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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 `BlockRngCore` trait and implementation helpers
    //!
    //! The [`BlockRngCore`] trait exists to assist in the implementation of RNGs
    //! which generate a block of data in a cache instead of returning generated
    //! values directly.
    //!
    //! Usage of this trait is optional, but provides two advantages:
    //! implementations only need to concern themselves with generation of the
    //! block, not the various [`RngCore`] methods (especially [`fill_bytes`], where
    //! the optimal implementations are not trivial), and this allows
    //! `ReseedingRng` (see [`rand`](https://docs.rs/rand) crate) perform periodic
    //! reseeding with very low overhead.
    //!
    //! # Example
    //!
    //! ```norun
    //! use rand_core::block::{BlockRngCore, BlockRng};
    //!
    //! struct MyRngCore;
    //!
    //! impl BlockRngCore for MyRngCore {
    //!     type Results = [u32; 16];
    //!
    //!     fn generate(&mut self, results: &mut Self::Results) {
    //!         unimplemented!()
    //!     }
    //! }
    //!
    //! impl SeedableRng for MyRngCore {
    //!     type Seed = unimplemented!();
    //!     fn from_seed(seed: Self::Seed) -> Self {
    //!         unimplemented!()
    //!     }
    //! }
    //!
    //! // optionally, also implement CryptoRng for MyRngCore
    //!
    //! // Final RNG.
    //! type MyRng = BlockRng<u32, MyRngCore>;
    //! ```
    //!
    //! [`BlockRngCore`]: crate::block::BlockRngCore
    //! [`fill_bytes`]: RngCore::fill_bytes

    use core::convert::AsRef;
    use core::{fmt, ptr};
    use {RngCore, CryptoRng, SeedableRng, Error};
    use impls::{fill_via_u32_chunks, fill_via_u64_chunks};

    /// A trait for RNGs which do not generate random numbers individually, but in
    /// blocks (typically `[u32; N]`). This technique is commonly used by
    /// cryptographic RNGs to improve performance.
    ///
    /// See the [module][crate::block] documentation for details.
    pub trait BlockRngCore {
        /// Results element type, e.g. `u32`.
        type Item;

        /// Results type. This is the 'block' an RNG implementing `BlockRngCore`
        /// generates, which will usually be an array like `[u32; 16]`.
        type Results: AsRef<[Self::Item]> + AsMut<[Self::Item]> + Default;

        /// Generate a new block of results.
        fn generate(&mut self, results: &mut Self::Results);
    }

    /// A wrapper type implementing [`RngCore`] for some type implementing
    /// [`BlockRngCore`] with `u32` array buffer; i.e. this can be used to implement
    /// a full RNG from just a `generate` function.
    ///
    /// The `core` field may be accessed directly but the results buffer may not.
    /// PRNG implementations can simply use a type alias
    /// (`pub type MyRng = BlockRng<MyRngCore>;`) but might prefer to use a
    /// wrapper type (`pub struct MyRng(BlockRng<MyRngCore>);`); the latter must
    /// re-implement `RngCore` but hides the implementation details and allows
    /// extra functionality to be defined on the RNG
    /// (e.g. `impl MyRng { fn set_stream(...){...} }`).
    ///
    /// `BlockRng` has heavily optimized implementations of the [`RngCore`] methods
    /// reading values from the results buffer, as well as
    /// calling [`BlockRngCore::generate`] directly on the output array when
    /// [`fill_bytes`] / [`try_fill_bytes`] is called on a large array. These methods
    /// also handle the bookkeeping of when to generate a new batch of values.
    ///
    /// No whole generated `u32` values are thown away and all values are consumed
    /// in-order. [`next_u32`] simply takes the next available `u32` value.
    /// [`next_u64`] is implemented by combining two `u32` values, least
    /// significant first. [`fill_bytes`] and [`try_fill_bytes`] consume a whole
    /// number of `u32` values, converting each `u32` to a byte slice in
    /// little-endian order. If the requested byte length is not a multiple of 4,
    /// some bytes will be discarded.
    ///
    /// See also [`BlockRng64`] which uses `u64` array buffers. Currently there is
    /// no direct support for other buffer types.
    ///
    /// For easy initialization `BlockRng` also implements [`SeedableRng`].
    ///
    /// [`next_u32`]: RngCore::next_u32
    /// [`next_u64`]: RngCore::next_u64
    /// [`fill_bytes`]: RngCore::fill_bytes
    /// [`try_fill_bytes`]: RngCore::try_fill_bytes
    #[derive(Clone)]
    #[cfg_attr(feature="serde1", derive(Serialize, Deserialize))]
    pub struct BlockRng<R: BlockRngCore + ?Sized> {
        results: R::Results,
        index: usize,
        /// The *core* part of the RNG, implementing the `generate` function.
        pub core: R,
    }

