ic-logging 0.17.5

//! Heap log admission, immutable rows, bounded retention and cursor paging.
//!
//! Callers own authorization, scheduling, time and any durable storage.

use alloc::{collections::VecDeque, string::String, vec::Vec};
use core::num::NonZeroUsize;

const TRUNCATION_SUFFIX: &str = "...[truncated]";

/// Severity ordering used by minimum-level filters.
#[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
pub enum Level {
    /// Development detail.
    Debug,
    /// Ordinary informational observation.
    Info,
    /// Successful outcome, ordered above informational observations.
    Ok,
    /// Recoverable or suspicious condition.
    Warn,
    /// Failed operation or invariant observation.
    Error,
}

/// Fixed limits chosen by the host before constructing a log.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct Limits {
    /// Maximum retained rows; count eviction removes the oldest row.
    pub max_entries: NonZeroUsize,
    /// Maximum retained message bytes, including any truncation suffix.
    pub max_message_bytes: usize,
    /// Maximum UTF-8 bytes in each source or topic label.
    pub max_label_bytes: usize,
}

/// Borrowed append input; bounded retained strings are copied only after admission.
#[derive(Clone, Copy, Debug)]
pub struct EntryInput<'a> {
    /// Caller-supplied whole seconds; may not regress within this log.
    pub created_at_secs: u64,
    /// Observation severity.
    pub level: Level,
    /// Application or component label.
    pub source: &'a str,
    /// Optional application-defined topic.
    pub topic: Option<&'a str>,
    /// Message to retain, truncated to the configured byte ceiling.
    pub message: &'a str,
}

/// Immutable retained row; sequence identities belong to one log instance.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Entry {
    /// Monotonic identity, starting at zero and never reused within one log.
    pub sequence: u64,
    /// Admitted caller-supplied timestamp.
    pub created_at_secs: u64,
    /// Observation severity.
    pub level: Level,
    /// Bounded application or component label.
    pub source: String,
    /// Optional bounded topic label.
    pub topic: Option<String>,
    /// Bounded message text.
    pub message: String,
    /// Whether the original message exceeded its byte limit.
    pub truncated: bool,
}

/// Label rejected by append admission.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum Label {
    /// Application or component source.
    Source,
    /// Optional topic.
    Topic,
}

/// Append refusal; every variant leaves entries and watermarks unchanged.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum AppendError {
    /// A label exceeded its configured byte ceiling.
    LabelTooLong {
        /// Rejected field.
        label: Label,
        /// Supplied UTF-8 byte length.
        actual_bytes: usize,
        /// Configured ceiling.
        max_bytes: usize,
    },
    /// The timestamp is older than the last admitted append, even after pruning.
    TimestampRegressed {
        /// Last admitted timestamp.
        previous: u64,
        /// Rejected timestamp.
        current: u64,
    },
    /// All sequence identities have been issued for this log instance.
    SequenceExhausted,
}

impl core::fmt::Display for AppendError {
    fn fmt(&self, formatter: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        match self {
            Self::LabelTooLong {
                label,
                actual_bytes,
                max_bytes,
            } => {
                let label = match label {
                    Label::Source => "source",
                    Label::Topic => "topic",
                };
                write!(
                    formatter,
                    "log {label} label exceeds {max_bytes} bytes ({actual_bytes} supplied)"
                )
            }
            Self::TimestampRegressed { previous, current } => write!(
                formatter,
                "log timestamp regressed from {previous} to {current} seconds"
            ),
            Self::SequenceExhausted => formatter.write_str("log sequence identities are exhausted"),
        }
    }
}

impl core::error::Error for AppendError {}

/// Optional exact label filters and a minimum severity; all supplied filters apply.
#[derive(Clone, Copy, Debug, Default)]
pub struct Filter<'a> {
    /// Match this exact source when present.
    pub source: Option<&'a str>,
    /// Match this exact topic when present.
    pub topic: Option<&'a str>,
    /// Include this severity and higher when present.
    pub min_level: Option<Level>,
}

impl Filter<'_> {
    fn matches(self, entry: &Entry) -> bool {
        let source_matches = self.source.is_none_or(|source| source == entry.source);
        let topic_matches = self
            .topic
            .is_none_or(|topic| Some(topic) == entry.topic.as_deref());
        let level_matches = self.min_level.is_none_or(|level| entry.level >= level);
        source_matches && topic_matches && level_matches
    }
}

