From c2ff924431d0645b74971931b520ede285ce02a0 Mon Sep 17 00:00:00 2001
From: Paul Buetow <paul@buetow.org>
Date: Mon, 20 Apr 2026 19:00:56 +0300
Subject: z4: encode Gemini narration PCM to MP3

---
 internal/tts/gemini.go | 87 +++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 79 insertions(+), 8 deletions(-)

(limited to 'internal')

diff --git a/internal/tts/gemini.go b/internal/tts/gemini.go
index 736e1d1..e4cec59 100644
--- a/internal/tts/gemini.go
+++ b/internal/tts/gemini.go
@@ -4,6 +4,9 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
 	"strings"
 
 	"google.golang.org/genai"
@@ -108,14 +111,8 @@ func (g *GeminiProvider) GenerateAudio(ctx context.Context, text, outputFile str
 	if err != nil {
 		return err
 	}
-	if strings.TrimSpace(outputFile) == "" {
-		return fmt.Errorf("output file is required")
-	}
-	if err := os.WriteFile(outputFile, data, 0o644); err != nil {
-		return fmt.Errorf("write audio: %w", err)
-	}
-	if mimeType != "" && !strings.HasPrefix(mimeType, "audio/") {
-		return fmt.Errorf("unexpected audio mime type %q", mimeType)
+	if err := writeAudioFile(data, mimeType, outputFile); err != nil {
+		return err
 	}
 	return nil
 }
@@ -132,6 +129,80 @@ func extractAudio(resp *genai.GenerateContentResponse) ([]byte, string, error) {
 	return nil, "", fmt.Errorf("audio payload missing")
 }
 
+func writeAudioFile(data []byte, mimeType, outputFile string) error {
+	if strings.TrimSpace(outputFile) == "" {
+		return fmt.Errorf("output file is required")
+	}
+	lowerMime := strings.ToLower(strings.TrimSpace(mimeType))
+	switch {
+	case strings.HasPrefix(lowerMime, "audio/mpeg"), strings.HasPrefix(lowerMime, "audio/mp3"):
+		if err := os.WriteFile(outputFile, data, 0o644); err != nil {
+			return fmt.Errorf("write audio: %w", err)
+		}
+		return nil
+	case strings.Contains(lowerMime, "audio/l16"), strings.Contains(lowerMime, "codec=pcm"), lowerMime == "":
+		return encodePCMToMP3(data, lowerMime, outputFile)
+	default:
+		return fmt.Errorf("unsupported audio mime type %q", mimeType)
+	}
+}
+
+func encodePCMToMP3(data []byte, mimeType, outputFile string) error {
+	ffmpegPath, err := exec.LookPath("ffmpeg")
+	if err != nil {
+		return fmt.Errorf("ffmpeg not found for audio conversion: %w", err)
+	}
+
+	rate := 24000
+	if idx := strings.Index(mimeType, "rate="); idx >= 0 {
+		value := mimeType[idx+len("rate="):]
+		for i, r := range value {
+			if r < '0' || r > '9' {
+				value = value[:i]
+				break
+			}
+		}
+		if parsed, parseErr := strconv.Atoi(value); parseErr == nil && parsed > 0 {
+			rate = parsed
+		}
+	}
+
+	tmpDir := filepath.Dir(outputFile)
+	rawFile, err := os.CreateTemp(tmpDir, "comicforge-tts-*.pcm")
+	if err != nil {
+		return fmt.Errorf("create temporary audio file: %w", err)
+	}
+	rawPath := rawFile.Name()
+	if _, err := rawFile.Write(data); err != nil {
+		_ = rawFile.Close()
+		_ = os.Remove(rawPath)
+		return fmt.Errorf("write temporary audio file: %w", err)
+	}
+	if err := rawFile.Close(); err != nil {
+		_ = os.Remove(rawPath)
+		return fmt.Errorf("close temporary audio file: %w", err)
+	}
+	defer func() {
+		_ = os.Remove(rawPath)
+	}()
+
+	cmd := exec.Command(ffmpegPath,
+		"-nostdin", "-hide_banner", "-loglevel", "error", "-y",
+		"-f", "s16le",
+		"-ar", fmt.Sprintf("%d", rate),
+		"-ac", "1",
+		"-i", rawPath,
+		"-codec:a", "libmp3lame",
+		"-q:a", "2",
+		outputFile,
+	)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("convert PCM audio to mp3: %w\n%s", err, strings.TrimSpace(string(out)))
+	}
+	return nil
+}
+
 func defaultOr(value, fallback string) string {
 	if strings.TrimSpace(value) != "" {
 		return value
-- 
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