Fix crash when deserializing from Stream into SourceMap (#85)

* Json.net is throwing an exception when trying to deserialize into an IReadOnlyList using a stream deserializer

* Remove ParsedMappings from serialization/deserialization code on SourceMap

src/SourcemapToolkit.SourcemapParser/SourceMap.cs

@@ -6,6 +6,36 @@ using Newtonsoft.Json;
 
 namespace SourcemapToolkit.SourcemapParser
 {
+	/// <summary>
+	/// A seemingly silly class, but unfortunately one which appears necessary.
+	/// While the SourceMap class is serializable, it is not deserializable directly.
+	/// The problem is that the IReadOnlyList generics throw the following exception:
+	/// 'Newtonsoft.Json.JsonSerializationException: Cannot create and populate list type'
+	/// when attempting to deserialize from a stream using a JsonTextReader.  This
+	/// intermediary class mitigates the problem by reading the data into a mutable list
+	/// first.
+	/// </summary>
+	internal class SourceMapDeserializable
+	{
+		/// <summary><see cref="SourceMap.Version"/></summary>
+		public int Version;
+
+		/// <summary><see cref="SourceMap.File"/></summary>
+		public string File;
+
+		/// <summary><see cref="SourceMap.Mappings"/></summary>
+		public string Mappings;
+
+		/// <summary><see cref="SourceMap.Sources"/></summary>
+		public List<string> Sources;
+
+		/// <summary><see cref="SourceMap.Names"/></summary>
+		public List<string> Names;
+
+		/// <summary><see cref="SourceMap.SourcesContent"/></summary>
+		public List<string> SourcesContent;
+	}
+
 	public class SourceMap
 	{
 		/// <summary>
@@ -42,6 +72,8 @@ namespace SourcemapToolkit.SourcemapParser
 		/// <summary>
 		/// Parsed version of the mappings string that is used for getting original names and source positions
 		/// </summary>
+		/// <remarks>Marked as JsonIgnore to because it isn't part of the actual format.  See: https://sourcemaps.info/spec.html#h.mofvlxcwqzej </remarks>
+		[JsonIgnore]
 		public IReadOnlyList<MappingEntry> ParsedMappings { get; }
 
 		/// <summary>

src/SourcemapToolkit.SourcemapParser/SourceMapParser.cs

@@ -27,26 +27,37 @@ namespace SourcemapToolkit.SourcemapParser
 			{
 				JsonSerializer serializer = new JsonSerializer();
 
-				SourceMap result = serializer.Deserialize<SourceMap>(jsonTextReader);
+				SourceMapDeserializable deserializedSourceMap = serializer.Deserialize<SourceMapDeserializable>(jsonTextReader);
 
 				// Since SourceMap is immutable we need to allocate a new one and copy over all the information
-				List<MappingEntry> parsedMappings = _mappingsListParser.ParseMappings(result.Mappings, result.Names, result.Sources);
+				List<MappingEntry> parsedMappings = _mappingsListParser.ParseMappings(deserializedSourceMap.Mappings, deserializedSourceMap.Names, deserializedSourceMap.Sources);
 
 				// Resize to free unused memory
-				parsedMappings.Capacity = parsedMappings.Count;
+				RemoveExtraSpaceFromList(parsedMappings);
+				RemoveExtraSpaceFromList(deserializedSourceMap.Sources);
+				RemoveExtraSpaceFromList(deserializedSourceMap.Names);
+				RemoveExtraSpaceFromList(deserializedSourceMap.SourcesContent);
 
-				result = new SourceMap(
-					version: result.Version,
-					file: result.File,
-					mappings: result.Mappings,
-					sources: result.Sources,
-					names: result.Names,
+				SourceMap result = new SourceMap(
+					version: deserializedSourceMap.Version,
+					file: deserializedSourceMap.File,
+					mappings: deserializedSourceMap.Mappings,
+					sources: deserializedSourceMap.Sources,
+					names: deserializedSourceMap.Names,
 					parsedMappings: parsedMappings,
-					sourcesContent: result.SourcesContent);
+					sourcesContent: deserializedSourceMap.SourcesContent);
 
 				sourceMapStream.Close();
 				return result;
 			}
 		}
+
+		private void RemoveExtraSpaceFromList<T>(List<T> list)
+		{
+			if (list != null)
+			{
+				list.Capacity = list.Count;
+			}
+		}
 	}
 }