Reimplement offset() and add range(). (#528)

* Reimplement offset()
* Implement range()

Fixes #526, thanks @felix9

README.md

@@ -454,6 +454,10 @@ The `start` and `end` properties both refer to the current parse position. The
 `offset` property contains an offset as a zero-based index and `line` and
 `column` properties contain a line and a column as one-based indices.
 
+Line and column are somewhat expensive to compute, so if you just need the
+offset, there's also a function `offset` that returns just the start offset,
+and a function `range` that returns the array `[start, end]` offsets.
+
 The code inside the predicate can also access options passed to the parser using
 the `options` variable.
 

lib/compiler/passes/generate-js.js

@@ -1035,6 +1035,14 @@ function generateJS(ast, options) {
       "    return input.substring(peg$savedPos, peg$currPos);",
       "  }",
       "",
+      "  function offset() {",
+      "    return peg$savedPos;",
+      "  }",
+      "",
+      "  function range() {",
+      "    return [peg$savedPos, peg$currPos];",
+      "  }",
+      "",
       "  function location() {",
       "    return peg$computeLocation(peg$savedPos, peg$currPos);",
       "  }",

test/behavior/generated-parser-behavior.spec.js

@@ -583,6 +583,24 @@ describe("generated parser behavior", function() {
             end: { offset: 3, line: 2, column: 1 }
           });
         });
+
+        it("|offset| returns current start offset", function() {
+          let parser = peg.generate([
+            "start = [0-9]+ val:mark { return val; }",
+            "mark = 'xx' { return offset(); }"
+          ].join("\n"), options);
+
+          expect(parser).to.parse("0123456xx", 7);
+        });
+
+        it("|range| returns current range", function() {
+          let parser = peg.generate([
+            "start = [0-9]+ val:mark { return val; }",
+            "mark = 'xx' { return range(); }"
+          ].join("\n"), options);
+
+          expect(parser).to.parse("0123456xx", [7, 9]);
+        });
       });
     });