feat(measurement): implement ICMP measurement engine

src/measurement/model.rs

@@ -28,6 +28,41 @@ pub enum OneWayClock {
     Calibrated,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct ProbeSchedule {
+    pub period: Duration,
+    pub offset: Duration,
+}
+
+impl ProbeSchedule {
+    pub(crate) fn next_probe_at(&self, origin: Instant, now: Instant) -> Instant {
+        const NANOS_PER_SEC: u128 = 1_000_000_000;
+
+        let first = origin + self.offset;
+
+        if now < first {
+            return first;
+        }
+
+        let elapsed_ns = now.duration_since(first).as_nanos();
+        let period_ns = self.period.as_nanos();
+        let remainder = elapsed_ns % period_ns;
+
+        let until_next = if remainder == 0 {
+            period_ns
+        } else {
+            period_ns - remainder
+        };
+
+        let until_next = Duration::new(
+            (until_next / NANOS_PER_SEC) as u64,
+            (until_next % NANOS_PER_SEC) as u32,
+        );
+
+        now + until_next
+    }
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub struct SignedDuration {
     nanos: i64,