Add XZ variable length helper methods

SabreTools.Serialization/Extensions/XZ.cs

@@ -0,0 +1,58 @@
+namespace SabreTools.Data.Extensions
+{
+    public static class XZ
+    {
+        /// <summary>
+        /// Decode a value from a variable-length integer
+        /// </summary>
+        /// <param name="value">Value to decode</param>
+        /// <param name="maxSize">Maximum number of bytes to parse</param>
+        /// <returns>UInt64 representing the decoded integer</returns>
+        /// <see href="https://tukaani.org/xz/xz-file-format.txt"/>
+        public static ulong DecodeVariableLength(this byte[] value, int maxSize)
+        {
+            if (maxSize <= 0)
+                return 0;
+
+            if (maxSize > 9)
+                maxSize = 9;
+
+            ulong output = value[0] & 0x7F;
+            int i = 0;
+
+            while (value[i++] & 0x80 != 0)
+            {
+                if (i >= maxSize || value[i] == 0x00)
+                    return 0;
+
+                output |= (value[i] & 0x7F) << (i * 7);
+            }
+
+            return output;
+        }
+
+        /// <summary>
+        /// Encode a value to a variable-length integer
+        /// </summary>
+        /// <param name="value">Value to encode</param>
+        /// <returns>Byte array representing the encoded integer</returns>
+        /// <see href="https://tukaani.org/xz/xz-file-format.txt"/>
+        public static byte[] EncodeVariableLength(this ulong value)
+        {
+            if (value > long.MaxValue / 2)
+                return [];
+
+            var output = new List<byte>();
+
+            int i = 0;
+            while (value >= 0x80)
+            {
+                output.Add((byte)value | 0x80);
+                value >>= 7;
+            }
+
+            output.Add((byte)value);
+            return [.. output];
+        }
+    }
+}