draco-core 1.0.3

Pure Rust core encoder and decoder for Draco geometry compression
Documentation 
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//! Shared rANS symbol-coding helpers.
//!
//! Defines [`RAnsSymbol`] (a symbol's probability and cumulative probability)
//! and the precision computations shared by the symbol encoder and decoder,
//! including the unique-symbol-bit-length to rANS-precision mapping. Port of
//! Draco's `rans_symbol_coding.h`.

#[derive(Debug, Clone, Copy, Default)]
pub struct RAnsSymbol {
    pub prob: u32,
    pub cum_prob: u32,
}

pub fn compute_rans_unclamped_precision(symbols_bit_length: u32) -> u32 {
    (3 * symbols_bit_length) / 2
}

pub fn compute_rans_precision_from_unique_symbols_bit_length(symbols_bit_length: u32) -> u32 {
    let prec = compute_rans_unclamped_precision(symbols_bit_length);
    prec.clamp(12, 20)
}

pub fn approximate_rans_frequency_table_bits(max_value: u32, num_unique_symbols: u32) -> u64 {
    let diff = max_value.saturating_sub(num_unique_symbols);
    let table_zero_frequency_bits = 8 * (num_unique_symbols + diff / 64);
    (8 * num_unique_symbols + table_zero_frequency_bits) as u64
}