Don't use an offset in UniformDuration

src/distributions/uniform.rs

@@ -693,18 +693,24 @@ uniform_float_impl! { f64x8, u64x8, f64, u64, 64 - 52 }
 #[cfg(feature = "std")]
 #[derive(Clone, Copy, Debug)]
 pub struct UniformDuration {
-    offset: Duration,
     mode: UniformDurationMode,
 }
 
 #[cfg(feature = "std")]
 #[derive(Debug, Copy, Clone)]
 enum UniformDurationMode {
     Small {
+        secs: u64,
+        nanos: Uniform<u32>,
+    },
+    Medium {
         nanos: Uniform<u64>,
     },
     Large {
-        size: Duration,
+        min_secs: u64,
+        min_nanos: u32,
+        max_secs: u64,
+        max_nanos: u32,
         secs: Uniform<u64>,
     }
 }
@@ -737,52 +743,65 @@ impl UniformSampler for UniformDuration {
         let low = *low_b.borrow();
         let high = *high_b.borrow();
         assert!(low <= high, "Uniform::new_inclusive called with `low > high`");
-        let size = high - low;
-        let nanos = size
-            .as_secs()
-            .checked_mul(1_000_000_000)
-            .and_then(|n| n.checked_add(size.subsec_nanos() as u64));
-
-        let mode = match nanos {
-            Some(nanos) => {
-                UniformDurationMode::Small {
-                    nanos: Uniform::new_inclusive(0, nanos),
-                }
+
+        let low_s = low.as_secs();
+        let low_n = low.subsec_nanos();
+        let high_s = high.as_secs();
+        let high_n = high.subsec_nanos();
+
+        let mode = if low_s == high_s {
+            UniformDurationMode::Small {
+                secs: low_s,
+                nanos: Uniform::new_inclusive(low_n, high_n),
             }
-            None => {
+        } else {
+            let max = high_s
+                .checked_mul(1_000_000_000)
+                .and_then(|n| n.checked_add(high_n as u64));
+
+            if let Some(higher_bound) = max {
+                let lower_bound = low_s * 1_000_000_000 + low_n as u64;
+                UniformDurationMode::Medium {
+                    nanos: Uniform::new_inclusive(lower_bound, higher_bound),
+                }
+            } else {
                 UniformDurationMode::Large {
-                    size: size,
-                    secs: Uniform::new_inclusive(0, size.as_secs()),
+                    min_secs: low_s,
+                    min_nanos: low_n,
+                    max_secs: high_s,
+                    max_nanos: high_n,
+                    secs: Uniform::new_inclusive(low_s, high_s),
                 }
             }
         };
-
         UniformDuration {
-            mode,
-            offset: low,
+            mode
         }
     }
 
     #[inline]
     fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> Duration {
-        let d = match self.mode {
-            UniformDurationMode::Small { nanos } => {
+        match self.mode {
+            UniformDurationMode::Small { secs, nanos } => {
+                let n = nanos.sample(rng);
+                Duration::new(secs, n)
+            }
+            UniformDurationMode::Medium { nanos } => {
                 let nanos = nanos.sample(rng);
                 Duration::new(nanos / 1_000_000_000, (nanos % 1_000_000_000) as u32)
             }
-            UniformDurationMode::Large { size, secs } => {
+            UniformDurationMode::Large { min_secs, min_nanos, max_secs, max_nanos, secs } => {
                 // constant folding means this is at least as fast as `gen_range`
                 let nano_range = Uniform::new(0, 1_000_000_000);
                 loop {
-                    let d = Duration::new(secs.sample(rng), nano_range.sample(rng));
-                    if d <= size {
-                        break d;
+                    let s = secs.sample(rng);
+                    let n = nano_range.sample(rng);
+                    if !((s == max_secs && n > max_nanos) || (s == min_secs && n < min_nanos))  {
+                        break Duration::new(s, n);
                     }
                 }
             }
-        };
-
-        self.offset + d
+        }
     }
 }