//! Statistics generator for [`slog`].
//!
//! This crate allows for statistics - counters, gauges, and bucket counters - to be automatically
//! calculated and reported based on logged events.  The logged events MUST implement the
//! [`ExtLoggable`] trait.
//!
//! To support this, the [`slog-extlog-derive`] crate can be used to link logs to a specific
//! statistic.   This generates fast, compile-time checking for statistics updates at the point
//! of logging.
//!
//! Users should use the [`define_stats`] macro to list their statistics.  They can then pass the
//! list (along with stats from any dependencies) to a [`StatisticsLogger`] wrapping an
//! [`slog::Logger`].  The statistics trigger function on the `ExtLoggable` objects then triggers
//! statistic updates based on logged values.
//!
//! Library users should export the result of `define_stats!`, so that binary developers can
//! track the set of stats from all dependent crates in a single tracker.
//!
//! Triggers should be added to [`ExtLoggable`] objects using the [`slog-extlog-derive`] crate.
//!
//! [`ExtLoggable`]: ../trait.ExtLoggable.html
//! [`define_stats`]: ../macro.define_stats.html
//! [`slog::Logger`]: ../../slog/struct.Logger.html
//! [`slog`]: ../../slog/index.html
//! [`slog-extlog-derive`]: ../../slog_extlog_derive/index.html
//! [`StatisticsLogger`]: ./struct.StatisticsLogger.html

use std::collections::HashMap;
use std::fmt;
use std::marker::PhantomData;
use std::ops::Deref;
use std::panic::RefUnwindSafe;
use std::sync::atomic::{AtomicIsize, Ordering};
use std::sync::Arc;
use std::sync::RwLock;

#[cfg(feature = "interval_logging")]
use std::time::Duration;

use slog::info;

//////////////////////////////////////////////////////
// Public types - stats definitions
//////////////////////////////////////////////////////

/// A configured statistic, defined in terms of the external logs that trigger it to change.
///
/// These definitions are provided at start of day to populate the tracker.
///
/// These should NOT be constructed directly but by using the
/// [`define_stats`](./macro.define_stats.html) macro.
///
pub trait StatDefinition: fmt::Debug {
    /// The name of this metric.  This name is reported in logs as the `metric_name` field.
    fn name(&self) -> &'static str;
    /// A human readable-description of the statistic, describing its meaning.  When logged this
    /// is the log message.
    fn description(&self) -> &'static str;
    /// The type of statistic.
    fn stype(&self) -> StatType;
    /// An optional list of field names to group the statistic by.
    fn group_by(&self) -> Vec<&'static str>;
    /// An optional set of numerical buckets to group the statistic by.
    fn buckets(&self) -> Option<Buckets>;
}

/// A macro to define the statistics that can be tracked by the logger.
/// Use of this macro requires [`StatDefinition`](trait.StatDefinition.html) to be in scope.
///
/// All statistics should be defined by their library using this macro.
///
/// The syntax is as follows:
///
/// ```text
///   define_stats!{
///      STATS_LIST_NAME = {
///          StatName(Type, "Description", ["tag1", "tag2", ...]),
///          StatName2(...),
///          ...
///      }
///   }
/// ```
///
/// The `STATS_LIST_NAME` is then created as a vector of definitions that can be passed in as the
/// `stats` field on a `StatsConfig` object.
///
/// Each definition in the list has the format above, with the fields as follows.
///
///   - `StatName` is the externally-facing metric name.
///   - `Type` is the `StatType` of this statistic, for example `Counter`.
///    Must be a valid subtype of that enum.
///   - `Description`  is a human readable description of the statistic.  This will be logged as
///   the log message,
///   - The list of `tags` define field names to group the statistic by.
///    A non-empty list indicates that this statistic should be split into groups,
///   counting the stat separately for each different value of these fields that is seen.
///   These might be a remote hostname, say, or a tag field.
///     - If multiple tags are provided, the stat is counted separately for all distinct
///       combinations of tag values.
///     - Use of this feature should be avoided for fields that can take very many values, such as
///   a subscriber number, or for large numbers of tags - each tag name and seen value adds a
///   performance dip and a small memory overhead that is never freed.
///   - If the `Type` field is set to `BucketCounter`, then a `BucketMethod`, bucket label and bucket limits must
///     also be provided like so:
///
/// ```text
///    define_stats!{
///      STATS_LIST_NAME = {
///          StatName(BucketCounter, "Description", ["tag1", "tag2", ...], (BucketMethod, "bucket_label", [1, 2, 3, ...])),
///          StatName2(...),
///          ...
///      }
///   }
/// ```
///
///   - The `BucketMethod` determines how the stat will be sorted into numerical buckets and should
///   - be a subtype of that enum.
///   - The bucket limits should be a list of `i64` values, each representing the upper bound of
///     that bucket.
///   - The bucket label should describe what the buckets measure and should be distinct from the tags.
///     Each stat log will be labelled with the pair `(bucket_label, bucket_value)` in addition to the tags,
///     where `bucket_value` is the numerical value of the bucket the log falls into.
#[macro_export]
macro_rules! define_stats {

    // Entry point - match each individual stat name and pass on the details for further parsing
    ($name:ident = {$($stat:ident($($details:tt),*)),*}) => {
        /// A vector of stats that can be passed in as the `stats` field on a `StatsConfig` object.
        pub static $name: $crate::stats::StatDefinitions = &[$(&$stat),*];

        mod inner_stats {
            $(
                #[derive(Debug, Clone)]
                // Prometheus metrics are snake_case, so allow non-camel-case types here.
                #[allow(non_camel_case_types)]
                pub struct $stat;
            )*
        }

