From 99209654e385415485c1008f6f93b7f0a2edca5a Mon Sep 17 00:00:00 2001
From: Dylan De Faoite <dylanseandefaoite@gmail.com>
Date: Sun, 26 Jul 2026 14:36:57 +0100
Subject: [PATCH] re-encode first segment

---
 .../services/media/StreamActionsService.java  | 71 ++++++++++++++-----
 1 file changed, 55 insertions(+), 16 deletions(-)

diff --git a/src/main/java/com/ddf/vodsystem/services/media/StreamActionsService.java b/src/main/java/com/ddf/vodsystem/services/media/StreamActionsService.java
index 2c81c6a..aeb8339 100644
--- a/src/main/java/com/ddf/vodsystem/services/media/StreamActionsService.java
+++ b/src/main/java/com/ddf/vodsystem/services/media/StreamActionsService.java
@@ -64,32 +64,31 @@ public class StreamActionsService {
             float duration,
             Path outputFile,
             ProgressTracker progressTracker
-    ) throws IOException {
+    ) {
+
         if (segments.isEmpty()) {
             return CompletableFuture.failedFuture(new IllegalArgumentException("Segment list cannot be empty"));
         }
 
-        Path firstSegment = segments.getFirst();
-        Path trimmedFirst = Files.createTempFile("trimmed-first-", ".ts");
-        try {
-            // First segment must be trimmed manually due to keyframe issues with the concat protocol
-            List<String> trimCommand = List.of(
-                    "ffmpeg", "-y",
-                    "-ss", String.valueOf(trimOffset),
-                    "-i", firstSegment.toAbsolutePath().toString(),
-                    "-c", "copy",
-                    trimmedFirst.toAbsolutePath().toString()
-            );
-            commandRunner.run(trimCommand);
+        if (trimOffset < 0 || duration <= 0) {
+            return CompletableFuture.failedFuture(new IllegalArgumentException(
+                    "trimOffset must be >= 0 and duration must be > 0 (got " + trimOffset + ", " + duration + ")"));
+        }
 
-            String concatInput = Stream.concat(Stream.of(trimmedFirst), segments.stream().skip(1))
+        Path headSegment = null;
+        try {
+            // Unique per invocation so concurrent saves by different users never
+            // collide on the same head fragment; cleaned up in the finally block.
+            headSegment = Files.createTempFile("reencode-section-", ".ts");
+            encodeHead(segments.getFirst(), trimOffset, headSegment);
+
+            String concatInput = Stream.concat(Stream.of(headSegment), segments.stream().skip(1))
                     .map(p -> p.toAbsolutePath().toString())
                     .collect(Collectors.joining("|"));
 
             List<String> command = List.of(
                     "ffmpeg",
                     "-y",
-                    "-ss", String.valueOf(trimOffset),
                     "-i", "concat:" + concatInput,
                     "-t", String.valueOf(duration),
                     "-c", "copy",
@@ -113,7 +112,13 @@ public class StreamActionsService {
             logger.error("Failed to save section to '{}': {}", outputFile, e.toString());
             return CompletableFuture.failedFuture(e);
         } finally {
-            Files.deleteIfExists(trimmedFirst);
+            if (headSegment != null) {
+                try {
+                    Files.deleteIfExists(headSegment);
+                } catch (IOException e) {
+                    logger.warn("Failed to delete temp head fragment '{}': {}", headSegment, e.toString());
+                }
+            }
         }
     }
 
@@ -162,4 +167,38 @@ public class StreamActionsService {
         // nginx hls_fragment_naming system uses seconds; convert to ms
         return value < 1_000_000_000_000L ? value * 1000L : value;
     }
+
+    /**
+     * Re-encodes {@code firstSegment} from {@code trimOffset} to its end,
+     * writing an MPEG-TS fragment that can be stream-copy concatenated with
+     * the untouched fragments that follow it.
+     */
+    private void encodeHead(Path firstSegment, float trimOffset, Path head)
+            throws IOException, InterruptedException {
+
+        List<String> command = List.of(
+                "ffmpeg",
+                "-y",
+                "-i", firstSegment.toAbsolutePath().toString(),
+                "-ss", String.valueOf(trimOffset),
+                "-c:v", "libx264",
+                "-preset", "veryfast",
+                "-crf", "20",
+                "-pix_fmt", "yuv420p",
+                "-c:a", "aac",
+                "-b:a", "160k",
+                "-muxdelay", "0",
+                "-muxpreload", "0",
+                "-f", "mpegts",
+                head.toAbsolutePath().toString()
+        );
+
+        logger.debug("Re-encoding head fragment '{}' from {}s", firstSegment, trimOffset);
+        commandRunner.run(command, line -> { /* head pass is short; not worth reporting */ });
+
+        if (!Files.exists(head) || Files.size(head) == 0) {
+            throw new IOException("Head re-encode produced no output — trimOffset " + trimOffset
+                    + "s may exceed the duration of " + firstSegment);
+        }
+    }
 }