//
//  SSEWriter.swift
//  Zyquo Router
//
//  Author: Simon-Pierre Boucher
//  Mail: contact@spboucher.ai
//
//  Spec-exact Server-Sent Events emission for /v1/chat/completions streaming:
//  each event is `data: <json>\n\n`, the stream ends with `data: [DONE]\n\n`.
//  Every write flushes; a write against a disconnected client throws, which
//  cancels the calling route task and, with it, the upstream provider call.
//

import Foundation
import NIOCore
import NIOHTTP1

struct SSEWriter {
    let outbound: NIOAsyncChannelOutboundWriter<HTTPServerResponsePart>

    private static let encoder: JSONEncoder = {
        let encoder = JSONEncoder()
        encoder.outputFormatting = [.withoutEscapingSlashes]
        return encoder
    }()

    /// Emits one `data:` event carrying an encodable payload (a chunk object).
    func send<T: Encodable>(_ payload: T) async throws {
        try await send(raw: Self.encoder.encode(payload))
    }

    /// Emits one `data:` event carrying pre-serialized JSON (pass-through path).
    func send(raw json: Data) async throws {
        var buffer = ByteBuffer()
        buffer.writeString("data: ")
        buffer.writeBytes(json)
        buffer.writeString("\n\n")
        try await outbound.write(.body(.byteBuffer(buffer)))
    }

    /// Emits an SSE comment line (keep-alive heartbeat).
    func sendHeartbeat() async throws {
        try await outbound.write(.body(.byteBuffer(ByteBuffer(string: ": keep-alive\n\n"))))
    }

    /// Terminates the stream per the OpenAI contract.
    func sendDone() async throws {
        try await outbound.write(.body(.byteBuffer(ByteBuffer(string: "data: [DONE]\n\n"))))
    }
}