From ed429d570e7494367cd8ef0135885547c39008a4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sebastian=20Dr=C3=B6ge?= <sebastian@centricular.com>
Date: Thu, 15 Dec 2022 18:45:19 +0200
Subject: [PATCH] mp4mux: Don't write gap edit lists if their duration would be
 zero

The track might start later than the earliest track by less than one
timescale units, in which case writing an empty gap edit list would be
useless and confusing.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1014>
---
 mux/mp4/src/mp4mux/boxes.rs | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/mux/mp4/src/mp4mux/boxes.rs b/mux/mp4/src/mp4mux/boxes.rs
index 219dc75d..4309c9cd 100644
--- a/mux/mp4/src/mp4mux/boxes.rs
+++ b/mux/mp4/src/mp4mux/boxes.rs
@@ -1646,24 +1646,30 @@ fn write_elst(
     let min_earliest_pts = header.streams.iter().map(|s| s.earliest_pts).min().unwrap();
 
     if min_earliest_pts != stream.earliest_pts {
-        // Entry count
-        v.extend(2u32.to_be_bytes());
-
-        // First entry for the gap
-
-        // Edit duration
         let gap = (stream.earliest_pts - min_earliest_pts)
             .nseconds()
             .mul_div_round(timescale as u64, gst::ClockTime::SECOND.nseconds())
             .context("too big gap")?;
-        v.extend(gap.to_be_bytes());
 
-        // Media time
-        v.extend((-1i64).to_be_bytes());
+        if gap > 0 {
+            // Entry count
+            v.extend(2u32.to_be_bytes());
 
-        // Media rate
-        v.extend(1u16.to_be_bytes());
-        v.extend(0u16.to_be_bytes());
+            // First entry for the gap
+
+            // Edit duration
+            v.extend(gap.to_be_bytes());
+
+            // Media time
+            v.extend((-1i64).to_be_bytes());
+
+            // Media rate
+            v.extend(1u16.to_be_bytes());
+            v.extend(0u16.to_be_bytes());
+        } else {
+            // Entry count
+            v.extend(1u32.to_be_bytes());
+        }
     } else {
         // Entry count
         v.extend(1u32.to_be_bytes());