use std::fs;
use std::path::Path;
use std::process::Stdio;
use std::time::Duration;

use anyhow::{anyhow, Context, Result};
use chainfire_client::Client;
use chrono::Utc;
use clap::Parser;
use deployer_types::{ClusterNodeRecord, DesiredSystemSpec, ObservedSystemState};
use tokio::process::Command;
use tokio::time::sleep;
use tracing::{info, warn};
use tracing_subscriber::EnvFilter;

fn cluster_prefix(cluster_namespace: &str, cluster_id: &str) -> String {
    format!("{}/clusters/{}/", cluster_namespace, cluster_id)
}

fn key_node(cluster_namespace: &str, cluster_id: &str, node_id: &str) -> Vec<u8> {
    format!(
        "{}nodes/{}",
        cluster_prefix(cluster_namespace, cluster_id),
        node_id
    )
    .into_bytes()
}

fn key_desired_system(cluster_namespace: &str, cluster_id: &str, node_id: &str) -> Vec<u8> {
    format!(
        "{}nodes/{}/desired-system",
        cluster_prefix(cluster_namespace, cluster_id),
        node_id
    )
    .into_bytes()
}

fn key_observed_system(cluster_namespace: &str, cluster_id: &str, node_id: &str) -> Vec<u8> {
    format!(
        "{}nodes/{}/observed-system",
        cluster_prefix(cluster_namespace, cluster_id),
        node_id
    )
    .into_bytes()
}

#[derive(Parser, Debug)]
#[command(author, version, about)]
struct Cli {
    #[arg(long, default_value = "http://127.0.0.1:7000")]
    chainfire_endpoint: String,

    #[arg(long, default_value = "photoncloud")]
    cluster_namespace: String,

    #[arg(long)]
    cluster_id: String,

    #[arg(long)]
    node_id: String,

    #[arg(long, default_value = "/etc/nixos")]
    flake_root: String,

    #[arg(long, default_value_t = 30)]
    interval_secs: u64,

    #[arg(long, default_value = "switch")]
    switch_action: String,

    #[arg(long, allow_hyphen_values = true)]
    health_check_command: Vec<String>,

    #[arg(long, default_value_t = false)]
    rollback_on_failure: bool,

    #[arg(long, default_value_t = false)]
    apply: bool,

    #[arg(long, default_value_t = false)]
    once: bool,
}

struct Agent {
    endpoint: String,
    cluster_namespace: String,
    cluster_id: String,
    node_id: String,
    flake_root: String,
    interval: Duration,
    switch_action: String,
    health_check_command: Vec<String>,
    rollback_on_failure: bool,
    apply: bool,
}

#[derive(Debug, Clone, PartialEq, Eq)]
struct ResolvedDesiredSystem {
    nixos_configuration: String,
    flake_ref: String,
    switch_action: String,
    health_check_command: Vec<String>,
    rollback_on_failure: bool,
}

impl Agent {
    fn new(cli: Cli) -> Self {
        Self {
            endpoint: cli.chainfire_endpoint,
            cluster_namespace: cli.cluster_namespace,
            cluster_id: cli.cluster_id,
            node_id: cli.node_id,
            flake_root: cli.flake_root,
            interval: Duration::from_secs(cli.interval_secs),
            switch_action: cli.switch_action,
            health_check_command: cli.health_check_command,
            rollback_on_failure: cli.rollback_on_failure,
            apply: cli.apply,
        }
    }

    async fn run_loop(&self) -> Result<()> {
        loop {
            if let Err(error) = self.tick().await {
                warn!(error = %error, "nix-agent tick failed");
            }
            sleep(self.interval).await;
        }
    }

    async fn tick(&self) -> Result<()> {
        let mut client = Client::connect(self.endpoint.clone()).await?;
        let node_key = key_node(&self.cluster_namespace, &self.cluster_id, &self.node_id);
        let node_raw = client.get_with_revision(&node_key).await?;
        let Some((node_bytes, _revision)) = node_raw else {
            warn!(
                cluster_id = %self.cluster_id,
                node_id = %self.node_id,
                "node definition not found; skipping nix reconciliation"
            );
            return Ok(());
        };

        let node: ClusterNodeRecord =
            serde_json::from_slice(&node_bytes).context("failed to parse node record")?;

