import Foundation

/// Minimal RTP header (RFC 3550) without extensions, mirroring the C++
/// `RtpHeader` and the Kotlin receiver.
struct RtpHeader: Equatable {
    var version = 2
    var padding = false
    var extensionHeader = false
    var csrcCount = 0
    var marker = false
    var payloadType = 96
    var sequenceNumber = 0
    var timestamp = 0
    var ssrc = 0

    /// Serializes the bare 12-byte header; requires a version-2, extension-less header.
    func serialize() -> [UInt8] {
        var out = [UInt8](repeating: 0, count: 12)
        out[0] = UInt8((version & 0x0F) << 6
                        | (padding ? 0x20 : 0)
                        | (extensionHeader ? 0x10 : 0)
                        | (csrcCount & 0x0F))
        out[1] = UInt8((marker ? 0x80 : 0) | (payloadType & 0x7F))
        out[2] = UInt8((sequenceNumber >> 8) & 0xFF)
        out[3] = UInt8(sequenceNumber & 0xFF)
        out[4] = UInt8((timestamp >> 24) & 0xFF)
        out[5] = UInt8((timestamp >> 16) & 0xFF)
        out[6] = UInt8((timestamp >> 8) & 0xFF)
        out[7] = UInt8(timestamp & 0xFF)
        out[8] = UInt8((ssrc >> 24) & 0xFF)
        out[9] = UInt8((ssrc >> 16) & 0xFF)
        out[10] = UInt8((ssrc >> 8) & 0xFF)
        out[11] = UInt8(ssrc & 0xFF)
        return out
    }

    /// Parses a 12-byte header from the start of a datagram.
    static func parse(_ input: [UInt8]) -> RtpHeader? {
        guard input.count >= 12 else { return nil }
        let b0 = Int(input[0])
        let b1 = Int(input[1])
        let version = b0 >> 6
        guard version == 2 else { return nil }
        return RtpHeader(
            version: version,
            padding: b0 & 0x20 != 0,
            extensionHeader: b0 & 0x10 != 0,
            csrcCount: b0 & 0x0F,
            marker: b1 & 0x80 != 0,
            payloadType: b1 & 0x7F,
            sequenceNumber: (Int(input[2]) << 8) | Int(input[3]),
            timestamp: (Int(input[4]) << 24) | (Int(input[5]) << 16) | (Int(input[6]) << 8) | Int(input[7]),
            ssrc: (Int(input[8]) << 24) | (Int(input[9]) << 16) | (Int(input[10]) << 8) | Int(input[11]),
        )
    }
}