        $(
            $crate::define_stats!{@single $stat, $($details),*}
        )*
    };

    // `BucketCounter`s require a `BucketMethod`, bucket label and bucket limits
    (@single $stat:ident, BucketCounter, $desc:expr, [$($tags:tt),*], ($bmethod:ident, $blabel:expr, [$($blimits:expr),*]) ) => {
        $crate::define_stats!{@inner $stat, BucketCounter, $desc, $bmethod, $blabel, [$($tags),*], [$($blimits),*]}
    };

    // Non `BucketCounter` stat types
    (@single $stat:ident, $stype:ident, $desc:expr, [$($tags:tt),*] ) => {
        $crate::define_stats!{@inner $stat, $stype, $desc, Freq, "", [$($tags),*], []}
    };

    // Retained for backwards-compatibility
    (@single $stat:ident, $stype:ident, $id:expr, $desc:expr, [$($tags:tt),*] ) => {
        $crate::define_stats!{@inner $stat, $stype, $desc, Freq, "", [$($tags),*], []}
    };

    // Trait impl for StatDefinition
    (@inner $stat:ident, $stype:ident, $desc:expr, $bmethod:ident, $blabel:expr, [$($tags:tt),*], [$($blimits:expr),*]) => {

        // Suppress the warning about cases - this value is never going to be seen
        #[allow(non_upper_case_globals)]
        static $stat : inner_stats::$stat = inner_stats::$stat;

        impl $crate::stats::StatDefinition for inner_stats::$stat {
            /// The name of this statistic.
            fn name(&self) -> &'static str { stringify!($stat) }
            /// A human readable-description of the statistic, describing its meaning.
            fn description(&self) -> &'static str { $desc }
            /// The type
            fn stype(&self) -> $crate::stats::StatType { $crate::stats::StatType::$stype }
            /// An optional list of field names to group the statistic by.
            fn group_by(&self) -> Vec<&'static str> { vec![$($tags),*] }
            /// The numerical buckets and bucketing method used to group the statistic.
            fn buckets(&self) -> Option<$crate::stats::Buckets> {
                match self.stype() {
                    $crate::stats::StatType::BucketCounter => {
                        Some($crate::stats::Buckets::new($crate::stats::BucketMethod::$bmethod,
                            $blabel,
                            &[$($blimits as i64),* ],
                        ))
                    },
                    _ => None
                }
            }
        }
    };
}

/// A stat definition, possibly filtered with some specific tag values.
pub struct StatDefinitionTagged {
    /// The statistic definition
    pub defn: &'static (dyn StatDefinition + Sync),
    /// THe fixed tag values.  The keys *must* match keys in `defn`.
    pub fixed_tags: &'static [(&'static str, &'static str)],
}

/// A trait indicating that this log can be used to trigger a statistics change.
pub trait StatTrigger {
    /// The list of stats that this trigger applies to.
    fn stat_list(&self) -> &[StatDefinitionTagged];
    /// The condition that must be satisfied for this stat to change
    fn condition(&self, _stat_id: &StatDefinitionTagged) -> bool {
        false
    }
    /// Get the associated tag value for this log.
    /// The value must be convertible to a string so it can be stored internally.
    fn tag_value(&self, stat_id: &StatDefinitionTagged, _tag_name: &'static str) -> String;
    /// The details of the change to make for this stat, if `condition` returned true.
    fn change(&self, _stat_id: &StatDefinitionTagged) -> Option<ChangeType> {
        None
    }
    /// The value to be used to sort the statistic into the correct bucket(s).
    fn bucket_value(&self, _stat_id: &StatDefinitionTagged) -> Option<f64> {
        None
    }
}

/// Types of changes made to a statistic.
// LCOV_EXCL_START not interesting to track automatic derive coverage
#[derive(Debug, Clone, Eq, PartialEq, Hash)]
pub enum ChangeType {
    /// Increment by a fixed amount.
    Incr(usize),
    /// Decrement by a fixed amount.
    Decr(usize),
    /// Set to a specific value.
    SetTo(isize),
}

/// Used to represent the upper limit of a bucket.
#[derive(Debug, Clone, Copy, serde::Serialize, PartialEq, PartialOrd, Eq, Ord)]
pub enum BucketLimit {
    /// A numerical upper limit.
    Num(i64),
    /// Represents a bucket with no upper limit.
    Unbounded,
}

impl slog::Value for BucketLimit {
    fn serialize(
        &self,
        _record: &::slog::Record<'_>,
        key: ::slog::Key,
        serializer: &mut dyn (::slog::Serializer),
    ) -> ::slog::Result {
        match *self {
            BucketLimit::Num(value) => serializer.emit_i64(key, value),
            BucketLimit::Unbounded => serializer.emit_str(key, "Unbounded"),
        }
    } // LCOV_EXCL_LINE Kcov bug?
}

impl fmt::Display for BucketLimit {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            BucketLimit::Num(val) => write!(f, "{}", val),
            BucketLimit::Unbounded => write!(f, "Unbounded"),
        }
    }
}

