Value forms for name are less special, can be overridden

src/Microsoft.TemplateEngine.Core/Matching/Trie.cs

@@ -48,11 +48,20 @@ public void AddPath(byte[] path, T terminal)
                         next.Terminals = new List<T>();
                     }
 
-                    next.Terminals.Add(terminal);
+                    int sameMatcherIndex = next.Terminals.FindIndex(t => t.Start == terminal.Start && t.End == terminal.End);
+
+                    if (sameMatcherIndex > -1)
+                    {   // this matching is identical to another terminal already added to the trie. Overwrite it.
+                        next.Terminals[sameMatcherIndex] = terminal;
+                    }
+                    else
+                    {
+                        next.Terminals.Add(terminal);
+                    }
                 }
 
                 current = next.NextNodes;
             }
         }
     }
-}
+}

src/Microsoft.TemplateEngine.Core/Matching/TrieEvaluator.cs

@@ -185,10 +185,7 @@ public bool TryGetNext(bool isFinal, ref int sequenceNumber, out TerminalLocatio
 
                             if (start >= _lastReturnedTerminalEndSequenceNumber)
                             {
-                                // Take the terminal for the match that started first & is after the end of the previous match
-                                // If multiple matches start at the same place, take the one that ends last.
-                                // If multiple matches start and end at the same place, take the later one in the EncounteredTerminals list (last in wins)
-                                if (start < minNonTerminatedPathStart && (best == null || start < minTerminalStart || start == minTerminalStart && terminal.End >= best.End))
+                                if (start < minNonTerminatedPathStart && (best == null || start < minTerminalStart || start == minTerminalStart && terminal.End > best.End))
                                 {
                                     bestPath = i;
                                     best = terminal;

src/Microsoft.TemplateEngine.Orchestrator.RunnableProjects/ParameterSymbol.cs

@@ -1,9 +1,9 @@
 using System;
 using System.Collections.Generic;
 using Microsoft.TemplateEngine.Abstractions;
+using Microsoft.TemplateEngine.Orchestrator.RunnableProjects.ValueForms;
 using Microsoft.TemplateEngine.Utils;
 using Newtonsoft.Json.Linq;
-using Microsoft.TemplateEngine.Orchestrator.RunnableProjects.ValueForms;
 
 namespace Microsoft.TemplateEngine.Orchestrator.RunnableProjects
 {
@@ -108,10 +108,12 @@ public static ISymbolModel FromJObject(JObject jObject, IParameterSymbolLocaliza
 
             if (!jObject.TryGetValue(nameof(symbol.Forms), StringComparison.OrdinalIgnoreCase, out JToken formsToken) || !(formsToken is JObject formsObject))
             {
-                symbol.Forms = SymbolValueFormsModel.Default;   // no value forms explicitly defined, use the default ("identity")
+                // no value forms explicitly defined, use the default ("identity")
+                symbol.Forms = SymbolValueFormsModel.Default;
             }
             else
             {
+                // the config defines forms for the symbol. Use them.
                 symbol.Forms = SymbolValueFormsModel.FromJObject(formsObject);
             }
 

src/Microsoft.TemplateEngine.Orchestrator.RunnableProjects/SimpleConfigModel.cs

@@ -925,7 +925,12 @@ private static ISymbolModel SetupDefaultNameSymbol(string sourceName)
   ""description"": ""The default name symbol"",
   ""datatype"": ""string"",
   ""forms"": {
-    ""global"": [ ""identity"", ""safe_name"", ""lower_safe_name"", ""safe_namespace"", ""lower_safe_namespace""]
+    ""global"": [ """ + IdentityValueForm.FormName
+                    + @""", """ + DefaultSafeNameValueFormModel.FormName
+                    + @""", """ + DefaultLowerSafeNameValueFormModel.FormName
+                    + @""", """ + DefaultSafeNamespaceValueFormModel.FormName
+                    + @""", """ + DefaultLowerSafeNamespaceValueFormModel.FormName
+                    + @"""]
   }
 ");
 

src/Microsoft.TemplateEngine.Orchestrator.RunnableProjects/ValueFormModel.cs

@@ -48,7 +48,7 @@ public static IValueForm GetForm(string name, JObject obj)
 
             if (!FormLookup.TryGetValue(identifier, out IValueForm value))
             {
-                return FormLookup["identity"].FromJObject(name, obj);
+                return FormLookup[IdentityValueForm.FormName].FromJObject(name, obj);
             }
 
             return value.FromJObject(name, obj);

src/Microsoft.TemplateEngine.Orchestrator.RunnableProjects/ValueForms/DefaultLowerSafeNameValueFormModel.cs