    // Custom Debug implementation that does not expose the contents of `results`.
    impl<R: BlockRngCore + fmt::Debug> fmt::Debug for BlockRng<R> {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("BlockRng")
               .field("core", &self.core)
               .field("result_len", &self.results.as_ref().len())
               .field("index", &self.index)
               .finish()
        }
    }

    impl<R: BlockRngCore> BlockRng<R> {
        /// Create a new `BlockRng` from an existing RNG implementing
        /// `BlockRngCore`. Results will be generated on first use.
        #[inline]
        pub fn new(core: R) -> BlockRng<R>{
            let results_empty = R::Results::default();
            BlockRng {
                core,
                index: results_empty.as_ref().len(),
                results: results_empty,
            }
        }

        /// Get the index into the result buffer.
        ///
        /// If this is equal to or larger than the size of the result buffer then
        /// the buffer is "empty" and `generate()` must be called to produce new
        /// results.
        #[inline(always)]
        pub fn index(&self) -> usize {
            self.index
        }

        /// Reset the number of available results.
        /// This will force a new set of results to be generated on next use.
        #[inline]
        pub fn reset(&mut self) {
            self.index = self.results.as_ref().len();
        }

        /// Generate a new set of results immediately, setting the index to the
        /// given value.
        #[inline]
        pub fn generate_and_set(&mut self, index: usize) {
            assert!(index < self.results.as_ref().len());
            self.core.generate(&mut self.results);
            self.index = index;
        }
    }

    impl<R: BlockRngCore<Item=u32>> RngCore for BlockRng<R>
    where <R as BlockRngCore>::Results: AsRef<[u32]> + AsMut<[u32]>
    {
        #[inline]
        fn next_u32(&mut self) -> u32 {
            if self.index >= self.results.as_ref().len() {
                self.generate_and_set(0);
            }

            let value = self.results.as_ref()[self.index];
            self.index += 1;
            value
        }

        #[inline]
        fn next_u64(&mut self) -> u64 {
            let read_u64 = |results: &[u32], index| {
                if cfg!(any(target_endian = "little")) {
                    // requires little-endian CPU
                    let ptr: *const u64 = results[index..=index+1].as_ptr() as *const u64;
                    unsafe { ptr::read_unaligned(ptr) }
                } else {
                    let x = u64::from(results[index]);
                    let y = u64::from(results[index + 1]);
                    (y << 32) | x
                }
            };

            let len = self.results.as_ref().len();

            let index = self.index;
            if index < len-1 {
                self.index += 2;
                // Read an u64 from the current index
                read_u64(self.results.as_ref(), index)
            } else if index >= len {
                self.generate_and_set(2);
                read_u64(self.results.as_ref(), 0)
            } else {
                let x = u64::from(self.results.as_ref()[len-1]);
                self.generate_and_set(1);
                let y = u64::from(self.results.as_ref()[0]);
                (y << 32) | x
            }
        }

        #[inline]
        fn fill_bytes(&mut self, dest: &mut [u8]) {
            let mut read_len = 0;
            while read_len < dest.len() {
                if self.index >= self.results.as_ref().len() {
                    self.generate_and_set(0);
                }
                let (consumed_u32, filled_u8) =
                    fill_via_u32_chunks(&self.results.as_ref()[self.index..],
                                        &mut dest[read_len..]);

                self.index += consumed_u32;
                read_len += filled_u8;
            }
        }

        #[inline(always)]
        fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
            Ok(self.fill_bytes(dest))
        }
    }

    impl<R: BlockRngCore + SeedableRng> SeedableRng for BlockRng<R> {
        type Seed = R::Seed;

        #[inline(always)]
        fn from_seed(seed: Self::Seed) -> Self {
            Self::new(R::from_seed(seed))
        }

        #[inline(always)]
        fn seed_from_u64(seed: u64) -> Self {
            Self::new(R::seed_from_u64(seed))
        }

        #[inline(always)]
        fn from_rng<S: RngCore>(rng: S) -> Result<Self, Error> {
            Ok(Self::new(R::from_rng(rng)?))
        }
    }

    /// A wrapper type implementing [`RngCore`] for some type implementing
    /// [`BlockRngCore`] with `u64` array buffer; i.e. this can be used to implement
    /// a full RNG from just a `generate` function.
    ///
    /// This is similar to [`BlockRng`], but specialized for algorithms that operate
    /// on `u64` values.
    ///
    /// No whole generated `u64` values are thrown away and all values are consumed
    /// in-order. [`next_u64`] simply takes the next available `u64` value.
    /// [`next_u32`] is however a bit special: half of a `u64` is consumed, leaving
    /// the other half in the buffer. If the next function called is [`next_u32`]
    /// then the other half is then consumed, however both [`next_u64`] and
    /// [`fill_bytes`] discard the rest of any half-consumed `u64`s when called.
    ///
    /// [`fill_bytes`] and [`try_fill_bytes`] consume a whole number of `u64`
    /// values. If the requested length is not a multiple of 8, some bytes will be
    /// discarded.
    ///
    /// [`next_u32`]: RngCore::next_u32
    /// [`next_u64`]: RngCore::next_u64
    /// [`fill_bytes`]: RngCore::fill_bytes
    /// [`try_fill_bytes`]: RngCore::try_fill_bytes
    #[derive(Clone)]
    #[cfg_attr(feature="serde1", derive(Serialize, Deserialize))]
    pub struct BlockRng64<R: BlockRngCore + ?Sized> {
        results: R::Results,
        index: usize,
        half_used: bool, // true if only half of the previous result is used
        /// The *core* part of the RNG, implementing the `generate` function.
        pub core: R,
    }

