use an enum instead of ushort for IccFormulaCurveElement.Type

src/ImageSharp/MetaData/Profiles/ICC/Curves/IccFormulaCurveElement.cs

@@ -8,18 +8,16 @@
         /// <summary>
         /// Initializes a new instance of the <see cref="IccFormulaCurveElement"/> class.
         /// </summary>
-        /// <param name="type">The type of this segment (0-2)</param>
+        /// <param name="type">The type of this segment</param>
         /// <param name="gamma">Gamma segment parameter</param>
         /// <param name="a">A segment parameter</param>
         /// <param name="b">B segment parameter</param>
         /// <param name="c">C segment parameter</param>
         /// <param name="d">D segment parameter</param>
         /// <param name="e">E segment parameter</param>
-        public IccFormulaCurveElement(ushort type, double gamma, double a, double b, double c, double d, double e)
+        public IccFormulaCurveElement(IccFormulaCurveType type, double gamma, double a, double b, double c, double d, double e)
             : base(IccCurveSegmentSignature.FormulaCurve)
         {
-            Guard.MustBeBetweenOrEqualTo(type, 0, 2, nameof(type));
-
             this.Type = type;
             this.Gamma = gamma;
             this.A = a;
@@ -32,7 +30,7 @@
         /// <summary>
         /// Gets the type of this curve
         /// </summary>
-        public ushort Type { get; }
+        public IccFormulaCurveType Type { get; }
 
         /// <summary>
         /// Gets the gamma curve parameter

src/ImageSharp/MetaData/Profiles/ICC/Enums/IccFormulaCurveType.cs

@@ -0,0 +1,28 @@
+// <copyright file="IccClutDataType.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageSharp
+{
+    /// <summary>
+    /// Formula curve segment type
+    /// </summary>
+    internal enum IccFormulaCurveType : ushort
+    {
+        /// <summary>
+        /// Type 1: Y = (a * X + b)^γ + c
+        /// </summary>
+        Type1 = 0,
+
+        /// <summary>
+        /// Type 1: Y = a * log10 (b * X^γ + c) + d
+        /// </summary>
+        Type2 = 1,
+
+        /// <summary>
+        /// Type 3: Y = a * b^(c * X + d) + e
+        /// </summary>
+        Type3 = 2
+    }
+}

src/ImageSharp/MetaData/Profiles/ICC/IccDataReader.cs

@@ -1588,29 +1588,26 @@ namespace ImageSharp
         /// <returns>The read segment</returns>
         public IccFormulaCurveElement ReadFormulaCurveElement()
         {
-            ushort type = this.ReadUInt16();
+            IccFormulaCurveType type = (IccFormulaCurveType)this.ReadUInt16();
             this.AddIndex(2);   // 2 bytes reserved
             double gamma, a, b, c, d, e;
             gamma = a = b = c = d = e = 0;
 
-            if (type == 0 || type == 1)
+            if (type == IccFormulaCurveType.Type1 || type == IccFormulaCurveType.Type2)
             {
                 gamma = this.ReadSingle();
             }
 
-            if (type >= 0 && type <= 2)
-            {
-                a = this.ReadSingle();
-                b = this.ReadSingle();
-                c = this.ReadSingle();
-            }
+            a = this.ReadSingle();
+            b = this.ReadSingle();
+            c = this.ReadSingle();
 
-            if (type == 1 || type == 2)
+            if (type == IccFormulaCurveType.Type2 || type == IccFormulaCurveType.Type3)
             {
                 d = this.ReadSingle();
             }
 
-            if (type == 2)
+            if (type == IccFormulaCurveType.Type3)
             {
                 e = this.ReadSingle();
             }

src/ImageSharp/MetaData/Profiles/ICC/IccDataWriter.cs

@@ -1772,27 +1772,24 @@ namespace ImageSharp
         /// <returns>The number of bytes written</returns>
         public int WriteFormulaCurveElement(IccFormulaCurveElement value)
         {
-            int count = this.WriteUInt16(value.Type);
+            int count = this.WriteUInt16((ushort)value.Type);
             count += this.WriteEmpty(2);
 
-            if (value.Type == 0 || value.Type == 1)
+            if (value.Type == IccFormulaCurveType.Type1 || value.Type == IccFormulaCurveType.Type2)
             {
                 count += this.WriteSingle((float)value.Gamma);
             }
 
-            if (value.Type >= 0 && value.Type <= 2)
-            {
-                count += this.WriteSingle((float)value.A);
-                count += this.WriteSingle((float)value.B);
-                count += this.WriteSingle((float)value.C);
-            }
+            count += this.WriteSingle((float)value.A);
+            count += this.WriteSingle((float)value.B);
+            count += this.WriteSingle((float)value.C);
 
-            if (value.Type == 1 || value.Type == 2)
+            if (value.Type == IccFormulaCurveType.Type2 || value.Type == IccFormulaCurveType.Type3)
             {
                 count += this.WriteSingle((float)value.D);
             }
 
-            if (value.Type == 2)
+            if (value.Type == IccFormulaCurveType.Type3)
             {
                 count += this.WriteSingle((float)value.E);
             }