        let desired = client
            .get(key_desired_system(
                &self.cluster_namespace,
                &self.cluster_id,
                &self.node_id,
            ))
            .await?
            .map(|bytes| serde_json::from_slice::<DesiredSystemSpec>(&bytes))
            .transpose()
            .context("failed to parse desired-system spec")?;

        let mut observed = self.base_observed_state(&node);
        let reconcile_result = self
            .reconcile_node(&node, desired.as_ref(), &mut observed)
            .await;
        if let Err(error) = reconcile_result {
            observed.status = Some("failed".to_string());
            observed.last_error = Some(error.to_string());
        }

        client
            .put(
                &key_observed_system(&self.cluster_namespace, &self.cluster_id, &self.node_id),
                &serde_json::to_vec(&observed)?,
            )
            .await?;

        Ok(())
    }

    fn base_observed_state(&self, node: &ClusterNodeRecord) -> ObservedSystemState {
        ObservedSystemState {
            node_id: node.node_id.clone(),
            current_system: read_symlink_target("/run/current-system"),
            booted_system: read_symlink_target("/run/booted-system"),
            ..ObservedSystemState::default()
        }
    }

    async fn reconcile_node(
        &self,
        node: &ClusterNodeRecord,
        desired: Option<&DesiredSystemSpec>,
        observed: &mut ObservedSystemState,
    ) -> Result<()> {
        match node.state.as_deref() {
            Some("failed") | Some("draining") => {
                observed.status = Some("paused".to_string());
                return Ok(());
            }
            _ => {}
        }

        let Some(desired) = resolve_desired_system(
            node,
            desired,
            &self.flake_root,
            &self.switch_action,
            &self.health_check_command,
            self.rollback_on_failure,
        ) else {
            observed.status = Some("idle".to_string());
            return Ok(());
        };

        observed.nixos_configuration = Some(desired.nixos_configuration.clone());
        observed.flake_root = Some(desired.flake_ref.clone());

        let previous_system = observed.current_system.clone();
        let target_system = self
            .build_target_system(&desired.flake_ref, &desired.nixos_configuration)
            .await
            .with_context(|| {
                format!(
                    "failed to build target system for {}",
                    desired.nixos_configuration
                )
            })?;
        observed.target_system = Some(target_system.clone());

        if observed.current_system.as_deref() == Some(target_system.as_str()) {
            observed.status = Some("active".to_string());
            observed.last_success = Some(Utc::now());
            return Ok(());
        }

        if !self.apply {
            observed.status = Some("pending".to_string());
            return Ok(());
        }

        observed.status = Some("reconciling".to_string());
        observed.last_attempt = Some(Utc::now());
        self.switch_to_target(&target_system, &desired.switch_action)
            .await?;

        observed.current_system = read_symlink_target("/run/current-system");
        observed.booted_system = read_symlink_target("/run/booted-system");

        if observed.current_system.as_deref() != Some(target_system.as_str()) {
            return Err(anyhow!(
                "switch completed but /run/current-system does not match target {}",
                target_system
            ));
        }

        self.run_health_check_and_maybe_rollback(&desired, previous_system.as_deref(), observed)
            .await?;

        observed.status = Some("active".to_string());
        observed.last_success = Some(Utc::now());
        observed.last_error = None;

        Ok(())
    }

    async fn build_target_system(&self, flake_ref: &str, configuration: &str) -> Result<String> {
        let flake_attr = target_flake_attr(flake_ref, configuration);
        let output = run_command(
            "nix",
            &["build", "--no-link", "--print-out-paths", flake_attr.as_str()],
        )
        .await?;
        let path = output
            .lines()
            .find(|line| !line.trim().is_empty())
            .map(str::trim)
            .ok_or_else(|| anyhow!("nix build returned no output path"))?;
        Ok(path.to_string())
    }

    async fn switch_to_target(&self, target_system: &str, switch_action: &str) -> Result<()> {
        let switch_bin = Path::new(target_system).join("bin/switch-to-configuration");
        if !switch_bin.exists() {
            return Err(anyhow!(
                "target system {} does not contain switch-to-configuration",
                target_system
            ));
        }

