From 0be617cd52a539c98f5183bda419e7309567b1ea Mon Sep 17 00:00:00 2001
From: Martin Algesten <martin@algesten.se>
Date: Fri, 10 Jan 2025 11:27:44 +0100
Subject: [PATCH] Set up simulcast send SSRC

---
 src/change/sdp.rs | 90 ++++++++++++++++++++++++++++++++++++++++-------
 src/sdp/mod.rs    |  5 +++
 2 files changed, 82 insertions(+), 13 deletions(-)

diff --git a/src/change/sdp.rs b/src/change/sdp.rs
index a3124485..ea8e83c2 100644
--- a/src/change/sdp.rs
+++ b/src/change/sdp.rs
@@ -269,17 +269,10 @@ impl<'a> SdpApi<'a> {
 
         let mut ssrcs = Vec::new();
 
-        let ssrc_group_count = if let Some(simulcast) = &simulcast {
-            if simulcast.force_ssrc_groups && dir.is_sending() {
-                simulcast.send.len()
-            } else {
-                0usize
-            }
-        } else {
-            1usize
-        };
+        // Main SSRC, not counting RTX.
+        let main_ssrc_count = simulcast.as_ref().map(|s| s.send.len()).unwrap_or(1);
 
-        for _ in 0..ssrc_group_count {
+        for _ in 0..main_ssrc_count {
             let rtx = kind.is_video().then(|| self.rtc.session.streams.new_ssrc());
             ssrcs.push((self.rtc.session.streams.new_ssrc(), rtx));
         }
@@ -1309,9 +1302,8 @@ impl AsSdpMediaLine for Media {
         }
 
         let count = ssrcs_tx.len();
-        #[allow(clippy::comparison_chain)]
-        for _ in 0..count {
-            let (ssrc, ssrc_rtx) = &ssrcs_tx[0];
+        for i in 0..count {
+            let (ssrc, ssrc_rtx) = &ssrcs_tx[i];
             if let Some(ssrc_rtx) = ssrc_rtx {
                 attrs.push(MediaAttribute::SsrcGroup {
                     semantics: "FID".to_string(),
@@ -1564,7 +1556,10 @@ impl Change {
 
 #[cfg(test)]
 mod test {
+    use sdp::RestrictionId;
+
     use crate::format::Codec;
+    use crate::media::Simulcast;
     use crate::sdp::RtpMap;
 
     use super::*;
@@ -1667,4 +1662,73 @@ mod test {
             "VP9 was not offered, so it should not be present in the answer"
         );
     }
+
+    #[test]
+    fn simulcast_ssrc_allocation() {
+        crate::init_crypto_default();
+
+        let mut rtc1 = Rtc::new();
+
+        let mut change = rtc1.sdp_api();
+        change.add_media(
+            MediaKind::Video,
+            Direction::SendOnly,
+            None,
+            None,
+            Some(Simulcast {
+                send: vec!["m".into(), "h".into(), "l".into()],
+                recv: vec![],
+            }),
+        );
+
+        let Change::AddMedia(am) = &change.changes[0] else {
+            panic!("Not AddMedia?!");
+        };
+
+        // these should be organized in order: m, h, l
+        let pending_ssrcs = am.ssrcs.clone();
+        assert_eq!(pending_ssrcs.len(), 3);
+
+        for p in &pending_ssrcs {
+            assert!(p.1.is_some()); // all should have rtx
+        }
+
+        let (offer, _) = change.apply().unwrap();
+        let sdp = offer.into_inner();
+        let line = &sdp.media_lines[0];
+
+        assert_eq!(
+            line.simulcast().unwrap().send,
+            SimulcastGroups(vec![
+                RestrictionId("m".into(), true),
+                RestrictionId("h".into(), true),
+                RestrictionId("l".into(), true),
+            ])
+        );
+
+        // Each SSRC, both regular and RTX get their own a=ssrc line.
+        assert_eq!(line.ssrc_info().len(), pending_ssrcs.len() * 2);
+
+        let fids: Vec<_> = line
+            .attrs
+            .iter()
+            .filter_map(|a| {
+                if let MediaAttribute::SsrcGroup { semantics, ssrcs } = a {
+                    // We don't have any other semantics right now.
+                    assert_eq!(semantics, "FID");
+                    assert_eq!(ssrcs.len(), 2);
+                    Some((ssrcs[0], ssrcs[1]))
+                } else {
+                    None
+                }
+            })
+            .collect();
+
+        assert_eq!(fids.len(), pending_ssrcs.len());
+
+        for (a, b) in fids.iter().zip(pending_ssrcs.iter()) {
+            assert_eq!(a.0, b.0);
+            assert_eq!(Some(a.1), b.1);
+        }
+    }
 }
diff --git a/src/sdp/mod.rs b/src/sdp/mod.rs
index 3c4d5a14..918c3655 100644
--- a/src/sdp/mod.rs
+++ b/src/sdp/mod.rs
@@ -42,6 +42,11 @@ impl SdpOffer {
     pub fn to_sdp_string(&self) -> String {
         self.0.to_string()
     }
+
+    #[cfg(test)]
+    pub(crate) fn into_inner(self) -> Sdp {
+        self.0
+    }
 }
 
 #[derive(Debug, PartialEq, Eq)]