/// One newest-first scan using an exclusive sequence cursor.
#[derive(Clone, Copy, Debug)]
pub struct PageRequest {
    /// Scan entries older than this sequence, or begin at the newest entry.
    pub before: Option<u64>,
    /// Maximum matching rows to return.
    pub max_results: NonZeroUsize,
    /// Maximum candidate rows to examine after binary-search positioning.
    pub max_scanned: NonZeroUsize,
}

/// Borrowed page; a continuation can be present even when no row matched.
#[derive(Debug)]
pub struct Page<'a> {
    /// Matching rows, newest first.
    pub entries: Vec<&'a Entry>,
    /// Number of candidate rows examined, not the total number of matches.
    pub scanned: usize,
    /// Exclusive cursor for the next older scan, if retained candidates remain.
    pub next_before: Option<u64>,
}

/// Result of a bounded age-retention call.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct RetentionBatch {
    /// Number of rows removed by this call.
    pub removed: usize,
    /// Whether the oldest remaining row is also expired.
    pub more_due: bool,
}

/// Complete passive checkpoint, with rows ordered oldest first.
///
/// This is not a byte format or an instance identity. Restore only through
/// [`Log::restore`]; consumers own durable encoding, identity and allocation.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct LogSnapshot {
    /// Fixed bounds of the original log.
    pub limits: Limits,
    /// Retained contiguous suffix of issued identities, oldest first.
    pub entries: Vec<Entry>,
    /// Next unissued identity; `None` means permanent sequence exhaustion.
    pub next_sequence: Option<u64>,
    /// Last admitted timestamp, retained even when no rows remain.
    pub last_created_at_secs: Option<u64>,
}

/// Refusal of a checkpoint that the maintained engine cannot produce.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum RestoreError {
    /// Retained row count exceeds the fixed limit.
    TooManyEntries,
    /// Rows are not contiguous or the next identity contradicts the retained tail.
    InvalidSequence,
    /// Row times regress or the timestamp watermark is inconsistent.
    InvalidTimestamp,
    /// A label/message exceeds its bound or truncated text is inconsistent.
    InvalidEntry,
}

impl core::fmt::Display for RestoreError {
    fn fmt(&self, formatter: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        formatter.write_str(match self {
            Self::TooManyEntries => "log checkpoint exceeds its entry limit",
            Self::InvalidSequence => "invalid log checkpoint sequence",
            Self::InvalidTimestamp => "invalid log checkpoint timestamp",
            Self::InvalidEntry => "invalid bounded log checkpoint entry",
        })
    }
}

impl core::error::Error for RestoreError {}

/// Caller-owned heap history with fixed limits and monotonic append watermarks.
#[derive(Debug)]
pub struct Log {
    limits: Limits,
    entries: VecDeque<Entry>,
    next_sequence: Option<u64>,
    last_created_at_secs: Option<u64>,
}

impl Log {
    /// Construct an empty log without preallocating the configured maximum.
    #[must_use]
    pub const fn new(limits: Limits) -> Self {
        Self {
            limits,
            entries: VecDeque::new(),
            next_sequence: Some(0),
            last_created_at_secs: None,
        }
    }

    /// Return the fixed bounds without copying retained messages.
    #[must_use]
    pub const fn limits(&self) -> Limits {
        self.limits
    }

    /// Copy all retained rows and both append watermarks for explicit checkpointing.
    /// Work and allocation scale with retained history, not a cursor-page limit.
    #[must_use]
    pub fn snapshot(&self) -> LogSnapshot {
        LogSnapshot {
            limits: self.limits,
            entries: self.entries.iter().cloned().collect(),
            next_sequence: self.next_sequence,
            last_created_at_secs: self.last_created_at_secs,
        }
    }

