Set up a helper method for converting fixed-point numbers to doubles.

Schema/src/binary/reader/BinaryReaderExtensions_FixedPoint.cs

@@ -0,0 +1,40 @@
+using System;
+
+using schema.util;
+
+namespace schema.binary;
+
+public static partial class BinaryReaderExtensions {
+  /// <summary>
+  ///   Shamelessly stolen from:
+  ///   https://github.com/scurest/apicula/blob/3d4e91e14045392a49c89e86dab8cb936225588c/src/util/fixed.rs#L5-L35
+  /// </summary>
+  public static double ReadDoubleFixedPoint(this IBinaryReader br,
+                                            byte signBits,
+                                            byte integerBits,
+                                            byte fractionBits) {
+    var totalBits = (uint) (signBits + integerBits + fractionBits);
+    var totalBytes = (int) BitLogic.BytesNeededToContainBits(totalBits);
+
+    var value = totalBytes switch {
+        1 => br.ReadByte(),
+        2 => br.ReadUInt16(),
+        3 => br.ReadUInt24(),
+        4 => br.ReadUInt32(),
+    };
+
+    double doubleValue;
+    if (signBits == 0) {
+      doubleValue = value;
+    } else {
+      var signMask = 1 << (integerBits + fractionBits);
+      if ((value & signMask) != 0) {
+        doubleValue = (double) (value | ~(signMask - 1));
+      } else {
+        doubleValue = value;
+      }
+    }
+
+    return doubleValue * Math.Pow(.5, fractionBits);
+  }
+}