@@ -4,7 +4,9 @@ namespace Microsoft.TemplateEngine.Orchestrator.RunnableProjects.ValueForms
 {
     public class DefaultLowerSafeNameValueFormModel : DefaultSafeNameValueFormModel
     {
-        public override string Identifier => "lower_safe_name";
+        public new static readonly string FormName = "lower_safe_name";
+
+        public override string Identifier => FormName;
 
         public override string Process(IReadOnlyDictionary<string, IValueForm> forms, string value)
         {

src/Microsoft.TemplateEngine.Orchestrator.RunnableProjects/ValueForms/DefaultLowerSafeNamespaceValueFormModel.cs

@@ -4,7 +4,9 @@ namespace Microsoft.TemplateEngine.Orchestrator.RunnableProjects.ValueForms
 {
     public class DefaultLowerSafeNamespaceValueFormModel : DefaultSafeNamespaceValueFormModel
     {
-        public override string Identifier => "lower_safe_namespace";
+        public new static readonly string FormName = "lower_safe_namespace";
+
+        public override string Identifier => FormName;
 
         public override string Process(IReadOnlyDictionary<string, IValueForm> forms, string value)
         {

src/Microsoft.TemplateEngine.Orchestrator.RunnableProjects/ValueForms/DefaultSafeNameValueFormModel.cs

@@ -11,7 +11,9 @@ public DefaultSafeNameValueFormModel()
         {
         }
 
-        public virtual string Identifier => "safe_name";
+        public static readonly string FormName = "safe_name";
+
+        public virtual string Identifier => FormName;
 
         public virtual string Name => Identifier;
 

src/Microsoft.TemplateEngine.Orchestrator.RunnableProjects/ValueForms/DefaultSafeNamespaceValueFormModel.cs

@@ -11,7 +11,9 @@ public DefaultSafeNamespaceValueFormModel()
         {
         }
 
-        public virtual string Identifier => "safe_namespace";
+        public static readonly string FormName = "safe_namespace";
+
+        public virtual string Identifier => FormName;
 
         public string Name => Identifier;
 

src/Microsoft.TemplateEngine.Orchestrator.RunnableProjects/ValueForms/IdentityValueForm.cs

@@ -5,7 +5,9 @@ namespace Microsoft.TemplateEngine.Orchestrator.RunnableProjects.ValueForms
 {
     public class IdentityValueForm : IValueForm
     {
-        public string Identifier => "identity";
+        public static readonly string FormName = "identity";
+
+        public string Identifier => FormName;
 
         public string Name { get; }
 

src/Microsoft.TemplateEngine.Orchestrator.RunnableProjects/ValueForms/SymbolValueFormsModel.cs

@@ -1,15 +1,19 @@
+using System;
 using System.Collections.Generic;
+using System.Linq;
 using Microsoft.TemplateEngine.Utils;
 using Newtonsoft.Json.Linq;
 
 namespace Microsoft.TemplateEngine.Orchestrator.RunnableProjects.ValueForms
 {
     public class SymbolValueFormsModel
     {
+        private static readonly string IdentityValueFormName = IdentityValueForm.FormName;
+
         public static SymbolValueFormsModel Empty { get; } = new SymbolValueFormsModel(Empty<string>.List.Value);
 
         // by default, symbols get the "identity" value form, for a direct replacement
-        public static SymbolValueFormsModel Default { get; } = new SymbolValueFormsModel(new List<string>() { "identity" });
+        public static SymbolValueFormsModel Default { get; } = new SymbolValueFormsModel(new List<string>() { IdentityValueFormName });
 
         public IReadOnlyList<string> GlobalForms { get; }
 
@@ -18,9 +22,56 @@ private SymbolValueFormsModel(IReadOnlyList<string> globalForms)
             GlobalForms = globalForms;
         }
 
