add IntervalData struct

CHANGES_UNRELEASED.md

@@ -30,3 +30,4 @@ Use the template below to make assigning a version number during the release cut
 
 ### What's Changed
   - Disabled Glean events recorded when the SDK is not ready for a feature ([#5185](https://github.com/mozilla/application-services/pull/5185))
+  - Add struct for IntervalData (behavioral targeting) ([#5205](https://github.com/mozilla/application-services/pull/5205))

components/nimbus/src/lib.rs

@@ -44,9 +44,10 @@ use once_cell::sync::OnceCell;
 use persistence::{Database, StoreId, Writer};
 use serde_derive::*;
 use serde_json::{Map, Value};
-use std::collections::HashSet;
+use std::collections::{HashSet, VecDeque};
 use std::path::{Path, PathBuf};
 use std::sync::{Arc, Mutex};
+use std::time::Instant;
 use updating::{read_and_remove_pending_experiments, write_pending_experiments};
 use uuid::Uuid;
 
@@ -796,6 +797,72 @@ impl NimbusTargetingHelper {
 
 type JsonObject = Map<String, Value>;
 
+pub struct IntervalData {
+    buckets: VecDeque<i32>,
+    starting_instant: Instant,
+}
+
+impl IntervalData {
+    pub fn new() -> IntervalData {
+        let mut data = IntervalData {
+            buckets: VecDeque::new(),
+            starting_instant: Instant::now(),
+        };
+        data.buckets.push_front(0);
+        return data;
+    }
+
+    fn increment(&mut self) {
+        match self.buckets.front_mut() {
+            Some(x) => *x += 1,
+            None => {
+                self.buckets.push_front(1);
+            }
+        };
+    }
+
+    fn rotate(&mut self, num_rotations: i32) {
+        for _ in 1..=num_rotations {
+            self.buckets.push_front(0)
+        }
+    }
+}
+
+#[cfg(test)]
+mod interval_data_unit_tests {
+    use super::*;
+
+    #[test]
+    fn increment_works_if_no_buckets_present() -> Result<()> {
+        let mut interval = IntervalData {
+            buckets: VecDeque::new(),
+            starting_instant: Instant::now(),
+        };
+        interval.increment();
+
+        assert!(matches!(interval.buckets[0], 1));
+        Ok(())
+    }
+
+    #[test]
+    fn increment_increments_front_bucket_if_it_exists() -> Result<()> {
+        let mut interval = IntervalData::new();
+        interval.increment();
+
+        assert!(matches!(interval.buckets[0], 1));
+        Ok(())
+    }
+
+    #[test]
+    fn rotate_adds_buckets_for_each_rotation() -> Result<()> {
+        let mut interval = IntervalData::new();
+        interval.rotate(3);
+
+        assert!(matches!(interval.buckets.len(), 4));
+        Ok(())
+    }
+}
+
 #[cfg(feature = "uniffi-bindings")]
 impl UniffiCustomTypeConverter for JsonObject {
     type Builtin = String;