1 files changed, 81 insertions, 0 deletions
diff --git a/third_party/rust/audio-mixer/src/lib.rs b/third_party/rust/audio-mixer/src/lib.rs
new file mode 100644
index 0000000000..10bf54603e
--- /dev/null
+++ b/third_party/rust/audio-mixer/src/lib.rs
@@ -0,0 +1,81 @@
+#[macro_use]
+extern crate bitflags;
+
+mod channel;
+mod coefficient;
+
+// Export Channel outside.
+pub use channel::Channel;
+use coefficient::{Coefficient, MixingCoefficient};
+
+use std::default::Default;
+use std::fmt::Debug;
+use std::ops::{AddAssign, Mul};
+
+// A mixer mixing M-channel input data to N-channel output data.
+// T::Coef is an associated type defined in MixingCoefficient, which indicates the type of the
+// mixing coefficient that would be used for type T. When T is f32, the T::Coef is f32. When T
+// is i16, the T::Coef is i32. When mixing data, a temporary variable with type T::Coef would be
+// created to hold the mixing result. Since the type of input and output audio data is T,
+// the methods provided from MixingCoefficient trait would be used to convert the value between
+// type T and T::Coef.
+#[derive(Debug)]
+pub struct Mixer<T>
+where
+    T: Copy + Debug + MixingCoefficient,
+    T::Coef: AddAssign + Copy + Debug + Default + Mul<T::Coef, Output = T::Coef>,
+{
+    coefficient: Coefficient<T>,
+}
+
+impl<T> Mixer<T>
+where
+    T: Copy + Debug + MixingCoefficient,
+    T::Coef: AddAssign + Copy + Debug + Default + Mul<T::Coef, Output = T::Coef>,
+{
+    pub fn new(input_channels: &[Channel], output_channels: &[Channel]) -> Self {
+        Self {
+            coefficient: Coefficient::create(input_channels, output_channels),
+        }
+    }
+
+    // To mix M-channel audio input data to N-channel output data, the data in output-channel i
+    // is the sum of product of data in input-channel j and the coefficient for mixing from
+    // input-channel j to output-channel i, for all j in M channels. That is,
+    // output_data(i) = Σ coefficient(j, i) * input_data(j), for all j in [0, M),
+    // where i is in [0, N) and coefficient is a function returning mixing coefficient from
+    // input channel j to output channel i.
+    pub fn mix(&self, input_buffer: &[T], output_buffer: &mut [T]) {
+        assert_eq!(
+            input_buffer.len(),
+            self.input_channels().len(),
+            "input slice must have the same size as the input channel's one."
+        );
+        assert_eq!(
+            output_buffer.len(),
+            self.output_channels().len(),
+            "output slice must have the same size as the output channel's one."
+        );
+        for (i, output) in output_buffer.iter_mut().enumerate() {
+            // T must implement Default that returns a zero value from default().
+            let mut value = T::Coef::default(); // Create a zero value.
+            for (j, input) in input_buffer.iter().enumerate() {
+                // T::Coef needs to implement `AddAssign` and `Mul` to make `+=` and `*` work.
+                // T needs to implement `Copy` so `*input` can be copied.
+                value += self.coefficient.get(j, i) * T::to_coefficient_value(*input);
+            }
+            *output = T::from_coefficient_value(
+                value,
+                self.coefficient.would_overflow_from_coefficient_value(),
+            );
+        }
+    }
+
+    pub fn input_channels(&self) -> &[Channel] {
+        &self.coefficient.input_channels()
+    }
+
+    pub fn output_channels(&self) -> &[Channel] {
+        &self.coefficient.output_channels()
+    }
+}