Add rubberBand #102

Sources/FluidInterfaceKit/Helper/Fluid.swift

@@ -166,5 +166,34 @@ public enum Fluid {
       return "<\(values.joined(separator: "; "))>"
     }
   }
+
+  /// https://gist.githubusercontent.com/jverkoey/3955471c6670386c54e1d62d9c965b53/raw/b618a3da1dab454195fd9ad66323cf7f36c93639/rubberBand.swift
+  public static func rubberBand(value: CGFloat, min: CGFloat, max: CGFloat, bandLength: CGFloat) -> CGFloat {
+    if value >= min && value <= max {
+      // While we're within range we don't rubber band the value.
+      return value
+    }
+
+    if bandLength <= 0 {
+      // The rubber band doesn't exist, return the minimum value so that we stay put.
+      return min
+    }
+
+    let rubberBandCoefficient: CGFloat = 0.55
+    // Accepts values from [0...+inf and ensures that f(x) < bandLength for all values.
+    let band: (CGFloat) -> CGFloat = { value in
+      let demoninator = value * rubberBandCoefficient / bandLength + 1
+      return bandLength * (1 - 1 / demoninator)
+    }
+    if (value > max) {
+      return band(value - max) + max;
+
+    } else if (value < min) {
+      return min - band(min - value);
+    }
+
+    return value;
+  }
+
 
 }