//! Optional standard [`log`] facade backed by single-attempt atomic ingresses.
//!
//! The ingress never allocates, masks interrupts, locks, spins, encodes
//! MAVLink, or accesses DMA. Flight images keep this feature disabled; a
//! future normal-task image must reject ISR/panic paths before formatting.

use core::{
    cell::UnsafeCell,
    sync::atomic::{AtomicBool, Ordering},
};

use log::{Level, LevelFilter, Log, Metadata, Record, SetLoggerError};

use crate::{ErrorAdditionalInformation, ErrorPath, LogFlags, LogRecord, LogText, Severity};

/// Saturating, process-local accounting for the optional global facade.
///
/// It is deliberately not serialized in a Kaptan diagnostic message; the
/// facade remains a small text ingress rather than a metrics protocol.
#[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
pub struct FacadeHealth {
    /// Calls rejected before formatting because their context is forbidden.
    pub forbidden_context: u32,
    /// Calls that lost the one-attempt ingress ownership race.
    pub contended: u32,
    /// Calls dropped because their bounded ingress lane was full.
    pub full: u32,
    /// Formatted messages truncated to the fixed text capacity.
    pub truncated: u32,
}

struct FacadeCounters {
    forbidden_context: core::sync::atomic::AtomicU32,
    contended: core::sync::atomic::AtomicU32,
    full: core::sync::atomic::AtomicU32,
    truncated: core::sync::atomic::AtomicU32,
}

impl FacadeCounters {
    const fn new() -> Self {
        Self {
            forbidden_context: core::sync::atomic::AtomicU32::new(0),
            contended: core::sync::atomic::AtomicU32::new(0),
            full: core::sync::atomic::AtomicU32::new(0),
            truncated: core::sync::atomic::AtomicU32::new(0),
        }
    }

    /// A single compare-exchange keeps this diagnostic counter non-wrapping
    /// without a retry loop. Under simultaneous facade callers it may
    /// undercount, but it never blocks or falsely wraps to zero.
    fn increment(counter: &core::sync::atomic::AtomicU32) {
        let current = counter.load(Ordering::Relaxed);
        if current != u32::MAX {
            let _ = counter.compare_exchange(
                current,
                current + 1,
                Ordering::Relaxed,
                Ordering::Relaxed,
            );
        }
    }

    fn snapshot(&self) -> FacadeHealth {
        FacadeHealth {
            forbidden_context: self.forbidden_context.load(Ordering::Relaxed),
            contended: self.contended.load(Ordering::Relaxed),
            full: self.full.load(Ordering::Relaxed),
            truncated: self.truncated.load(Ordering::Relaxed),
        }
    }
}

#[derive(Clone, Copy)]
struct IngressRecord {
    severity: Severity,
    flags: LogFlags,
    body: IngressBody,
}

#[derive(Clone, Copy)]
enum IngressBody {
    Log {
        message: LogText,
    },
    Error {
        path: ErrorPath,
        additional_information: ErrorAdditionalInformation,
    },
}

impl IngressBody {
    const fn is_truncated(self) -> bool {
        match self {
            Self::Log { message } => message.is_truncated(),
            Self::Error {
                path,
                additional_information,
            } => path.is_truncated() || additional_information.is_truncated(),
        }
    }
}

struct IngressQueue<const N: usize> {
    entries: [Option<IngressRecord>; N],
    head: usize,
    len: usize,
}

impl<const N: usize> IngressQueue<N> {
    const fn new() -> Self {
        assert!(N > 0, "facade ingress must be nonempty");
        Self {
            entries: [None; N],
            head: 0,
            len: 0,
        }
    }

    fn push(&mut self, record: IngressRecord) -> bool {
        if self.len == N {
            return false;
        }
        let index = (self.head + self.len) % N;
        self.entries[index] = Some(record);
        self.len += 1;
        true
    }

    fn pop(&mut self) -> Option<IngressRecord> {
        if self.len == 0 {
            return None;
        }
        let record = self.entries[self.head].take();
        self.head = (self.head + 1) % N;
        self.len -= 1;
        record
    }
}

