// Package rsa implements crypto/rsa
package rsa

import (
	"bytes"
	"crypto"
	"crypto/rand"
	"crypto/rsa"
	"fmt"
	"math/big"

	"github.com/golang-module/dongle/openssl"
)

var (
	// returns an invalid public key error.
	// 返回无效的公钥错误
	invalidPublicKeyError = func() error {
		return fmt.Errorf("rsa: invalid public key, please make sure the public key is valid")
	}
	// returns an invalid private key error.
	// 返回无效的私钥错误
	invalidPrivateKeyError = func() error {
		return fmt.Errorf("rsa: invalid private key, please make sure the private key is valid")
	}
	// returns an unsupported hash function error.
	// 返回不支持的哈希函数错误
	unsupportedHashError = func() error {
		return fmt.Errorf("rsa: invalid hash function, the hash function is unsupported")
	}
)

// KeyPair defines a KeyPair struct.
// 定义 KeyPair 结构体
type KeyPair struct {
	publicKey  []byte
	privateKey []byte
	hash       crypto.Hash
}

// NewKeyPair returns a new KeyPair instance.
// 初始化 keyPair 结构体
func NewKeyPair() *KeyPair {
	return &KeyPair{}
}

// SetPublicKey sets public key.
// 设置公钥
func (k *KeyPair) SetPublicKey(publicKey []byte) {
	k.publicKey = publicKey
}

// SetPrivateKey sets private key.
// 设置私钥
func (k *KeyPair) SetPrivateKey(privateKey []byte) {
	k.privateKey = privateKey
}

// SetHash sets hash algorithm.
// 设置哈希算法
func (k *KeyPair) SetHash(hash crypto.Hash) {
	k.hash = hash
}

// EncryptByPublicKey encrypts by public key.
// 通过公钥加密
func (k *KeyPair) EncryptByPublicKey(src []byte) (dst []byte, err error) {
	dst = []byte("")
	if len(src) == 0 {
		return
	}
	if !openssl.RSA.IsPublicKey(k.publicKey) {
		err = invalidPublicKeyError()
		return
	}
	pub, err := openssl.RSA.ParsePublicKey(k.publicKey)
	if err != nil {
		err = invalidPublicKeyError()
		return
	}
	buffer := bytes.NewBufferString("")
	for _, chunk := range bytesSplit(src, pub.Size()-11) {
		dst, err = rsa.EncryptPKCS1v15(rand.Reader, pub, chunk)
		buffer.Write(dst)
	}
	dst = buffer.Bytes()
	return
}

// DecryptByPrivateKey encrypts by private key.
// 通过私钥解密
func (k *KeyPair) DecryptByPrivateKey(src []byte) (dst []byte, err error) {
	dst = []byte("")
	if len(src) == 0 {
		return
	}
	if !openssl.RSA.IsPrivateKey(k.privateKey) {
		err = invalidPrivateKeyError()
		return
	}
	pri, err := openssl.RSA.ParsePrivateKey(k.privateKey)
	if err != nil {
		err = invalidPrivateKeyError()
		return
	}
	buffer := bytes.NewBufferString("")
	for _, chunk := range bytesSplit(src, pri.Size()) {
		dst, err = rsa.DecryptPKCS1v15(rand.Reader, pri, chunk)
		buffer.Write(dst)
	}
	dst = buffer.Bytes()
	return
}

// EncryptByPrivateKey encrypts by private key.
// 通过私钥加密
func (k *KeyPair) EncryptByPrivateKey(src []byte) (dst []byte, err error) {
	dst = []byte("")
	if len(src) == 0 {
		return
	}

	if !openssl.RSA.IsPrivateKey(k.privateKey) {
		err = invalidPrivateKeyError()
		return
	}
	pri, err := openssl.RSA.ParsePrivateKey(k.privateKey)
	if err != nil {
		err = invalidPrivateKeyError()
		return
	}
	buffer := bytes.NewBufferString("")
	for _, chunk := range bytesSplit(src, pri.Size()-11) {
		dst, err = rsa.SignPKCS1v15(nil, pri, crypto.Hash(0), chunk)
		buffer.Write(dst)
	}
	dst = buffer.Bytes()
	return
}

