// -*- go -*-
//
// Copyright (c) 2020-2021 Markku Rossi
//
// All rights reserved.
//

// Package sha256 implements the he SHA-224 and SHA-256 cryptographic
// hash functions.
package sha256

const (
	// The size of a SHA256 checksum in bytes.
	Size = 32

	// The blocksize of SHA256 and SHA224 in bytes.
	BlockSize = 64

	init = 0x6a09e667bb67ae853c6ef372a54ff53a510e527f9b05688c1f83d9ab5be0cd19
)

// Sum256 returns the SHA256 checksum of the data.
func Sum256(data []byte) [Size]byte {
	var state uint256 = init
	var block uint512
	var hash [Size]byte

	var pad [BlockSize]byte
	pad[0] = 0x80

	for i := 0; i < len(data); i++ {
		block <<= 8
		block = block | uint512(data[i])

		if (i+1)%BlockSize == 0 {
			state = Block(block, state)
			block = uint512(0)
		}
	}
	if len(data)%BlockSize < 56 {
		for i := len(data) % BlockSize; i < 56; i++ {
			block <<= 8
			block |= uint512(pad[i-len(data)%BlockSize])
		}
	} else {
		for i := len(data) % BlockSize; i < BlockSize; i++ {
			block <<= 8
			block |= uint512(pad[i-len(data)%BlockSize])
		}
		state = Block(block, state)
		block = uint512(0)
	}
	// Length in bits.
	block <<= 64
	block |= uint512(len(data) << 3)

	state = Block(block, state)

	for i := 0; i < Size; i++ {
		hash[Size-i-1] = byte(state & 0xff)
		state >>= 8
	}
	return hash
}

// Block adds a new SHA-256 block to the state.
func Block(block uint512, state uint256) uint256 {
	return native("sha256.circ", block, state)
}