/// Fixed global-facade ingress. The normal task/runtime owner takes each
/// transport-neutral record and fans it out to independently filtered sinks.
pub struct FacadeIngress<const N: usize> {
    /// One dedicated critical slot keeps error records independent from the
    /// bounded ordinary FIFO. It is deliberately one slot: producers never
    /// spin, and a later error may be dropped only when an earlier error is
    /// already retained.
    critical: UnsafeCell<Option<IngressRecord>>,
    critical_busy: AtomicBool,
    queue: UnsafeCell<IngressQueue<N>>,
    queue_busy: AtomicBool,
    counters: FacadeCounters,
}

// SAFETY: each interior-mutability cell is accessed only while its matching
// atomic busy bit is acquired by one compare-exchange. The operation never
// waits or retries; contention leaves the corresponding bounded lane intact.
unsafe impl<const N: usize> Sync for FacadeIngress<N> {}

impl<const N: usize> FacadeIngress<N> {
    /// Creates an empty fixed ingress.
    pub const fn new() -> Self {
        Self {
            critical: UnsafeCell::new(None),
            critical_busy: AtomicBool::new(false),
            queue: UnsafeCell::new(IngressQueue::new()),
            queue_busy: AtomicBool::new(false),
            counters: FacadeCounters::new(),
        }
    }

    fn push(&self, record: IngressRecord) {
        if record.severity <= Severity::Error {
            self.push_critical(record);
            return;
        }
        match self.with_queue(|queue| queue.push(record)) {
            Some(true) => {}
            Some(false) => FacadeCounters::increment(&self.counters.full),
            None => FacadeCounters::increment(&self.counters.contended),
        }
    }

    fn push_critical(&self, record: IngressRecord) {
        match self.with_critical(|slot| {
            if slot.is_none() {
                *slot = Some(record);
                true
            } else {
                false
            }
        }) {
            Some(true) => {}
            Some(false) => FacadeCounters::increment(&self.counters.full),
            None => FacadeCounters::increment(&self.counters.contended),
        }
    }

    /// Process-local bounded-ingress accounting.
    pub fn health(&self) -> FacadeHealth {
        self.counters.snapshot()
    }

    /// Takes at most one preformatted event for composition-level fan-out.
    ///
    /// The ingress deliberately applies no sink policy. A caller may submit
    /// the returned copy to MAVLink, blackbox, or any later sink without one
    /// sink's verbosity selection suppressing another's event.
    pub fn take(&self) -> Option<LogRecord> {
        let record = match self.with_critical(Option::take) {
            Some(Some(record)) => record,
            Some(None) => self.with_queue(IngressQueue::pop).flatten()?,
            None => return None,
        };
        Some(match record.body {
            IngressBody::Log { message } => LogRecord::new(record.severity, record.flags, message),
            IngressBody::Error {
                path,
                additional_information,
            } => LogRecord::error(record.flags, path, additional_information),
        })
    }

    fn with_critical<R>(
        &self,
        operation: impl FnOnce(&mut Option<IngressRecord>) -> R,
    ) -> Option<R> {
        if self
            .critical_busy
            .compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed)
            .is_err()
        {
            return None;
        }
        // SAFETY: `critical_busy` grants exclusive access to the one fixed
        // critical slot. No call to `operation` escapes its mutable reference.
        let result = operation(unsafe { &mut *self.critical.get() });
        self.critical_busy.store(false, Ordering::Release);
        Some(result)
    }

    fn with_queue<R>(&self, operation: impl FnOnce(&mut IngressQueue<N>) -> R) -> Option<R> {
        if self
            .queue_busy
            .compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed)
            .is_err()
        {
            return None;
        }
        // SAFETY: `queue_busy` grants this caller exclusive access. No call
        // to `operation` escapes the mutable reference and it only performs
        // fixed-array queue work.
        let result = operation(unsafe { &mut *self.queue.get() });
        self.queue_busy.store(false, Ordering::Release);
        Some(result)
    }
}

impl<const N: usize> Default for FacadeIngress<N> {
    fn default() -> Self {
        Self::new()
    }
}

