Add ability to save the current offset as a variable

README.md

@@ -273,6 +273,30 @@ current object. `options` is an object which can have the following keys:
 ### skip(length)
 Skip parsing for `length` bytes.
 
+### saveOffset(name, options)
+Save the current buffer offset as key `name`. This function is only useful when called after another function which would advance the internal buffer offset.
+
+```javascript
+var parser = new Parser()
+  // this call advances the buffer offset by
+  // a variable (i.e. unknown to us) number of bytes
+  .string('name', {
+    zeroTerminated: true
+  })
+  // this variable points to an absolute position
+  // in the buffer
+  .uint32('seekOffset')
+  // now, save the "current" offset in the stream
+  // as the variable "currentOffset"
+  .saveOffset('currentOffset')
+  // finally, use the saved offset to figure out
+  // how many bytes we need to skip
+  .skip(function() {
+    return this.seekOffset - this.currentOffset;
+  })
+  ... // the parser would continue here
+```
+
 ### endianess(endianess)
 Define what endianess to use in this parser. `endianess` can be either
 `"little"` or `"big"`. The default endianess of `Parser` is set to big-endian.

lib/binary_parser.js

@@ -29,7 +29,8 @@ var SPECIAL_TYPES = {
   Skip: null,
   Choice: null,
   Nest: null,
-  Bit: null
+  Bit: null,
+  SaveOffset: null
 };
 
 var aliasRegistry = {};
@@ -116,6 +117,10 @@ Parser.prototype.skip = function(length, options) {
   return this.setNextParser("skip", "", { length: length });
 };
 
+Parser.prototype.saveOffset = function(varName, options) {
+  return this.setNextParser("saveoffset", varName, options);
+};
+
 Parser.prototype.string = function(varName, options) {
   if (!options.zeroTerminated && !options.length && !options.greedy) {
     throw new Error(
@@ -359,7 +364,7 @@ Parser.prototype.sizeOf = function() {
     // if this is a nested parser
   } else if (this.type === "Nest") {
     size = this.options.type.sizeOf();
-  } else if (!this.type) {
+  } else if (this.type === "SaveOffset" || !this.type) {
     size = 0;
   }
 
@@ -528,6 +533,11 @@ Parser.prototype.generateSkip = function(ctx) {
   ctx.pushCode("offset += {0};", length);
 };
 
+Parser.prototype.generateSaveOffset = function(ctx) {
+  var name = ctx.generateVariable(this.varName);
+  ctx.pushCode("{0} = offset;", name);
+};
+
 Parser.prototype.generateString = function(ctx) {
   var name = ctx.generateVariable(this.varName);
   var start = ctx.generateTmpVariable();