opus: implement range decoder.

symphonia-codec-opus/src/lib.rs

@@ -25,6 +25,9 @@ use symphonia_core::formats::Packet;
 use symphonia_core::io::{BufReader, ReadBytes};
 use symphonia_core::support_codec;
 
+mod range_decoder;
+use range_decoder::RangeDecoder;
+
 #[allow(dead_code)]
 pub struct OpusDecoder {
     ident_header: IdentificationHeader,
@@ -43,6 +46,21 @@ enum Mode {
     Hybrid,
 }
 
+/// The bandwidth of the current audio packet.
+/// See RFC 6716 Section 2 Table 1 and RFC 6716 Section 4.3 Table 55.
+enum Bandwidth {
+    /// 4 kHz, index 12
+    NarrowBand,
+    /// 6 kHz, index 16
+    MediumBand,
+    /// 8 kHz, index 16
+    WideBand,
+    /// 12 kHz, index 18
+    SuperWideBand,
+    /// 20 kHz, index 20
+    FullBand,
+}
+
 impl Decoder for OpusDecoder {
     fn try_new(params: &CodecParameters, _: &DecoderOptions) -> Result<Self> {
         let extra_data = match params.extra_data.as_ref() {
@@ -82,9 +100,36 @@ impl Decoder for OpusDecoder {
             16..=31 => Mode::Celt,
             _ => unreachable!(),
         };
+        let bandwidth = match config {
+            0..=3 => Bandwidth::NarrowBand,
+            4..=7 => Bandwidth::MediumBand,
+            8..=11 => Bandwidth::WideBand,
+            12..=13 => Bandwidth::SuperWideBand,
+            14..=15 => Bandwidth::FullBand,
+            16..=19 => Bandwidth::NarrowBand,
+            20..=23 => Bandwidth::WideBand,
+            24..=27 => Bandwidth::SuperWideBand,
+            28..=31 => Bandwidth::FullBand,
+            _ => unreachable!(),
+        };
+
         let stereo_flag = toc & 0b00000100;
         let frame_count_code = toc & 0b00000011;
 
+        // RFC 7845 state that samples are counted "assumming a 48 khz decoding rate".
+        // RFC 6716 record frame sizes in milliseconds.
+        // 48000Hz == 48000/(1second) == 48000/(1000millseconds) == 48/(1millisecond).
+        // See RFC 6716 Section 3.1 Table 2 and RFC 7845 Section 4.
+        let frame_size = match config {
+            0..=11 => [10 * 48, 20 * 48, 40 * 48, 60 * 48][(config % 4) as usize],
+            12..=15 => [10 * 48, 20 * 48][(config % 2) as usize],
+            // Separate 2.5 * 48 into 2 * 48 + 0.5 * 48 to avoid float and division.
+            16..=31 => [2 * 48 + 48 >> 1, 5 * 48, 10 * 48, 20 * 48][(config % 4) as usize],
+            _ => unreachable!(),
+        };
+
+        let mut range_decoder = RangeDecoder::try_new(&mut reader);
+
         Ok(self.buffer.as_audio_buffer_ref())
     }
 

symphonia-codec-opus/src/range_decoder.rs

@@ -0,0 +1,103 @@
+// Symphonia
+// Copyright (c) 2019-2021 The Project Symphonia Developers.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+#![warn(rust_2018_idioms)]
+#![forbid(unsafe_code)]
+// The following lints are allowed in all Symphonia crates. Please see clippy.toml for their
+// justification.
+#![allow(clippy::comparison_chain)]
+#![allow(clippy::excessive_precision)]
+#![allow(clippy::identity_op)]
+#![allow(clippy::manual_range_contains)]
+// Disable to better express the specification.
+#![allow(clippy::collapsible_else_if)]
+
+use symphonia_core::errors::Result;
+use symphonia_core::io::{BufReader, ReadBytes};
+
+/// The range decoder for the Opus Decoder.
+/// See RFC 6716 Section 4.1, https://tools.ietf.org/pdf/rfc7845.pdf.
+pub struct RangeDecoder<'a> {
+    range: u32,
+    value: u32,
+    number_of_bits: u32,
+    reader: &'a mut BufReader<'a>,
+    previous_byte: u8,
+}
+
+impl RangeDecoder<'_> {
+    /// Create a new range decoder that reads from a `BufReader`.
+    /// See RFC 6716 Section 4.1.1.    
+    pub fn try_new<'a>(reader: &'a mut BufReader<'a>) -> Result<RangeDecoder<'a>> {
+        let previous_byte = reader.read_byte()?;
+        let range = 128;
+        let value: u32 = 127u32 - u32::from(previous_byte >> 1);
+
+        // number_of_bits can also be initialised to 33 after the first renormalization.
+        // See RFC 6716 Section 4.1.6.
+        let mut range_decoder =
+            RangeDecoder { range, value, number_of_bits: 9, reader, previous_byte };
+        range_decoder.normalize();
+        Ok(range_decoder)
+    }
+
+    /// Decodes the current symbol given a frequency table and the sum of that frequency table.
+    /// See RFC 6716 Section 4.1.2.       
+    fn decode_symbol(&mut self, symbol_frequencies: &[u32], total: u32) -> u32 {
+        use std::cmp;
+
+        // current_symbol_frequency == fs.
+        // lower_symbol_frequency_threshold = fl.
+        // higher_symbol_frequency_threshold = fh.
+        // index_of_current_symbol = k.
+        // See RFC 6716 Section 4.1.2 for more details on fs, fl, fh and k.
+        let current_symbol_frequency =
+            total - cmp::min(self.value / (self.range / total) + 1, total);
+        let mut lower_symbol_frequency_threshold = 0;
+        let mut higher_symbol_frequency_threshold = 0;
+        let mut index_of_current_symbol = 0;
+        for frequency in symbol_frequencies {
+            higher_symbol_frequency_threshold = lower_symbol_frequency_threshold + frequency;
+            if lower_symbol_frequency_threshold <= current_symbol_frequency
+                && current_symbol_frequency < lower_symbol_frequency_threshold + frequency
+            {
+                break;
+            }
+            lower_symbol_frequency_threshold += frequency;
+            index_of_current_symbol += 1;
+        }
+
+        self.value = self.value - self.range / total * (total - higher_symbol_frequency_threshold);
+        if lower_symbol_frequency_threshold > 0 {
+            self.range = self.range / total
+                * (higher_symbol_frequency_threshold - lower_symbol_frequency_threshold);
+        } else {
+            self.range =
+                self.range - self.range / total * (total - higher_symbol_frequency_threshold);
+        }
+
+        self.normalize();
+
+        index_of_current_symbol
+    }
+
+    // Normalizes the range decoder's range.
+    // See RFC 6716 Section 4.1.2.1.
+    fn normalize(&mut self) -> Result<()> {
+        while self.range <= (1 << 23) {
+            self.number_of_bits += 8;
+            self.range <<= 8;
+            let carry_bit = self.previous_byte & 1;
+            let current_byte = self.reader.read_byte()?;
+            self.value = ((self.value << 8)
+                + u32::from(255 - (carry_bit << 7) | current_byte >> 1))
+                & 0x7FFFFFF;
+            self.previous_byte = current_byte;
+        }
+        Ok(())
+    }
+}