From c5cafcc95699d900499730a862a48f0c6086fc6b Mon Sep 17 00:00:00 2001
From: "Devgru @ nemetz" <git@devg.ru>
Date: Tue, 30 Jul 2013 20:28:45 +0400
Subject: [PATCH] custom BEB128 implementation

---
 src/beb.coffee     | 32 ++++++++++++++++++++++++++++++++
 src/compose.coffee |  6 +++---
 src/leb.coffee     | 34 ----------------------------------
 src/parse.coffee   |  4 ++--
 test/beb.coffee    | 14 ++++++++++++++
 5 files changed, 51 insertions(+), 39 deletions(-)
 create mode 100644 src/beb.coffee
 delete mode 100644 src/leb.coffee
 create mode 100644 test/beb.coffee

diff --git a/src/beb.coffee b/src/beb.coffee
new file mode 100644
index 0000000..05ee2ee
--- /dev/null
+++ b/src/beb.coffee
@@ -0,0 +1,32 @@
+###
+BEB128 implementation. Tarantool uses BEB128 and calls it LEB128 somewhy.
+
+Big-endian base-128 uses 7 bits for value and most significant bit (MSB) for flag
+If flag is 1 — there is more bytes, continue reading.
+So, 127 is 0x7f, 128 is 0x81 0x00, 129 is 0x81 0x01 and so on.
+###
+
+encode = (value) ->
+    bytes = []
+
+    bytes.push value >> 28 | 0x80 if value >= 1 << 28
+    bytes.push value >> 21 | 0x80 if value >= 1 << 21
+    bytes.push value >> 14 | 0x80 if value >= 1 << 14
+    bytes.push value >>  7 | 0x80 if value >= 1 <<  7
+    bytes.push value & 0x7F
+
+    new Buffer bytes
+
+decode = (buffer, position = 0) ->
+    value = 0
+    loop # stops after reading byte with MSB=0
+        byte = buffer[position++]
+        value = (value << 7) + (byte & 0x7F)
+        break unless byte & 0x80
+    
+    value: value
+    nextIndex: position
+
+module.exports =
+    encode: encode
+    decode: decode
diff --git a/src/compose.coffee b/src/compose.coffee
index 12b4673..ac6248a 100644
--- a/src/compose.coffee
+++ b/src/compose.coffee
@@ -1,4 +1,4 @@
-leb = require './leb'
+beb = require './beb'
 
 module.exports = compose =
     ###
@@ -21,14 +21,14 @@ module.exports = compose =
     stringField: (value) ->
         # what about string encoding?
         stringBuffer = new Buffer value, 'utf-8' # default
-        lengthBuffer = leb.encodeUInt32 stringBuffer.length
+        lengthBuffer = beb.encode stringBuffer.length
         
         Buffer.concat [lengthBuffer, stringBuffer]
     
     tuple: (tuple) ->
         buffers = [compose.int32s tuple.length]
         for field in tuple
-            buffers.push leb.encodeUInt32 field.length
+            buffers.push beb.encode field.length
             buffers.push field
         
         Buffer.concat buffers
diff --git a/src/leb.coffee b/src/leb.coffee
deleted file mode 100644
index 8cebcd3..0000000
--- a/src/leb.coffee
+++ /dev/null
@@ -1,34 +0,0 @@
-# this file fixes LEB128 behaviour to be compatible with Tarantool
-
-leb = require 'leb'
-
-fix = (result) ->
-    len = result.length
-    # we have to reverse it
-    reversed = new Buffer len
-    for i in [0...len]
-        reversed[i] = result[len - 1 - i]
-    
-    # and toggle #7 bit in first and last bytes
-    reversed[0] ^= 0x80
-    reversed[len - 1] ^= 0x80
-    
-    reversed
-
-encode = (data) -> fix leb.encodeUInt32 data
-
-decode = (buf, pos = 0) ->
-    # we have to know what amount of bytes to read
-    temp = leb.decodeUInt32 buf, pos
-
-    # then slice the buffer, remembering position
-    buf = buf.slice pos, temp.nextIndex
-    result = leb.decodeUInt32 fix buf
-
-    # fix the position
-    result.nextIndex += pos
-    result
-
-module.exports =
-    encodeUInt32: encode
-    decodeUInt32: decode
diff --git a/src/parse.coffee b/src/parse.coffee
index 5c52e7b..ed18780 100644
--- a/src/parse.coffee
+++ b/src/parse.coffee
@@ -1,4 +1,4 @@
-leb = require './leb'
+beb = require './beb'
 
 module.exports = parse =
     ###
@@ -29,7 +29,7 @@ module.exports = parse =
         fields = []
         
         while count > 0
-            sizeLeb = leb.decodeUInt32 tuple, bytesRead
+            sizeLeb = beb.decode tuple, bytesRead
             bytesRead = sizeLeb.nextIndex
             size = sizeLeb.value
             fields.push tuple.slice bytesRead, bytesRead += size
diff --git a/test/beb.coffee b/test/beb.coffee
new file mode 100644
index 0000000..b4c3f95
--- /dev/null
+++ b/test/beb.coffee
@@ -0,0 +1,14 @@
+beb = require '../src/beb'
+
+exports['encode'] = (test) ->
+    test.equalBuffer = (b1, b2) ->
+        @equal b1.toString('hex'), b2.toString('hex')
+    test.equalBuffer (new Buffer [0x80 | 1, 0]), beb.encode 128
+    test.equalBuffer (new Buffer [0x80 | 1, 1]), beb.encode 129
+    do test.done
+    
+exports['fuzz'] = (test) ->
+    for i in [0...(1<<15)] by 1<<5
+        test.equal i, beb.decode(beb.encode(i)).value
+    do test.done
+