rand-bits 0.1.1

//! Utility functions for generating random numbers with a fixed number of set bits (ones).
//!
//! # Example
//!
//! ```rust
//! use rand::thread_rng;
//! use rand_bits::RngBits;
//!
//! let mut rng = thread_rng();
//! let x: u8 = rng.gen_bits(4); // generates a u8 with 4 set bits
//! assert_eq!(x.count_ones(), 4);
//! let y: u16 = rng.gen_bits(15); // generates a u16 with 15 set bits
//! assert_eq!(y.count_ones(), 15);
//! let z: u64 = rng.gen_bits(1); // generates a u64 with 1 set bits
//! assert_eq!(z.count_ones(), 1);
//! ```
//!
//! # License
//!
//! This crate is licensed under the MIT License.

#![forbid(unsafe_code)]

use std::cmp::min;

use phf::{phf_map, Map};
use rand::Rng;

const MAPPING: Map<u32, &'static [u8]> = phf_map! {
    1u32 => &[0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80],
    2u32 => &[0x03, 0x05, 0x06, 0x09, 0x0A, 0x0C, 0x11, 0x12, 0x14, 0x18, 0x21, 0x22, 0x24, 0x28, 0x30, 0x41, 0x42, 0x44, 0x48, 0x50, 0x60, 0x81, 0x82, 0x84, 0x88, 0x90, 0xA0, 0xC0],
    3u32 => &[0x07, 0x0B, 0x0D, 0x0E, 0x13, 0x15, 0x16, 0x19, 0x1A, 0x1C, 0x23, 0x25, 0x26, 0x29, 0x2A, 0x2C, 0x31, 0x32, 0x34, 0x38, 0x43, 0x45, 0x46, 0x49, 0x4A, 0x4C, 0x51, 0x52, 0x54, 0x58, 0x61, 0x62, 0x64, 0x68, 0x70, 0x83, 0x85, 0x86, 0x89, 0x8A, 0x8C, 0x91, 0x92, 0x94, 0x98, 0xA1, 0xA2, 0xA4, 0xA8, 0xB0, 0xC1, 0xC2, 0xC4, 0xC8, 0xD0, 0xE0],
    4u32 => &[0x0F, 0x17, 0x1B, 0x1D, 0x1E, 0x27, 0x2B, 0x2D, 0x2E, 0x33, 0x35, 0x36, 0x39, 0x3A, 0x3C, 0x47, 0x4B, 0x4D, 0x4E, 0x53, 0x55, 0x56, 0x59, 0x5A, 0x5C, 0x63, 0x65, 0x66, 0x69, 0x6A, 0x6C, 0x71, 0x72, 0x74, 0x78, 0x87, 0x8B, 0x8D, 0x8E, 0x93, 0x95, 0x96, 0x99, 0x9A, 0x9C, 0xA3, 0xA5, 0xA6, 0xA9, 0xAA, 0xAC, 0xB1, 0xB2, 0xB4, 0xB8, 0xC3, 0xC5, 0xC6, 0xC9, 0xCA, 0xCC, 0xD1, 0xD2, 0xD4, 0xD8, 0xE1, 0xE2, 0xE4, 0xE8, 0xF0],
};

/// A generic random value distribution, implemented for many primitive types.
/// Usually generates values with a numerically uniform distribution, and with a
/// range appropriate to the type.
///
/// Based on [`rand::distributions::Standard`].
pub struct Standard;

/// Types (distributions) that can be used to create a random instance of `T`.
///
/// Based on [`rand::distributions::Distribution`].
pub trait Distribution<T> {
    /// Generate a random value of `T`, using `rng` as the source of randomness.
    fn sample<R>(&self, rng: &mut R, bits: u32) -> T
    where
        R: Rng + ?Sized;
}

impl Distribution<u8> for Standard {
    fn sample<R>(&self, rng: &mut R, bits: u32) -> u8
    where
        R: Rng + ?Sized,
    {
        match bits {
            0 => u8::MIN,
            u8::BITS => u8::MAX,
            1..=4 => {
                let values = MAPPING.get(&bits).expect("bits count out of range");
                let index = rng.gen_range(0..values.len());
                values[index]
            },
            5..=7 => {
                let bits = u8::BITS - bits;
                Distribution::<u8>::sample(self, rng, bits) ^ 0xFF
            },
            _ => panic!("bits count out of range"),
        }
    }
}

impl Distribution<u16> for Standard {
    fn sample<R>(&self, rng: &mut R, bits: u32) -> u16
    where
        R: Rng + ?Sized,
    {
        match bits {
            0 => u16::MIN,
            u16::BITS => u16::MAX,
            bits if (1..u16::BITS).contains(&bits) => {
                let min_high_bits = bits.checked_sub(u8::BITS).unwrap_or_default();
                let max_high_bits = min(bits, u8::BITS);
                let high_bits = rng.gen_range(min_high_bits..=max_high_bits);
                let low_bits = bits - high_bits;