    // Custom Debug implementation that does not expose the contents of `results`.
    impl<R: BlockRngCore + fmt::Debug> fmt::Debug for BlockRng64<R> {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("BlockRng64")
               .field("core", &self.core)
               .field("result_len", &self.results.as_ref().len())
               .field("index", &self.index)
               .field("half_used", &self.half_used)
               .finish()
        }
    }

    impl<R: BlockRngCore> BlockRng64<R> {
        /// Create a new `BlockRng` from an existing RNG implementing
        /// `BlockRngCore`. Results will be generated on first use.
        #[inline]
        pub fn new(core: R) -> BlockRng64<R>{
            let results_empty = R::Results::default();
            BlockRng64 {
                core,
                index: results_empty.as_ref().len(),
                half_used: false,
                results: results_empty,
            }
        }

        /// Get the index into the result buffer.
        ///
        /// If this is equal to or larger than the size of the result buffer then
        /// the buffer is "empty" and `generate()` must be called to produce new
        /// results.
        #[inline(always)]
        pub fn index(&self) -> usize {
            self.index
        }

        /// Reset the number of available results.
        /// This will force a new set of results to be generated on next use.
        #[inline]
        pub fn reset(&mut self) {
            self.index = self.results.as_ref().len();
            self.half_used = false;
        }

        /// Generate a new set of results immediately, setting the index to the
        /// given value.
        #[inline]
        pub fn generate_and_set(&mut self, index: usize) {
            assert!(index < self.results.as_ref().len());
            self.core.generate(&mut self.results);
            self.index = index;
            self.half_used = false;
        }
    }

    impl<R: BlockRngCore<Item=u64>> RngCore for BlockRng64<R>
    where <R as BlockRngCore>::Results: AsRef<[u64]> + AsMut<[u64]>
    {
        #[inline]
        fn next_u32(&mut self) -> u32 {
            let mut index = self.index * 2 - self.half_used as usize;
            if index >= self.results.as_ref().len() * 2 {
                self.core.generate(&mut self.results);
                self.index = 0;
                // `self.half_used` is by definition `false`
                self.half_used = false;
                index = 0;
            }

            self.half_used = !self.half_used;
            self.index += self.half_used as usize;

            // Index as if this is a u32 slice.
            unsafe {
                let results =
                    &*(self.results.as_ref() as *const [u64] as *const [u32]);
                if cfg!(target_endian = "little") {
                    *results.get_unchecked(index)
                } else {
                    *results.get_unchecked(index ^ 1)
                }
            }
        }

        #[inline]
        fn next_u64(&mut self) -> u64 {
            if self.index >= self.results.as_ref().len() {
                self.core.generate(&mut self.results);
                self.index = 0;
            }

            let value = self.results.as_ref()[self.index];
            self.index += 1;
            self.half_used = false;
            value
        }

        #[inline]
        fn fill_bytes(&mut self, dest: &mut [u8]) {
            let mut read_len = 0;
            self.half_used = false;
            while read_len < dest.len() {
                if self.index as usize >= self.results.as_ref().len() {
                    self.core.generate(&mut self.results);
                    self.index = 0;
                }

                let (consumed_u64, filled_u8) =
                    fill_via_u64_chunks(&self.results.as_ref()[self.index as usize..],
                                        &mut dest[read_len..]);

                self.index += consumed_u64;
                read_len += filled_u8;
            }
        }

        #[inline(always)]
        fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
            Ok(self.fill_bytes(dest))
        }
    }

    impl<R: BlockRngCore + SeedableRng> SeedableRng for BlockRng64<R> {
        type Seed = R::Seed;

        #[inline(always)]
        fn from_seed(seed: Self::Seed) -> Self {
            Self::new(R::from_seed(seed))
        }

        #[inline(always)]
        fn seed_from_u64(seed: u64) -> Self {
            Self::new(R::seed_from_u64(seed))
        }

        #[inline(always)]
        fn from_rng<S: RngCore>(rng: S) -> Result<Self, Error> {
            Ok(Self::new(R::from_rng(rng)?))
        }
    }

    impl<R: BlockRngCore + CryptoRng> CryptoRng for BlockRng<R> {}

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