Add a gain to decibel conversion function

nih_plug/src/util.rs

@@ -16,7 +16,7 @@
 
 pub const MINUS_INFINITY_DB: f32 = -100.0;
 
-/// Convert decibels to gain, treating anything below -100 dB as minus infinity.
+/// Convert decibels to a voltage gain ratio, treating anything below -100 dB as minus infinity.
 pub fn db_to_gain(dbs: f32) -> f32 {
     if dbs > MINUS_INFINITY_DB {
         10.0f32.powf(dbs * 0.05)
@@ -25,6 +25,16 @@ pub fn db_to_gain(dbs: f32) -> f32 {
     }
 }
 
+/// Convert a voltage gain ratio to decibels. Gain ratios that aren't positive will be treated as
+/// [MINUS_INFINITY_DB].
+pub fn gain_to_db(gain: f32) -> f32 {
+    if gain > 0.0 {
+        gain.log10() * 20.0
+    } else {
+        MINUS_INFINITY_DB
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -43,4 +53,24 @@ mod tests {
     fn test_db_to_gain_minus_infinity() {
         assert_eq!(db_to_gain(-100.0), 0.0);
     }
+
+    #[test]
+    fn test_gain_to_db_positive() {
+        assert_eq!(gain_to_db(4.0), 12.041201);
+    }
+
+    #[test]
+    fn test_gain_to_db_negative() {
+        assert_eq!(gain_to_db(0.25), -12.041201);
+    }
+
+    #[test]
+    fn test_gain_to_db_minus_infinity_zero() {
+        assert_eq!(gain_to_db(0.0), MINUS_INFINITY_DB);
+    }
+
+    #[test]
+    fn test_gain_to_db_minus_infinity_negative() {
+        assert_eq!(gain_to_db(-2.0), MINUS_INFINITY_DB);
+    }
 }