/// A set of numerical buckets together with a method for sorting values into them.
#[derive(Debug, Clone, serde::Serialize, PartialEq)]
pub struct Buckets {
    /// The method to use to sort values into buckets.
    pub method: BucketMethod,
    /// Label name describing what the buckets measure.
    pub label_name: &'static str,
    /// The upper bounds of the buckets.
    limits: Vec<BucketLimit>,
}

impl Buckets {
    /// Create a new Buckets instance.
    pub fn new(method: BucketMethod, label_name: &'static str, limits: &[i64]) -> Buckets {
        let mut limits: Vec<BucketLimit> = limits.iter().map(|f| BucketLimit::Num(*f)).collect();
        limits.push(BucketLimit::Unbounded);
        Buckets {
            method,
            label_name,
            limits,
        }
    }

    /// return a vector containing the indices of the buckets that should be updated
    pub fn assign_buckets(&self, value: f64) -> Vec<usize> {
        match self.method {
            BucketMethod::CumulFreq => self
                .limits
                .iter()
                .enumerate()
                .filter(|(_, limit)| match limit {
                    BucketLimit::Num(b) => (value <= *b as f64),
                    BucketLimit::Unbounded => true,
                })
                .map(|(i, _)| i)
                .collect(),
            BucketMethod::Freq => {
                let mut min_limit_index = self.limits.len() - 1;
                for (i, limit) in self.limits.iter().enumerate() {
                    if let BucketLimit::Num(b) = limit {
                        if value <= *b as f64 && *limit <= self.limits[min_limit_index] {
                            min_limit_index = i
                        }
                    }
                }
                vec![min_limit_index]
            }
        }
    }
    /// The number of buckets.
    pub fn len(&self) -> usize {
        self.limits.len()
    }
    /// Whether or not the number of buckets is greater than zero.
    pub fn is_empty(&self) -> bool {
        self.limits.is_empty()
    }
    /// Get the bound of an individual bucket by index.
    pub fn get(&self, index: usize) -> Option<BucketLimit> {
        self.limits.get(index).cloned()
    }
}

/// Used to determine which buckets to update when a BucketCounter stat is updated
#[derive(Debug, Clone, Copy, serde::Serialize, PartialEq)]
pub enum BucketMethod {
    /// When a value is recorded, only update the bucket it lands in
    Freq,
    /// When a value us recorded, update its bucket and every higher bucket
    CumulFreq,
}

/// Types of statistics.  Automatically determined from the `StatDefinition`.
#[derive(Debug, Clone, Copy, serde::Serialize, PartialEq)]
pub enum StatType {
    /// A counter - a value that only increments.
    Counter,
    /// A gauge - a value that represents a current value and can go up or down.
    Gauge,
    /// A counter that is additionally grouped into numerical buckets
    BucketCounter,
}
// LCOV_EXCL_STOP

impl slog::Value for StatType {
    fn serialize(
        &self,
        _record: &::slog::Record<'_>,
        key: ::slog::Key,
        serializer: &mut dyn (::slog::Serializer),
    ) -> ::slog::Result {
        match *self {
            StatType::Counter => serializer.emit_str(key, "counter"),
            StatType::Gauge => serializer.emit_str(key, "gauge"),
            StatType::BucketCounter => serializer.emit_str(key, "bucket counter"),
        }
    } // LCOV_EXCL_LINE Kcov bug?
}

/////////////////////////////////////////////////////////////////////////////////
// The statistics tracker and related fields.
/////////////////////////////////////////////////////////////////////////////////

/// An object that tracks statistics and can be asked to log them
// LCOV_EXCL_START not interesting to track automatic derive coverage
#[derive(Debug)]
pub struct StatsTracker<T: StatisticsLogFormatter> {
    // The list of statistics, mapping from stat name to value.
    stats: HashMap<&'static str, Stat>,

    // The callback to make for logging the statistic.  This is a marker type so store it
    // as phantom.
    stat_formatter: PhantomData<T>,
}
// LCOV_EXCL_STOP

impl<T> Default for StatsTracker<T>
where
    T: StatisticsLogFormatter,
{
    fn default() -> Self {
        StatsTracker {
            stats: HashMap::new(),
            stat_formatter: PhantomData,
        }
    }
}

impl<T> StatsTracker<T>
where
    T: StatisticsLogFormatter,
{
    /// Create a new tracker with the given formatter.
    pub fn new() -> Self {
        Default::default()
    }

    /// Add a new statistic to this tracker.
    pub fn add_statistic(&mut self, defn: &'static (dyn StatDefinition + Sync + RefUnwindSafe)) {
        let stat = Stat {
            defn,
            is_grouped: !defn.group_by().is_empty(),
            group_values: RwLock::new(HashMap::new()),
            stat_type_data: StatTypeData::new(defn),
            value: StatValue::new(0, 1),
        }; // LCOV_EXCL_LINE Kcov bug?

        self.stats.insert(defn.name(), stat);
    } // LCOV_EXCL_LINE Kcov bug

    /// Update the statistics for the current log.
    ///
    /// This checks for any configured stats that are triggered by this log, and
    /// updates their value appropriately.
    fn update_stats(&self, log: &dyn StatTrigger) {
        for stat_def in log.stat_list() {
            if log.condition(stat_def) {
                let stat = &self.stats.get(stat_def.defn.name()).unwrap_or_else(|| {
                    panic!(
                        "No statistic found with name {}, did you try writing a log through a
                         logger which wasn't initialized with your stats definitions?",
                        stat_def.defn.name()
                    )
                });

                stat.update(stat_def, log)
            }
        }
    }

    /// Log all statistics.
    ///
    /// This function is usually just called on a timer by the logger directly.
    pub fn log_all(&self, logger: &StatisticsLogger<T>) {
        for stat in self.stats.values() {
            // Log all the grouped and bucketed values.
            let outputs = stat.get_tagged_vals();

