export type SSERequestInit = {

    /**
     * The HTTP method to use.
     */
    method: 'GET' | 'POST';

    /**
     * Request headers sent on every connect and reconnect.
     */
    headers?: Record<string, string>;

    /**
     * Request body for POST-based SSE endpoints.
     */
    body?: string | FormData;
};

/**
 * The fetch function to use.
 *
 * NOTE: This is compatible with Browser/Node's native "fetch" function.
 *       We use this in place of "typeof fetch" so that we can accept non-standard URLs ("url" is a "string" here).
 *       For example, a LibP2P adapter might not use a standardized URL format (and might only include "path").
 *       This would cause a type error as native fetch expects type "URL".
 */
export type SSERequestFunction = {
    fetch: (url: string, options: RequestInit) => Promise<Response>;
};

/**
 * Lifecycle hooks invoked by {@link SSESession} during connect, read, and teardown.
 */
export type SSESessionCallbacks = {

    /**
     * Called before each fetch so callers can attach auth headers, cookies, or
     * a `Last-Event-ID` for resume semantics.
     */
    onRequest: (request: RequestInit) => Promise<RequestInit>;

    /**
     * Called after the HTTP stream is established and before body reading begins.
     */
    onConnected: () => void;

    /**
     * Called when the active transport ends — including {@link SSESession.abort},
     * server stream completion, and errors. Not paired with {@link SSESessionCallbacks.onConnected}
     * when the initial connect never succeeds.
     */
    onDisconnected: () => void;

    /**
     * Called on fetch or read failures. Not invoked for intentional
     * {@link SSESession.abort} aborts.
     */
    onError: (error: Error) => void;
};

export type SSESessionRetryInterface = {

    /**
     * Retry policy used while establishing the HTTP connection in
     * {@link SSESession.connect}. Defaults to {@link ExponentialBackoff} with
     * unlimited attempts.
     */
    retry: {
        run<T>(fn: () => Promise<T>, onError?: (error: Error) => void): Promise<T>;
    };
};

export interface SSEParser {

    /**
     * Incremental SSE frame parser for the response body.
     *
     * {@link SSEEventParser.reset} is called by the session when abandoning a
     * transport so partial frames do not carry over to the next connection.
     */
    eventParser: {
        parseEvents(buffer: Uint8Array): SSEvent[];
        reset(): void;
    };
}

export type SSELifecycleOptions = {

    /**
     * When true, {@link SSESession} calls {@link SSESession.connect} again after
     * a transport **error** (not an intentional abort).
     */
    attemptReconnect: boolean;

    /**
     * When true, {@link SSESession} calls {@link SSESession.connect} again after
     * the **server** closes the stream normally (`done`).
     */
    persistent: boolean;
};

/**
 * Events emitted by {@link SSESession}.
 *
 * - `"connected"` — HTTP stream established.
 * - `"message"` — A complete SSE event was parsed.
 * - `"disconnected"` — The active transport ended (including {@link SSESession.abort}).
 * - `"error"` — An unexpected fetch or read failure.
 * - `"closed"` — {@link SSESession.disconnect} was called; visibility handling is detached.
 */
export type SSESessionEventMap = {
    connected: void;
    disconnected: void;
    error: Error;
    message: SSEvent;
    closed: void;
};

/**
 * Configuration for {@link SSESession}.
 */
export type SSESessionOptions = SSESessionCallbacks
    & SSERequestInit
    & SSERequestFunction
    & SSESessionRetryInterface
    & SSELifecycleOptions
    & SSEParser;

/**
 * Represents a Server-Sent Event.
 */
export interface SSEvent {

    /**
     * Event data.
     */
    data: string;

    /**
     * Event type.
     * This value is optionally sent by the server. Traditional EventSource allows listeners for specific event types.
     * The SSE Session collapses all event types into "message" event.
     */
    event?: string;

    /**
     * Event ID.
     * This value is optionally sent by the server as a "checkpoint" the client can use to resume from using the Last-Event-ID header.
     */
    id?: string;

    /**
     * Reconnection time in milliseconds.
     * This value is optionally sent by the server to indicate the server's preferred time before reconnecting
     */
    retry?: number;
}