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+// Copyright 2018 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.
+
+//! The Bernoulli distribution.
+
+use crate::distributions::Distribution;
+use crate::Rng;
+use core::{fmt, u64};
+
+#[cfg(feature = "serde1")]
+use serde::{Serialize, Deserialize};
+/// The Bernoulli distribution.
+///
+/// This is a special case of the Binomial distribution where `n = 1`.
+///
+/// # Example
+///
+/// ```rust
+/// use rand::distributions::{Bernoulli, Distribution};
+///
+/// let d = Bernoulli::new(0.3).unwrap();
+/// let v = d.sample(&mut rand::thread_rng());
+/// println!("{} is from a Bernoulli distribution", v);
+/// ```
+///
+/// # Precision
+///
+/// This `Bernoulli` distribution uses 64 bits from the RNG (a `u64`),
+/// so only probabilities that are multiples of 2<sup>-64</sup> can be
+/// represented.
+#[derive(Clone, Copy, Debug, PartialEq)]
+#[cfg_attr(feature = "serde1", derive(Serialize, Deserialize))]
+pub struct Bernoulli {
+    /// Probability of success, relative to the maximal integer.
+    p_int: u64,
+}
+
+// To sample from the Bernoulli distribution we use a method that compares a
+// random `u64` value `v < (p * 2^64)`.
+//
+// If `p == 1.0`, the integer `v` to compare against can not represented as a
+// `u64`. We manually set it to `u64::MAX` instead (2^64 - 1 instead of 2^64).
+// Note that  value of `p < 1.0` can never result in `u64::MAX`, because an
+// `f64` only has 53 bits of precision, and the next largest value of `p` will
+// result in `2^64 - 2048`.
+//
+// Also there is a 100% theoretical concern: if someone consistently wants to
+// generate `true` using the Bernoulli distribution (i.e. by using a probability
+// of `1.0`), just using `u64::MAX` is not enough. On average it would return
+// false once every 2^64 iterations. Some people apparently care about this
+// case.
+//
+// That is why we special-case `u64::MAX` to always return `true`, without using
+// the RNG, and pay the performance price for all uses that *are* reasonable.
+// Luckily, if `new()` and `sample` are close, the compiler can optimize out the
+// extra check.
+const ALWAYS_TRUE: u64 = u64::MAX;
+
+// This is just `2.0.powi(64)`, but written this way because it is not available
+// in `no_std` mode.
+const SCALE: f64 = 2.0 * (1u64 << 63) as f64;
+
+/// Error type returned from `Bernoulli::new`.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum BernoulliError {
+    /// `p < 0` or `p > 1`.
+    InvalidProbability,
+}
+
+impl fmt::Display for BernoulliError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            BernoulliError::InvalidProbability => "p is outside [0, 1] in Bernoulli distribution",
+        })
+    }
+}
+
+#[cfg(feature = "std")]
+impl ::std::error::Error for BernoulliError {}
+
+impl Bernoulli {
+    /// Construct a new `Bernoulli` with the given probability of success `p`.
+    ///
+    /// # Precision
+    ///
+    /// For `p = 1.0`, the resulting distribution will always generate true.
+    /// For `p = 0.0`, the resulting distribution will always generate false.
+    ///
+    /// This method is accurate for any input `p` in the range `[0, 1]` which is
+    /// a multiple of 2<sup>-64</sup>. (Note that not all multiples of
+    /// 2<sup>-64</sup> in `[0, 1]` can be represented as a `f64`.)
+    #[inline]
+    pub fn new(p: f64) -> Result<Bernoulli, BernoulliError> {
+        if !(0.0..1.0).contains(&p) {
+            if p == 1.0 {
+                return Ok(Bernoulli { p_int: ALWAYS_TRUE });
+            }
+            return Err(BernoulliError::InvalidProbability);
+        }
+        Ok(Bernoulli {
+            p_int: (p * SCALE) as u64,
+        })
+    }
+
+    /// Construct a new `Bernoulli` with the probability of success of
+    /// `numerator`-in-`denominator`. I.e. `new_ratio(2, 3)` will return
+    /// a `Bernoulli` with a 2-in-3 chance, or about 67%, of returning `true`.