                let mut value = Distribution::<u8>::sample(self, rng, high_bits) as u16;
                value <<= u8::BITS;
                value |= Distribution::<u8>::sample(self, rng, low_bits) as u16;
                value
            },
            _ => panic!("bits count out of range"),
        }
    }
}

impl Distribution<u32> for Standard {
    fn sample<R>(&self, rng: &mut R, bits: u32) -> u32
    where
        R: Rng + ?Sized,
    {
        match bits {
            0 => u32::MIN,
            u32::BITS => u32::MAX,
            bits if (1..u32::BITS).contains(&bits) => {
                let min_high_bits = bits.checked_sub(u16::BITS).unwrap_or_default();
                let max_high_bits = min(bits, u16::BITS);
                let high_bits = rng.gen_range(min_high_bits..=max_high_bits);
                let low_bits = bits - high_bits;

                let mut value = Distribution::<u16>::sample(self, rng, high_bits) as u32;
                value <<= u16::BITS;
                value |= Distribution::<u16>::sample(self, rng, low_bits) as u32;
                value
            },
            _ => panic!("bits count out of range"),
        }
    }
}

impl Distribution<u64> for Standard {
    fn sample<R>(&self, rng: &mut R, bits: u32) -> u64
    where
        R: Rng + ?Sized,
    {
        match bits {
            0 => u64::MIN,
            u64::BITS => u64::MAX,
            bits if (1..u64::BITS).contains(&bits) => {
                let min_high_bits = bits.checked_sub(u32::BITS).unwrap_or_default();
                let max_high_bits = min(bits, u32::BITS);
                let high_bits = rng.gen_range(min_high_bits..=max_high_bits);
                let low_bits = bits - high_bits;

                let mut value = Distribution::<u32>::sample(self, rng, high_bits) as u64;
                value <<= u32::BITS;
                value |= Distribution::<u32>::sample(self, rng, low_bits) as u64;
                value
            },
            _ => panic!("bits count out of range"),
        }
    }
}

impl Distribution<u128> for Standard {
    fn sample<R>(&self, rng: &mut R, bits: u32) -> u128
    where
        R: Rng + ?Sized,
    {
        match bits {
            0 => u128::MIN,
            u128::BITS => u128::MAX,
            bits if (1..u128::BITS).contains(&bits) => {
                let min_high_bits = bits.checked_sub(u64::BITS).unwrap_or_default();
                let max_high_bits = min(bits, u64::BITS);
                let high_bits = rng.gen_range(min_high_bits..=max_high_bits);
                let low_bits = bits - high_bits;

                let mut value = Distribution::<u64>::sample(self, rng, high_bits) as u128;
                value <<= u64::BITS;
                value |= Distribution::<u64>::sample(self, rng, low_bits) as u128;
                value
            },
            _ => panic!("bits count out of range"),
        }
    }
}

/// An automatically-implemented extension trait on [`rand::Rng`].
///
/// # Example:
///
/// ```rust
/// # use rand::thread_rng;
/// use rand_bits::RngBits;
///
/// fn foo<R>(rng: &mut R) -> u16
/// where
///     R: RngBits + ?Sized,
/// {
///     rng.gen_bits(16)
/// }
///
/// # let v = foo(&mut thread_rng());
/// ```
pub trait RngBits: Rng {
    /// Return a random value supporting the [`Standard`] distribution with a chosen
    /// number of bits set to active.
    ///
    /// # Example
    ///
    /// ```rust
    /// use rand::thread_rng;
    /// use rand_bits::RngBits;
    ///
    /// let mut rng = thread_rng();
    /// let x: u32 = rng.gen_bits(11);
    /// println!("{}", x);
    /// ```
    fn gen_bits<T>(&mut self, bits: u32) -> T
    where
        Standard: Distribution<T>,
    {
        Standard.sample(self, bits)
    }
}

impl<R> RngBits for R where R: Rng {}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn u8() {
        let mut rng = rand::thread_rng();
        for i in 0..=u8::BITS {
            let n: u8 = rng.gen_bits(i);
            assert_eq!(n.count_ones(), i);
        }
    }

    #[test]
    fn u16() {
        let mut rng = rand::thread_rng();
        for i in 0..=u16::BITS {
            let n: u16 = rng.gen_bits(i);
            assert_eq!(n.count_ones(), i);
        }
    }

    #[test]
    fn u32() {
        let mut rng = rand::thread_rng();
        for i in 0..=u32::BITS {
            let n: u32 = rng.gen_bits(i);
            assert_eq!(n.count_ones(), i);
        }
    }

    #[test]
    fn u64() {
        let mut rng = rand::thread_rng();
        for i in 0..=u64::BITS {
            let n: u64 = rng.gen_bits(i);
            assert_eq!(n.count_ones(), i);
        }
    }

    #[test]
    fn u128() {
        let mut rng = rand::thread_rng();
        for i in 0..=u128::BITS {
            let n: u128 = rng.gen_bits(i);
            assert_eq!(n.count_ones(), i);
        }
    }
}