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    //! Benchmark metrics.
    use std::collections::BTreeMap;

    #[derive(Clone, PartialEq, Debug, Copy)]
    pub struct Metric {
        value: f64,
        noise: f64,
    }

    impl Metric {
        pub fn new(value: f64, noise: f64) -> Metric {
            Metric { value, noise }
        }
    }

    #[derive(Clone, PartialEq)]
    pub struct MetricMap(BTreeMap<String, Metric>);

    impl MetricMap {
        pub fn new() -> MetricMap {
            MetricMap(BTreeMap::new())
        }

        /// Insert a named `value` (+/- `noise`) metric into the map. The value
        /// must be non-negative. The `noise` indicates the uncertainty of the
        /// metric, which doubles as the "noise range" of acceptable
        /// pairwise-regressions on this named value, when comparing from one
        /// metric to the next using `compare_to_old`.
        ///
        /// If `noise` is positive, then it means this metric is of a value
        /// you want to see grow smaller, so a change larger than `noise` in the
        /// positive direction represents a regression.
        ///
        /// If `noise` is negative, then it means this metric is of a value
        /// you want to see grow larger, so a change larger than `noise` in the
        /// negative direction represents a regression.
        pub fn insert_metric(&mut self, name: &str, value: f64, noise: f64) {
            let m = Metric { value, noise };
            self.0.insert(name.to_owned(), m);
        }

        pub fn fmt_metrics(&self) -> String {
            let v = self
                .0
                .iter()
                .map(|(k, v)| format!("{}: {} (+/- {})", *k, v.value, v.noise))
                .collect::<Vec<_>>();
            v.join(", ")
        }
    }