feat: add range based exponential buckets in histogram (#233)

Signed-off-by: Sidhant Kohli <[email protected]>

Author: kohlisid

Diff for: CHANGELOG.md

@@ -34,12 +34,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Supoort `Arc<String>` for `EncodeLabelValue`.
   See [PR 217].
 
+- Add `histogram::exponential_buckets_range`.
+  See [PR 233].
+
 - Added `get` method to `Family`.
   See [PR 234].
 
 [PR 173]: https://github.com/prometheus/client_rust/pull/173
 [PR 216]: https://github.com/prometheus/client_rust/pull/216
 [PR 217]: https://github.com/prometheus/client_rust/pull/217
+[PR 233]: https://github.com/prometheus/client_rust/pull/233
 [PR 234]: https://github.com/prometheus/client_rust/pull/234
 
 ### Fixed

Diff for: src/metrics/histogram.rs

@@ -122,6 +122,30 @@ pub fn exponential_buckets(start: f64, factor: f64, length: u16) -> impl Iterato
         .take(length.into())
 }
 
+/// Exponential bucket distribution within a range
+///
+/// Creates `length` buckets, where the lowest bucket is `min` and the highest bucket is `max`.
+///
+/// If `length` is less than 1, or `min` is less than or equal to 0, an empty iterator is returned.
+pub fn exponential_buckets_range(min: f64, max: f64, length: u16) -> impl Iterator<Item = f64> {
+    let mut len_observed = length;
+    let mut min_bucket = min;
+    // length needs a positive length and min needs to be greater than 0
+    // set len_observed to 0 and min_bucket to 1.0
+    // this will return an empty iterator in the result
+    if length < 1 || min <= 0.0 {
+        len_observed = 0;
+        min_bucket = 1.0;
+    }
+    // We know max/min and highest bucket. Solve for growth_factor.
+    let growth_factor = (max / min_bucket).powf(1.0 / (len_observed as f64 - 1.0));
+
+    iter::repeat(())
+        .enumerate()
+        .map(move |(i, _)| min_bucket * growth_factor.powf(i as f64))
+        .take(len_observed.into())
+}
+
 /// Linear bucket distribution.
 pub fn linear_buckets(start: f64, width: f64, length: u16) -> impl Iterator<Item = f64> {
     iter::repeat(())
@@ -166,4 +190,21 @@ mod tests {
             linear_buckets(0.0, 1.0, 10).collect::<Vec<_>>()
         );
     }
+
+    #[test]
+    fn exponential_range() {
+        assert_eq!(
+            vec![1.0, 2.0, 4.0, 8.0, 16.0, 32.0],
+            exponential_buckets_range(1.0, 32.0, 6).collect::<Vec<_>>()
+        );
+    }
+
+    #[test]
+    fn exponential_range_incorrect() {
+        let res = exponential_buckets_range(1.0, 32.0, 0).collect::<Vec<_>>();
+        assert!(res.is_empty());
+
+        let res = exponential_buckets_range(0.0, 32.0, 6).collect::<Vec<_>>();
+        assert!(res.is_empty());
+    }
 }