From d04d04801ffc56c01e2ea78e1d3c06260dd390ae Mon Sep 17 00:00:00 2001
From: "J. David Lee" <johnl@crumpington.com>
Date: Sun, 1 Dec 2019 22:12:58 +0100
Subject: [PATCH] flac: initial functionality complete

---
 lib/flac/flac.go      | 105 ++++++++++++++++++++++++++++++++++++++++--
 lib/flac/flac_test.go |  30 ++++++++++++
 2 files changed, 131 insertions(+), 4 deletions(-)

diff --git a/lib/flac/flac.go b/lib/flac/flac.go
index fa6c6dc..f368389 100644
--- a/lib/flac/flac.go
+++ b/lib/flac/flac.go
@@ -3,8 +3,11 @@ package flac
 import (
 	"bytes"
 	"fmt"
+	"io"
+	"math"
 	"os/exec"
 	"strconv"
+	"unsafe"
 )
 
 type Metadata struct {
@@ -12,6 +15,7 @@ type Metadata struct {
 	BPS        int64
 	Channels   int64
 	NumSamples int64 // Total length of file in samples (per channel).
+	MD5Sum     string
 }
 
 func GetMetadata(path string) (md Metadata, err error) {
@@ -21,6 +25,7 @@ func GetMetadata(path string) (md Metadata, err error) {
 		"--show-bps",
 		"--show-channels",
 		"--show-total-samples",
+		"--show-md5sum",
 		path)
 
 	out, err := cmd.Output()
@@ -29,10 +34,10 @@ func GetMetadata(path string) (md Metadata, err error) {
 	}
 
 	l := bytes.Split(out, []byte{'\n'})
-	if len(l) != 5 {
+	if len(l) != 6 {
 		return md, fmt.Errorf("Output not understood:\n%s", out)
 	}
-	l = l[:4]
+	l = l[:5]
 
 	md.SampleRate, err = strconv.ParseInt(string(l[0]), 10, 64)
 	if err != nil {
@@ -53,11 +58,103 @@ func GetMetadata(path string) (md Metadata, err error) {
 	if err != nil {
 		return md, fmt.Errorf("Invalid length: %w", err)
 	}
+	md.MD5Sum = string(l[4])
 
 	return md, nil
 }
 
-func Load(path string, nSamples int64) (L []float32, R []float32, err error) {
-	// TODO!
+// Load nSamples of data from the given file.
+func Load(path string) (L []float32, R []float32, err error) {
+	md, err := GetMetadata(path)
+	if err != nil {
+		return nil, nil, err
+	}
+	if md.BPS != 16 || md.Channels != 2 {
+		return nil, nil, fmt.Errorf("Only 16 bit stereo flac files are supported.")
+	}
+
+	cmd := exec.Command(
+		"flac",
+		"--decode",
+		"--stdout",
+		"--force-raw-format",
+		"--endian=little",
+		"--sign=signed",
+		path)
+	r, err := cmd.StdoutPipe()
+	if err != nil {
+		return nil, nil, fmt.Errorf("Failed to connect to flac process: %w", err)
+	}
+	defer r.Close()
+
+	if err := cmd.Start(); err != nil {
+		return nil, nil, fmt.Errorf("Failed to start flac process: %w", err)
+	}
+
+	// Allocate L and R, and fill.
+	L = make([]float32, md.NumSamples)
+	R = make([]float32, md.NumSamples)
+
+	written := 0
+	buf := make([]byte, 4096)
+	sampleVals := (*[1 << 44]int16)(unsafe.Pointer(&buf[0]))[:2048]
+
+	for {
+		n, err := r.Read(buf)
+		if n != 0 {
+			for i := range sampleVals[:n/4] {
+				L[written] = float32(sampleVals[2*i]) / math.MaxInt16
+				R[written] = float32(sampleVals[2*i+1]) / math.MaxInt16
+				written++
+			}
+		}
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			return L, R, fmt.Errorf("Failed to read from flac process: %w", err)
+		}
+	}
+
 	return
 }
+
+// Save to path, assuming L and R are normalized to [-1,1]
+func Save(path string, L, R []float32) error {
+	cmd := exec.Command(
+		"flac",
+		"--best",
+		"--force",
+		"--no-padding",
+		"--force-raw-format",
+		"--endian=little",
+		"--sign=signed",
+		"--channels=2",
+		"--bps=16",
+		"--sample-rate=48000",
+		"--input-size="+fmt.Sprintf("%d", 4*len(L)), // In bytes.
+		"-",
+		"-o", path)
+	w, err := cmd.StdinPipe()
+	if err != nil {
+		return fmt.Errorf("Failed to connect to flac process: %w", err)
+	}
+	defer w.Close()
+
+	if err := cmd.Start(); err != nil {
+		return fmt.Errorf("Failed to start flac process: %w", err)
+	}
+
+	iBuf := make([]int16, 2*len(L))
+	for i := range L {
+		iBuf[2*i] = int16(math.MaxInt16 * L[i])
+		iBuf[2*i+1] = int16(math.MaxInt16 * R[i])
+	}
+
+	buf := (*[1 << 44]byte)(unsafe.Pointer(&iBuf[0]))[:4*len(L)]
+	if _, err := w.Write(buf); err != nil {
+		return fmt.Errorf("Failed to write flac data: %w", err)
+	}
+
+	return nil
+}
diff --git a/lib/flac/flac_test.go b/lib/flac/flac_test.go
index 4c49dec..61e0e96 100644
--- a/lib/flac/flac_test.go
+++ b/lib/flac/flac_test.go
@@ -2,6 +2,7 @@ package flac
 
 import (
 	"testing"
+	"time"
 )
 
 func TestGetMetadata(t *testing.T) {
@@ -14,8 +15,37 @@ func TestGetMetadata(t *testing.T) {
 		BPS:        16,
 		Channels:   2,
 		NumSamples: 174564,
+		MD5Sum:     "c4f2c7de2f0f952354bfec3a0809c00e",
 	}
 	if md != expected {
 		t.Fatalf("%v != %v", md, expected)
 	}
 }
+
+func TestLoad(t *testing.T) {
+	l, r, err := Load("test_files/audio.flac")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err := Save("test_files/out.flac", l, r); err != nil {
+		t.Fatal(err)
+	}
+
+	// Apparently necessary.
+	time.Sleep(time.Second)
+
+	md1, err := GetMetadata("test_files/audio.flac")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	md2, err := GetMetadata("test_files/out.flac")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if md1 != md2 {
+		t.Fatalf("%v != %v", md2, md1)
+	}
+}