    /// Consume the log into a checkpoint without cloning retained entries.
    /// Fixed limits, row order and both append watermarks are preserved, including
    /// empty histories and permanent sequence exhaustion.
    #[must_use]
    pub fn into_snapshot(self) -> LogSnapshot {
        LogSnapshot {
            limits: self.limits,
            entries: self.entries.into(),
            next_sequence: self.next_sequence,
            last_created_at_secs: self.last_created_at_secs,
        }
    }

    /// Admit a complete checkpoint without replaying entries or reissuing identities.
    ///
    /// Empty retained history preserves its original append watermarks. Admission
    /// proves internal consistency, not source authenticity or host continuity.
    /// Byte decoders must bound allocation before constructing the checkpoint.
    ///
    /// # Errors
    /// Rejects bounds, row ordering, watermarks and truncation states that cannot
    /// result from this engine's append/retention operations.
    pub fn restore(snapshot: LogSnapshot) -> Result<Self, RestoreError> {
        let LogSnapshot {
            limits,
            entries,
            next_sequence,
            last_created_at_secs,
        } = snapshot;
        if entries.len() > limits.max_entries.get() {
            return Err(RestoreError::TooManyEntries);
        }
        if (next_sequence == Some(0)) != last_created_at_secs.is_none() {
            return Err(RestoreError::InvalidTimestamp);
        }
        // The last row cannot expire at the final representable timestamp, and
        // count eviction always leaves a newer row in this nonzero-capacity log.
        if entries.is_empty() && last_created_at_secs == Some(u64::MAX) {
            return Err(RestoreError::InvalidTimestamp);
        }
        let mut previous: Option<&Entry> = None;
        for entry in &entries {
            if entry.source.len() > limits.max_label_bytes
                || entry
                    .topic
                    .as_ref()
                    .is_some_and(|topic| topic.len() > limits.max_label_bytes)
                || !valid_retained_message(entry, limits.max_message_bytes)
            {
                return Err(RestoreError::InvalidEntry);
            }
            if let Some(previous) = previous {
                if previous.sequence.checked_add(1) != Some(entry.sequence) {
                    return Err(RestoreError::InvalidSequence);
                }
                if entry.created_at_secs < previous.created_at_secs {
                    return Err(RestoreError::InvalidTimestamp);
                }
            }
            previous = Some(entry);
        }
        if let Some(last) = previous {
            if last.sequence.checked_add(1) != next_sequence {
                return Err(RestoreError::InvalidSequence);
            }
            if Some(last.created_at_secs) != last_created_at_secs {
                return Err(RestoreError::InvalidTimestamp);
            }
        }
        Ok(Self {
            limits,
            entries: entries.into(),
            next_sequence,
            last_created_at_secs,
        })
    }

    /// Return the retained row count.
    #[must_use]
    pub fn len(&self) -> usize {
        self.entries.len()
    }

    /// Whether this log currently has no retained rows.
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.entries.is_empty()
    }

    /// Admit one row and evict the oldest if the count ceiling is reached.
    ///
    /// # Errors
    /// Returns a typed refusal for oversized labels, regressed time or exhausted
    /// sequence identities. Validation completes before any eviction or mutation.
    pub fn append(&mut self, input: EntryInput<'_>) -> Result<u64, AppendError> {
        self.validate_label(Label::Source, input.source)?;
        if let Some(topic) = input.topic {
            self.validate_label(Label::Topic, topic)?;
        }
        if let Some(previous) = self.last_created_at_secs
            && input.created_at_secs < previous
        {
            return Err(AppendError::TimestampRegressed {
                previous,
                current: input.created_at_secs,
            });
        }
        let sequence = self.next_sequence.ok_or(AppendError::SequenceExhausted)?;
        let entry = Entry {
            sequence,
            created_at_secs: input.created_at_secs,
            level: input.level,
            source: String::from(input.source),
            topic: input.topic.map(String::from),
            message: bounded_message(input.message, self.limits.max_message_bytes),
            truncated: input.message.len() > self.limits.max_message_bytes,
        };
        if self.entries.len() == self.limits.max_entries.get() {
            self.entries.pop_front();
        }
        self.entries.push_back(entry);
        self.next_sequence = sequence.checked_add(1);
        self.last_created_at_secs = Some(input.created_at_secs);
        Ok(sequence)
    }

