Fix back-tracking

sebastienros · Jan 2, 2021 · 1ba6600 · 1ba6600
1 parent 1c17b47
commit 1ba6600
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Showing 5 changed files with 51 additions and 0 deletions.
diff --git a/src/Parlot/Fluent/OneOf.cs b/src/Parlot/Fluent/OneOf.cs
@@ -25,13 +25,18 @@ public override bool Parse(ParseContext context, ref ParseResult<ParseResult<obj
 
             var parsed = new ParseResult<object>();
 
+            var start = context.Scanner.Cursor.Position;
+
             for (var i = 0; i < Parsers.Count; i++)
             {
                 if (Parsers[i].Parse(context, ref parsed))
                 {
                     result.Set(parsed.Buffer, parsed.Start, parsed.End, parsed);
                     return true;
                 }
+
+                // If the choice as a subset of its parsers that succeeded, it might have advanced the cursor
+                context.Scanner.Cursor.ResetPosition(start);
             }
 
             return false;
@@ -61,12 +66,17 @@ public override bool Parse(ParseContext context, ref ParseResult<T> result)
                 return false;
             }
 
+            var start = context.Scanner.Cursor.Position;
+
             for (var i = 0; i < Parsers.Count; i++)
             {
                 if (Parsers[i].Parse(context, ref result))
                 {
                     return true;
                 }
+
+                // If the choice as a subset of its parsers that succeeded, it might have advanced the cursor
+                context.Scanner.Cursor.ResetPosition(start);
             }
 
             return false;

diff --git a/src/Parlot/Fluent/Sequence.cs b/src/Parlot/Fluent/Sequence.cs
@@ -28,6 +28,8 @@ public override bool Parse(ParseContext context, ref ParseResult<ValueTuple<T1,
                     result.Set(parseResult1.Buffer, parseResult1.Start, parseResult2.End, new ValueTuple<T1, T2>(parseResult1.Value, parseResult2.Value));
                     return true;
                 }
+
+                context.Scanner.Cursor.ResetPosition(parseResult1.Start);
             }
 
             return false;
@@ -69,6 +71,8 @@ public override bool Parse(ParseContext context, ref ParseResult<ValueTuple<T1,
                     result.Set(tupleResult.Buffer, tupleResult.Start, lastResult.End, tuple);
                     return true;
                 }
+
+                context.Scanner.Cursor.ResetPosition(tupleResult.Start);
             }
 
             return false;
@@ -108,6 +112,8 @@ public override bool Parse(ParseContext context, ref ParseResult<ValueTuple<T1,
                     result.Set(tupleResult.Buffer, tupleResult.Start, lastResult.End, tuple);
                     return true;
                 }
+
+                context.Scanner.Cursor.ResetPosition(tupleResult.Start);
             }
 
             return false;
@@ -148,6 +154,8 @@ public override bool Parse(ParseContext context, ref ParseResult<ValueTuple<T1,
                     result.Set(tupleResult.Buffer, tupleResult.Start, lastResult.End, tuple);
                     return true;
                 }
+
+                context.Scanner.Cursor.ResetPosition(tupleResult.Start);
             }
 
             return false;
@@ -189,6 +197,8 @@ public override bool Parse(ParseContext context, ref ParseResult<ValueTuple<T1,
                     result.Set(tupleResult.Buffer, tupleResult.Start, lastResult.End, tuple);
                     return true;
                 }
+
+                context.Scanner.Cursor.ResetPosition(tupleResult.Start);
             }
 
             return false;
@@ -232,6 +242,8 @@ public override bool Parse(ParseContext context, ref ParseResult<ValueTuple<T1,
                     result.Set(tupleResult.Buffer, tupleResult.Start, lastResult.End, tuple);
                     return true;
                 }
+
+                context.Scanner.Cursor.ResetPosition(tupleResult.Start);
             }
 
             return false;
@@ -261,6 +273,8 @@ public override bool Parse(ParseContext context, ref ParseResult<IList<ParseResu
 
             var parsed = new ParseResult<object>();
 
+            var start = context.Scanner.Cursor.Position;
+
             for (var i = 0; i < _parsers.Length; i++)
             {
                 if (!_parsers[i].Parse(context, ref parsed))
@@ -279,6 +293,7 @@ public override bool Parse(ParseContext context, ref ParseResult<IList<ParseResu
             }
             else
             {
+                context.Scanner.Cursor.ResetPosition(start);
                 return false;
             }
         }

diff --git a/src/Parlot/Fluent/SkipAnd.cs b/src/Parlot/Fluent/SkipAnd.cs
@@ -27,6 +27,8 @@ public override bool Parse(ParseContext context, ref ParseResult<T> result)
                     result.Set(parseResult1.Buffer, parseResult1.Start, parseResult2.End, parseResult2.Value);
                     return true;
                 }
+
+                context.Scanner.Cursor.ResetPosition(parseResult1.Start);
             }
 
             return false;

diff --git a/src/Parlot/Fluent/TextBefore.cs b/src/Parlot/Fluent/TextBefore.cs
@@ -26,6 +26,12 @@ public override bool Parse(ParseContext context, ref ParseResult<TextSpan> resul
 
             var parsed = new ParseResult<T>();
 
+            if (_delimiter.Parse(context, ref parsed))
+            {
+                context.Scanner.Cursor.ResetPosition(start);
+                return false;
+            }
+
             while (true)
             {
                 if (_delimiter.Parse(context, ref parsed) || (!_failOnEof && context.Scanner.Cursor.Eof))
@@ -88,6 +94,12 @@ public override bool Parse(ParseContext context, ref ParseResult<TextSpan> resul
 
             var parsed = new ParseResult<object>();
 
+            if (_delimiter.Parse(context, ref parsed))
+            {
+                context.Scanner.Cursor.ResetPosition(start);
+                return false;
+            }
+
             while (true)
             {
                 if (_delimiter.Parse(context, ref parsed) || (!_failOnEof && context.Scanner.Cursor.Eof))

diff --git a/test/Parlot.Tests/FluentTests.cs b/test/Parlot.Tests/FluentTests.cs
@@ -248,5 +248,17 @@ public void NumbersShouldAcceptSignIfAllowed()
             Assert.Equal(-123, Terms.Decimal(NumberOptions.AllowSign).Parse("-123"));
             Assert.Equal(-123, Terms.Integer(NumberOptions.AllowSign).Parse("-123"));
         }
+
+        [Fact]
+        public void OneOfShouldRestorePosition()
+        {
+            var choice = OneOf(
+                Literals.Char('a').And(Literals.Char('b')).And(Literals.Char('c')).And(Literals.Char('d')),
+                Literals.Char('a').And(Literals.Char('b')).And(Literals.Char('e')).And(Literals.Char('d'))
+                ).Then(x => x.Item1.ToString() + x.Item2.ToString() + x.Item3.ToString() + x.Item4.ToString());
+
+            Assert.Equal("abcd", choice.Parse("abcd"));
+            Assert.Equal("abed", choice.Parse("abed"));
+        }
     }
 }