//! System metrics for the admin dashboard.
//!
//! Disk is `statvfs(storage_dir)` so the number reflects the volume the
//! app actually writes to (not the root filesystem of the host). When the
//! storage backend doesn't expose a local path (e.g. a future S3 impl)
//! the disk fields are `null` rather than fabricated.
//!
//! Memory and CPU come from `sysinfo`. CPU requires two refreshes with
//! at least 200ms between them to compute a meaningful delta; the
//! handler eats the 250ms wall-clock cost on each request. Admin
//! traffic is low-volume so a background cache isn't worth the moving
//! parts yet — revisit if polling becomes frequent.

use std::path::Path;
use std::time::Duration;

use axum::extract::State;
use axum::routing::get;
use axum::{Json, Router};
use serde::Serialize;
use sysinfo::{CpuRefreshKind, MemoryRefreshKind, RefreshKind, System};

use crate::app::AppState;
use crate::auth::extractor::RequireAdmin;
use crate::error::AppResult;

const ALERT_THRESHOLD_PERCENT: f64 = 90.0;

pub fn routes() -> Router<AppState> {
    Router::new().route("/admin/system", get(system))
}

#[derive(Debug, Serialize)]
pub struct SystemStats {
    pub disk: Option<DiskStats>,
    pub memory: MemoryStats,
    pub cpu: CpuStats,
    pub alerts: Vec<Alert>,
}

#[derive(Debug, Serialize)]
pub struct DiskStats {
    pub total_bytes: u64,
    pub used_bytes: u64,
    pub free_bytes: u64,
    pub percent_used: f64,
}

#[derive(Debug, Serialize)]
pub struct MemoryStats {
    pub total_bytes: u64,
    pub used_bytes: u64,
    pub percent_used: f64,
}

#[derive(Debug, Serialize)]
pub struct CpuStats {
    pub percent_used: f64,
}

#[derive(Debug, Serialize)]
pub struct Alert {
    pub level: AlertLevel,
    pub message: String,
}

#[derive(Debug, Serialize, Clone, Copy)]
#[serde(rename_all = "snake_case")]
pub enum AlertLevel {
    Warning,
}

async fn system(
    State(state): State<AppState>,
    _admin: RequireAdmin,
) -> AppResult<Json<SystemStats>> {
    let disk = state.storage.local_root().and_then(disk_stats_for);
    let (memory, cpu) = memory_and_cpu().await;
    let mut alerts = Vec::new();
    if let Some(d) = &disk {
        if d.percent_used >= ALERT_THRESHOLD_PERCENT {
            alerts.push(Alert {
                level: AlertLevel::Warning,
                message: format!(
                    "disk near full ({:.0}% used)",
                    d.percent_used
                ),
            });
        }
    }
    if memory.percent_used >= ALERT_THRESHOLD_PERCENT {
        alerts.push(Alert {
            level: AlertLevel::Warning,
            message: format!(
                "memory near full ({:.0}% used)",
                memory.percent_used
            ),
        });
    }
    Ok(Json(SystemStats {
        disk,
        memory,
        cpu,
        alerts,
    }))
}

fn disk_stats_for(root: &Path) -> Option<DiskStats> {
    let s = nix::sys::statvfs::statvfs(root).ok()?;
    // statvfs reports `f_frsize * f_blocks` for total bytes. `f_bavail`
    // is "free to non-root callers" which is what an operator actually
    // cares about — `f_bfree` includes blocks reserved for root.
    let block = s.fragment_size();
    let total = block * s.blocks();
    let avail = block * s.blocks_available();
    let used = total.saturating_sub(avail);
    let percent_used = if total > 0 {
        (used as f64) * 100.0 / (total as f64)
    } else {
        0.0
    };
    Some(DiskStats {
        total_bytes: total,
        used_bytes: used,
        free_bytes: avail,
        percent_used,
    })
}

async fn memory_and_cpu() -> (MemoryStats, CpuStats) {
    // sysinfo's CPU sampling needs two refreshes with a delay between
    // them — the first seeds the delta counters, the second measures.
    // We do this once per request; admin traffic is low enough that the
    // 250ms cost is invisible.
    let mut sys = System::new_with_specifics(
        RefreshKind::new()
            .with_cpu(CpuRefreshKind::everything())
            .with_memory(MemoryRefreshKind::everything()),
    );
    sys.refresh_cpu_all();
    // Yield the runtime instead of blocking it for the gap.
    tokio::time::sleep(Duration::from_millis(250)).await;
    sys.refresh_cpu_all();
    sys.refresh_memory();

    let total = sys.total_memory();
    let used = sys.used_memory();
    let mem_pct = if total > 0 {
        (used as f64) * 100.0 / (total as f64)
    } else {
        0.0
    };
    let memory = MemoryStats {
        total_bytes: total,
        used_bytes: used,
        percent_used: mem_pct,
    };

    let cpu = CpuStats {
        percent_used: sys.global_cpu_usage() as f64,
    };
    (memory, cpu)
}