use std::sync::OnceLock;

use opentelemetry::global;
use tracing::warn;

// 进程指标只描述 api-server 自身，不携带请求、用户或作品维度，避免 OTLP 指标高基数膨胀。
pub(crate) fn register_process_metrics() {
    static REGISTERED: OnceLock<()> = OnceLock::new();
    REGISTERED.get_or_init(register_process_metrics_once);
}

fn register_process_metrics_once() {
    let meter = global::meter("genarrative-api");

    meter
        .i64_observable_up_down_counter("process.memory.usage")
        .with_unit("By")
        .with_description("api-server process physical memory usage")
        .with_callback(|observer| {
            let Some(snapshot) = ProcessMetricsSnapshot::collect() else {
                return;
            };
            observer.observe(to_i64(snapshot.rss_bytes), &[]);
        })
        .build();

    meter
        .i64_observable_up_down_counter("process.memory.virtual")
        .with_unit("By")
        .with_description("api-server committed virtual memory")
        .with_callback(|observer| {
            let Some(snapshot) = ProcessMetricsSnapshot::collect() else {
                return;
            };
            if let Some(virtual_bytes) = snapshot.virtual_bytes {
                observer.observe(to_i64(virtual_bytes), &[]);
            }
        })
        .build();

    meter
        .i64_observable_up_down_counter("genarrative.process.memory.private")
        .with_unit("By")
        .with_description("api-server private memory for local diagnostics")
        .with_callback(|observer| {
            let Some(snapshot) = ProcessMetricsSnapshot::collect() else {
                return;
            };
            if let Some(private_bytes) = snapshot.private_bytes {
                observer.observe(to_i64(private_bytes), &[]);
            }
        })
        .build();

    meter
        .i64_observable_up_down_counter("process.thread.count")
        .with_unit("{thread}")
        .with_description("api-server process thread count")
        .with_callback(|observer| {
            let Some(snapshot) = ProcessMetricsSnapshot::collect() else {
                return;
            };
            observer.observe(to_i64(snapshot.thread_count), &[]);
        })
        .build();

    meter
        .i64_observable_up_down_counter("process.windows.handle.count")
        .with_unit("{handle}")
        .with_description("api-server process handle count on Windows")
        .with_callback(|observer| {
            let Some(snapshot) = ProcessMetricsSnapshot::collect() else {
                return;
            };
            if let Some(handle_count) = snapshot.windows_handle_count {
                observer.observe(to_i64(handle_count), &[]);
            }
        })
        .build();

    meter
        .i64_observable_up_down_counter("process.unix.file_descriptor.count")
        .with_unit("{file_descriptor}")
        .with_description("api-server process file descriptor count on Unix")
        .with_callback(|observer| {
            let Some(snapshot) = ProcessMetricsSnapshot::collect() else {
                return;
            };
            if let Some(fd_count) = snapshot.unix_fd_count {
                observer.observe(to_i64(fd_count), &[]);
            }
        })
        .build();
}

fn to_i64(value: u64) -> i64 {
    value.min(i64::MAX as u64) as i64
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct ProcessMetricsSnapshot {
    rss_bytes: u64,
    private_bytes: Option<u64>,
    virtual_bytes: Option<u64>,
    thread_count: u64,
    windows_handle_count: Option<u64>,
    unix_fd_count: Option<u64>,
}

impl ProcessMetricsSnapshot {
    fn collect() -> Option<Self> {
        collect_process_metrics()
            .inspect_err(|error| {
                warn!(%error, "采集 api-server 进程内存指标失败");
            })
            .ok()
    }
}

#[cfg(windows)]
fn collect_process_metrics() -> Result<ProcessMetricsSnapshot, String> {
    use windows_sys::Win32::{
        System::{
            ProcessStatus::{GetProcessMemoryInfo, PROCESS_MEMORY_COUNTERS_EX},
            Threading::{GetCurrentProcess, GetCurrentProcessId, GetProcessHandleCount},
        },
    };

    let handle = unsafe { GetCurrentProcess() };
    let mut counters = PROCESS_MEMORY_COUNTERS_EX {
        cb: std::mem::size_of::<PROCESS_MEMORY_COUNTERS_EX>() as u32,
        ..Default::default()
    };
    let ok = unsafe {
        GetProcessMemoryInfo(
            handle,
            std::ptr::addr_of_mut!(counters).cast(),
            counters.cb,
        )
    };
    if ok == 0 {
        return Err("GetProcessMemoryInfo returned false".to_string());
    }

    let mut handle_count = 0_u32;
    let handle_count = if unsafe { GetProcessHandleCount(handle, &mut handle_count) } == 0 {
        None
    } else {
        Some(u64::from(handle_count))
    };

    Ok(ProcessMetricsSnapshot {
        rss_bytes: counters.WorkingSetSize as u64,
        private_bytes: Some(counters.PrivateUsage as u64),
        virtual_bytes: Some(counters.PrivateUsage as u64),
        thread_count: u64::from(unsafe { GetCurrentProcessId() }.thread_count()?),
        windows_handle_count: handle_count,
        unix_fd_count: None,
    })
}

#[cfg(windows)]
trait WindowsProcessThreadCount {
    fn thread_count(self) -> Result<u32, String>;
}

