Add save/restore (and rewind)

Closes #221

Author: tjvr

lib/nearley.js

@@ -321,6 +321,11 @@ Parser.prototype.feed = function(chunk) {
             throw err;
         }
 
+        // maybe save lexer state
+        if (this.options.keepHistory) {
+          column.lexerState = lexer.save()
+        }
+
         this.current++;
     }
     if (column) {
@@ -334,6 +339,33 @@ Parser.prototype.feed = function(chunk) {
     return this;
 };
 
+Parser.prototype.save = function() {
+    var column = this.table[this.current];
+    column.lexerState = this.lexerState;
+    return column;
+};
+
+Parser.prototype.restore = function(column) {
+    var index = column.index;
+    this.current = index;
+    this.table[index] = column;
+    this.table.splice(index + 1);
+    this.lexerState = column.lexerState;
+
+    // Incrementally keep track of results
+    this.results = this.finish();
+};
+
+// nb. deprecated: use save/restore instead!
+Parser.prototype.rewind = function(index) {
+    if (!this.options.keepHistory) {
+        throw new Error('set option `keepHistory` to enable rewinding')
+    }
+    // nb. recall column (table) indicies fall between token indicies.
+    //        col 0   --   token 0   --   col 1
+    this.restore(this.table[index]);
+};
+
 Parser.prototype.finish = function() {
     // Return the possible parsings
     var considerations = [];

test/launch.js

@@ -168,11 +168,20 @@ describe('Parser', function() {
       )
     })
 
-    // TODO: save/restore
-
-    /*
     var tosh = compile(read("examples/tosh.ne"));
 
+    it('can save state', function() {
+        let first = "say 'hello'";
+        let second = " for 2 secs";
+        let p = new nearley.Parser(tosh, { keepHistory: true });
+        p.feed(first);
+        p.current.should.equal(11)
+        p.table.length.should.equal(12)
+        var col = p.save();
+        col.index.should.equal(11)
+        col.lexerState.col.should.equal(first.length)
+    });
+
     it('can rewind', function() {
         let first = "say 'hello'";
         let second = " for 2 secs";
@@ -184,6 +193,7 @@ describe('Parser', function() {
         p.feed(second);
 
         p.rewind(first.length);
+
         p.current.should.equal(11)
         p.table.length.should.equal(12)
 
@@ -194,6 +204,36 @@ describe('Parser', function() {
         let p = new nearley.Parser(tosh, {});
         p.rewind.should.throw();
     })
-    */
+
+    it('restores line numbers', function() {
+      let p = new nearley.Parser(testGrammar);
+      p.feed('abc\n')
+      p.save().lexerState.line.should.equal(2)
+      p.feed('123\n')
+      var col = p.save();
+      col.lexerState.line.should.equal(3)
+      p.feed('q')
+      p.restore(col);
+      p.lexer.line.should.equal(3)
+      p.feed('z')
+    });
+
+    it('restores column number', function() {
+      let p = new nearley.Parser(testGrammar);
+      p.feed('foo\nbar')
+      var col = p.save();
+      col.lexerState.line.should.equal(2)
+      col.lexerState.col.should.equal(3)
+      p.feed('123');
+      p.lexerState.col.should.equal(6)
+
+      p.restore(col);
+      p.lexerState.line.should.equal(2)
+      p.lexerState.col.should.equal(3)
+      p.feed('456')
+      p.lexerState.col.should.equal(6)
+    });
+
+    // TODO: moo save/restore
 
 });