-        public static SymbolValueFormsModel FromJObject(JObject obj)
+        // Sets up the value forms for a symbol, based on configuration from template.json
+        // There are two acceptable configuration formats for each forms specification.
+        //
+        // Note: in the examples below, "global" is used. But we'll be extending this to allow
+        //  conditional forms, which will have other names.
+        //  The same format will be used for other named form definitions.
+        //
+        // Simple:
+        // "forms": {
+        //   "global": [ <strings representing value form names> ]
+        // }
+        //
+        // Detailed:
+        // "forms" {
+        //   "global": {
+        //     "forms": [ <strings representing value form names> ],
+        //     "addIdentity": <bool default true>,
+        //     // other future extensions, e.g. conditionals
+        //   },
+        //
+        // If the symbol doesn't include an "identity" form and the addIdentity flag isn't false,
+        // an identity specification is added to the beginning of the symbol's value form list.
+        // If there is an identity form listed, its position remains intact irrespective of the addIdentity flag.
+        public static SymbolValueFormsModel FromJObject(JObject configJson)
         {
-            IReadOnlyList<string> globalForms = obj.ArrayAsStrings("global");
+            JToken globalConfig = configJson.Property("global").Value;
+            List<string> globalForms;
+            bool addIdentity;
+            if (globalConfig.Type == JTokenType.Array)
+            {
+                // config is just an array of form names.
+                globalForms = globalConfig.ArrayAsStrings().ToList();
+                addIdentity = true; // default value
+            }
+            else if (globalConfig.Type == JTokenType.Object)
+            {
+                // config is an object.
+                globalForms = globalConfig.ArrayAsStrings("forms").ToList();
+                addIdentity = globalConfig.ToBool("addIdentity");
+            }
+            else
+            {
+                throw new Exception("Malformed global value forms.");
+            }
+
+            if (addIdentity && !globalForms.Contains(IdentityValueFormName, StringComparer.OrdinalIgnoreCase))
+            {
+                globalForms.Insert(0, IdentityValueFormName);
+            }
+
             return new SymbolValueFormsModel(globalForms);
         }
     }

test/Microsoft.TemplateEngine.Core.UnitTests/OperationTrieTests.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.IO;
 using Microsoft.TemplateEngine.Core.Contracts;
@@ -97,6 +97,29 @@ public void VerifyOperationTrieFindsTokenAtEnd()
             Assert.Equal(buffer.Length, currentBufferPosition);
         }
 
+        [Fact(DisplayName = nameof(VerifyLastInWinsForIdenticalMatching))]
+        public void VerifyLastInWinsForIdenticalMatching()
+        {
+            OperationTrie trie = OperationTrie.Create(new IOperation[]
+            {
+                new MockOperation("TestOp1", null, true, TokenConfig.LiteralToken(new byte[] { 5, 5, 5 })),
+                new MockOperation("TestOp2", null, true, TokenConfig.LiteralToken(new byte[] { 2, 3, 4, 5 })),
+                new MockOperation("TestOp3", null, true, TokenConfig.LiteralToken(new byte[] { 7, 7, 7 })),
+                new MockOperation("TestOp4", null, true, TokenConfig.LiteralToken(new byte[] { 9, 9, 9, 9 }),
+                                                        TokenConfig.LiteralToken(new byte[] { 2, 3, 4, 5 })
+                ),
+            });
+
+            byte[] buffer = { 9, 8, 9, 8, 7, 2, 3, 4, 5 };
+            int currentBufferPosition = 0;
+            IOperation match = trie.GetOperation(buffer, buffer.Length, ref currentBufferPosition, out int token);
+
+            Assert.NotNull(match);
+            Assert.Equal("TestOp4", match.Id);
+            Assert.Equal(1, token);
+            Assert.Equal(buffer.Length, currentBufferPosition);
+        }
+
         private class OperationTrie : Trie<OperationTerminal>
         {
             public static OperationTrie Create(IEnumerable<IOperation> operations)