//
//  PKCE.swift
//  react-native-zalo-auth
//
//

import Foundation
#if os(Linux)
import Crypto
#else
import CommonCrypto
#endif

public func generateState(withLength len: Int) -> String {
    let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    let length = UInt32(letters.count)

    var randomString = ""
    for _ in 0..<len {
        let rand = arc4random_uniform(length)
        let idx = letters.index(letters.startIndex, offsetBy: Int(rand))
        let letter = letters[idx]
        randomString += String(letter)
    }
    return randomString
}

public func generateCodeVerifier() -> String? {
    var buffer = [UInt8](repeating: 0, count: 32)
    _ = SecRandomCopyBytes(kSecRandomDefault, buffer.count, &buffer)
   let codeVerifier = Data(buffer).base64EncodedString()
        .replacingOccurrences(of: "+", with: "-")
        .replacingOccurrences(of: "/", with: "_")
        .replacingOccurrences(of: "=", with: "")
        .trimmingCharacters(in: .whitespaces)

    return codeVerifier
}

public func generateCodeChallenge(codeVerifier: String?) -> String? {
    guard let verifier = codeVerifier, let data = verifier.data(using: .utf8) else { return nil }

    #if !os(Linux)
    var buffer = [UInt8](repeating: 0,  count: Int(CC_SHA256_DIGEST_LENGTH))
    data.withUnsafeBytes {
        _ = CC_SHA256($0.baseAddress, CC_LONG(data.count), &buffer)
    }
    let hash = Data(buffer)
    #else
    let buffer = [UInt8](repeating: 0,  count: SHA256.byteCount)
    let sha = Array(HMAC<SHA256>.authenticationCode(for: buffer, using: SymmetricKey(size: .bits256)))
    let hash = Data(sha)
    #endif

    let challenge = hash.base64EncodedString()
        .replacingOccurrences(of: "+", with: "-")
        .replacingOccurrences(of: "/", with: "_")
        .replacingOccurrences(of: "=", with: "")
        .trimmingCharacters(in: .whitespaces)

    return challenge
}