package imgutil

import "bytes"

// 与管理后台 ShowImg / imgCode.js 保持一致。
var (
	encryptMagicNumber = []byte{0x88, 0xA8, 0x30, 0xCB, 0x10, 0x76}
	decryptKey         = []byte{50, 48, 49, 57, 121, 115, 97, 112, 112, 55, 53, 50, 55} // 2019ysapp7527
	plainImageHeaders  = [][]byte{
		{0xFF, 0xD8, 0xFF}, // jpeg
		{0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A}, // png
		{0x47, 0x49, 0x46}, // gif
	}
)

const (
	encryptedPrefixLen = 100
	encryptXORAllKey   = 0xA3
)

// DecryptImage 按后台 handleVerAutoImg 的规则解密图床图片。
// 已是 jpg/png/gif 则原样返回。
func DecryptImage(data []byte) []byte {
	if len(data) == 0 {
		return data
	}
	out := append([]byte(nil), data...)
	if hasEncryptMagic(out) {
		return xorAllLength(out)
	}
	if !isPlainImage(out) {
		xorPrefix(out, decryptKey, encryptedPrefixLen)
	}
	return out
}

func hasEncryptMagic(data []byte) bool {
	return len(data) >= len(encryptMagicNumber) && bytes.Equal(data[:len(encryptMagicNumber)], encryptMagicNumber)
}

func isPlainImage(data []byte) bool {
	for _, header := range plainImageHeaders {
		if len(data) >= len(header) && bytes.Equal(data[:len(header)], header) {
			return true
		}
	}
	return false
}

func xorPrefix(src, key []byte, length int) {
	if len(key) == 0 || len(src) == 0 {
		return
	}
	if length <= 0 || length > len(src) {
		length = len(src)
	}
	for i := 0; i < length; {
		for j := 0; j < len(key) && i < length; j++ {
			src[i] ^= key[j]
			i++
		}
	}
}

func xorAllLength(src []byte) []byte {
	out := make([]byte, 0, len(src))
	for i, b := range src {
		if i < len(encryptMagicNumber) && b == encryptMagicNumber[i] {
			continue
		}
		out = append(out, b^encryptXORAllKey)
	}
	return out
}