/// Installed standard logger. The context probe must return true for
/// ISR/panic/hard-fault/shutdown contexts; those calls are rejected before
/// [`Record::args`] is formatted.
pub struct GlobalLogger<const N: usize> {
    ingress: &'static FacadeIngress<N>,
    forbidden_context: fn() -> bool,
}

impl<const N: usize> GlobalLogger<N> {
    /// Creates a logger tied to one static bounded ingress.
    pub const fn new(ingress: &'static FacadeIngress<N>, forbidden_context: fn() -> bool) -> Self {
        Self {
            ingress,
            forbidden_context,
        }
    }

    /// Installs this logger once for the process/image and enables all Rust
    /// levels compiled into the image. Each sink applies its policy after it
    /// receives a record from the ingress.
    pub fn install(&'static self) -> Result<(), SetLoggerError> {
        log::set_logger(self)?;
        log::set_max_level(LevelFilter::Trace);
        Ok(())
    }
}

impl<const N: usize> Log for GlobalLogger<N> {
    fn enabled(&self, _: &Metadata<'_>) -> bool {
        if (self.forbidden_context)() {
            FacadeCounters::increment(&self.ingress.counters.forbidden_context);
            return false;
        }
        true
    }

    fn log(&self, record: &Record<'_>) {
        if (self.forbidden_context)() {
            FacadeCounters::increment(&self.ingress.counters.forbidden_context);
            return;
        }
        let severity = severity(record.level());
        let flags = if record.level() == Level::Trace {
            LogFlags::TRACE
        } else {
            LogFlags::NONE
        };
        let body = if severity <= Severity::Error {
            IngressBody::Error {
                path: ErrorPath::from_message(record.target()),
                additional_information: ErrorAdditionalInformation::from_arguments(format_args!(
                    "{}",
                    record.args()
                )),
            }
        } else {
            IngressBody::Log {
                message: LogText::from_arguments(format_args!(
                    "[{}] {}",
                    record.target(),
                    record.args()
                )),
            }
        };
        if body.is_truncated() {
            FacadeCounters::increment(&self.ingress.counters.truncated);
        }
        self.ingress.push(IngressRecord {
            severity,
            flags,
            body,
        });
    }

    fn flush(&self) {}
}

const fn severity(level: Level) -> Severity {
    match level {
        Level::Error => Severity::Error,
        Level::Warn => Severity::Warning,
        Level::Info => Severity::Info,
        Level::Debug | Level::Trace => Severity::Debug,
    }
}

#[cfg(test)]
mod tests {
    use core::fmt;
    use std::sync::Once;

    use super::{FacadeIngress, GlobalLogger};
    use crate::{RecordChannel, Severity};
    use log::{Level, Log, Metadata, Record};

    static GLOBAL_INGRESS: FacadeIngress<2> = FacadeIngress::new();
    static GLOBAL_LOGGER: GlobalLogger<2> = GlobalLogger::new(&GLOBAL_INGRESS, never_forbidden);
    static INSTALL_GLOBAL: Once = Once::new();

    fn never_forbidden() -> bool {
        false
    }

    fn always_forbidden() -> bool {
        true
    }

    #[test]
    fn rejects_forbidden_context_before_formatting() {
        static INGRESS: FacadeIngress<2> = FacadeIngress::new();
        let logger = GlobalLogger::new(&INGRESS, always_forbidden);
        let metadata = Metadata::builder()
            .level(Level::Error)
            .target("test")
            .build();
        assert!(!logger.enabled(&metadata));
        assert_eq!(INGRESS.health().forbidden_context, 1);

        static UNFILTERED: FacadeIngress<2> = FacadeIngress::new();
        let unfiltered_logger = GlobalLogger::new(&UNFILTERED, never_forbidden);
        let debug = Metadata::builder()
            .level(Level::Debug)
            .target("test")
            .build();
        assert!(unfiltered_logger.enabled(&debug));
        unfiltered_logger.log(
            &Record::builder()
                .metadata(debug)
                .args(format_args!("sink decides whether to keep this"))
                .build(),
        );
        assert_eq!(
            UNFILTERED
                .take()
                .expect("debug event reaches the unfiltered ingress")
                .severity(),
            Severity::Debug
        );
    }