+    ///
+    /// return `true`. If `numerator == 0` it will always return `false`.
+    /// For `numerator > denominator` and `denominator == 0`, this returns an
+    /// error. Otherwise, for `numerator == denominator`, samples are always
+    /// true; for `numerator == 0` samples are always false.
+    #[inline]
+    pub fn from_ratio(numerator: u32, denominator: u32) -> Result<Bernoulli, BernoulliError> {
+        if numerator > denominator || denominator == 0 {
+            return Err(BernoulliError::InvalidProbability);
+        }
+        if numerator == denominator {
+            return Ok(Bernoulli { p_int: ALWAYS_TRUE });
+        }
+        let p_int = ((f64::from(numerator) / f64::from(denominator)) * SCALE) as u64;
+        Ok(Bernoulli { p_int })
+    }
+}
+
+impl Distribution<bool> for Bernoulli {
+    #[inline]
+    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> bool {
+        // Make sure to always return true for p = 1.0.
+        if self.p_int == ALWAYS_TRUE {
+            return true;
+        }
+        let v: u64 = rng.gen();
+        v < self.p_int
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::Bernoulli;
+    use crate::distributions::Distribution;
+    use crate::Rng;
+
+    #[test]
+    #[cfg(feature="serde1")]
+    fn test_serializing_deserializing_bernoulli() {
+        let coin_flip = Bernoulli::new(0.5).unwrap();
+        let de_coin_flip : Bernoulli = bincode::deserialize(&bincode::serialize(&coin_flip).unwrap()).unwrap();
+
+        assert_eq!(coin_flip.p_int, de_coin_flip.p_int);
+    }
+
+    #[test]
+    fn test_trivial() {
+        // We prefer to be explicit here.
+        #![allow(clippy::bool_assert_comparison)]
+
+        let mut r = crate::test::rng(1);
+        let always_false = Bernoulli::new(0.0).unwrap();
+        let always_true = Bernoulli::new(1.0).unwrap();
+        for _ in 0..5 {
+            assert_eq!(r.sample::<bool, _>(&always_false), false);
+            assert_eq!(r.sample::<bool, _>(&always_true), true);
+            assert_eq!(Distribution::<bool>::sample(&always_false, &mut r), false);
+            assert_eq!(Distribution::<bool>::sample(&always_true, &mut r), true);
+        }
+    }
+
+    #[test]
+    #[cfg_attr(miri, ignore)] // Miri is too slow
+    fn test_average() {
+        const P: f64 = 0.3;
+        const NUM: u32 = 3;
+        const DENOM: u32 = 10;
+        let d1 = Bernoulli::new(P).unwrap();
+        let d2 = Bernoulli::from_ratio(NUM, DENOM).unwrap();
+        const N: u32 = 100_000;
+
+        let mut sum1: u32 = 0;
+        let mut sum2: u32 = 0;
+        let mut rng = crate::test::rng(2);
+        for _ in 0..N {
+            if d1.sample(&mut rng) {
+                sum1 += 1;
+            }
+            if d2.sample(&mut rng) {
+                sum2 += 1;
+            }
+        }
+        let avg1 = (sum1 as f64) / (N as f64);
+        assert!((avg1 - P).abs() < 5e-3);
+
+        let avg2 = (sum2 as f64) / (N as f64);
+        assert!((avg2 - (NUM as f64) / (DENOM as f64)).abs() < 5e-3);
+    }
+
+    #[test]
+    fn value_stability() {
+        let mut rng = crate::test::rng(3);
+        let distr = Bernoulli::new(0.4532).unwrap();
+        let mut buf = [false; 10];
+        for x in &mut buf {
+            *x = rng.sample(&distr);
+        }
+        assert_eq!(buf, [
+            true, false, false, true, false, false, true, true, true, true
+        ]);
+    }
+
+    #[test]
+    fn bernoulli_distributions_can_be_compared() {
+        assert_eq!(Bernoulli::new(1.0), Bernoulli::new(1.0));
+    }
+}