            // The `outputs` is a vector of tuples containing the (tag value, bucket_index, stat value).
            for (tag_values, val) in outputs {
                // The tags require a vector of (tag name, tag value) types, so get these if present.
                let tags = stat.get_tag_pairs(&tag_values);

                T::log_stat(
                    &logger,
                    &StatLogData {
                        stype: stat.defn.stype(),
                        name: stat.defn.name(),
                        description: stat.defn.description(),
                        value: val,
                        tags,
                    },
                ); // LCOV_EXCL_LINE Kcov bug?
            }
        }
    }

    /// Retrieve the current values of all stats tracked by this logger.
    pub fn get_stats(&self) -> Vec<StatSnapshot> {
        self.stats
            .values()
            .map(|stat| stat.get_snapshot())
            .collect::<Vec<_>>()
    }
}

////////////////////////////////////
// Types to help integrate the tracker with loggers.
////////////////////////////////////

/// The default period between logging all statistics.
pub const DEFAULT_LOG_INTERVAL_SECS: u64 = 300;

/// Type alias for the return of [`define_stats`](../macro.define_stats.html).
pub type StatDefinitions = &'static [&'static (dyn StatDefinition + Sync + RefUnwindSafe)];

/// A builder to allow customization of stats config.  This gives flexibility when the other
/// methods are insufficient.
///
/// Create the builder using `new()` and chain other methods as required, ending with `fuse()` to
/// return the `StatsConfig`.
///
/// # Example
/// Creating a config with a custom stats interval and the default formatter.
///
/// ```
/// # use slog_extlog::stats::*;
/// # #[tokio::main]
/// # async fn main() {
///
/// slog_extlog::define_stats! {
///     MY_STATS = {
///         SomeStat(Counter, "A test counter", []),
///         SomeOtherStat(Counter, "Another test counter", [])
///     }
/// }
///
/// let full_stats = vec![MY_STATS];
/// let logger = slog::Logger::root(slog::Discard, slog::o!());
/// let stats = StatsLoggerBuilder::<DefaultStatisticsLogFormatter>::default()
///     .with_stats(full_stats)
///     .fuse(logger);
///
/// # }
/// ```
#[derive(Debug)]
pub struct StatsLoggerBuilder<T>
where
    T: StatisticsLogFormatter,
{
    /// The period, in seconds, to log the generated metrics into the log stream.  One log will be
    /// generated for each metric value.  A value of `None` indicates stats should never be logged.
    /// Defaults to `None` (no interval logging).
    #[cfg(feature = "interval_logging")]
    pub interval_secs: Option<u64>,
    /// The list of statistics to track.  This MUST be created using the
    /// [`define_stats`](../macro.define_stats.html) macro.
    pub stats: Vec<StatDefinitions>,
    /// An object that handles formatting the individual statistic values into a log.
    pub stat_formatter: PhantomData<T>,
}

impl<T> Default for StatsLoggerBuilder<T>
where
    T: StatisticsLogFormatter,
{
    fn default() -> Self {
        Self {
            #[cfg(feature = "interval_logging")]
            interval_secs: None,
            stats: vec![],
            stat_formatter: PhantomData,
        }
    }
}

impl<T: StatisticsLogFormatter> StatsLoggerBuilder<T> {
    /// Set the list of statistics to track.
    pub fn with_stats(mut self, stats: Vec<StatDefinitions>) -> Self {
        self.stats = stats;
        self
    }

    /// Enable emitting of logs for each registered statistic on a regular interval (in seconds).
    #[cfg(feature = "interval_logging")]
    pub fn with_log_interval(mut self, interval_secs: u64) -> Self {
        self.interval_secs = Some(interval_secs);
        self
    }

    /// Construct the StatisticsLogger - this will start the interval logging if requested.
    pub fn fuse(self, logger: slog::Logger) -> StatisticsLogger<T> {
        let mut tracker = StatsTracker::new();
        for set in self.stats {
            for s in set {
                tracker.add_statistic(*s)
            }
        }

        // Wrap the tracker in an Arc so we can pass it to the Logger and to the timer (if enabled).
        let tracker = Arc::new(tracker);

        #[cfg(feature = "interval_logging")]
        {
            // Clone the logger and tracker for using on the timer - we may not need them,
            // but the clones are cheap.
            let timer_tracker = Arc::clone(&tracker);
            let timer_logger = logger.clone();

            let timer_full_logger = StatisticsLogger {
                logger: timer_logger.clone(),
                tracker: timer_tracker,
            };

            // Kick off a timer to repeatedly log stats, if requested.
            if let Some(interval_secs) = self.interval_secs {
                tokio::spawn(async move {
                    let mut interval = tokio::time::interval(Duration::from_secs(interval_secs));

                    // The first tick completes immediately, so we skip it.
                    interval.tick().await;

                    loop {
                        interval.tick().await;
                        timer_full_logger.tracker.log_all(&timer_full_logger);
                    }
                });
            }
        }
        StatisticsLogger { logger, tracker }
    }
}