        run_command(
            switch_bin
                .to_str()
                .ok_or_else(|| anyhow!("invalid switch path"))?,
            &[switch_action],
        )
        .await?;
        Ok(())
    }

    async fn run_health_check_and_maybe_rollback(
        &self,
        desired: &ResolvedDesiredSystem,
        previous_system: Option<&str>,
        observed: &mut ObservedSystemState,
    ) -> Result<()> {
        if desired.health_check_command.is_empty() {
            return Ok(());
        }

        if let Err(error) = run_vec_command(&desired.health_check_command).await {
            let error_message = format!("health check failed after activation: {error}");
            if desired.rollback_on_failure {
                self.rollback_to_previous(previous_system).await?;
                observed.current_system = read_symlink_target("/run/current-system");
                observed.booted_system = read_symlink_target("/run/booted-system");
                observed.status = Some("rolled-back".to_string());
                observed.last_error = Some(error_message);
                return Ok(());
            }

            return Err(anyhow!(error_message));
        }

        Ok(())
    }

    async fn rollback_to_previous(&self, previous_system: Option<&str>) -> Result<()> {
        let previous_system = previous_system
            .filter(|value| !value.is_empty())
            .ok_or_else(|| anyhow!("rollback requested but no previous system is known"))?;
        self.switch_to_target(previous_system, "switch").await
    }
}

fn resolve_desired_system(
    node: &ClusterNodeRecord,
    desired: Option<&DesiredSystemSpec>,
    local_flake_root: &str,
    local_switch_action: &str,
    local_health_check_command: &[String],
    local_rollback_on_failure: bool,
) -> Option<ResolvedDesiredSystem> {
    let nixos_configuration = desired
        .and_then(|spec| spec.nixos_configuration.clone())
        .or_else(|| {
            node.install_plan
                .as_ref()
                .and_then(|plan| plan.nixos_configuration.clone())
        })?;

    Some(ResolvedDesiredSystem {
        nixos_configuration,
        flake_ref: desired
            .and_then(|spec| spec.flake_ref.clone())
            .unwrap_or_else(|| local_flake_root.to_string()),
        switch_action: desired
            .and_then(|spec| spec.switch_action.clone())
            .unwrap_or_else(|| local_switch_action.to_string()),
        health_check_command: desired
            .map(|spec| spec.health_check_command.clone())
            .filter(|command| !command.is_empty())
            .unwrap_or_else(|| local_health_check_command.to_vec()),
        rollback_on_failure: desired
            .and_then(|spec| spec.rollback_on_failure)
            .unwrap_or(local_rollback_on_failure),
    })
}

fn target_flake_attr(flake_root: &str, configuration: &str) -> String {
    format!(
        "{}#nixosConfigurations.{}.config.system.build.toplevel",
        flake_root, configuration
    )
}

fn read_symlink_target(path: &str) -> Option<String> {
    fs::read_link(path)
        .ok()
        .map(|value| value.to_string_lossy().into_owned())
}

async fn run_command(program: &str, args: &[&str]) -> Result<String> {
    let output = Command::new(program)
        .args(args)
        .stdin(Stdio::null())
        .stdout(Stdio::piped())
        .stderr(Stdio::piped())
        .output()
        .await
        .with_context(|| format!("failed to execute {}", program))?;

    if output.status.success() {
        Ok(String::from_utf8_lossy(&output.stdout).trim().to_string())
    } else {
        let stderr = String::from_utf8_lossy(&output.stderr).trim().to_string();
        let stdout = String::from_utf8_lossy(&output.stdout).trim().to_string();
        Err(anyhow!(
            "{} {:?} failed with status {}: stdout='{}' stderr='{}'",
            program,
            args,
            output.status,
            stdout,
            stderr
        ))
    }
}

async fn run_vec_command(command: &[String]) -> Result<String> {
    let (program, args) = command
        .split_first()
        .ok_or_else(|| anyhow!("command vector is empty"))?;
    let arg_refs = args.iter().map(String::as_str).collect::<Vec<_>>();
    run_command(program, &arg_refs).await
}

