From 776111de59b567ef586114e18cede66c5fd4bbb2 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Tue, 17 Nov 2020 08:07:18 -0500
Subject: [PATCH] Avoid unnecessary LINQ and string joins in
 JsonConfigurationFileParser

---
 .../src/JsonConfigurationFileParser.cs        | 40 +++++++++----------
 1 file changed, 18 insertions(+), 22 deletions(-)

diff --git a/src/libraries/Microsoft.Extensions.Configuration.Json/src/JsonConfigurationFileParser.cs b/src/libraries/Microsoft.Extensions.Configuration.Json/src/JsonConfigurationFileParser.cs
index 17d8ea4a60f591..d8cdbc4438ad31 100644
--- a/src/libraries/Microsoft.Extensions.Configuration.Json/src/JsonConfigurationFileParser.cs
+++ b/src/libraries/Microsoft.Extensions.Configuration.Json/src/JsonConfigurationFileParser.cs
@@ -3,27 +3,24 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.IO;
-using System.Linq;
 using System.Text.Json;
 
 namespace Microsoft.Extensions.Configuration.Json
 {
-    internal class JsonConfigurationFileParser
+    internal sealed class JsonConfigurationFileParser
     {
         private JsonConfigurationFileParser() { }
 
-        private readonly IDictionary<string, string> _data = new SortedDictionary<string, string>(StringComparer.OrdinalIgnoreCase);
-        private readonly Stack<string> _context = new Stack<string>();
-        private string _currentPath;
+        private readonly SortedDictionary<string, string> _data = new SortedDictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+        private readonly Stack<string> _paths = new Stack<string>();
 
         public static IDictionary<string, string> Parse(Stream input)
             => new JsonConfigurationFileParser().ParseStream(input);
 
         private IDictionary<string, string> ParseStream(Stream input)
         {
-            _data.Clear();
-
             var jsonDocumentOptions = new JsonDocumentOptions
             {
                 CommentHandling = JsonCommentHandling.Skip,
@@ -55,22 +52,26 @@ private void VisitElement(JsonElement element)
                 ExitContext();
             }
 
-            if (isEmpty && _currentPath != null)
+            if (isEmpty && _paths.Count > 0)
             {
-                _data[_currentPath] = null;
+                _data[_paths.Peek()] = null;
             }
         }
 
         private void VisitValue(JsonElement value)
         {
-            switch (value.ValueKind) {
+            Debug.Assert(_paths.Count > 0);
+
+            switch (value.ValueKind)
+            {
                 case JsonValueKind.Object:
                     VisitElement(value);
                     break;
 
                 case JsonValueKind.Array:
                     int index = 0;
-                    foreach (JsonElement arrayElement in value.EnumerateArray()) {
+                    foreach (JsonElement arrayElement in value.EnumerateArray())
+                    {
                         EnterContext(index.ToString());
                         VisitValue(arrayElement);
                         ExitContext();
@@ -83,7 +84,7 @@ private void VisitValue(JsonElement value)
                 case JsonValueKind.True:
                 case JsonValueKind.False:
                 case JsonValueKind.Null:
-                    string key = _currentPath;
+                    string key = _paths.Peek();
                     if (_data.ContainsKey(key))
                     {
                         throw new FormatException(SR.Format(SR.Error_KeyIsDuplicated, key));
@@ -96,16 +97,11 @@ private void VisitValue(JsonElement value)
             }
         }
 
-        private void EnterContext(string context)
-        {
-            _context.Push(context);
-            _currentPath = ConfigurationPath.Combine(_context.Reverse());
-        }
+        private void EnterContext(string context) =>
+            _paths.Push(_paths.Count > 0 ?
+                _paths.Peek() + ConfigurationPath.KeyDelimiter + context :
+                context);
 
-        private void ExitContext()
-        {
-            _context.Pop();
-            _currentPath = ConfigurationPath.Combine(_context.Reverse());
-        }
+        private void ExitContext() => _paths.Pop();
     }
 }