/// Data and callback type for actually generating the log.
///
/// This allows the user to decide what format to actually log the stats in.
// LCOV_EXCL_START not interesting to track automatic derive coverage
#[derive(Debug)]
pub struct StatLogData<'a> {
    /// The description, as provided on the definition.
    pub description: &'static str,
    /// The statistic type, automatically determined from the definition.
    pub stype: StatType,
    /// The statistic name, as provided on the definition.
    pub name: &'static str,
    /// The current value.
    pub value: f64,
    /// The groups and name.
    pub tags: Vec<(&'static str, &'a str)>,
}

/// Structure to use for the default implementation of `StatisticsLogFormatter`.
#[derive(Debug, Clone)]
pub struct DefaultStatisticsLogFormatter;
// LCOV_EXCL_STOP

/// The log identifier for the default formatter.
pub static DEFAULT_LOG_ID: &str = "STATS-1";

impl StatisticsLogFormatter for DefaultStatisticsLogFormatter {
    /// The formatting callback.  This default implementation just logs each field.
    fn log_stat(logger: &StatisticsLogger<Self>, stat: &StatLogData<'_>)
    where
        Self: Sized,
    {
        // A realistic implementation would use `xlog`.  However, since the derivation of
        // `ExtLoggable` depends on this crate, we can't use it here!
        //
        // So just log the value manually using the `slog` macros.
        info!(logger, "New statistic value";
               "log_id" => DEFAULT_LOG_ID,
               "name" => stat.name,
               "metric_type" => stat.stype,
               "description" => stat.description,
               "value" => stat.value,
               "tags" => stat.tags.iter().
                   map(|x| format!("{}={}", x.0, x.1)).collect::<Vec<_>>().join(","))
    }
}

/// A trait object to allow users to customise the format of stats when logged.
pub trait StatisticsLogFormatter: Sync + Send + 'static {
    /// The formatting callback.  This should take the statistic information and log it through the
    /// provided logger in the relevant format.
    ///
    /// The `DefaultStatisticsLogFormatter` provides a basic format, or users can override the
    /// format of the generated logs by providing an object that implements this trait in the
    /// `StatsConfig`.
    fn log_stat(logger: &StatisticsLogger<Self>, stat: &StatLogData<'_>)
    where
        Self: Sized;
}

/// A logger with statistics tracking.
///
/// This should only be created through the `new` method.
#[derive(Debug)]
pub struct StatisticsLogger<T: StatisticsLogFormatter> {
    /// The logger that receives the logs.
    logger: slog::Logger,
    /// The stats tracker.
    tracker: Arc<StatsTracker<T>>,
}

// Manually impl clone because the automatically derived type requires that `T:Clone`,
// which isn't needed.
//
// See https://github.com/rust-lang/rust/issues/26925 for details.
impl<T: StatisticsLogFormatter> Clone for StatisticsLogger<T> {
    fn clone(&self) -> Self {
        StatisticsLogger {
            logger: self.logger.clone(),
            tracker: self.tracker.clone(),
        } // LCOV_EXCL_LINE Kcov bug
    }
}

impl<T: StatisticsLogFormatter> Deref for StatisticsLogger<T> {
    type Target = slog::Logger;
    fn deref(&self) -> &Self::Target {
        &self.logger
    }
}

impl<T> StatisticsLogger<T>
where
    T: StatisticsLogFormatter + Send + Sync + 'static,
{
    /// Build a child logger with new parameters.
    ///
    /// This is essentially a wrapper around `slog::Logger::new()`.
    pub fn with_params<P>(&self, params: slog::OwnedKV<P>) -> Self
    where
        P: slog::SendSyncRefUnwindSafeKV + 'static,
    {
        StatisticsLogger {
            logger: self.logger.new(params),
            tracker: self.tracker.clone(),
        } // LCOV_EXCL_LINE Kcov bug
    }

    /// Update the statistics for the current log.
    pub fn update_stats(&self, log: &dyn StatTrigger) {
        self.tracker.update_stats(log)
    }

    /// Modify the logger field without changing the stats tracker
    pub fn set_slog_logger(&mut self, logger: slog::Logger) {
        self.logger = logger;
    }

    /// Retrieve the current values of all stats tracked by this logger.
    pub fn get_stats(&self) -> Vec<StatSnapshot> {
        self.tracker.get_stats()
    }
}

/// The values contained in a `StatSnapshot` for each stat type.
#[derive(Debug)]
pub enum StatSnapshotValues {
    /// `StatSnapshot` values for the Counter stat type.
    Counter(Vec<StatSnapshotValue>),
    /// `StatSnapshot` values for the Gauge stat type.
    Gauge(Vec<StatSnapshotValue>),
    /// Bucket description, and `StatSnapshot` values by bucket for the BucketCounter stat type.
    BucketCounter(Buckets, Vec<(StatSnapshotValue, BucketLimit)>),
}

impl StatSnapshotValues {
    /// Returns true if self contains no StatSnapshotValue entries.
    pub fn is_empty(&self) -> bool {
        match self {
            StatSnapshotValues::Counter(ref vals) | StatSnapshotValues::Gauge(ref vals) => {
                vals.is_empty()
            }
            StatSnapshotValues::BucketCounter(_, ref vals) => vals.is_empty(),
        }
    }
}

