1 files changed, 96 insertions, 0 deletions
diff --git a/getrandom/src/util.rs b/getrandom/src/util.rs
new file mode 100644
index 0000000..e0e9307
--- /dev/null
+++ b/getrandom/src/util.rs
@@ -0,0 +1,96 @@
+// Copyright 2019 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
+
+// This structure represents a lazily initialized static usize value. Useful
+// when it is preferable to just rerun initialization instead of locking.
+// Both unsync_init and sync_init will invoke an init() function until it
+// succeeds, then return the cached value for future calls.
+//
+// Both methods support init() "failing". If the init() method returns UNINIT,
+// that value will be returned as normal, but will not be cached.
+//
+// Users should only depend on the _value_ returned by init() functions.
+// Specifically, for the following init() function:
+//      fn init() -> usize {
+//          a();
+//          let v = b();
+//          c();
+//          v
+//      }
+// the effects of c() or writes to shared memory will not necessarily be
+// observed and additional synchronization methods with be needed.
+pub struct LazyUsize(AtomicUsize);
+
+impl LazyUsize {
+    pub const fn new() -> Self {
+        Self(AtomicUsize::new(Self::UNINIT))
+    }
+
+    // The initialization is not completed.
+    pub const UNINIT: usize = usize::max_value();
+    // The initialization is currently running.
+    pub const ACTIVE: usize = usize::max_value() - 1;
+
+    // Runs the init() function at least once, returning the value of some run
+    // of init(). Multiple callers can run their init() functions in parallel.
+    // init() should always return the same value, if it succeeds.
+    pub fn unsync_init(&self, init: impl FnOnce() -> usize) -> usize {
+        // Relaxed ordering is fine, as we only have a single atomic variable.
+        let mut val = self.0.load(Relaxed);
+        if val == Self::UNINIT {
+            val = init();
+            self.0.store(val, Relaxed);
+        }
+        val
+    }
+
+    // Synchronously runs the init() function. Only one caller will have their
+    // init() function running at a time, and exactly one successful call will
+    // be run. init() returning UNINIT or ACTIVE will be considered a failure,
+    // and future calls to sync_init will rerun their init() function.
+    pub fn sync_init(&self, init: impl FnOnce() -> usize, mut wait: impl FnMut()) -> usize {
+        // Common and fast path with no contention. Don't wast time on CAS.
+        match self.0.load(Relaxed) {
+            Self::UNINIT | Self::ACTIVE => {}
+            val => return val,
+        }
+        // Relaxed ordering is fine, as we only have a single atomic variable.
+        loop {
+            match self.0.compare_and_swap(Self::UNINIT, Self::ACTIVE, Relaxed) {
+                Self::UNINIT => {
+                    let val = init();
+                    self.0.store(
+                        match val {
+                            Self::UNINIT | Self::ACTIVE => Self::UNINIT,
+                            val => val,
+                        },
+                        Relaxed,
+                    );
+                    return val;
+                }
+                Self::ACTIVE => wait(),
+                val => return val,
+            }
+        }
+    }
+}
+
+// Identical to LazyUsize except with bool instead of usize.
+pub struct LazyBool(LazyUsize);
+
+impl LazyBool {
+    pub const fn new() -> Self {
+        Self(LazyUsize::new())
+    }
+
+    pub fn unsync_init(&self, init: impl FnOnce() -> bool) -> bool {
+        self.0.unsync_init(|| init() as usize) != 0
+    }
+}