#[cfg(windows)]
impl WindowsProcessThreadCount for u32 {
    fn thread_count(self) -> Result<u32, String> {
        use windows_sys::Win32::{
            Foundation::{CloseHandle, INVALID_HANDLE_VALUE},
            System::Diagnostics::ToolHelp::{
                CreateToolhelp32Snapshot, PROCESSENTRY32, Process32First, Process32Next,
                TH32CS_SNAPPROCESS,
            },
        };

        let snapshot = unsafe { CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0) };
        if snapshot == INVALID_HANDLE_VALUE {
            return Err("CreateToolhelp32Snapshot returned INVALID_HANDLE_VALUE".to_string());
        }

        let mut entry = PROCESSENTRY32 {
            dwSize: std::mem::size_of::<PROCESSENTRY32>() as u32,
            ..Default::default()
        };
        let mut found = None;
        let mut ok = unsafe { Process32First(snapshot, &mut entry) };
        while ok != 0 {
            if entry.th32ProcessID == self {
                found = Some(entry.cntThreads);
                break;
            }
            ok = unsafe { Process32Next(snapshot, &mut entry) };
        }
        unsafe {
            CloseHandle(snapshot);
        }

        found.ok_or_else(|| format!("process {self} not found in ToolHelp snapshot"))
    }
}

#[cfg(target_os = "linux")]
fn collect_process_metrics() -> Result<ProcessMetricsSnapshot, String> {
    let status = std::fs::read_to_string("/proc/self/status")
        .map_err(|error| format!("read /proc/self/status failed: {error}"))?;
    let statm = std::fs::read_to_string("/proc/self/statm")
        .map_err(|error| format!("read /proc/self/statm failed: {error}"))?;
    let page_size = linux_page_size_bytes()?;

    let rss_bytes = parse_status_kb(&status, "VmRSS:")
        .map(|value| value * 1024)
        .or_else(|| parse_statm_pages(&statm, 1).map(|value| value * page_size))
        .ok_or_else(|| "missing VmRSS/statm resident field".to_string())?;
    let virtual_bytes = parse_status_kb(&status, "VmSize:")
        .map(|value| value * 1024)
        .or_else(|| parse_statm_pages(&statm, 0).map(|value| value * page_size))
        .ok_or_else(|| "missing VmSize/statm size field".to_string())?;
    let private_bytes = parse_status_kb(&status, "VmData:").map(|value| value * 1024);
    let thread_count = parse_status_u64(&status, "Threads:")
        .ok_or_else(|| "missing Threads field".to_string())?;

    Ok(ProcessMetricsSnapshot {
        rss_bytes,
        private_bytes,
        virtual_bytes: Some(virtual_bytes),
        thread_count,
        windows_handle_count: None,
        unix_fd_count: linux_fd_count(),
    })
}

#[cfg(target_os = "linux")]
fn linux_page_size_bytes() -> Result<u64, String> {
    let output = std::process::Command::new("getconf")
        .arg("PAGESIZE")
        .output()
        .map_err(|error| format!("getconf PAGESIZE failed: {error}"))?;
    if !output.status.success() {
        return Err(format!("getconf PAGESIZE exited with {}", output.status));
    }
    let text = String::from_utf8(output.stdout)
        .map_err(|error| format!("getconf PAGESIZE output is not utf8: {error}"))?;
    text.trim()
        .parse::<u64>()
        .map_err(|error| format!("parse PAGESIZE failed: {error}"))
}

#[cfg(target_os = "linux")]
fn linux_fd_count() -> Option<u64> {
    let entries = std::fs::read_dir("/proc/self/fd").ok()?;
    Some(entries.filter_map(Result::ok).count() as u64)
}

#[cfg(target_os = "linux")]
fn parse_status_kb(status: &str, key: &str) -> Option<u64> {
    parse_status_u64(status, key)
}

#[cfg(target_os = "linux")]
fn parse_status_u64(status: &str, key: &str) -> Option<u64> {
    status.lines().find_map(|line| {
        let rest = line.strip_prefix(key)?.trim();
        rest.split_whitespace().next()?.parse::<u64>().ok()
    })
}

#[cfg(target_os = "linux")]
fn parse_statm_pages(statm: &str, index: usize) -> Option<u64> {
    statm
        .split_whitespace()
        .nth(index)?
        .parse::<u64>()
        .ok()
}

#[cfg(not(any(windows, target_os = "linux")))]
fn collect_process_metrics() -> Result<ProcessMetricsSnapshot, String> {
    Err("process metrics are only implemented for Windows and Linux".to_string())
}

#[cfg(test)]
mod tests {
    #[cfg(target_os = "linux")]
    use super::{parse_statm_pages, parse_status_kb, parse_status_u64};

    #[cfg(target_os = "linux")]
    #[test]
    fn parses_linux_proc_status_memory_fields() {
        let status = "Name:\tapi-server\nVmSize:\t  123456 kB\nVmRSS:\t   7890 kB\nVmData:\t   3456 kB\nThreads:\t37\n";

        assert_eq!(parse_status_kb(status, "VmRSS:"), Some(7890));
        assert_eq!(parse_status_kb(status, "VmSize:"), Some(123456));
        assert_eq!(parse_status_kb(status, "VmData:"), Some(3456));
        assert_eq!(parse_status_u64(status, "Threads:"), Some(37));
    }

    #[cfg(target_os = "linux")]
    #[test]
    fn parses_linux_statm_pages() {
        assert_eq!(parse_statm_pages("100 20 0 0 0 0 0", 0), Some(100));
        assert_eq!(parse_statm_pages("100 20 0 0 0 0 0", 1), Some(20));
        assert_eq!(parse_statm_pages("100 20", 7), None);
    }
}