/// A snapshot of the current values for a particular stat.
// LCOV_EXCL_START not interesting to track automatic derive coverage
#[derive(Debug)]
pub struct StatSnapshot {
    /// A configured statistic, defined in terms of the external logs that trigger it to change.
    pub definition: &'static dyn StatDefinition,
    /// The values contained in a `StatSnapshot` for each stat type.
    pub values: StatSnapshotValues,
}
// LCOV_EXCL_STOP

impl StatSnapshot {
    /// Create a new snapshot of a stat. The StatSnapshotValues enum variant passed
    /// should match the stat type in the definition.
    pub fn new(definition: &'static dyn StatDefinition, values: StatSnapshotValues) -> Self {
        StatSnapshot { definition, values }
    }
}

/// A snapshot of a current (possibly grouped) value for a stat.
// LCOV_EXCL_START not interesting to track automatic derive coverage
#[derive(Debug)]
pub struct StatSnapshotValue {
    /// A vec of the set of tags that this value belongs to. A group can have several tags
    /// and the stat is counted separately for all distinct combinations of tag values.
    /// This may be an empty vec is the stat is not grouped.
    pub group_values: Vec<String>,
    /// The value of the stat with the above combination of groups (note that this may be bucketed).
    pub value: f64,
}
// LCOV_EXCL_STOP

impl StatSnapshotValue {
    /// Create a new snapshot value.
    pub fn new(group_values: Vec<String>, value: f64) -> Self {
        StatSnapshotValue {
            group_values,
            value,
        }
    }
}

///////////////////////////
// Private types and private methods.
///////////////////////////

/// A struct used to store any data internal to a stat that is specific to the
/// stat type.
#[derive(Debug)]
enum StatTypeData {
    Counter,
    Gauge,
    BucketCounter(BucketCounterData),
}

impl StatTypeData {
    /// Create a new `StatTypeData`
    fn new(defn: &'static dyn StatDefinition) -> Self {
        match defn.stype() {
            StatType::Counter => StatTypeData::Counter,
            StatType::Gauge => StatTypeData::Gauge,
            StatType::BucketCounter => {
                let is_grouped = !defn.group_by().is_empty();
                StatTypeData::BucketCounter(BucketCounterData::new(
                    defn.buckets().expect(
                        "Stat definition with type BucketCounter did not contain bucket info",
                    ),
                    is_grouped,
                ))
            }
        }
    }

    /// Update the stat values
    fn update(&self, defn: &StatDefinitionTagged, trigger: &dyn StatTrigger) {
        if let StatTypeData::BucketCounter(ref bucket_counter_data) = self {
            bucket_counter_data.update(defn, trigger);
        }
    }

    /// Get all the tags for this stat as a vector of `(name, value)` tuples.
    fn get_tag_pairs<'a, 'b, 'c>(
        &'a self,
        tag_values: &'b str,
        defn: &'c dyn StatDefinition,
    ) -> Option<Vec<(&'static str, &'b str)>> {
        if let StatTypeData::BucketCounter(ref bucket_counter_data) = self {
            Some(bucket_counter_data.get_tag_pairs(tag_values, defn))
        } else {
            None
        }
    }

    /// Get all the tagged value names currently tracked.
    fn get_tagged_vals(&self) -> Option<Vec<(String, f64)>> {
        if let StatTypeData::BucketCounter(ref bucket_counter_data) = self {
            Some(bucket_counter_data.get_tagged_vals())
        } else {
            None
        }
    }
}

/// Contains data that is specific to the `BucketCounter` stat type.
#[derive(Debug)]
struct BucketCounterData {
    buckets: Buckets,
    bucket_values: Vec<StatValue>,
    bucket_group_values: RwLock<HashMap<String, Vec<StatValue>>>,
    is_grouped: bool,
}

impl BucketCounterData {
    fn new(buckets: Buckets, is_grouped: bool) -> Self {
        let buckets_len = buckets.len();
        let mut bucket_values = Vec::new();
        bucket_values.reserve_exact(buckets_len);
        for _ in 0..buckets_len {
            bucket_values.push(StatValue::new(0, 1));
        }

        BucketCounterData {
            buckets,
            bucket_values,
            bucket_group_values: RwLock::new(HashMap::new()),
            is_grouped,
        }
    }

    /// Update the stat values.
    fn update(&self, defn: &StatDefinitionTagged, trigger: &dyn StatTrigger) {
        // Update the bucketed values.
        let bucket_value = trigger.bucket_value(defn).expect("Bad log definition");
        let buckets_to_update = self.buckets.assign_buckets(bucket_value);

        for index in &buckets_to_update {
            self.bucket_values
                .get(*index)
                .expect("Invalid bucket index")
                .update(&trigger.change(defn).expect("Bad log definition"));
        }

        if self.is_grouped {
            // Update the grouped and bucketed values.
            self.update_grouped(defn, trigger, &buckets_to_update);
        }
    }

    /// Update the grouped stat values.
    fn update_grouped(
        &self,
        defn: &StatDefinitionTagged,
        trigger: &dyn StatTrigger,
        buckets_to_update: &[usize],
    ) {
        let change = trigger.change(defn).expect("Bad log definition");
        let tag_values = defn
            .defn
            .group_by()
            .iter()
            .map(|n| trigger.tag_value(defn, n))
            .collect::<Vec<String>>()
            .join(","); // LCOV_EXCL_LINE Kcov bug?

