Merge pull request #7 from dominictarr/encoding-length

Encoding length

README.md

@@ -31,6 +31,11 @@ if you also require the length (number of bytes) that were required to decode th
 
 similar to `bytesRead` when encoding a number it can be useful to know how many bytes where written (especially if you pass an output array). you can access this via `varint.encode.bytesWritten` which holds the number of bytes written in the last encode.
 
+
+### varint.encodingLength(num)
+
+returns the number of bytes this number will be encoded as, up to a maximum of 8.
+
 ## usage notes
 
 if you are using this to decode buffers from a streaming source it's up to you to make sure that you send 'complete' buffers into `varint.decode`. the maximum number of bytes that varint will need to decode is 8, so all you have to do is make sure you are sending buffers that are at least 8 bytes long from the point at which you know a varint range begins.

index.js

@@ -1,4 +1,5 @@
 module.exports = {
     encode: require('./encode.js')
   , decode: require('./decode.js')
-}
+  , encodingLength: require('./length.js')
+}

length.js

@@ -0,0 +1,21 @@
+
+var N1 = Math.pow(2,  7)
+var N2 = Math.pow(2, 14)
+var N3 = Math.pow(2, 21)
+var N4 = Math.pow(2, 28)
+var N5 = Math.pow(2, 35)
+var N6 = Math.pow(2, 42)
+var N7 = Math.pow(2, 49)
+
+module.exports = function (value) {
+  return (
+    value < N1 ? 1
+  : value < N2 ? 2
+  : value < N3 ? 3
+  : value < N4 ? 4
+  : value < N5 ? 5
+  : value < N6 ? 6
+  : value < N7 ? 7
+  :              8
+  )
+}

test.js

@@ -2,6 +2,7 @@ var varint = require('./index')
   , test = require('tape')
   , decode = varint.decode
   , encode = varint.encode
+  , encodingLength = varint.encodingLength
 
 test('fuzz test', function(assert) {
   var expect
@@ -106,6 +107,15 @@ test('fuzz test - big', function(assert) {
   assert.end()
 })
 
+test('encodingLength', function (assert) {
+
+  for(var i = 0; i <= 53; i++) {
+    var n = Math.pow(2, i)
+    assert.equal(encode(n).length, encodingLength(n))
+  }
+
+  assert.end()
+})
 
 function randint(range) {
   return Math.floor(Math.random() * range)