//
//  UUIDv7.swift
//  UUIDv7
//
//  Created by Yoshimasa Niwa on 7/25/25.
//

import Foundation
import Security

extension UUID {
    public static func v7() -> Self {
        v7(timestamp: Date())
    }

    // A simple UUIDv7 implementation.
    // It is only provides monotonicity up to milliseconds granularity.
    // Note that RFC 9415 defines optional implementation about increased monotonicity,
    // which can be implemented.
    // See section 5.7 and 6.2 in <https://www.rfc-editor.org/rfc/rfc9562.txt>.
    static func v7(timestamp: Date) -> Self {
        // 48-bit UNIX Epoc timestamp in milliseconds
        let timestamp = UInt64(timestamp.timeIntervalSince1970 * 1000)

        // 80-bit cryptographically secure pseudo random number.
        var randomBytes = [UInt8](repeating: 0, count: 10)
        if SecRandomCopyBytes(kSecRandomDefault, randomBytes.count, &randomBytes) != errSecSuccess {
            fatalError("Failed to generate secure random bytes")
        }

        let uuid = (
            UInt8((timestamp >> 40) & 0xFF),
            UInt8((timestamp >> 32) & 0xFF),
            UInt8((timestamp >> 24) & 0xFF),
            UInt8((timestamp >> 16) & 0xFF),
            UInt8((timestamp >> 8) & 0xFF),
            UInt8(timestamp & 0xFF),
            randomBytes[0] & 0x0F | 0x70, // version
            randomBytes[1],
            randomBytes[2] & 0x3F | 0x80, // variant
            randomBytes[3],
            randomBytes[4],
            randomBytes[5],
            randomBytes[6],
            randomBytes[7],
            randomBytes[8],
            randomBytes[9],
        )

        return UUID(uuid: uuid)
    }

    var version: UInt8 {
        (uuid.6 & 0xF0) >> 4
    }

    public var timestamp: Date? {
        guard version == 0x07 else {
            return nil
        }

        let timestamp =
            UInt64(uuid.0) << 40 |
            UInt64(uuid.1) << 32 |
            UInt64(uuid.2) << 24 |
            UInt64(uuid.3) << 16 |
            UInt64(uuid.4) << 8 |
            UInt64(uuid.5)

        return Date(timeIntervalSince1970: Double(timestamp) / 1000.0)
    }
}