Musubi/Packages/MusubiKit/Sources/Intercom/RTC/SDP.swift

import Foundation

/// What Musubi needs from a browser's SDP offer (Unified Plan, BUNDLE, rtcp-mux).
struct SDPOffer: Equatable {
    struct Media: Equatable {
        var kind: String
        var mid: String
        var port: Int
        var payloadTypes: [Int]
        var rtpmap: [Int: String] = [:]
        var fmtp: [Int: String] = [:]
    }

    var iceUfrag: String
    var icePwd: String
    /// `sha-256` fingerprint, uppercase hex with colons.
    var fingerprint: String
    var media: [Media]

    enum ParseError: Error, Equatable { case missing(String) }

    init(_ sdp: String) throws {
        var sessionUfrag: String?, sessionPwd: String?, sessionFingerprint: String?
        var mediaUfrag: String?, mediaPwd: String?, mediaFingerprint: String?
        var media: [Media] = []

        for raw in sdp.split(whereSeparator: \.isNewline) {
            let line = raw.trimmingCharacters(in: .whitespaces)
            if line.hasPrefix("m=") {
                let parts = line.dropFirst(2).split(separator: " ")
                guard parts.count >= 3 else { continue }
                media.append(Media(kind: String(parts[0]), mid: "\(media.count)", port: Int(parts[1]) ?? 0,
                                   payloadTypes: parts.dropFirst(3).compactMap { Int($0) }))
                continue
            }
            guard line.hasPrefix("a=") else { continue }
            let attribute = line.dropFirst(2)
            let (name, value) = attribute.firstIndex(of: ":").map {
                (String(attribute[..<$0]), String(attribute[attribute.index(after: $0)...]))
            } ?? (String(attribute), "")
            let inMedia = !media.isEmpty

            switch name {
            case "ice-ufrag": if inMedia { mediaUfrag = mediaUfrag ?? value } else { sessionUfrag = value }
            case "ice-pwd": if inMedia { mediaPwd = mediaPwd ?? value } else { sessionPwd = value }
            case "fingerprint":
                let parts = value.split(separator: " ")
                guard parts.count == 2, parts[0].lowercased() == "sha-256" else { continue }
                if inMedia { mediaFingerprint = mediaFingerprint ?? parts[1].uppercased() } else { sessionFingerprint = parts[1].uppercased() }
            case "mid" where inMedia:
                media[media.count - 1].mid = value
            case "rtpmap" where inMedia, "fmtp" where inMedia:
                guard let space = value.firstIndex(of: " "), let pt = Int(value[..<space]) else { continue }
                let rest = String(value[value.index(after: space)...])
                if name == "rtpmap" { media[media.count - 1].rtpmap[pt] = rest } else { media[media.count - 1].fmtp[pt] = rest }
            default: break
            }
        }

        guard let ufrag = sessionUfrag ?? mediaUfrag else { throw ParseError.missing("ice-ufrag") }
        guard let pwd = sessionPwd ?? mediaPwd else { throw ParseError.missing("ice-pwd") }
        guard let fingerprint = sessionFingerprint ?? mediaFingerprint else { throw ParseError.missing("fingerprint") }
        iceUfrag = ufrag
        icePwd = pwd
        self.fingerprint = fingerprint
        self.media = media
    }

    /// Payload type for Opus at 48 kHz.
    func opusPayloadType(in media: Media) -> Int? {
        media.payloadTypes.first { media.rtpmap[$0]?.lowercased().hasPrefix("opus/48000") == true }
    }

    /// Payload type for H.264 with packetization-mode=1 in (constrained) baseline profile.
    func h264PayloadType(in media: Media) -> Int? {
        let candidates = media.payloadTypes.filter { pt in
            guard media.rtpmap[pt]?.uppercased().hasPrefix("H264/90000") == true, let fmtp = media.fmtp[pt]?.lowercased() else { return false }
            return fmtp.contains("packetization-mode=1")
        }
        func profile(_ pt: Int) -> String {
            media.fmtp[pt]?.lowercased().split(separator: ";")
                .first { $0.trimmingCharacters(in: .whitespaces).hasPrefix("profile-level-id=") }
                .map { String($0.split(separator: "=").last ?? "") } ?? ""
        }
        return candidates.first { profile($0).hasPrefix("42e0") } ?? candidates.first { profile($0).hasPrefix("4200") }
    }
}

/// Builds the ICE-lite, DTLS-passive, send-only answer.
struct SDPAnswer {
    struct Track {
        var mid: String
        var kind: String
        /// nil rejects the section (port 0).
        var payloadType: Int?
        var ssrc: UInt32
    }

    var iceUfrag: String
    var icePwd: String
    var fingerprint: String
    var address: String
    var port: UInt16
    var tracks: [Track]

    var text: String {
        let accepted = tracks.filter { $0.payloadType != nil }
        var lines = [
            "v=0",
            "o=- \(UInt32.random(in: 1...UInt32.max)) 2 IN IP4 \(address)",
            "s=Musubi",
            "t=0 0",
            "a=ice-lite",
            "a=group:BUNDLE \(accepted.map(\.mid).joined(separator: " "))",
            "a=msid-semantic: WMS musubi",
        ]
        for track in tracks {
            guard let pt = track.payloadType else {
                lines += ["m=\(track.kind) 0 UDP/TLS/RTP/SAVPF 0", "c=IN IP4 0.0.0.0", "a=mid:\(track.mid)", "a=inactive"]
                continue
            }
            lines += [
                "m=\(track.kind) \(port) UDP/TLS/RTP/SAVPF \(pt)",
                "c=IN IP4 \(address)",
                "a=rtcp:\(port) IN IP4 \(address)",
                "a=ice-ufrag:\(iceUfrag)",
                "a=ice-pwd:\(icePwd)",
                "a=fingerprint:sha-256 \(fingerprint)",
                "a=setup:passive",
                "a=mid:\(track.mid)",
                "a=sendonly",
                "a=rtcp-mux",
                "a=msid:musubi \(track.kind)",
            ]
            if track.kind == "audio" {
                lines += ["a=rtpmap:\(pt) opus/48000/2", "a=fmtp:\(pt) minptime=10;useinbandfec=1"]
            } else {
                lines += [
                    "a=rtpmap:\(pt) H264/90000",
                    "a=rtcp-fb:\(pt) nack pli",
                    "a=rtcp-fb:\(pt) ccm fir",
                    "a=fmtp:\(pt) level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=42e01f",
                ]
            }
            lines += [
                "a=ssrc:\(track.ssrc) cname:musubi",
                "a=ssrc:\(track.ssrc) msid:musubi \(track.kind)",
                "a=candidate:1 1 udp 2130706431 \(address) \(port) typ host",
                "a=end-of-candidates",
            ]
        }
        return lines.joined(separator: "\r\n") + "\r\n"
    }
}