        let found_values = {
            let inner_vals = self.bucket_group_values.read().expect("Poisoned lock");
            if let Some(tagged_bucket_vals) = inner_vals.get(&tag_values) {
                update_bucket_values(tagged_bucket_vals, buckets_to_update, &change);
                true
            } else {
                false
            }
        };

        if !found_values {
            // we didn't find bucketed values for this tag combination. Create them now.
            let mut new_bucket_vals = Vec::new();
            let bucket_len = self.buckets.len();
            new_bucket_vals.reserve_exact(bucket_len);
            for _ in 0..bucket_len {
                new_bucket_vals.push(StatValue::new(0, 1));
            }

            let mut inner_vals = self.bucket_group_values.write().expect("Poisoned lock");
            // It's possible that while we were waiting for the write lock another thread got
            // in and created the bucketed entries, so check again.
            let vals = inner_vals
                .entry(tag_values)
                .or_insert_with(|| new_bucket_vals);

            update_bucket_values(vals, buckets_to_update, &change);
        }
    }

    /// Get all the tags for this stat as a vector of `(name, value)` tuples.
    fn get_tag_pairs<'a, 'b, 'c>(
        &'a self,
        tag_values: &'b str,
        defn: &'c dyn StatDefinition,
    ) -> Vec<(&'static str, &'b str)> {
        let mut tag_names = defn.group_by();
        // Add the bucket label name as an additional tag name.
        tag_names.push(self.buckets.label_name);
        tag_names
            .iter()
            .cloned()
            .zip(tag_values.split(','))
            .collect::<Vec<_>>()
    }

    /// Get all the tagged values currently tracked.
    fn get_tagged_vals(&self) -> Vec<(String, f64)> {
        if self.is_grouped {
            let mut tag_bucket_vals = Vec::new();

            {
                // Only hold the read lock long enough to get the keys, bucket indices, and values.
                let inner_vals = self.bucket_group_values.read().expect("Poisoned lock");

                for (group_values_str, bucket_values) in inner_vals.iter() {
                    for (index, val) in bucket_values.iter().enumerate() {
                        tag_bucket_vals.push((group_values_str.to_string(), index, val.as_float()));
                    }
                }
            }

            tag_bucket_vals
                .into_iter()
                .map(|(mut tag_values, index, val)| {
                    let bucket = self.buckets.get(index).expect("Invalid bucket index");
                    // Add the bucket label value as an additional tag value.
                    tag_values.push_str(&format!(",{}", bucket.to_string()));
                    (tag_values, val)
                })
                .collect()
        } else {
            self.bucket_values
                .iter()
                .enumerate()
                .map(|(index, val)| {
                    let bucket = self.buckets.get(index).expect("Invalid bucket index");
                    (bucket.to_string(), val.as_float())
                })
                .collect()
        }
    }

    /// Get a snapshot of the current stat values.
    fn get_snapshot_values(&self) -> Vec<(StatSnapshotValue, BucketLimit)> {
        if self.is_grouped {
            let mut tag_bucket_vals = Vec::new();

            {
                // Only hold the read lock long enough to get the keys, bucket indices, and values.
                let inner_vals = self.bucket_group_values.read().expect("Poisoned lock");

                for (group_values_str, bucket_values) in inner_vals.iter() {
                    for (index, val) in bucket_values.iter().enumerate() {
                        tag_bucket_vals.push((group_values_str.to_string(), index, val.as_float()));
                    }
                }
            }

            tag_bucket_vals
                .into_iter()
                .map(|(tag_values, index, val)| {
                    let bucket = self.buckets.get(index).expect("Invalid bucket index");
                    let group_values = tag_values
                        .split(',')
                        .map(|group| group.to_string())
                        .collect::<Vec<_>>();

                    (StatSnapshotValue::new(group_values, val), bucket)
                })
                .collect()
        } else {
            self.bucket_values
                .iter()
                .enumerate()
                .map(|(index, val)| {
                    let bucket = self.buckets.get(index).expect("Invalid bucket index");
                    (StatSnapshotValue::new(vec![], val.as_float()), bucket)
                })
                .collect()
        }
    }
}

// LCOV_EXCL_START not interesting to track automatic derive coverage
/// The internal representation of a tracked statistic.
#[derive(Debug)]
struct Stat {
    // The definition fields, as a trait object.
    defn: &'static (dyn StatDefinition + Sync + RefUnwindSafe),
    // The value - if grouped, this is the total value across all statistics.
    value: StatValue,
    // Does this stat use groups.  Cached here for efficiency.
    is_grouped: bool,
    // The fields the stat is grouped by.  If empty, then there is no grouping.
    group_values: RwLock<HashMap<String, StatValue>>,
    // Data specific to the stat type.
    stat_type_data: StatTypeData,
}
// LCOV_EXCL_STOP

impl Stat {
    // Get all the tags for this stat as a vector of `(name, value)` tuples.
    fn get_tag_pairs<'a, 'b>(&'a self, tag_values: &'b str) -> Vec<(&'static str, &'b str)> {
        // if the stat type has its own `get_tag_pairs` method use that, otherwise
        // use the default.
        self.stat_type_data
            .get_tag_pairs(tag_values, self.defn)
            .unwrap_or_else(|| {
                self.defn
                    .group_by()
                    .iter()
                    .cloned()
                    .zip(tag_values.split(','))
                    .collect::<Vec<_>>()
            })
    }

