From 1dd4c9fb83cbe7a3ea927220e13f7456d99fb8da Mon Sep 17 00:00:00 2001
From: Gwilym Inzani <email@gwilym.dev>
Date: Sun, 16 Jul 2023 22:57:12 +0100
Subject: [PATCH] Add a concept of a restart point

---
 agb/src/sound/mixer/mod.rs      | 17 +++++++++++++++++
 agb/src/sound/mixer/sw_mixer.rs |  6 +++---
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/agb/src/sound/mixer/mod.rs b/agb/src/sound/mixer/mod.rs
index 0bff6c50..1a7fe1c4 100644
--- a/agb/src/sound/mixer/mod.rs
+++ b/agb/src/sound/mixer/mod.rs
@@ -226,6 +226,7 @@ pub struct SoundChannel {
     data: &'static [u8],
     pos: Num<u32, 8>,
     should_loop: bool,
+    restart_point: Num<u32, 8>,
 
     playback_speed: Num<u32, 8>,
     volume: Num<i16, 4>, // between 0 and 1
@@ -276,6 +277,7 @@ impl SoundChannel {
             priority: SoundPriority::Low,
             volume: 1.into(),
             is_stereo: false,
+            restart_point: 0.into(),
         }
     }
 
@@ -319,6 +321,7 @@ impl SoundChannel {
             priority: SoundPriority::High,
             volume: 1.into(),
             is_stereo: false,
+            restart_point: 0.into(),
         }
     }
 
@@ -330,6 +333,20 @@ impl SoundChannel {
         self
     }
 
+    /// Sets the point at which the sample should restart once it loops. Does nothing
+    /// unless you also call [`should_loop()`].
+    ///
+    /// Useful if your song has an introduction or similar.
+    #[inline(always)]
+    pub fn restart_point(&mut self, restart_point: impl Into<Num<u32, 8>>) -> &mut Self {
+        self.restart_point = restart_point.into();
+        assert!(
+            self.restart_point.floor() as usize <= self.data.len(),
+            "restart point must be shorter than the length of the sample"
+        );
+        self
+    }
+
     /// Sets the speed at which this should channel should be played. Defaults
     /// to 1 with values between 0 and 1 being slower above 1 being faster.
     ///
diff --git a/agb/src/sound/mixer/sw_mixer.rs b/agb/src/sound/mixer/sw_mixer.rs
index 6a14b5b3..55617aac 100644
--- a/agb/src/sound/mixer/sw_mixer.rs
+++ b/agb/src/sound/mixer/sw_mixer.rs
@@ -441,7 +441,7 @@ impl MixerBuffer {
             >= channel.data.len() as u32
         {
             if channel.should_loop {
-                channel.pos = 0.into();
+                channel.pos = channel.restart_point * 2;
             } else {
                 channel.is_done = true;
                 return;
@@ -466,7 +466,7 @@ impl MixerBuffer {
         let channel_len = Num::<u32, 8>::new(channel.data.len() as u32);
         let mut playback_speed = channel.playback_speed;
 
-        while playback_speed >= channel_len {
+        while playback_speed >= channel_len - channel.restart_point {
             playback_speed -= channel_len;
         }
 
@@ -484,7 +484,7 @@ impl MixerBuffer {
         for i in 0..self.frequency.buffer_size() {
             if channel.pos >= channel_len {
                 if channel.should_loop {
-                    channel.pos -= channel_len;
+                    channel.pos -= channel_len + channel.restart_point;
                 } else {
                     channel.is_done = true;
                     break;