code: function _derive (values, digestFn) {
    var length = values.length
    var results = []
  
    for (var i = 0; i < length; i += 2) {
      var left = values[i]
      var right = i + 1 === length ? left : values[i + 1]
      var data = Buffer.concat([left, right])
  
      results.push(digestFn(data))
    }
  
    return results
  }
docs: returns an array of hashes of length: values.length / 2 + (values.length % 2)