// Package ioutil contains various constructs for io operations. package ioutil import ( "io" ) // An function alias type that implements io.Writer. type WriterFunc func([]byte) (int, error) // Delegates the call to the WriterFunc while implementing io.Writer. func (w WriterFunc) Write(b []byte) (int, error) { return w(b) } // An function alias type that implements io.Reader. type ReaderFunc func([]byte) (int, error) // Delegates the call to the WriterFunc while implementing io.Reader. func (r ReaderFunc) Read(b []byte) (int, error) { return r(b) } // Returns a writer that delegates calls to Write(...) while ensuring // that it is never called with less bytes than the specified amount. // // Calls with fewer bytes are buffered while a call with a nil slice // causes the buffer to be flushed to the underlying writer. func SizedWriter(writer io.Writer, size int) io.Writer { var buffer []byte = make([]byte, 0, size) var write WriterFunc write = func(input []byte) (int, error) { var ( count int err error ) // Flush the buffer when called with no bytes to write if input == nil { // Call the writer with whatever we have in store.. count, err = writer.Write(buffer) // Advance the buffer buffer = buffer[:copy(buffer, buffer[count:])] return 0, err } // Delegate to the writer if the size is right if len(buffer) == 0 && len(input) >= size { return writer.Write(input) } // Append data to the buffer count = copy(buffer[len(buffer):size], input) buffer = buffer[:len(buffer)+count] // Return if we don't have enough bytes to write if len(buffer) < size { return len(input), nil } // Flush the buffer as it is filled _, err = write(nil) if err != nil { return count, err } // Handle the rest of the input return write(input[count:]) } return write } // Returns a reader that delegates calls to Read(...) while ensuring // that the output buffer is never smaller than the required size // and is downsized to a multiple of the required size if larger. func SizedReader(reader io.Reader, size int) io.Reader { var buffer []byte = make([]byte, 0, size) return ReaderFunc(func(output []byte) (int, error) { var ( count int err error ) start: // Reply with the buffered data if there is any if len(buffer) > 0 { count = copy(output, buffer) // Advance the data in the buffer buffer = buffer[:copy(buffer, buffer[count:])] // Return count and error if we have read the whole buffer if len(buffer) == 0 { return count, err } // Do not propagate an error until the buffer is exhausted return count, nil } // Delegate if the buffer is empty and the destination buffer is large enough if len(output) >= size { return reader.Read(output[:(len(output)/size)*size]) } // Perform a read into the buffer count, err = reader.Read(buffer[:size]) // Size the buffer down to the read data size // and restart if we have successfully read some bytes buffer = buffer[:count] if len(buffer) > 0 { goto start } // Returning on err/misbehaving noop reader return 0, err }) }