// DecryptByPublicKey encrypts by public key.
// 通过公钥解密
func (k *KeyPair) DecryptByPublicKey(src []byte) (dst []byte, err error) {
	dst = []byte("")
	if len(src) == 0 {
		return
	}

	if !openssl.RSA.IsPublicKey(k.publicKey) {
		err = invalidPublicKeyError()
		return
	}
	pub, err := openssl.RSA.ParsePublicKey(k.publicKey)
	if err != nil {
		err = invalidPublicKeyError()
		return
	}
	buffer := bytes.NewBufferString("")
	bigInt := new(big.Int)
	for _, chunk := range bytesSplit(src, pub.Size()) {
		bigInt.Exp(new(big.Int).SetBytes(chunk), big.NewInt(int64(pub.E)), pub.N)
		dst = leftUnPad(leftPad(bigInt.Bytes(), pub.Size()))
		buffer.Write(dst)
	}
	dst = buffer.Bytes()
	return
}

// SignByPrivateKey signs by private key.
// 通过私钥签名
func (k *KeyPair) SignByPrivateKey(src []byte) (dst []byte, err error) {
	dst = []byte("")
	pri, err := openssl.RSA.ParsePrivateKey(k.privateKey)
	if err != nil {
		err = invalidPrivateKeyError()
		return
	}
	if !k.IsSupportedHash() {
		err = unsupportedHashError()
		return
	}
	hasher := k.hash.New()
	hasher.Write(src)
	hashed := hasher.Sum(nil)
	dst, err = rsa.SignPKCS1v15(rand.Reader, pri, k.hash, hashed)
	return
}

// VerifyByPublicKey verify by public key.
// 通过公钥验签
func (k *KeyPair) VerifyByPublicKey(src, sign []byte) (err error) {
	pub, err := openssl.RSA.ParsePublicKey(k.publicKey)
	if err != nil {
		err = invalidPublicKeyError()
		return
	}
	if !k.IsSupportedHash() {
		err = unsupportedHashError()
		return
	}
	hasher := k.hash.New()
	hasher.Write(src)
	hashed := hasher.Sum(nil)
	return rsa.VerifyPKCS1v15(pub, k.hash, hashed, sign)
}

// IsPublicKey whether is a public key.
// 是否是公钥
func (k *KeyPair) IsPublicKey() bool {
	return openssl.RSA.IsPublicKey(k.publicKey)
}

// IsPrivateKey whether is a private key.
// 是否是私钥
func (k *KeyPair) IsPrivateKey() bool {
	return openssl.RSA.IsPrivateKey(k.privateKey)
}

// IsSupportedHash whether is a supported hash algorithm.
// 判断是否是支持的哈希算法
func (k *KeyPair) IsSupportedHash() bool {
	hashes := []crypto.Hash{
		crypto.MD5, crypto.SHA1, crypto.SHA224, crypto.SHA256, crypto.SHA384, crypto.SHA512, crypto.RIPEMD160,
	}
	for _, hash := range hashes {
		if hash == k.hash {
			return true
		}
	}
	return false
}

// left padding.
// 左补码
func leftPad(src []byte, size int) (dst []byte) {
	dst = make([]byte, size)
	copy(dst[len(dst)-len(src):], src)
	return
}

// remove left padding.
// 左减码
func leftUnPad(src []byte) (dst []byte) {
	n := len(src)
	t := 2
	for i := 2; i < n; i++ {
		if src[i] == 0xff {
			t = t + 1
		} else {
			if src[i] == src[0] {
				t = t + int(src[1])
			}
			break
		}
	}
	dst = make([]byte, n-t)
	copy(dst, src[t:])
	return
}

// split the byte slice by the specified size.
// 按照指定长度分割字节切片
func bytesSplit(buf []byte, size int) [][]byte {
	var chunk []byte
	chunks := make([][]byte, 0, len(buf)/size+1)
	for len(buf) >= size {
		chunk, buf = buf[:size], buf[size:]
		chunks = append(chunks, chunk)
	}
	if len(buf) > 0 {
		chunks = append(chunks, buf[:])
	}
	return chunks
}