    #[test]
    fn formats_once_then_exposes_a_transport_neutral_record() {
        static INGRESS: FacadeIngress<2> = FacadeIngress::new();
        let logger = GlobalLogger::new(&INGRESS, never_forbidden);
        let record = Record::builder()
            .metadata(
                Metadata::builder()
                    .level(Level::Error)
                    .target("test")
                    .build(),
            )
            .args(format_args!("link failed"))
            .build();
        logger.log(&record);
        let pending = INGRESS.take().expect("facade event retained");
        assert_eq!(pending.error_path().unwrap().as_str(), "test");
        assert_eq!(
            pending.additional_information().unwrap().as_str(),
            "link failed"
        );
        assert_eq!(pending.severity(), Severity::Error);
        assert_eq!(pending.channel(), RecordChannel::Error);
    }

    struct MustNotFormat;

    impl fmt::Display for MustNotFormat {
        fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
            panic!("forbidden-context log arguments must not format")
        }
    }

    #[test]
    fn standard_log_macros_enqueue_bounded_targeted_text() {
        INSTALL_GLOBAL.call_once(|| GLOBAL_LOGGER.install().expect("one global test logger"));
        log::info!("armed configuration loaded");
        assert!(
            GLOBAL_INGRESS
                .take()
                .expect("macro event retained")
                .message()
                .unwrap()
                .as_str()
                .ends_with("armed configuration loaded")
        );
    }

    #[test]
    fn forbidden_context_skips_argument_formatting() {
        static INGRESS: FacadeIngress<2> = FacadeIngress::new();
        let logger = GlobalLogger::new(&INGRESS, always_forbidden);
        logger.log(
            &Record::builder()
                .metadata(
                    Metadata::builder()
                        .level(Level::Error)
                        .target("test")
                        .build(),
                )
                .args(format_args!("{}", MustNotFormat))
                .build(),
        );
        assert_eq!(INGRESS.health().forbidden_context, 1);
    }

    #[test]
    fn bounded_ingress_exposes_local_drop_and_truncation_accounting() {
        static INGRESS: FacadeIngress<1> = FacadeIngress::new();
        let logger = GlobalLogger::new(&INGRESS, never_forbidden);
        logger.log(
            &Record::builder()
                .metadata(
                    Metadata::builder()
                        .level(Level::Info)
                        .target("test")
                        .build(),
                )
                .args(format_args!("x"))
                .build(),
        );
        logger.log(
            &Record::builder()
                .metadata(
                    Metadata::builder()
                        .level(Level::Info)
                        .target("test")
                        .build(),
                )
                .args(format_args!("y"))
                .build(),
        );
        let long = "z".repeat(crate::MAXIMUM_LOG_TEXT_BYTES + 1);
        logger.log(
            &Record::builder()
                .metadata(
                    Metadata::builder()
                        .level(Level::Info)
                        .target("test")
                        .build(),
                )
                .args(format_args!("{long}"))
                .build(),
        );
        let health = INGRESS.health();
        assert_eq!(health.full, 2);
        assert_eq!(health.truncated, 1);
    }

    #[test]
    fn full_optional_ingress_retains_an_error_for_the_error_channel() {
        static INGRESS: FacadeIngress<1> = FacadeIngress::new();
        let logger = GlobalLogger::new(&INGRESS, never_forbidden);
        logger.log(
            &Record::builder()
                .metadata(
                    Metadata::builder()
                        .level(Level::Info)
                        .target("test")
                        .build(),
                )
                .args(format_args!("optional"))
                .build(),
        );
        logger.log(
            &Record::builder()
                .metadata(
                    Metadata::builder()
                        .level(Level::Error)
                        .target("test")
                        .build(),
                )
                .args(format_args!("critical"))
                .build(),
        );

        let error = INGRESS
            .take()
            .expect("error retained ahead of optional text");
        assert_eq!(error.channel(), RecordChannel::Error);
        assert_eq!(error.error_path().unwrap().as_str(), "test");
        assert_eq!(error.additional_information().unwrap().as_str(), "critical");
        assert_eq!(INGRESS.health().full, 0);
    }
}