fix edge case where first segment is longer than offset

src/main/java/com/ddf/vodsystem/services/media/StreamActionsService.java

@@ -15,6 +15,7 @@ import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.List;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
@@ -31,9 +32,11 @@ import java.util.stream.Stream;
 public class StreamActionsService {
     private static final Logger logger = LoggerFactory.getLogger(StreamActionsService.class);
     private final CommandRunner commandRunner;
+    private final MetadataService metadataService;
 
-    public StreamActionsService(CommandRunner commandRunner) {
+    public StreamActionsService(CommandRunner commandRunner, MetadataService metadataService) {
         this.commandRunner = commandRunner;
+        this.metadataService = metadataService;
     }
 
     /**
@@ -141,6 +144,7 @@ public class StreamActionsService {
 
             for (int i = 0; i < streamFiles.size(); i++) {
                 Path streamFile = streamFiles.get(i);
+
                 long segmentStart = parseTimestampMs(streamFile);
                 long segmentEnd = (i + 1 < streamFiles.size())
                         ? parseTimestampMs(streamFiles.get(i + 1))
@@ -155,6 +159,59 @@ public class StreamActionsService {
         return results;
     }
 
+    /**
+     * The segments backing a save and the offset into the first one at which
+     * the requested section begins.
+     *
+     * @param segments   the ordered segments to concatenate, in playback order
+     * @param trimOffset the offset, in seconds, into {@code segments.getFirst()}
+     *                   at which the requested section starts
+     */
+    public record SegmentSelection(List<Path> segments, float trimOffset) {}
+
+    /**
+     * Selects the recorded {@code .ts} segments overlapping {@code [startTime, endTime)}
+     * and computes the trim offset into the first segment.
+     * <p>
+     * If {@code startTime} falls in a stream gap after the earliest overlapping segment
+     * (so that segment holds no footage for the requested start), the segment is dropped
+     * and the selection begins at the first genuinely-recorded footage instead.
+     *
+     * @throws IllegalArgumentException if no segments hold footage for the requested range
+     * @throws IOException              if listing the stream directory or probing a segment fails
+     */
+    public SegmentSelection selectSegments(Path streamDirectory, Instant startTime, Instant endTime) throws IOException {
+        List<Path> segments = getSegmentsInRange(streamDirectory, startTime, endTime);
+        if (segments.isEmpty()) {
+            throw new IllegalArgumentException("No stream segments found in the given time range");
+        }
+
+        long firstMs = parseTimestampMs(segments.getFirst());
+        float trimOffset = Math.max(0f, (startTime.toEpochMilli() - firstMs) / 1000f);
+
+        // if the first segment is longer than the trim-offset, then ggs
+        if (trimOffset > 0 && probeDurationSeconds(segments.getFirst()) < trimOffset) {
+            segments.removeFirst();
+            if (segments.isEmpty()) {
+                throw new IllegalArgumentException("No recorded footage at the requested start time");
+            }
+            trimOffset = 0f;
+        }
+
+        return new SegmentSelection(segments, trimOffset);
+    }
+
+    private float probeDurationSeconds(Path segment) throws IOException {
+        try {
+            return metadataService.getVideoMetadata(segment).get().getDuration();
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+            throw new IOException("Interrupted while probing segment duration: " + segment, e);
+        } catch (ExecutionException e) {
+            throw new IOException("Failed to probe segment duration: " + segment, e);
+        }
+    }
+
     public long parseTimestampMs(Path path) {
         String fullName = path.getFileName().toString();