Implemented color functions from my Demeton project (#770)

TODO.md

@@ -1,8 +1,9 @@
 # TODO
 
 - https://github.com/sledilnik/website/issues/770
-    - tooltips
     - try using different colors
+        - using color mixing function
+    - tooltips
 
 - new OWID export
     - return back the official URL, once it has been pushed to production

src/visualizations/App.fsproj

@@ -9,6 +9,7 @@
         <Compile Include="IntersectionObserver.fs" />
         <Compile Include="ShareButton.fs" />
         <Compile Include="Highcharts.fs" />
+        <Compile Include="Colors.fs" />
         <Compile Include="Utils.fs" />
         <Compile Include="Days.fs" />
         <Compile Include="Statistics.fs" />

src/visualizations/Colors.fs

@@ -0,0 +1,37 @@
+module Colors
+
+type RgbaColor = uint32
+
+let inline r(color: RgbaColor) = byte (color >>> 24)
+let inline g(color: RgbaColor) = byte (color >>> 16)
+let inline b(color: RgbaColor) = byte (color >>> 8)
+let inline a(color: RgbaColor) = byte color
+
+/// <summary>
+/// Constructs the <see cref="RgbaColor" /> from R, G, B and alpha
+/// components.
+/// </summary>
+let inline rgbaColor (r: byte) (g: byte) (b: byte) (a: byte): RgbaColor =
+    (uint32 r <<< 24) ||| (uint32 g <<< 16) ||| (uint32 b <<< 8) ||| (uint32 a)
+
+let inline mixColors colorA colorB mixRatio =
+    let mixByteValues (v1:byte) (v2: byte): byte =
+        let v1Float = float v1
+        let mixedFloat = (float v2 - v1Float) * mixRatio + v1Float
+        byte mixedFloat
+
+    match mixRatio with
+    | 0. -> colorA
+    | 1. -> colorB
+    | _ ->
+        rgbaColor
+            (mixByteValues (r colorA) (r colorB))
+            (mixByteValues (g colorA) (g colorB))
+            (mixByteValues (b colorA) (b colorB))
+            (mixByteValues (a colorA) (a colorB))
+
+let inline toHex(color: RgbaColor): string =
+    let alpha = a color
+    match alpha with
+    | 0xffuy -> sprintf "#%02x%02x%02x" (r color) (g color) (b color)
+    | _ -> sprintf "#%02x%02x%02x%02x" alpha (r color) (g color) (b color)