1 files changed, 218 insertions, 0 deletions
diff --git a/src/global_rng.rs b/src/global_rng.rs
new file mode 100644
index 0000000..f994902
--- /dev/null
+++ b/src/global_rng.rs
@@ -0,0 +1,218 @@
+//! A global, thread-local random number generator.
+
+use crate::Rng;
+
+use std::cell::Cell;
+use std::ops::RangeBounds;
+
+// Chosen by fair roll of the dice.
+const DEFAULT_RNG_SEED: u64 = 0xef6f79ed30ba75a;
+
+impl Default for Rng {
+    /// Initialize the `Rng` from the system's random number generator.
+    ///
+    /// This is equivalent to [`Rng::new()`].
+    #[inline]
+    fn default() -> Rng {
+        Rng::new()
+    }
+}
+
+impl Rng {
+    /// Creates a new random number generator.
+    #[inline]
+    pub fn new() -> Rng {
+        try_with_rng(Rng::fork).unwrap_or_else(|_| Rng::with_seed(0x4d595df4d0f33173))
+    }
+}
+
+thread_local! {
+    static RNG: Cell<Rng> = Cell::new(Rng(random_seed().unwrap_or(DEFAULT_RNG_SEED)));
+}
+
+/// Run an operation with the current thread-local generator.
+#[inline]
+fn with_rng<R>(f: impl FnOnce(&mut Rng) -> R) -> R {
+    RNG.with(|rng| {
+        let current = rng.replace(Rng(0));
+
+        let mut restore = RestoreOnDrop { rng, current };
+
+        f(&mut restore.current)
+    })
+}
+
+/// Try to run an operation with the current thread-local generator.
+#[inline]
+fn try_with_rng<R>(f: impl FnOnce(&mut Rng) -> R) -> Result<R, std::thread::AccessError> {
+    RNG.try_with(|rng| {
+        let current = rng.replace(Rng(0));
+
+        let mut restore = RestoreOnDrop { rng, current };
+
+        f(&mut restore.current)
+    })
+}
+
+/// Make sure the original RNG is restored even on panic.
+struct RestoreOnDrop<'a> {
+    rng: &'a Cell<Rng>,
+    current: Rng,
+}
+
+impl Drop for RestoreOnDrop<'_> {
+    fn drop(&mut self) {
+        self.rng.set(Rng(self.current.0));
+    }
+}
+
+/// Initializes the thread-local generator with the given seed.
+#[inline]
+pub fn seed(seed: u64) {
+    with_rng(|r| r.seed(seed));
+}
+
+/// Gives back **current** seed that is being held by the thread-local generator.
+#[inline]
+pub fn get_seed() -> u64 {
+    with_rng(|r| r.get_seed())
+}
+
+/// Generates a random `bool`.
+#[inline]
+pub fn bool() -> bool {
+    with_rng(|r| r.bool())
+}
+
+/// Generates a random `char` in ranges a-z and A-Z.
+#[inline]
+pub fn alphabetic() -> char {
+    with_rng(|r| r.alphabetic())
+}
+
+/// Generates a random `char` in ranges a-z, A-Z and 0-9.
+#[inline]
+pub fn alphanumeric() -> char {
+    with_rng(|r| r.alphanumeric())
+}
+
+/// Generates a random `char` in range a-z.
+#[inline]
+pub fn lowercase() -> char {
+    with_rng(|r| r.lowercase())
+}
+
+/// Generates a random `char` in range A-Z.
+#[inline]
+pub fn uppercase() -> char {
+    with_rng(|r| r.uppercase())
+}
+
+/// Choose an item from an iterator at random.
+///
+/// This function may have an unexpected result if the `len()` property of the
+/// iterator does not match the actual number of items in the iterator. If
+/// the iterator is empty, this returns `None`.
+#[inline]
+pub fn choice<I>(iter: I) -> Option<I::Item>
+where
+    I: IntoIterator,
+    I::IntoIter: ExactSizeIterator,
+{
+    with_rng(|r| r.choice(iter))
+}
+
+/// Generates a random digit in the given `base`.
+///
+/// Digits are represented by `char`s in ranges 0-9 and a-z.
+///
+/// Panics if the base is zero or greater than 36.
+#[inline]
+pub fn digit(base: u32) -> char {
+    with_rng(|r| r.digit(base))
+}
+
+/// Shuffles a slice randomly.
+#[inline]
+pub fn shuffle<T>(slice: &mut [T]) {
+    with_rng(|r| r.shuffle(slice))
+}
+
+macro_rules! integer {
+    ($t:tt, $doc:tt) => {
+        #[doc = $doc]
+        ///
+        /// Panics if the range is empty.
+        #[inline]
+        pub fn $t(range: impl RangeBounds<$t>) -> $t {
+            with_rng(|r| r.$t(range))
+        }
+    };
+}
+
+integer!(u8, "Generates a random `u8` in the given range.");
+integer!(i8, "Generates a random `i8` in the given range.");
+integer!(u16, "Generates a random `u16` in the given range.");
+integer!(i16, "Generates a random `i16` in the given range.");
+integer!(u32, "Generates a random `u32` in the given range.");
+integer!(i32, "Generates a random `i32` in the given range.");
+integer!(u64, "Generates a random `u64` in the given range.");
+integer!(i64, "Generates a random `i64` in the given range.");
+integer!(u128, "Generates a random `u128` in the given range.");
+integer!(i128, "Generates a random `i128` in the given range.");
+integer!(usize, "Generates a random `usize` in the given range.");
+integer!(isize, "Generates a random `isize` in the given range.");
+integer!(char, "Generates a random `char` in the given range.");
+
+/// Generates a random `f32` in range `0..1`.
+pub fn f32() -> f32 {
+    with_rng(|r| r.f32())
+}
+
+/// Generates a random `f64` in range `0..1`.
+pub fn f64() -> f64 {
+    with_rng(|r| r.f64())
+}
+
+/// Collects `amount` values at random from the iterator into a vector.
+pub fn choose_multiple<T: Iterator>(source: T, amount: usize) -> Vec<T::Item> {
+    with_rng(|rng| rng.choose_multiple(source, amount))
+}
+
+#[cfg(not(all(
+    any(target_arch = "wasm32", target_arch = "wasm64"),
+    target_os = "unknown"
+)))]
+fn random_seed() -> Option<u64> {
+    use std::collections::hash_map::DefaultHasher;
+    use std::hash::{Hash, Hasher};
+    use std::thread;
+    use std::time::Instant;
+
+    let mut hasher = DefaultHasher::new();
+    Instant::now().hash(&mut hasher);
+    thread::current().id().hash(&mut hasher);
+    let hash = hasher.finish();
+    Some((hash << 1) | 1)
+}
+
+#[cfg(all(
+    any(target_arch = "wasm32", target_arch = "wasm64"),
+    target_os = "unknown",
+    feature = "js"
+))]
+fn random_seed() -> Option<u64> {
+    // TODO(notgull): Failures should be logged somewhere.
+    let mut seed = [0u8; 8];
+    getrandom::getrandom(&mut seed).ok()?;
+    Some(u64::from_ne_bytes(seed))
+}
+
+#[cfg(all(
+    any(target_arch = "wasm32", target_arch = "wasm64"),
+    target_os = "unknown",
+    not(feature = "js")
+))]
+fn random_seed() -> Option<u64> {
+    None
+}