feat: 语法分析的详细错误信息 (#63)

Reviewed-on: PostGuard/Canon#63

Canon.Core/Exceptions/GrammarException.cs

@@ -1,12 +1,71 @@
+using System.Text;
+using Canon.Core.Abstractions;
+using Canon.Core.GrammarParser;
+using Canon.Core.LexicalParser;
+
 namespace Canon.Core.Exceptions;
+
 /// <summary>
 /// 语法分析中引发的异常
 /// </summary>
 public class GrammarException : Exception
 {
-    public GrammarException()  { }
+    public override string Message { get; }
 
-    public GrammarException(string message) : base(message) { }
+    /// <summary>
+    /// 语法分析错误时的分析状态
+    /// </summary>
+    public ITransformer CurrentState { get; }
 
-    public GrammarException(string message, Exception innerException) : base(message, innerException) { }
+    /// <summary>
+    /// 语法分析错误时的输入符号
+    /// </summary>
+    public SemanticToken CurrentToken { get; }
+
+    public GrammarException(ITransformer currentState)
+    {
+        CurrentState = currentState;
+        CurrentToken = SemanticToken.End;
+
+        StringBuilder builder = new();
+        builder.Append("Except ");
+
+        foreach (TerminatorBase terminatorBase in ListNextTerminators(CurrentState))
+        {
+            builder.Append('\'').Append(terminatorBase).Append("' ");
+        }
+
+        Message = builder.ToString();
+    }
+
+    public GrammarException(ITransformer currentState, SemanticToken currentToken)
+    {
+        CurrentState = currentState;
+        CurrentToken = currentToken;
+
+        StringBuilder builder = new();
+        builder.Append("Expect ");
+
+        foreach (TerminatorBase terminatorBase in ListNextTerminators(CurrentState))
+        {
+            builder.Append('\'').Append(terminatorBase).Append("',");
+        }
+
+        if (!CurrentToken.Equals(SemanticToken.End))
+        {
+            builder.Append("but '").Append(CurrentToken.LiteralValue).Append("' found.");
+        }
+
+        Message = builder.ToString();
+    }
+
+    private static List<TerminatorBase> ListNextTerminators(ITransformer state)
+    {
+        List<TerminatorBase> result = [];
+
+        result.AddRange(state.ShiftTable.Keys);
+        result.AddRange(state.ReduceTable.Keys);
+
+        return result;
+    }
 }