package com.example.aiapp

/**
 * The frame name the server uses to say a cursor was too far behind to continue from. Must match
 * `send_backlog` in the backend's routes.rs.
 */
private const val RESET_EVENT = "reset"

/**
 * The SSE half of the API: one long-lived GET per open session screen, replaying the transcript
 * after a cursor and then following it live.
 *
 * The connection and its framing belong to [Sse]; what stays here is what this stream's frames
 * mean. [close] from any thread ends it, and the caller owns reconnecting -- with the last seq it
 * saw as the new cursor.
 */
class EventStream(settings: ServerSettings, private val address: TranscriptAddress) {
    private val stream = Sse(settings)

    fun close() = stream.close()

    /**
     * Streams events after [after] into [onEvent] until the stream drops.
     *
     * [onReset] fires when the server answers that the cursor is too far behind to continue from:
     * everything already displayed is stale and the events that follow are a fresh window, so the
     * caller drops what it holds and rebuilds. It arrives before those events, so a caller that
     * clears on it stays in order.
     */
    fun run(
        after: Long,
        onOpen: () -> Unit,
        onReset: () -> Unit,
        // The frame's own text as well as the event parsed from it: the transcript cache stores the
        // one and the screen folds the other, and they have to be the same line.
        onEvent: (raw: String, event: SeqEvent) -> Unit,
    ) {
        stream.run("/${address.urlPath}/events?after=$after", onOpen) { name, data ->
            // A named frame carries no payload and a data frame has no name.
            if (name == RESET_EVENT) onReset()
            else if (data.isNotEmpty()) onEvent(data, parseSeqEvent(data))
        }
    }
}