From 1b811cb18d47a463fdcd35216ab1cb3630d5df73 Mon Sep 17 00:00:00 2001
From: Oliver Booth <me@olivr.me>
Date: Wed, 10 Mar 2021 12:17:23 +0000
Subject: [PATCH] (#38) Add Single/Double.LerpFrom/To/With

---
 X10D/src/DoubleExtensions/DoubleExtensions.cs | 53 +++++++++++++++++++
 X10D/src/SingleExtensions/SingleExtensions.cs | 45 ++++++++++++++++
 2 files changed, 98 insertions(+)

diff --git a/X10D/src/DoubleExtensions/DoubleExtensions.cs b/X10D/src/DoubleExtensions/DoubleExtensions.cs
index 94bcfa3..c859e58 100644
--- a/X10D/src/DoubleExtensions/DoubleExtensions.cs
+++ b/X10D/src/DoubleExtensions/DoubleExtensions.cs
@@ -53,6 +53,51 @@ namespace X10D.DoubleExtensions
             return !number.IsEven();
         }
 
+        /// <summary>
+        ///     Linearly interpolates to the current value from a specified source using a specified alpha.
+        /// </summary>
+        /// <param name="target">The interpolation target.</param>
+        /// <param name="value">The interpolation source.</param>
+        /// <param name="alpha">The interpolation alpha.</param>
+        /// <returns>
+        ///     The interpolation result as determined by <c>(1 - <paramref name="alpha" />) * <paramref name="value" /> +
+        ///     <paramref name="alpha" /> * <paramref name="target" /></c>.
+        /// </returns>
+        public static double LerpFrom(this double target, double value, double alpha)
+        {
+            return LerpInternal(value, target, alpha);
+        }
+
+        /// <summary>
+        ///     Linearly interpolates from the current value to a specified target using a specified alpha.
+        /// </summary>
+        /// <param name="value">The interpolation source.</param>
+        /// <param name="target">The interpolation target.</param>
+        /// <param name="alpha">The interpolation alpha.</param>
+        /// <returns>
+        ///     The interpolation result as determined by <c>(1 - <paramref name="alpha" />) * <paramref name="value" /> +
+        ///     <paramref name="alpha" /> * <paramref name="target" /></c>.
+        /// </returns>
+        public static double LerpTo(this double value, double target, double alpha)
+        {
+            return LerpInternal(value, target, alpha);
+        }
+
+        /// <summary>
+        ///     Linearly interpolates to a specified target from a specified source, using the current value as the alpha value.
+        /// </summary>
+        /// <param name="alpha">The interpolation alpha.</param>
+        /// <param name="value">The interpolation source.</param>
+        /// <param name="target">The interpolation target.</param>
+        /// <returns>
+        ///     The interpolation result as determined by <c>(1 - <paramref name="alpha" />) * <paramref name="value" /> +
+        ///     <paramref name="alpha" /> * <paramref name="target" /></c>.
+        /// </returns>
+        public static double LerpWith(this double alpha, double value, double target)
+        {
+            return LerpInternal(value, target, alpha);
+        }
+
         /// <summary>
         ///     Converts an angle from radians to degrees.
         /// </summary>
@@ -73,5 +118,13 @@ namespace X10D.DoubleExtensions
         {
             return Math.Round(v / nearest) * nearest;
         }
+
+        private static double LerpInternal(double a, double b, double t)
+        {
+            // rookie mistake: a + t * (b - a)
+            
+            // "precise" method: (1 - t) * a + t * b
+            return (1 - t) * a + t * b;
+        }
     }
 }
diff --git a/X10D/src/SingleExtensions/SingleExtensions.cs b/X10D/src/SingleExtensions/SingleExtensions.cs
index 89be04f..45d092d 100644
--- a/X10D/src/SingleExtensions/SingleExtensions.cs
+++ b/X10D/src/SingleExtensions/SingleExtensions.cs
@@ -54,6 +54,51 @@ namespace X10D.SingleExtensions
             return !number.IsEven();
         }
 
+        /// <summary>
+        ///     Linearly interpolates to the current value from a specified source using a specified alpha.
+        /// </summary>
+        /// <param name="target">The interpolation target.</param>
+        /// <param name="value">The interpolation source.</param>
+        /// <param name="alpha">The interpolation alpha.</param>
+        /// <returns>
+        ///     The interpolation result as determined by <c>(1 - <paramref name="alpha" />) * <paramref name="value" /> +
+        ///     <paramref name="alpha" /> * <paramref name="target" /></c>.
+        /// </returns>
+        public static float LerpFrom(this float target, float value, float alpha)
+        {
+            return (float)((double)target).LerpFrom(value, alpha);
+        }
+
+        /// <summary>
+        ///     Linearly interpolates from the current value to a specified target using a specified alpha.
+        /// </summary>
+        /// <param name="value">The interpolation source.</param>
+        /// <param name="target">The interpolation target.</param>
+        /// <param name="alpha">The interpolation alpha.</param>
+        /// <returns>
+        ///     The interpolation result as determined by <c>(1 - <paramref name="alpha" />) * <paramref name="value" /> +
+        ///     <paramref name="alpha" /> * <paramref name="target" /></c>.
+        /// </returns>
+        public static float LerpTo(this float value, float target, float alpha)
+        {
+            return (float)((double)value).LerpTo(target, alpha);
+        }
+
+        /// <summary>
+        ///     Linearly interpolates to a specified target from a specified source, using the current value as the alpha value.
+        /// </summary>
+        /// <param name="alpha">The interpolation alpha.</param>
+        /// <param name="value">The interpolation source.</param>
+        /// <param name="target">The interpolation target.</param>
+        /// <returns>
+        ///     The interpolation result as determined by <c>(1 - <paramref name="alpha" />) * <paramref name="value" /> +
+        ///     <paramref name="alpha" /> * <paramref name="target" /></c>.
+        /// </returns>
+        public static float LerpWith(this float alpha, float value, float target)
+        {
+            return (float)((double)alpha).LerpWith(value, target);
+        }
+
         /// <summary>
         ///     Converts an angle from radians to degrees.
         /// </summary>