// 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)
		}

		// 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)
		goto start
	})
}

package ioutil

import (
	diff "0dev.org/diff"
	"bytes"
	"io"
	"testing"
)

func TestWriter(t *testing.T) {
	var (
		input  []byte = []byte{0, 1, 2, 3, 4, 5, 6, 7}
		output []byte

	// Diff the result against the initial input
	delta := diff.Diff(diff.D{len(input), len(output),
		func(i, j int) bool { return input[i] == output[j] }})
	if len(delta.Added) > 0 || len(delta.Removed) > 0 {
		t.Error("Differences detected ", delta)
	}
}

func TestMinReader(t *testing.T) {
	var (
		input  []byte = []byte{0, 1, 2, 3, 4, 5, 6, 7}
		output []byte = make([]byte, 16)

		reader *bytes.Reader = bytes.NewReader(input)
		min    io.Reader     = MinReader(reader, 4)
	)

	// Expecting a read count of 2
	count, err := min.Read(output[:2])
	if count != 2 {
		t.Error("Invalid read count from MinReader", count)
	}
	if err != nil {
		t.Error("Unexpected error from MinReader", err)
	}

	// Expecting a read count of 2 as it should have 2 bytes in its buffer
	count, err = min.Read(output[:3])
	if count != 2 {
		t.Error("Invalid read count from MinReader", count)
	}
	if err != nil {
		t.Error("Unexpected error from MinReader", err)
	}

	// Expecting a read count of 4 as the buffer should be empty
	count, err = min.Read(output[:4])
	if count != 4 {
		t.Error("Invalid read count from MinReader", count)
	}
	if err != nil {
		t.Error("Unexpected error from MinReader", err)
	}

	// Expecting a read count of 0 with an EOF as the buffer should be empty
	count, err = min.Read(output[:4])
	if count != 0 {
		t.Error("Invalid read count from MinReader", count)
	}
	if err != io.EOF {
		t.Error("Unexpected error from MinReader", err)
	}
}