From bebad3784d1e5c13d80bb4c77f77a08af00c0727 Mon Sep 17 00:00:00 2001
From: Chui Tey <teyc@cognoware.com>
Date: Mon, 6 Feb 2023 06:45:19 +1000
Subject: [PATCH] #121 Failed parsing nested case statements

---
 .../Parsers/TSQLCaseExpressionParser.cs       | 126 +++++++++++++++---
 .../Tokens/Parsers/TSQLTokenParserHelper.cs   |   6 +
 2 files changed, 115 insertions(+), 17 deletions(-)

diff --git a/TSQL_Parser/TSQL_Parser/Expressions/Parsers/TSQLCaseExpressionParser.cs b/TSQL_Parser/TSQL_Parser/Expressions/Parsers/TSQLCaseExpressionParser.cs
index 794fa5b..c32e047 100644
--- a/TSQL_Parser/TSQL_Parser/Expressions/Parsers/TSQLCaseExpressionParser.cs
+++ b/TSQL_Parser/TSQL_Parser/Expressions/Parsers/TSQLCaseExpressionParser.cs
@@ -1,9 +1,5 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-
+using TSQL.Elements;
 using TSQL.Tokens;
 using TSQL.Tokens.Parsers;
 
@@ -22,21 +18,110 @@ public TSQLCaseExpression Parse(ITSQLTokenizer tokenizer)
 
 			caseExpression.Tokens.Add(tokenizer.Current);
 
-			TSQLTokenParserHelper.ReadUntilStop(
+			tokenizer.MoveNext();
+
+			TSQLTokenParserHelper.ReadCommentsAndWhitespace(
 				tokenizer,
-				caseExpression,
-				new List<TSQLFutureKeywords>() { },
-				new List<TSQLKeywords>() {
-					TSQLKeywords.END
-				},
-				lookForStatementStarts: false);
-
-			// this is different than the other clauses because the
-			// stop word is still part of the expression instead of
-			// being part of the next expression or clause like in
-			// the other parsers
+				caseExpression.Tokens);
+
+			var nextToken = tokenizer.Current ?? throw new TSQLParseException("CASE expression is incomplete. Expect WHEN or an input expression");
+
+			TSQLToken whenToken = null;
+
+			if (!nextToken.IsKeyword(TSQLKeywords.WHEN))
+			{
+				caseExpression.IsSimpleCaseExpression = true;
+				var valueExpr = new TSQLValueExpressionParser().Parse(tokenizer);
+				caseExpression.Tokens.AddRange(valueExpr.Tokens);
+
+				TSQLTokenParserHelper.ReadCommentsAndWhitespace(
+					tokenizer,
+					caseExpression.Tokens);
+
+				whenToken = tokenizer.Current;
+			}
+			else
+			{
+				caseExpression.IsSimpleCaseExpression = false;
+				whenToken = nextToken;
+			}
+
+			if (!whenToken.IsKeyword(TSQLKeywords.WHEN))
+			{
+				throw new TSQLParseException("CASE expression is incorrect. It should have a WHEN keyword, instead we have: " + whenToken.Text);
+			}
+
+			// 'WHEN' keyword
+			caseExpression.Tokens.Add(whenToken);
+
+			do
+			{
+				if (!tokenizer.MoveNext())
+				{
+					throw new TSQLParseException("CASE expression is incomplete. After WHEN, there should be an expression");
+				}
+
+				TSQLTokenParserHelper.ReadCommentsAndWhitespace(
+					tokenizer,
+					caseExpression.Tokens);
+
+				var whenExpr = new TSQLValueExpressionParser().Parse(tokenizer);
+				caseExpression.Tokens.AddRange(whenExpr.Tokens);
+
+				if (!tokenizer.Current.IsKeyword(TSQLKeywords.THEN))
+				{
+					throw new TSQLParseException(
+						"CASE expression is incomplete. After WHEN, there should be a THEN keyword");
+				}
+
+				// 'THEN' keyword
+				caseExpression.Tokens.Add(tokenizer.Current);
+
+				if (!tokenizer.MoveNext())
+				{
+					throw new TSQLParseException(
+						"CASE expression is incomplete. After THEN, there should be an expression");
+				}
+
+				TSQLTokenParserHelper.ReadCommentsAndWhitespace(
+					tokenizer,
+					caseExpression.Tokens);
+
+				var thenExpr = new TSQLValueExpressionParser().Parse(tokenizer);
+				caseExpression.Tokens.AddRange(thenExpr.Tokens);
+
+			} while (tokenizer.Current.IsKeyword(TSQLKeywords.WHEN));
+
+			if (!tokenizer.Current.IsKeyword(TSQLKeywords.ELSE))
+			{
+				throw new TSQLParseException(
+					"CASE expression is incomplete. There should be an ELSE keyword");
+			}
+
+			caseExpression.Tokens.Add(tokenizer.Current);
+
+			if (!tokenizer.MoveNext())
+			{
+				throw new TSQLParseException(
+					"CASE expression incomplete. There should be an expression after ELSE keyword");
+			}
+
+			TSQLTokenParserHelper.ReadCommentsAndWhitespace(
+				tokenizer,
+				caseExpression.Tokens);
+
+			var elseExpr = new TSQLValueExpressionParser().Parse(tokenizer);
+			caseExpression.Tokens.AddRange(elseExpr.Tokens);
+
+			if (!tokenizer.Current.IsKeyword(TSQLKeywords.END))
+			{
+				throw new TSQLParseException("CASE expression incomplete. There should be an END keyword");
+			}
+
+			// 'END' keyword
 			caseExpression.Tokens.Add(tokenizer.Current);
 
+			// move past the END keyword
 			tokenizer.MoveNext();
 
 			TSQLTokenParserHelper.ReadCommentsAndWhitespace(
@@ -45,5 +130,12 @@ public TSQLCaseExpression Parse(ITSQLTokenizer tokenizer)
 
 			return caseExpression;
 		}
+
+		private TSQLToken ReadNextToken(ITSQLTokenizer tokenizer)
+		{
+			if (!tokenizer.MoveNext()) return null;
+			return tokenizer.Current;
+		}
+
 	}
 }
diff --git a/TSQL_Parser/TSQL_Parser/Tokens/Parsers/TSQLTokenParserHelper.cs b/TSQL_Parser/TSQL_Parser/Tokens/Parsers/TSQLTokenParserHelper.cs
index 378b2a5..7292a88 100644
--- a/TSQL_Parser/TSQL_Parser/Tokens/Parsers/TSQLTokenParserHelper.cs
+++ b/TSQL_Parser/TSQL_Parser/Tokens/Parsers/TSQLTokenParserHelper.cs
@@ -104,6 +104,12 @@ public static void ReadCommentsAndWhitespace(
 				element.Tokens);
 		}
 
+		/// <summary>
+		/// Precondition: tokenizer.Current has not been consumed
+		/// Postcondition: tokenizer advanced if token consumed
+		/// </summary>
+		/// <param name="tokenizer"></param>
+		/// <param name="savedTokens"></param>
 		public static void ReadCommentsAndWhitespace(
 			ITSQLTokenizer tokenizer,
 			List<TSQLToken> savedTokens)