[kahrendt]

On a big-endian host, Opus audio reaches the listener with every sample's bytes reversed, and an Opus source sends the server byte-swapped samples. libopus reads and writes host-order opus_int16, but the player's on_audio_write() output and the source's write_audio() input are little-endian.

The samples are now swapped after opus_decode() and before opus_encode(). The swap is behind if constexpr (std::endian::native == std::endian::big), so on ESP32 and other little-endian hosts it compiles to nothing.

Fixes #132.

Notes

• New private helper src/pcm_byte_order.h (pcm16_le_to_from_host()), shared by the decoder and the source's Opus encoder. It swaps byte by byte, so it needs no alignment.
• The encoder swaps its pcm_scratch_ in place. SourceTask refills the scratch completely for each chunk, so nothing reads the swapped data afterwards. warm_up() encodes silence and needs no swap.
• Host micro-flac pin: v0.1.1 -> v0.3.0. v0.3.0's decoder output is little-endian on big-endian hosts (write_samples(): optimized paths emit native-endian PCM, so big-endian output depends on buffer alignment esphome-libs/micro-flac#36), which covers FLAC for host consumers such as sendspin-cli. The only API change that touches this code is [[nodiscard]] on decode(), and every call already uses the result.
• idf_component.yml stays at >=0.1.1 until ESPHome moves to micro-flac v0.3.0. ESP targets are little-endian, so they are unaffected either way.
• on_audio_write()'s doc now states the data is interleaved little-endian, matching write_audio().

Testing

• 703/703 host tests pass with ASan against micro-flac v0.3.0.
• No new test: on a little-endian host the swap is a constexpr no-op, so a host test can't catch a regression. A standalone run with the big-endian branch forced on turns the bytes ff d3 from the issue's big-endian harness into -45, the value libopus wrote there.
• Not yet run on a big-endian target; @mguaylam offered to test on MIPS hardware.

[Copilot]

🟡 Changes recommended

The integration guide must also document the updated public listener contract.

1 open finding

• Document interleaved little-endian PCM format in integration guide · New

What changed in this PR

Ensures Opus PCM remains little-endian on big-endian hosts.

Changes:

• Adds shared in-place PCM16 byte-order conversion.
• Applies conversion around Opus decoding and encoding.
• Updates the player contract and micro-flac dependency.

File	Description
src/​pcm_byte_order.h	Adds endian conversion helper.
src/​decoder.cpp	Converts decoded Opus PCM to little-endian.
src/​source_encoder_opus.cpp	Converts source PCM to host order before encoding.
include/​sendspin/​player_role.h	Documents listener PCM byte order.
cmake/​host.cmake	Updates micro-flac to v0.3.0.

🧠 Review effort: Balanced

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