add saveoffset method

Author: Fabian Wiles

README.md

@@ -289,6 +289,30 @@ Jump to `offset`, execute parser for `type` and rewind to previous offset.
     start of the input buffer. Can be a string `[u]int{8, 16, 32, 64}{le, be}`
     or an user defined Parser object.
 
+### 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
+```
+
 ### skip(length)
 Skip parsing for `length` bytes.
 

lib/binary_parser.ts

@@ -35,6 +35,7 @@ type ComplexTypes =
   | 'nest'
   | 'skip'
   | 'pointer'
+  | 'saveoffset'
   | '';
 
 type Endianess = 'be' | 'le';
@@ -151,6 +152,7 @@ const CAPITILIZED_TYPE_NAMES: { [key in Types]: string } = {
   nest: 'Nest',
   skip: 'Skip',
   pointer: 'Pointer',
+  saveoffset: 'SaveOffset',
   '': '',
 };
 
@@ -539,6 +541,10 @@ export class Parser {
     return this.setNextParser('pointer', varName, options);
   }
 
+  saveoffset(varName: string, options?: ParserOptions) {
+    return this.setNextParser('saveoffset', varName, options);
+  }
+
   endianess(endianess: 'little' | 'big') {
     switch (endianess.toLowerCase()) {
       case 'little':
@@ -762,6 +768,9 @@ export class Parser {
         case 'pointer':
           this.generatePointer(ctx);
           break;
+        case 'saveoffset':
+          this.generateSaveOffset(ctx);
+          break;
       }
       this.generateAssert(ctx);
     }
@@ -1108,4 +1117,9 @@ export class Parser {
     // Restore offset
     ctx.pushCode(`offset = ${tempVar};`);
   }
+
+  private generateSaveOffset(ctx: Context) {
+    const varName = ctx.generateVariable(this.varName);
+    ctx.pushCode(`${varName} = offset`);
+  }
 }

test/composite_parser.js

@@ -995,6 +995,40 @@ describe('Composite parser', function() {
     });
   });
 
+  describe('SaveOffset', () => {
+    it('should save the offset', () => {
+      const buff = Buffer.from([0x01, 0x00, 0x02]);
+      const parser = Parser.start().int8('a').int16('b').saveoffset('bytesRead');
+
+      assert.deepEqual(parser.parse(buff), {
+        a: 1,
+        b: 2,
+        bytesRead: 3,
+      })
+    })
+
+    it('should save the offset if not at end', () => {
+      const buff = Buffer.from([0x01, 0x00, 0x02]);
+      const parser = Parser.start().int8('a').saveoffset('bytesRead').int16('b');
+
+      assert.deepEqual(parser.parse(buff), {
+        a: 1,
+        b: 2,
+        bytesRead: 1,
+      })
+    })
+
+    it('should save the offset with a dynamic parser', () => {
+      const buff = Buffer.from([0x74, 0x65, 0x73, 0x74, 0x00]);
+      const parser = Parser.start().string('name', {zeroTerminated: true}).saveoffset('bytesRead');
+
+      assert.deepEqual(parser.parse(buff), {
+        name: 'test',
+        bytesRead: 5,
+      })
+    })
+  })
+
   describe('Utilities', function() {
     it('should count size for fixed size structs', function() {
       var parser = Parser.start()