split subtitle service helpers

internal/service/subtitle_convert.go

@@ -0,0 +1,66 @@
+package service
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+// srtToVTT performs the minimal SRT -> WebVTT transformation: prepend
+// "WEBVTT\n\n" and replace ',' with '.' in the timecode separators.
+func srtToVTT(body string) string {
+	body = strings.ReplaceAll(body, "\r\n", "\n")
+	out := strings.Builder{}
+	out.WriteString("WEBVTT\n\n")
+	for _, line := range strings.Split(body, "\n") {
+		if strings.Contains(line, "-->") {
+			line = strings.ReplaceAll(line, ",", ".")
+		}
+		out.WriteString(line)
+		out.WriteByte('\n')
+	}
+	return out.String()
+}
+
+// assToVTT extracts the dialogue lines from an ASS/SSA subtitle. Styling
+// is dropped so the browser <track> element can still display text.
+func assToVTT(body string) string {
+	out := strings.Builder{}
+	out.WriteString("WEBVTT\n\n")
+	for i, line := range strings.Split(body, "\n") {
+		line = strings.TrimSpace(line)
+		if !strings.HasPrefix(line, "Dialogue:") {
+			continue
+		}
+		parts := strings.SplitN(line, ",", 10)
+		if len(parts) < 10 {
+			continue
+		}
+		fmt.Fprintf(&out, "%d\n%s --> %s\n%s\n\n",
+			i,
+			normaliseTimecode(parts[1]),
+			normaliseTimecode(parts[2]),
+			stripASSTags(parts[9]),
+		)
+	}
+	return out.String()
+}
+
+func normaliseTimecode(t string) string {
+	t = strings.TrimSpace(t)
+	parts := strings.Split(t, ":")
+	if len(parts) != 3 {
+		return t
+	}
+	hh := parts[0]
+	if len(hh) == 1 {
+		hh = "0" + hh
+	}
+	return hh + ":" + parts[1] + ":" + strings.ReplaceAll(parts[2], ".", ".")
+}
+
+var assTag = regexp.MustCompile(`\{[^}]*\}`)
+
+func stripASSTags(s string) string {
+	return assTag.ReplaceAllString(s, "")
+}