//! Host-level command handler: request-reply on `corrosion.{license}.host.cmd`.
//!
//! One subscriber; each message handled in its own task so a slow command
//! never blocks the dispatch loop. Phase 0 commands: ping, probe, sysinfo.

use futures::StreamExt;
use serde::Deserialize;
use serde_json::json;
use std::sync::Arc;
use sysinfo::System;

use crate::agent::Agent;
use crate::prober;
use crate::subjects;
use crate::telemetry;
use crate::update;
use crate::version;

#[derive(Debug, Deserialize)]
struct HostCommand {
    func: String,
    /// Signed-update artifact URL (for func = "update").
    #[serde(default)]
    url: Option<String>,
}

pub async fn run(agent: Arc<Agent>) -> anyhow::Result<()> {
    let subject = subjects::host_cmd(&agent.cfg.license_id);
    let mut sub = agent.nats.subscribe(subject.clone()).await?;
    tracing::info!("host command handler listening on {subject}");

    let cancel = agent.shutdown.clone();
    loop {
        tokio::select! {
            msg = sub.next() => {
                match msg {
                    Some(msg) => {
                        let agent = agent.clone();
                        tokio::spawn(async move { handle(agent, msg).await });
                    }
                    None => {
                        tracing::warn!("host command subscription ended");
                        break;
                    }
                }
            }
            _ = cancel.cancelled() => {
                tracing::info!("host command handler stopping");
                break;
            }
        }
    }
    Ok(())
}

async fn handle(agent: Arc<Agent>, msg: async_nats::Message) {
    let Some(reply) = msg.reply.clone() else {
        tracing::warn!("host command without reply subject ignored");
        return;
    };

    let cmd = match serde_json::from_slice::<HostCommand>(&msg.payload) {
        Ok(cmd) => cmd,
        Err(e) => {
            publish(&agent, &reply, json!({ "status": "error", "message": format!("invalid command payload: {e}") })).await;
            return;
        }
    };

    // Self-update is special: it must reply BEFORE relaunching, because the
    // relaunch replaces this process and nothing after it would run.
    if cmd.func == "update" {
        let Some(url) = cmd.url else {
            publish(&agent, &reply, json!({ "status": "error", "message": "update requires a 'url'" })).await;
            return;
        };
        match update::download_verify_swap(&url).await {
            Ok(_) => {
                publish(&agent, &reply, json!({ "status": "success", "func": "update", "message": "verified and swapped; relaunching" })).await;
                let _ = agent.nats.flush().await;
                update::relaunch_and_exit();
            }
            Err(e) => {
                publish(&agent, &reply, json!({ "status": "error", "func": "update", "message": format!("{e:#}") })).await;
            }
        }
        return;
    }

    let response = dispatch(&agent, &cmd.func).await;
    publish(&agent, &reply, response).await;
}

async fn publish(agent: &Arc<Agent>, reply: &async_nats::Subject, value: serde_json::Value) {
    match serde_json::to_vec(&value) {
        Ok(bytes) => {
            if let Err(e) = agent.nats.publish(reply.clone(), bytes.into()).await {
                tracing::warn!("response publish failed: {e}");
            }
        }
        Err(e) => tracing::error!("response serialize failed: {e}"),
    }
}

async fn dispatch(agent: &Arc<Agent>, func: &str) -> serde_json::Value {
    match func {
        "ping" => json!({
            "status": "success",
            "func": "ping",
            "version": version::VERSION,
            "commit": version::GIT_HASH,
            "uptime_seconds": agent.started.elapsed().as_secs(),
        }),
        "probe" => {
            let report = prober::run_probe(&agent.cfg.probe_targets).await;
            *agent.last_probe.write().await = Some(report.clone());
            match serde_json::to_value(&report) {
                Ok(report_json) => json!({
                    "status": "success",
                    "func": "probe",
                    "report": report_json,
                }),
                Err(e) => json!({ "status": "error", "message": format!("probe serialize: {e}") }),
            }
        }
        "sysinfo" => {
            let mut sys = System::new();
            sys.refresh_cpu_usage();
            tokio::time::sleep(std::time::Duration::from_millis(250)).await;
            let payload = telemetry::collect(agent, &mut sys).await;
            match serde_json::to_value(&payload) {
                Ok(snapshot) => json!({
                    "status": "success",
                    "func": "sysinfo",
                    "snapshot": snapshot,
                }),
                Err(e) => json!({ "status": "error", "message": format!("sysinfo serialize: {e}") }),
            }
        }
        other => json!({
            "status": "error",
            "message": format!("unknown func '{other}' (supported: ping, probe, sysinfo)"),
        }),
    }
}