buffer: add method readBits for reading slices of a byte

lib/buffer.js

@@ -616,6 +616,21 @@ function checkOffset(offset, ext, length) {
 }
 
 
+Buffer.prototype.readBits = function(position, offset, bitLength, noAssert) {
+  position = position >>> 0;
+  offset = offset >>> 0;
+  bitLength = bitLength >>> 0;
+  if (!noAssert) {
+    if (position < 0 || position >= this.length)
+      throw new RangeError('index out of range');
+    if (bitLength > 8)
+      throw new RangeError('bit length out of range');
+  }
+
+  return (this[position] >> offset) & ((1 << bitLength) - 1);
+};
+
+
 Buffer.prototype.readUIntLE = function(offset, byteLength, noAssert) {
   offset = offset >>> 0;
   byteLength = byteLength >>> 0;

test/parallel/test-buffer.js

@@ -1172,3 +1172,7 @@ Buffer.poolSize = ps;
 assert.throws(function() {
   Buffer(10).copy();
 });
+
+var b = new Buffer(1).fill(1);
+assert.equal(b.readBits(0, 0, 4), 1);
+assert.equal(b.readBits(0, 4, 4), 0);