#[tokio::main]
async fn main() -> Result<()> {
    tracing_subscriber::fmt()
        .with_env_filter(EnvFilter::from_default_env().add_directive("info".parse()?))
        .init();

    let cli = Cli::parse();
    let once = cli.once;
    let agent = Agent::new(cli);

    info!(
        cluster_id = %agent.cluster_id,
        node_id = %agent.node_id,
        flake_root = %agent.flake_root,
        apply = agent.apply,
        "starting nix-agent"
    );

    if once {
        agent.tick().await?;
    } else {
        agent.run_loop().await?;
    }

    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;
    use deployer_types::{DesiredSystemSpec, InstallPlan};

    fn test_node() -> ClusterNodeRecord {
        ClusterNodeRecord {
            node_id: "node01".to_string(),
            machine_id: None,
            ip: "10.0.0.10".to_string(),
            hostname: "node01".to_string(),
            roles: vec!["control-plane".to_string()],
            labels: Default::default(),
            pool: Some("control".to_string()),
            node_class: Some("control-plane".to_string()),
            failure_domain: Some("rack-a".to_string()),
            nix_profile: Some("profiles/control-plane".to_string()),
            install_plan: Some(InstallPlan {
                nixos_configuration: Some("node01".to_string()),
                disko_config_path: Some("nix/nodes/vm-cluster/node01/disko.nix".to_string()),
            }),
            state: Some("active".to_string()),
            last_heartbeat: None,
        }
    }

    #[test]
    fn resolve_desired_system_falls_back_to_install_plan() {
        let resolved = resolve_desired_system(
            &test_node(),
            None,
            "/opt/plasmacloud-src",
            "switch",
            &[],
            true,
        )
        .expect("desired system should resolve");
        assert_eq!(resolved.nixos_configuration, "node01");
        assert_eq!(resolved.flake_ref, "/opt/plasmacloud-src");
        assert_eq!(resolved.switch_action, "switch");
        assert!(resolved.rollback_on_failure);
    }

    #[test]
    fn resolve_desired_system_prefers_chainfire_spec() {
        let desired = DesiredSystemSpec {
            node_id: "node01".to_string(),
            nixos_configuration: Some("node01-next".to_string()),
            flake_ref: Some("github:centra/cloud".to_string()),
            switch_action: Some("boot".to_string()),
            health_check_command: vec!["true".to_string()],
            rollback_on_failure: Some(true),
        };

        let resolved = resolve_desired_system(
            &test_node(),
            Some(&desired),
            "/opt/plasmacloud-src",
            "switch",
            &[],
            false,
        )
        .expect("desired system should resolve");
        assert_eq!(resolved.nixos_configuration, "node01-next");
        assert_eq!(resolved.flake_ref, "github:centra/cloud");
        assert_eq!(resolved.switch_action, "boot");
        assert_eq!(resolved.health_check_command, vec!["true".to_string()]);
        assert!(resolved.rollback_on_failure);
    }

    #[test]
    fn resolve_desired_system_uses_local_health_check_defaults_when_spec_omits_them() {
        let desired = DesiredSystemSpec {
            node_id: "node01".to_string(),
            nixos_configuration: Some("node01-next".to_string()),
            flake_ref: None,
            switch_action: None,
            health_check_command: Vec::new(),
            rollback_on_failure: None,
        };

        let resolved = resolve_desired_system(
            &test_node(),
            Some(&desired),
            "/opt/plasmacloud-src",
            "switch",
            &["systemctl".to_string(), "is-system-running".to_string()],
            true,
        )
        .expect("desired system should resolve");

        assert_eq!(resolved.flake_ref, "/opt/plasmacloud-src");
        assert_eq!(resolved.switch_action, "switch");
        assert_eq!(
            resolved.health_check_command,
            vec!["systemctl".to_string(), "is-system-running".to_string()]
        );
        assert!(resolved.rollback_on_failure);
    }

    #[test]
    fn target_flake_attr_is_rendered_from_root_and_configuration() {
        assert_eq!(
            target_flake_attr("/opt/plasmacloud-src", "node01"),
            "/opt/plasmacloud-src#nixosConfigurations.node01.config.system.build.toplevel"
        );
    }

    #[test]
    fn read_symlink_target_returns_none_for_missing_path() {
        assert_eq!(read_symlink_target("/tmp/photoncloud-nix-agent-missing-link"), None);
    }
}