export interface SlackPermalink {
    originalUrl: string;
    workspaceUrl: string;
    channelId: string;
    messageTs: string;
    threadTs: string;
}

const SLACK_TIMESTAMP = /^\d+\.\d{1,6}$/;

function timestampFromPath(digits: string): string {
    if (digits.length <= 6) {
        throw new Error("Slack permalink has an invalid message timestamp.");
    }

    return `${digits.slice(0, -6)}.${digits.slice(-6)}`;
}

export function parseSlackPermalink(input: string): SlackPermalink {
    let url: URL;
    try {
        url = new URL(input.trim());
    } catch {
        throw new Error("Expected a valid Slack message permalink.");
    }

    if (url.protocol !== "https:" || (url.hostname !== "slack.com" && !url.hostname.endsWith(".slack.com"))) {
        throw new Error("Expected an HTTPS permalink hosted on slack.com.");
    }

    const match = url.pathname.match(/^\/archives\/([A-Z][A-Z0-9]+)\/p(\d+)\/?$/i);
    if (!match) {
        throw new Error("Expected a Slack message permalink containing /archives/<channel>/p<timestamp>.");
    }

    const channelId = match[1].toUpperCase();
    const messageTs = timestampFromPath(match[2]);
    const requestedThreadTs = url.searchParams.get("thread_ts");

    if (requestedThreadTs && !SLACK_TIMESTAMP.test(requestedThreadTs)) {
        throw new Error("Slack permalink has an invalid thread_ts parameter.");
    }

    return {
        originalUrl: url.toString(),
        workspaceUrl: `${url.protocol}//${url.host}`,
        channelId,
        messageTs,
        threadTs: requestedThreadTs ?? messageTs,
    };
}

export function buildSlackReplyPermalink(target: SlackPermalink, messageTs: string): string {
    if (!SLACK_TIMESTAMP.test(messageTs)) {
        throw new Error("Slack returned an invalid message timestamp.");
    }

    const permalink = new URL(`/archives/${target.channelId}/p${messageTs.replace(".", "")}`, target.workspaceUrl);
    permalink.searchParams.set("thread_ts", target.threadTs);
    permalink.searchParams.set("cid", target.channelId);
    return permalink.toString();
}