package inetcksum

import (
	`io`
)

/*
Aligned returns true if the current underlying buffer in an InetChecksum 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, but will not be written
to the persistent underlying storage.

If aligned's underlying buffer/storage is empty or nil, aligned will be true.

Aligned will also force-set the internal state's aligned status.
*/
func (i *InetChecksum) Aligned() (aligned bool) {

	i.alignLock.Lock()
	defer i.alignLock.Unlock()

	i.bufLock.RLock()
	aligned = i.buf.Len()&2 == 0
	i.bufLock.RUnlock()

	i.aligned = aligned

	return
}

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

	blockSize = 1

	return
}

/*
Bytes returns teh bytes currently in the internal storage.

curBuf will be nil if the internal storage has not yet been initialized.
*/
func (i *InetChecksum) Bytes() (curBuf []byte) {

	i.bufLock.RLock()
	defer i.bufLock.RUnlock()

	if i.buf.Len() != 0 {
		curBuf = i.buf.Bytes()
	}

	return
}

// Clear empties the internal buffer (but does not affect the checksum state).
func (i *InetChecksum) Clear() {

	i.bufLock.Lock()
	defer i.bufLock.Unlock()

	i.buf.Reset()
}

/*
DisablePersist disables the internal persistence of an InetChecksum.

This is recommended for integrations that desire the concurrency safety
of an InetChecksum but want a smaller memory footprint and do not need a copy
of data that was hashed.

Any data existing in the buffer will NOT be cleared out if DisablePersist is called.
You must call [InetChecksum.Clear] to do that.

Persistence CANNOT be reenabled once disabled. [InetChecksum.Reset]
must be called to re-enable persistence.
*/
func (i *InetChecksum) DisablePersist() {

	i.bufLock.Lock()
	defer i.bufLock.Unlock()

	i.disabledBuf = true
}

// Len returns the current amount of bytes stored in this InetChecksum's internal buffer.
func (i *InetChecksum) Len() (l int) {

	i.bufLock.RLock()
	defer i.bufLock.RUnlock()
	l = i.buf.Len()

	return
}

/*
Reset resets the internal buffer/storage to an empty state.

If persistence was disabled ([InetChecksum.DisablePersist]),
this method will re-enable it with an empty buffer.
If you wish the buffer to be disabled, you must invoke [InetChecksum.DisablePersist]
again.

If you only wish to clear the buffer without losing the checksum state,
use [InetChecksum.Clear].
*/
func (i *InetChecksum) Reset() {

	i.alignLock.Lock()
	i.bufLock.Lock()
	i.sumLock.Lock()
	i.lastLock.Lock()

	i.aligned = false
	i.alignLock.Unlock()

	i.buf.Reset()
	i.disabledBuf = false
	i.bufLock.Unlock()

	i.last = 0x00
	i.lastLock.Unlock()

	i.sum = 0
	i.sumLock.Unlock()
}

// Size returns how many bytes a checksum is. (It will always return 2.)
func (i *InetChecksum) 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 *InetChecksum) 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 *InetChecksum) Sum16() (cksum uint16) {

	var thisSum uint32

	i.alignLock.RLock()
	i.lastLock.RLock()
	i.sumLock.RLock()

	thisSum = i.sum
	i.sumLock.RUnlock()

	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
	}
	i.lastLock.RUnlock()
	i.alignLock.RUnlock()

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

	return
}

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

	var sum16 uint16 = i.Sum16()

	cksum = make([]byte, 2)

	ord.PutUint16(cksum, sum16)

	return
}

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

If this operation returns an error, you MUST call [InetChecksum.Reset] as the instance
being used can no longer be considered to be in a consistent state.

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

Write is concurrency safe; a copy of p is made first and all hashing/internal
storage writing is performed on/which that copy.
*/
func (i *InetChecksum) Write(p []byte) (n int, err error) {

	var idx int
	var bufLen int
	var buf []byte
	var iter int
	var origLast byte
	var origAligned bool
	var origSum uint32

	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)

	i.alignLock.Lock()
	defer i.alignLock.Unlock()
	i.bufLock.Lock()
	defer i.bufLock.Unlock()
	i.sumLock.Lock()
	defer i.sumLock.Unlock()
	i.lastLock.Lock()
	defer i.lastLock.Unlock()

	origLast = i.last
	origAligned = i.aligned
	origSum = i.sum

	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
		}
	}

	if !i.disabledBuf {
		if n, err = i.buf.Write(buf); err != nil {
			i.sum = origSum
			i.aligned = origAligned
			i.last = origLast
			return
		}
	}

	return
}

// WriteByte writes a single byte to the underlying storage. It conforms to [io.ByteWriter].
func (i *InetChecksum) WriteByte(c byte) (err error) {

	var origLast byte
	var origAligned bool
	var origSum uint32

	i.alignLock.Lock()
	defer i.alignLock.Unlock()
	i.bufLock.Lock()
	defer i.bufLock.Unlock()
	i.sumLock.Lock()
	defer i.sumLock.Unlock()
	i.lastLock.Lock()
	defer i.lastLock.Unlock()

	origLast = i.last
	origAligned = i.aligned
	origSum = i.sum

	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
	}

	if !i.disabledBuf {
		if err = i.WriteByte(c); err != nil {
			i.sum = origSum
			i.aligned = origAligned
			i.last = origLast
			return
		}
	}

	return
}

// WriteString writes a string to the underlying storage. It conforms to [io.StringWriter].
func (i *InetChecksum) WriteString(s string) (n int, err error) {

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

	return
}

// WriteTo writes the current contents of the underlying buffer to w. The contents are not drained. Noop if persistence is disabled.
func (i *InetChecksum) WriteTo(w io.Writer) (n int64, err error) {

	var wrtn int

	if i.disabledBuf {
		return
	}

	i.bufLock.RLock()
	defer i.bufLock.RUnlock()

	if wrtn, err = w.Write(i.buf.Bytes()); err != nil {
		n = int64(wrtn)
		return
	}
	n = int64(wrtn)

	return
}