hexops
flac: 'encoding wrapper is super buggy and doesn't work yet'
The encoding implementation in mach-flac should be brought up to standards and there should be an example of encoding a flac file.
Hello!
I spent some time looking into this and with some tweaks, was able to encode a working FLAC file.
> file output.flac examples/assets/center.flac
output.flac: FLAC audio bitstream data, 16 bit, stereo, 48 kHz, 576000 samples
examples/assets/center.flac: FLAC audio bitstream data, 16 bit, 3 channels, 48 kHz, 288000 samples
I've outlined my findings below and if you're interested, I can open PRs in hexops/mach-flac and hexops/flac to help integrate these fixes :)
Background
As noted by @alichraghi in their comment above encodeStream
/// encoder is buggy and doesn't work yet (UB?)
The encoder crashes when compiled with Zig safety rules turned on (e.g., ReleaseFast doesn't crash):
> zig build -Doptimize=Debug
...
Illegal instruction at address 0x11fd22c
flac/src/libFLAC/stream_encoder.c:4130:28: 0x11fd22c in find_best_partition_order_ (.../stream_encoder.c)
raw_bits_per_partition+sum,
...
When this crash happens, raw_bits_per_partition is a null pointer and I'm assuming Zig considers pointer arithmetic with a null pointer an illegal instruction. Makes sense!
Through code inspection, raw_bits_per_partition only seems relevant when dealing with something called "escape coding" in FLAC files and it looks like escape code support was dropped in xiph/flac@3b5f471837a2d7ae4ec59770eebbb5ef215e56d7.
A Fix
I patched the C code with the following:
if(!
set_partitioned_rice_(
#ifdef EXACT_RICE_BITS_CALCULATION
residual,
#endif
abs_residual_partition_sums+sum,
- raw_bits_per_partition+sum,
+ do_escape_coding ? raw_bits_per_partition + sum : raw_bits_per_partition,
residual_samples,
predictor_order,
rice_parameter_limit,
rice_parameter_search_dist,
(uint32_t)partition_order,
do_escape_coding,
&private_->partitioned_rice_contents_extra[!best_parameters_index],
&residual_bits
)
)
And now if I run my demo program, it produces a valid file:
> zig build demo -Doptimize=Debug
bad samples: 0
Decoded FLAC: Channels: 2, Sample Rate: 48000, Bits per Sample: 16, Samples: 576000
Re-encoded FLAC successfully.
Bonus Decoder Fix
The decoder was shifting the samples in writeCallback:
const shift_amount: u5 = @intCast(32 - data.bits_per_sample);
if (frame.*.header.channels == 3) {
for (0..frame.*.header.blocksize) |i| {
const center = @divExact(buffer[2][i] << shift_amount, 2);
const left = (buffer[0][i] << shift_amount) + center;
const right = (buffer[1][i] << shift_amount) + center;
This was causing samples values to far exceed the expected value range for 16-bit samples. I removed the shift.
Demo
Here's the demo to decode and encode the file:
const std = @import("std");
const mach_flac = @import("mach-flac");
const flac_file = @embedFile("assets/center.flac");
pub fn main() !void {
const allocator = std.heap.page_allocator;
// Open the original FLAC file
const buffer = std.io.fixedBufferStream(flac_file);
const input_stream: std.io.StreamSource = .{ .const_buffer = buffer };
// Decode the original FLAC file
const decoded_data = try mach_flac.decodeStream(
allocator,
input_stream,
);
defer allocator.free(decoded_data.samples);
// with the decoder shifting samples, many go out of range
var invalid_sample_count: usize = 0;
for (decoded_data.samples) |sample| {
if (sample > 32767 or sample < -32768) {
invalid_sample_count += 1;
}
}
std.debug.print("bad samples: {d}\n", .{invalid_sample_count});
std.debug.print("Decoded FLAC: Channels: {}, Sample Rate: {}, Bits per Sample: {}, Samples: {}\n", .{
decoded_data.channels,
decoded_data.sample_rate,
decoded_data.bits_per_sample,
decoded_data.samples.len,
});
// Re-encode into a new FLAC file
const output_flac = try std.fs.cwd().createFile("output.flac", .{});
defer output_flac.close();
const output_stream: std.io.StreamSource = .{ .file = output_flac };
const aligned_samples = try allocator.alignedAlloc(i32, 32, decoded_data.samples.len);
@memcpy(aligned_samples, decoded_data.samples);
defer allocator.free(aligned_samples);
try mach_flac.encodeStream(
output_stream,
decoded_data.channels,
decoded_data.bits_per_sample,
decoded_data.sample_rate,
aligned_samples,
5,
);
std.debug.print("Re-encoded FLAC successfully.\n", .{});
}
