = unchecked_value & mask;
let value = if small_value < low_threshold {
br.return_bits(1);
small_value
} else if unchecked_value > mask {
unchecked_value - low_threshold
} else {
unchecked_value
};
//println!("{}, {}, {}", self.symbol_probablilities.len(), unchecked_value, value);
let prob = (value as i32) - 1;
self.symbol_probabilities.push(prob);
if prob != 0 {
if prob > 0 {
probability_counter += prob as u32;
} else {
// probability -1 counts as 1
assert!(prob == -1);
probability_counter += 1;
}
} else {
//fast skip further zero probabilities
loop {
let skip_amount = br.get_bits(2)? as usize;
self.symbol_probabilities
.resize(self.symbol_probabilities.len() + skip_amount, 0);
if skip_amount != 3 {
break;
}
}
}
}
if probability_counter != probability_sum {
return Err(FSETableError::ProbabilityCounterMismatch {
got: probability_counter,
expected_sum: probability_sum,
symbol_probabilities: self.symbol_probabilities.clone(),
});
}
if self.symbol_probabilities.len() > self.max_symbol as usize + 1 {
return Err(FSETableError::TooManySymbols {
got: self.symbol_probabilities.len(),
});
}
let bytes_read = if br.bits_read() % 8 == 0 {
br.bits_read() / 8
} else {
(br.bits_read() / 8) + 1
};
Ok(bytes_read)
}
}
/// A single entry in an FSE table.
#[derive(Copy, Clone, Debug)]
pub struct Entry {
/// This value is used as an offset value, and it is added
/// to a value read from the stream to determine the next state value.
pub base_line: u32,
/// How many bits should be read from the stream when decoding this entry.
pub num_bits: u8,
/// The byte that should be put in the decode output when encountering this state.
pub symbol: u8,
}
/// This value is added to the first 4 bits of the stream to determine the
/// `Accuracy_Log`
const ACC_LOG_OFFSET: u8 = 5;
fn highest_bit_set(x: u32) -> u32 {
assert!(x > 0);
u32::BITS - x.leading_zeros()
}
//utility functions for building the decoding table from probabilities
/// Calculate the position of the next entry of the table given the current
/// position and size of the table.
fn next_position(mut p: usize, table_size: usize) -> usize {
p += (table_size >> 1) + (table_size >> 3) + 3;
p &= table_size - 1;
p
}
fn calc_baseline_and_numbits(
num_states_total: u32,
num_states_symbol: u32,
state_number: u32,
) -> (u32, u8) {
if num_states_symbol == 0 {
return (0, 0);
}
let num_state_slices = if 1 << (highest_bit_set(num_states_symbol) - 1) == num_states_symbol {
num_states_symbol
} else {
1 << (highest_bit_set(num_states_symbol))
}; //always power of two
let num_double_width_state_slices = num_state_slices - num_states_symbol; //leftovers to the power of two need to be distributed
let num_single_width_state_slices = num_states_symbol - num_double_width_state_slices; //these will not receive a double width slice of states
let slice_width = num_states_total / num_state_slices; //size of a single width slice of states
let num_bits = highest_bit_set(slice_width) - 1; //number of bits needed to read for one slice
if state_number < num_double_width_state_slices {
let baseline = num_single_width_state_slices * slice_width + state_number * slice_width * 2;
(baseline, num_bits as u8 + 1)
} else {
let index_shifted = state_number - num_double_width_state_slices;
((index_shifted * slice_width), num_bits as u8)
}
}