You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: decoder.rs
    path of file: ./repos/Symphonia/symphonia-bundle-mp3/src
    
    the code of the file until where you have to start completion: // Symphonia
// Copyright (c) 2019-2022 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/.

use symphonia_core::audio::{AsAudioBufferRef, AudioBuffer, AudioBufferRef, Signal};
use symphonia_core::codecs::{CodecDescriptor, CodecParameters, CodecType};
use symphonia_core::codecs::{Decoder, DecoderOptions, FinalizeResult};
use symphonia_core::errors::{decode_error, unsupported_error, Result};
use symphonia_core::formats::Packet;
use symphonia_core::io::FiniteStream;
use symphonia_core::support_codec;

#[cfg(feature = "mp1")]
use symphonia_core::codecs::CODEC_TYPE_MP1;
#[cfg(feature = "mp2")]
use symphonia_co
    
    fn decode_inner(&mut self, packet: &Packet) -> Result<()> {
        let mut reader = packet.as_buf_reader();

        let header = header::read_frame_header(&mut reader)?;

        // The packet should be the size stated in the header.
        if header.frame_size != reader.bytes_available() as usize {
            return decode_error("mpa: invalid packet length");
        }

        // The audio buffer can only be created after the first frame is decoded.
        if self.buf.is_unused() {
            self.buf = AudioBuffer::new(1152, header.spec());
        }
        else {
            // Ensure the packet contains an audio frame with the same signal specification as the
            // buffer.
            //
            // TODO: Is it worth it to support changing signal specifications?
            if self.buf.spec() != &header.spec() {
                return decode_error("mpa: invalid audio buffer signal spec for packet");
            }
        }

        // Clear the audio buffer.
        self.buf.clear();

        // Choose the decode step based on the MPEG layer and the current codec type.
        match &mut self.state {
            #[cfg(feature = "mp1")]
            State::Layer1(layer) if header.layer == MpegLayer::Layer1 => {
                layer.decode(&mut reader, &header, &mut self.buf)?;
            }
            #[cfg(feature = "mp2")]
            State::Layer2(layer) if header.layer == MpegLayer::Layer2 => {
                layer.decode(&mut reader, &header, &mut self.buf)?;
            }
            #[cfg(feature = "mp3")]
            State::Layer3(layer) if header.layer == MpegLayer::Layer3 => {
                layer.decode(&mut reader, &header, &mut self.buf)?;
            }
            _ => return decode_error("mpa: invalid mpeg audio layer"),
        }

        self.buf.trim(packet.trim_start() as usize, packet.trim_end() as usize);

        Ok(())
    }