package inetcksum

/*
Aligned returns true if the current checksum for an InetChecksumSimple is
aligned to the algorithm's requirement for an even number of bytes.

Note that if Aligned returns false, a single null pad byte will be applied
to the underlying data buffer at time of a Sum* call.
*/
func (i *InetChecksumSimple) Aligned() (aligned bool) {

	aligned = i.aligned

	return
}

// BlockSize returns the number of bytes at a time that InetChecksumSimple operates on. (It will always return 1.)
func (i *InetChecksumSimple) BlockSize() (blockSize int) {

	blockSize = 1

	return
}

// Reset resets the state of an InetChecksumSimple.
func (i *InetChecksumSimple) Reset() {

	i.last = 0x00
	i.sum = 0
	i.last = 0x00

}

// Size returns how many bytes a checksum is. (It will always return 2.)
func (i *InetChecksumSimple) Size() (bufSize int) {

	bufSize = 2

	return
}

// Sum computes the checksum cksum of the current buffer and appends it as big-endian bytes to b.
func (i *InetChecksumSimple) Sum(b []byte) (cksumAppended []byte) {

	var sum16 []byte = i.Sum16Bytes()

	cksumAppended = append(b, sum16...)

	return
}

/*
Sum16 computes the checksum of the current buffer and returns it as a uint16.

This is the native number used in the IPv4 header.
All other Sum* methods wrap this method.

If the underlying buffer is empty or nil, cksum will be 0xffff (65535)
in line with common implementations.
*/
func (i *InetChecksumSimple) Sum16() (cksum uint16) {

	var thisSum uint32

	thisSum = i.sum

	if !i.aligned {
		/*
			"Pad" at the end of the additive ops - a bitshift is used on the sum integer itself
			instead of a binary.Append() or append() or such to avoid additional memory allocation.
		*/
		thisSum += uint32(i.last) << padShift
	}

	// Fold the "carried ones".
	for thisSum > cksumMask {
		thisSum = (thisSum & cksumMask) + (thisSum >> cksumShift)
	}
	cksum = ^uint16(thisSum)

	return
}

/*
Sum16Bytes is a convenience wrapper around [InetChecksumSimple.Sum16]
which returns a slice of the uint16 as a 2-byte-long slice instead.
*/
func (i *InetChecksumSimple) Sum16Bytes() (cksum []byte) {

	var sum16 uint16 = i.Sum16()

	cksum = make([]byte, 2)

	ord.PutUint16(cksum, sum16)

	return
}

/*
Write writes data to the underlying InetChecksumSimple buffer. It conforms to [io.Writer].

p may be nil or empty; no error will be returned and n will be 0 if so.

A copy of p is made first and all hashing operations are performed on that copy.
*/
func (i *InetChecksumSimple) Write(p []byte) (n int, err error) {

	var idx int
	var bufLen int
	var buf []byte
	var iter int

	if p == nil || len(p) == 0 {
		return
	}

	// The TL;DR here is the checksum boils down to:
	// cksum = cksum + ((high << 8) | low)

	bufLen = len(p)
	buf = make([]byte, bufLen)
	copy(buf, p)

	if !i.aligned {
		// Last write was unaligned, so pair i.last in.
		i.sum += (uint32(i.last) << padShift) | uint32(buf[0])
		i.aligned = true
		idx = 1
	}

	// Operate on bytepairs.
	// Note that idx is set to either 0 or 1 depending on if
	// buf[0] has already been summed in.
	for iter = idx; iter < bufLen; iter += 2 {
		if iter+1 < bufLen {
			// Technically could use "i.sum += uint32(ord.Uint16(buf[iter:iter+2))" here instead.
			i.sum += (uint32(buf[iter]) << padShift) | uint32(buf[iter+1])
		} else {
			i.last = buf[iter]
			i.aligned = false
			break
		}
	}

	return
}

// WriteByte checksums a single byte. It conforms to [io.ByteWriter].
func (i *InetChecksumSimple) WriteByte(c byte) (err error) {

	if i.aligned {
		// Since it's a single byte, we just set i.last and unalign.
		i.last = c
		i.aligned = false
	} else {
		// It's unaligned, so join with i.last and align.
		i.sum += (uint32(i.last) << padShift) | uint32(c)
		i.aligned = true
	}

	return
}

// WriteString checksums a string. It conforms to [io.StringWriter].
func (i *InetChecksumSimple) WriteString(s string) (n int, err error) {

	if n, err = i.Write([]byte(s)); err != nil {
		return
	}

	return
}