File src/0dev.org/ioutil/ioutil.go artifact 4d4fd7d642 part of check-in 4195e7817d

// 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 reader that will delegate calls to Read(...) while ensuring
// that the output buffer will never be smaller than the required size
func MinReader(reader io.Reader, size int) io.Reader {
	var buffer []byte = make([]byte, 0, size)

	return ReaderFunc(func(output []byte) (readCount int, e error) {
		var (
			bufferLength int = len(buffer)
			err          error
		)
	start:
		// Reply with the buffered data if there is any
		if bufferLength > 0 {
			readCount = copy(output, buffer)

			if readCount < bufferLength {
				// Advance the data in the buffer
				buffer = buffer[:copy(buffer, buffer[readCount:])]
			} else {
				// Clear the buffer
				buffer = buffer[:0]
			}
			// Stage any error for returning
			e, err = err, nil
			return readCount, e
		}

		// Delegate if the buffer is empty and the destination buffer is large enough
		if len(output) >= size {
			return reader.Read(output[:len(output)/size])
		}

		// Extend the buffer up to the desired size and perform a Read
		buffer = buffer[:size]
		readCount, err = reader.Read(buffer)

		// Size the buffer down to the read data size and restart
		buffer = buffer[:readCount]
		bufferLength = len(buffer)

		if bufferLength > 0 {
			goto start
		}
		// Stage any error for returning
		e, err = err, nil
		return 0, e
	})
}