math: add exp_decay to interpolation.v (#24545)

vlib/math/interpolation.v

@@ -11,6 +11,16 @@ pub fn mix[T](start T, end T, t T) T {
 	return start * (1 - t) + end * t
 }
 
+// exp_decay returns a frame independent exponential decay value between `a` and `b` using `delta_time_seconds`.
+// `decay` is supposed to be useful in the range 1.0 to 25.0. From slow to fast.
+// The function is a frame rate independent (approximation) of the well-known `lerp` or `mix` (linear interpolation) function.
+// It is ported to V from the pseudo code shown towards the end of the video https://youtu.be/LSNQuFEDOyQ?t=2977
+// NOTE: Thanks to Freya Holmér for the function and the work done in this field.
+@[inline]
+pub fn exp_decay[T](a T, b T, decay f64, delta_time_seconds f64) T {
+	return T(f64(b) + (f64(a) - f64(b)) * exp(-decay * delta_time_seconds))
+}
+
 // clip constrain the given value `x`, to lie between two further values `min_value` and `max_value`.
 // See: https://registry.khronos.org/OpenGL-Refpages/gl4/html/clamp.xhtml
 // Also: https://en.wikipedia.org/wiki/Clamp_(function)

vlib/math/interpolation_test.v

@@ -1,5 +1,7 @@
 import math
 
+const decay = 100
+
 fn test_mix() {
 	assert math.mix(0.0, 100.0, 0.0) == 0.0
 	assert math.mix(0.0, 100.0, 0.1) == 10.0
@@ -18,6 +20,20 @@ fn test_mix() {
 	assert math.mix(100.0, 500.0, 1.0) == 500.0
 }
 
+fn test_exp_decay() {
+	assert math.exp_decay(0.0, 100.0, decay, 0.0) == 0.0
+	assert math.exp_decay(0.0, 100.0, decay, 1.0) == 100.0
+
+	assert math.exp_decay(100.0, 500.0, decay, 0.0) == 100.0
+	assert math.exp_decay(100.0, 500.0, decay, 1.0) == 500.0
+
+	assert math.exp_decay(0, 100, decay, 0.0) == 0
+	assert math.exp_decay(0, 100, decay, 1.0) == 100
+
+	assert math.exp_decay(100, 500, decay, 0.0) == 100
+	assert math.exp_decay(100, 500, decay, 1.0) == 500
+}
+
 fn test_clip() {
 	assert math.clip(0.0, 10.0, 50.0) == 10.0
 	assert math.clip(5.5, 10.0, 50.0) == 10.0