    /// Remove at most `max_remove` entries strictly older than the age cutoff.
    /// An entry remains available through its full configured age second.
    pub fn prune_expired(
        &mut self,
        now_secs: u64,
        max_age_secs: u64,
        max_remove: usize,
    ) -> RetentionBatch {
        let cutoff = now_secs.saturating_sub(max_age_secs);
        let mut removed = 0;
        while removed < max_remove && self.expired_before(cutoff) {
            self.entries.pop_front();
            removed += 1;
        }
        RetentionBatch {
            removed,
            more_due: self.expired_before(cutoff),
        }
    }

    /// First second at which the oldest row expires; `None` for empty history or overflow.
    #[must_use]
    pub fn next_expiry_secs(&self, max_age_secs: u64) -> Option<u64> {
        self.entries
            .front()?
            .created_at_secs
            .checked_add(max_age_secs)?
            .checked_add(1)
    }

    /// Scan newest-first without cloning retained messages or counting every match.
    /// Follow `next_before` with the same filters, including after an empty page.
    #[must_use]
    pub fn page(&self, filter: Filter<'_>, request: PageRequest) -> Page<'_> {
        let mut cursor = request.before.map_or(self.entries.len(), |sequence| {
            self.entries
                .binary_search_by_key(&sequence, |entry| entry.sequence)
                .unwrap_or_else(|index| index)
        });
        let mut entries = Vec::new();
        let mut scanned = 0;
        let mut last_scanned = None;
        while cursor > 0
            && scanned < request.max_scanned.get()
            && entries.len() < request.max_results.get()
        {
            cursor -= 1;
            let entry = &self.entries[cursor];
            scanned += 1;
            last_scanned = Some(entry.sequence);
            if filter.matches(entry) {
                entries.push(entry);
            }
        }
        Page {
            entries,
            scanned,
            next_before: if cursor > 0 { last_scanned } else { None },
        }
    }

    const fn validate_label(&self, label: Label, text: &str) -> Result<(), AppendError> {
        if text.len() > self.limits.max_label_bytes {
            return Err(AppendError::LabelTooLong {
                label,
                actual_bytes: text.len(),
                max_bytes: self.limits.max_label_bytes,
            });
        }
        Ok(())
    }

    fn expired_before(&self, cutoff: u64) -> bool {
        self.entries
            .front()
            .is_some_and(|entry| entry.created_at_secs < cutoff)
    }
}

const fn truncation_suffix(max_bytes: usize) -> &'static str {
    if max_bytes > TRUNCATION_SUFFIX.len() {
        TRUNCATION_SUFFIX
    } else {
        ""
    }
}

fn valid_retained_message(entry: &Entry, max_bytes: usize) -> bool {
    if entry.message.len() > max_bytes {
        return false;
    }
    if !entry.truncated {
        return true;
    }
    let suffix = truncation_suffix(max_bytes);
    let Some(prefix) = entry.message.strip_suffix(suffix) else {
        return false;
    };
    let budget = max_bytes - suffix.len();
    budget.saturating_sub(prefix.len()) < 4
}

fn bounded_message(message: &str, max_bytes: usize) -> String {
    if message.len() <= max_bytes {
        return String::from(message);
    }
    let suffix = truncation_suffix(max_bytes);
    let mut end = max_bytes - suffix.len();
    while !message.is_char_boundary(end) {
        end -= 1;
    }
    // Reserve the final payload once, including the suffix, to avoid growth.
    let mut retained = String::with_capacity(end + suffix.len());
    retained.push_str(&message[..end]);
    retained.push_str(suffix);
    retained
}

#[cfg(test)]
mod tests;