    /// Get all the tagged value names currently tracked.
    fn get_tagged_vals(&self) -> Vec<(String, f64)> {
        // if the stat type has its own `get_tagged_vals` method use that, otherwise
        // use the default.
        self.stat_type_data.get_tagged_vals().unwrap_or_else(|| {
            if self.is_grouped {
                // Only hold the read lock long enough to get the keys and values.
                let inner_vals = self.group_values.read().expect("Poisoned lock");
                inner_vals
                    .iter()
                    .map(|(group_values_str, value)| {
                        (group_values_str.to_string(), value.as_float())
                    })
                    .collect()
            } else {
                vec![("".to_string(), self.value.as_float())]
            }
        })
    }

    /// Update the stat's value(s) according to the given `StatTrigger` and `StatDefinition`.
    fn update(&self, defn: &StatDefinitionTagged, trigger: &dyn StatTrigger) {
        // update the stat value
        self.value
            .update(&trigger.change(defn).expect("Bad log definition"));

        // If the stat is grouped, update the grouped values.
        if self.is_grouped {
            self.update_grouped(defn, trigger)
        }

        // Update type-specific stat data.
        self.stat_type_data.update(defn, trigger);
    }

    fn update_grouped(&self, defn: &StatDefinitionTagged, trigger: &dyn StatTrigger) {
        let change = trigger.change(defn).expect("Bad log definition");

        let tag_values = self
            .defn
            .group_by()
            .iter()
            .map(|n| trigger.tag_value(defn, n))
            .collect::<Vec<String>>()
            .join(","); // LCOV_EXCL_LINE Kcov bug?

        let found_values = {
            let inner_vals = self.group_values.read().expect("Poisoned lock");
            if let Some(val) = inner_vals.get(&tag_values) {
                val.update(&change);
                true
            } else {
                false
            }
        };

        if !found_values {
            // We didn't find a grouped value.  Get the write lock on the map so we can add it.
            let mut inner_vals = self.group_values.write().expect("Poisoned lock");
            // It's possible that while we were waiting for the write lock another thread got
            // in and created the stat entry, so check again.
            let val = inner_vals
                .entry(tag_values)
                .or_insert_with(|| StatValue::new(0, 1));

            val.update(&change);
        }
    }

    /// Get the current values for this stat as a StatSnapshot.
    fn get_snapshot(&self) -> StatSnapshot {
        let stat_snapshot_values = match self.stat_type_data {
            StatTypeData::BucketCounter(ref bucket_counter_data) => {
                StatSnapshotValues::BucketCounter(
                    bucket_counter_data.buckets.clone(),
                    bucket_counter_data.get_snapshot_values(),
                )
            }
            StatTypeData::Counter => StatSnapshotValues::Counter(self.get_snapshot_values()),
            StatTypeData::Gauge => StatSnapshotValues::Gauge(self.get_snapshot_values()),
        };

        StatSnapshot::new(self.defn, stat_snapshot_values)
    }

    /// Get a snapshot of the current stat values.
    fn get_snapshot_values(&self) -> Vec<StatSnapshotValue> {
        self.get_tagged_vals()
            .iter()
            .map(|(group_values_str, value)| {
                let group_values = if !group_values_str.is_empty() {
                    group_values_str
                        .split(',')
                        .map(|group| group.to_string())
                        .collect::<Vec<_>>()
                } else {
                    vec![]
                };
                StatSnapshotValue::new(group_values, *value)
            })
            .collect()
    }
}

// LCOV_EXCL_START not interesting to track automatic derive coverage
/// A single statistic value.
#[derive(Debug)]
struct StatValue {
    // The tracked integer value.
    num: AtomicIsize,
    // A divisor for printing the stat value only - currently this is always 1 but is here
    // to allow in future for, say, percentages.
    divisor: u64,
}
// LCOV_EXCL_STOP

impl StatValue {
    /// Create a new value.
    fn new(num: isize, divisor: u64) -> Self {
        StatValue {
            num: AtomicIsize::new(num),
            divisor,
        }
    }

    /// Update the stat and return whether it has changed.
    fn update(&self, change: &ChangeType) -> bool {
        match *change {
            ChangeType::Incr(i) => {
                self.num.fetch_add(i as isize, Ordering::Relaxed);
                true
            }
            ChangeType::Decr(d) => {
                self.num.fetch_sub(d as isize, Ordering::Relaxed);
                true
            }

            ChangeType::SetTo(v) => self.num.swap(v, Ordering::Relaxed) != v,
        }
    }

    /// Return the statistic value as a float, for use in display.
    fn as_float(&self) -> f64 {
        (self.num.load(Ordering::Relaxed) as f64) / (self.divisor as isize as f64)
    }
}

fn update_bucket_values(
    bucket_values: &[StatValue],
    buckets_to_update: &[usize],
    change: &ChangeType,
) {
    for index in buckets_to_update.iter() {
        bucket_values
            .get(*index)
            .expect("Invalid bucket index")
            .update(change);
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[allow(dead_code)]
    struct DummyNonCloneFormatter;
    impl StatisticsLogFormatter for DummyNonCloneFormatter {
        fn log_stat(_logger: &StatisticsLogger<Self>, _stat: &StatLogData<'_>)
        where
            Self: Sized,
        {
        }
    }

    #[test]
    // Check that loggers can be cloned even if the formatter can't.
    fn check_clone() {
        let builder = StatsLoggerBuilder::<DummyNonCloneFormatter>::default();
        let logger = builder.fuse(slog::Logger::root(slog::Discard, slog::o!()));
        fn is_clone<T: Clone>(_: &T) {}
        is_clone(&logger);
    }
}