Function decode [src]

Prototype

 pub fn decode( bit_reader: *BitReader, expected_symbol_count: usize, max_accuracy_log: u4, entries: []Table.Fse, ) !usize

Parameters

bit_reader: *BitReaderexpected_symbol_count: usizemax_accuracy_log: u4entries: []Table.Fse

Source

  pub fn decode(
    bit_reader: *BitReader,
    expected_symbol_count: usize,
    max_accuracy_log: u4,
    entries: []Table.Fse,
) !usize {
    const accuracy_log_biased = try bit_reader.readBitsNoEof(u4, 4);
    if (accuracy_log_biased > max_accuracy_log -| 5) return error.MalformedAccuracyLog;
    const accuracy_log = accuracy_log_biased + 5;

    var values: [256]u16 = undefined;
    var value_count: usize = 0;

    const total_probability = @as(u16, 1) << accuracy_log;
    var accumulated_probability: u16 = 0;

    while (accumulated_probability < total_probability) {
        // WARNING: The RFC is poorly worded, and would suggest std.math.log2_int_ceil is correct here,
        //          but power of two (remaining probabilities + 1) need max bits set to 1 more.
        const max_bits = std.math.log2_int(u16, total_probability - accumulated_probability + 1) + 1;
        const small = try bit_reader.readBitsNoEof(u16, max_bits - 1);

        const cutoff = (@as(u16, 1) << max_bits) - 1 - (total_probability - accumulated_probability + 1);

        const value = if (small < cutoff)
            small
        else value: {
            const value_read = small + (try bit_reader.readBitsNoEof(u16, 1) << (max_bits - 1));
            break :value if (value_read < @as(u16, 1) << (max_bits - 1))
                value_read
            else
                value_read - cutoff;
        };

        accumulated_probability += if (value != 0) value - 1 else 1;

        values[value_count] = value;
        value_count += 1;

        if (value == 1) {
            while (true) {
                const repeat_flag = try bit_reader.readBitsNoEof(u2, 2);
                if (repeat_flag + value_count > 256) return error.MalformedFseTable;
                for (0..repeat_flag) |_| {
                    values[value_count] = 1;
                    value_count += 1;
                }
                if (repeat_flag < 3) break;
            }
        }
        if (value_count == 256) break;
    }
    bit_reader.alignToByte();

    if (value_count < 2) return error.MalformedFseTable;
    if (accumulated_probability != total_probability) return error.MalformedFseTable;
    if (value_count > expected_symbol_count) return error.MalformedFseTable;

    const table_size = total_probability;

    try build(values[0..value_count], entries[0..table_size]);
    return table_size;
}  