OTP/OTPKit/OTPGenerator.swift

//
//  OTPGenerator.swift
//  OTPKit
//
//  Created by Shadowfacts on 8/21/21.
//

import Foundation
import CryptoKit

public struct OTPGenerator {
    
    private init() {}
    
    public static func generate(key: OTPKey, counter: Int64) -> String {
        let data = withUnsafeBytes(of: counter.bigEndian) {
            Data($0)
        }
        
        let mac = HMAC<Insecure.SHA1>.authenticationCode(for: data, using: SymmetricKey(data: key.secret))
        
        let uint32Code = mac.withUnsafeBytes { (ptr) -> UInt32 in
            let offset = Int(ptr.last! & 0x0F)
            let offsetPtr = ptr.baseAddress!.advanced(by: offset)
            return offsetPtr.assumingMemoryBound(to: UInt32.self).pointee.bigEndian & 0x7FFFFFFF
        }
        
        let truncated = uint32Code % UInt32(pow(10, Double(key.digits)))
        
        var s = truncated.description
        if s.count < key.digits {
            s = String(repeating: "0", count: key.digits - s.count) + s
        }
        return s
    }
    
    public static func generate(key: TOTPKey) -> TOTPCode {
        let now = Date()
        let counter = Int64(now.timeIntervalSince1970 / Double(key.period))
        let start = Date(timeIntervalSince1970: Double(counter * Int64(key.period)))
        let code = generate(key: key, counter: counter)
        return TOTPCode(code: code, validFrom: start, validInterval: key.period)
    }
    
}