scepter 0.1.5

Composable primitives for planet-scale time-series routing, indexing, and aggregation.
Documentation 
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use std::collections::HashMap;
use std::hash::Hash;

use std::convert::Infallible;

use crate::distribution::{Distribution, DistributionError};

/// A type that can absorb inputs and produce a partial result for distributed
/// aggregation.
pub trait PartialAggregate<Input> {
    /// Final aggregate output type.
    type Output;

    /// Adds one input value to the aggregate.
    fn add(&mut self, input: Input);
    /// Finishes aggregation and returns the output value.
    fn finish(self) -> Self::Output;
}

/// A type that can be combined with another value of the same type.
pub trait Mergeable {
    /// Error returned when values cannot be merged.
    type Error;

    /// Merges `other` into `self`.
    fn merge_from(&mut self, other: Self) -> Result<(), Self::Error>;
}

impl<T: Clone> Mergeable for Distribution<T> {
    type Error = DistributionError;

    fn merge_from(&mut self, other: Self) -> Result<(), Self::Error> {
        self.merge(&other)
    }
}

/// Summing partial aggregate.
#[derive(Debug, Clone, Copy, Default, PartialEq)]
pub struct Sum<T> {
    value: T,
}

impl PartialAggregate<u64> for Sum<u64> {
    type Output = u64;

    fn add(&mut self, input: u64) {
        self.value += input;
    }

    fn finish(self) -> Self::Output {
        self.value
    }
}

/// Aggregates deltas keyed by a bucket such as `(target-prefix, metric-name)`.
#[derive(Debug, Clone, Default)]
pub struct CollectionAggregator<K, V> {
    values: HashMap<K, V>,
}

impl<K, V> CollectionAggregator<K, V>
where
    K: Eq + Hash,
    V: Mergeable,
{
    /// Creates an empty collection aggregator.
    pub fn new() -> Self {
        Self {
            values: HashMap::new(),
        }
    }

    /// Adds a value to the aggregate bucket identified by `key`.
    pub fn add(&mut self, key: K, value: V) -> Result<(), V::Error> {
        match self.values.get_mut(&key) {
            Some(existing) => existing.merge_from(value)?,
            None => {
                self.values.insert(key, value);
            }
        }
        Ok(())
    }

    /// Returns the aggregated values by key.
    pub fn into_inner(self) -> HashMap<K, V> {
        self.values
    }
}

impl Mergeable for u64 {
    type Error = Infallible;

    fn merge_from(&mut self, other: Self) -> Result<(), Self::Error> {
        *self += other;
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::distribution::{Bucket, Distribution};

    #[test]
    fn collection_aggregator_merges_values_by_key() {
        let left = Distribution::<()>::new(vec![Bucket {
            range: 0.0..10.0,
            count: 1,
        }]);
        let right = Distribution::<()>::new(vec![Bucket {
            range: 0.0..10.0,
            count: 2,
        }]);

        let mut aggregator = CollectionAggregator::new();
        aggregator.add("rpc", left).unwrap();
        aggregator.add("rpc", right).unwrap();

        assert_eq!(aggregator.into_inner()["rpc"].total_count(), 3);
    }
}