Add |offset| property to exceptions thrown by parsers

Based on a patch by Marcin Stefaniuk (marcin@stefaniuk.info).

README.md

@@ -70,7 +70,7 @@ To get parser’s source code, call the `toSource` method on the parser.
 Using the Parser
 ----------------
 
-Using the generated parser is simple — just call its `parse` method and pass an input string as a parameter. The method will return a parse result (the exact value depends on the grammar used to build the parser) or throw an exception if the input is invalid. The exception will contain `line`, `column` and `message` properties with more details about the error.
+Using the generated parser is simple — just call its `parse` method and pass an input string as a parameter. The method will return a parse result (the exact value depends on the grammar used to build the parser) or throw an exception if the input is invalid. The exception will contain `offset`, `line`, `column` and `message` properties with more details about the error.
 
     parser.parse("abba"); // returns ["a", "b", "b", "a"]
 

src/emitter.js

@@ -443,6 +443,7 @@ PEG.compiler.emitter = function(ast) {
             '        var errorPosition = computeErrorPosition();',
             '        throw new this.SyntaxError(',
             '          buildErrorMessage(),',
+            '          Math.max(pos, rightmostFailuresPos),',
             '          errorPosition.line,',
             '          errorPosition.column',
             '        );',
@@ -457,9 +458,10 @@ PEG.compiler.emitter = function(ast) {
             '  ',
             '  /* Thrown when a parser encounters a syntax error. */',
             '  ',
-            '  result.SyntaxError = function(message, line, column) {',
+            '  result.SyntaxError = function(message, offset, line, column) {',
             '    this.name = "SyntaxError";',
             '    this.message = message;',
+            '    this.offset = offset;',
             '    this.line = line;',
             '    this.column = column;',
             '  };',

src/parser.js

@@ -3598,6 +3598,7 @@ PEG.parser = (function(){
         var errorPosition = computeErrorPosition();
         throw new this.SyntaxError(
           buildErrorMessage(),
+          Math.max(pos, rightmostFailuresPos),
           errorPosition.line,
           errorPosition.column
         );
@@ -3612,9 +3613,10 @@ PEG.parser = (function(){
   
   /* Thrown when a parser encounters a syntax error. */
   
-  result.SyntaxError = function(message, line, column) {
+  result.SyntaxError = function(message, offset, line, column) {
     this.name = "SyntaxError";
     this.message = message;
+    this.offset = offset;
     this.line = line;
     this.column = column;
   };

test/compiler-test.js

@@ -447,10 +447,10 @@ test("error positions", function() {
   var simpleParser = PEG.buildParser('start = "a"');
 
   /* Regular match failure */
-  doesNotParseWithPos(simpleParser, "b", 1, 1);
+  doesNotParseWithPos(simpleParser, "b", 0, 1, 1);
 
   /* Trailing input */
-  doesNotParseWithPos(simpleParser, "ab", 1, 2);
+  doesNotParseWithPos(simpleParser, "ab", 1, 1, 2);
 
   var digitsParser = PEG.buildParser([
     'start  = line (("\\r" / "\\n" / "\\u2028" / "\\u2029")+ line)*',
@@ -458,16 +458,16 @@ test("error positions", function() {
     'digits = digits:[0-9]+ { return digits.join(""); }'
   ].join("\n"));
 
-  doesNotParseWithPos(digitsParser, "1\n2\n\n3\n\n\n4 5 x", 7, 5);
+  doesNotParseWithPos(digitsParser, "1\n2\n\n3\n\n\n4 5 x", 13, 7, 5);
 
   /* Non-Unix newlines */
-  doesNotParseWithPos(digitsParser, "1\rx", 2, 1);   // Old Mac
-  doesNotParseWithPos(digitsParser, "1\r\nx", 2, 1); // Windows
-  doesNotParseWithPos(digitsParser, "1\n\rx", 3, 1); // mismatched
+  doesNotParseWithPos(digitsParser, "1\rx", 2, 2, 1);   // Old Mac
+  doesNotParseWithPos(digitsParser, "1\r\nx", 3, 2, 1); // Windows
+  doesNotParseWithPos(digitsParser, "1\n\rx", 3, 3, 1); // mismatched
 
   /* Strange newlines */
-  doesNotParseWithPos(digitsParser, "1\u2028x", 2, 1); // line separator
-  doesNotParseWithPos(digitsParser, "1\u2029x", 2, 1); // paragraph separator
+  doesNotParseWithPos(digitsParser, "1\u2028x", 2, 2, 1); // line separator
+  doesNotParseWithPos(digitsParser, "1\u2029x", 2, 2, 1); // paragraph separator
 });
 
 test("start rule", function() {

test/helpers.js

@@ -19,11 +19,12 @@ doesNotParseWithMessage = function(parser, input, message) {
   );
 };
 
-doesNotParseWithPos = function(parser, input, line, column) {
+doesNotParseWithPos = function(parser, input, offset, line, column) {
   raises(
     function() { parser.parse(input); },
     function(e) {
       return e instanceof parser.SyntaxError
+        && e.offset === offset
         && e.line === line
         && e.column === column;
     }