// 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 {
			reduced := (len(input) / size) * size
			count, err = writer.Write(input[:reduced])
			if count < reduced || err != nil {
				return count, err
			}

			// Stage any remaining data in the buffer
			buffer = append(buffer, input[count:]...)
			return len(input), nil
		}

		// 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
	})
}
