ai-app/app/androidApp/src/main/kotlin/com/example/aiapp/Sse.kt

package com.example.aiapp

import java.io.IOException
import java.net.HttpURLConnection
import java.net.URL

/**
 * How long to wait before opening a dropped stream again.
 *
 * Shared by every screen that follows one, so a reconnect is not paced differently depending on
 * which stream dropped. Short enough that a tunnel coming back is not noticed, long enough that a
 * server which is genuinely down is not being asked several times a second.
 */
const val RECONNECT_DELAY_MS = 1500L

/**
 * One server-sent-events connection, framed.
 *
 * The framing is the part worth having once: `data:` and `event:` lines accumulate until a blank
 * line ends the frame, comments (keep-alives) start with `:`, and a frame is either named with no
 * payload or a payload with no name. Two screens follow two different streams — a session's
 * transcript and what a machine's import list is doing — and neither should be re-deriving that.
 *
 * Blocking: [run] occupies its thread until the stream ends. [close], from any thread, is the
 * cancellation path — it disconnects the socket, which unblocks the read, and [run] then returns
 * rather than throwing, so a deliberate close is not reported as a connection error. Reconnecting
 * belongs to the caller, which is the only one that knows where to resume from.
 */
class Sse(private val settings: ServerSettings) {
    @Volatile private var connection: HttpURLConnection? = null
    @Volatile private var closed = false

    fun close() {
        closed = true
        connection?.disconnect()
    }

    /**
     * Follows the stream at [path], handing each frame to [onFrame] as its name (null for an
     * ordinary data frame) and its payload. The path is given here rather than at construction
     * because a caller that reconnects usually resumes from somewhere new -- a cursor it has
     * advanced past -- and that lives in the query string.
     *
     * [onOpen] fires once the server has accepted the connection. That is the measured moment the
     * stream is live, and the only honest thing to clear a previous failure on: clearing on the
     * first *event* instead left an idle stream displaying a connection error it had already
     * recovered from, indefinitely.
     */
    fun run(path: String, onOpen: () -> Unit, onFrame: (name: String?, data: String) -> Unit) {
        // Opening is inside the try, not before it. Everything this method can fail at owes the
        // caller the same kind of failure -- both callers retry an [ApiException] and let anything
        // else reach the top of the app -- and a connection that could not even be constructed
        // used to escape as a raw `IOException` from a line no `catch` covered.
        var connection: HttpURLConnection? = null
        try {
            connection =
                (URL("${settings.baseUrl}$path").openConnection() as HttpURLConnection).also {
                    this.connection = it
                }
            connection.applyPinnedTls()
            connection.connectTimeout = CONNECT_TIMEOUT_MS
            // No read timeout: between events there is nothing to read for as long as the thing
            // being followed is idle; the server's keep-alives and a dead socket erroring out are
            // the liveness story.
            connection.readTimeout = 0
            connection.setRequestProperty("Authorization", "Bearer ${settings.token}")
            connection.setRequestProperty("Accept", "text/event-stream")
            if (connection.responseCode != 200) {
                val detail = connection.errorStream?.bufferedReader()?.readText()?.trim()
                throw ApiException(detail ?: "HTTP ${connection.responseCode} for the event stream")
            }

            onOpen()
            val reader = connection.inputStream.bufferedReader()
            val data = StringBuilder()
            var name: String? = null
            while (true) {
                val line = reader.readLine() ?: break
                when {
                    line.isEmpty() -> {
                        if (name != null || data.isNotEmpty()) onFrame(name, data.toString())
                        data.clear()
                        name = null
                    }
                    line.startsWith("data:") -> data.append(line.removePrefix("data:").trim())
                    line.startsWith("event:") -> name = line.removePrefix("event:").trim()
                    else -> {} // id:, comments -- nothing to do
                }
            }
        } catch (e: ApiException) {
            throw e
        } catch (e: IOException) {
            if (!closed) {
                throw ApiException(
                    "Can't reach the server -- retrying. (${e.message ?: e::class.simpleName})",
                    cause = e,
                )
            }
        } finally {
            connection?.disconnect()
            this.connection = null
        }
    }
}