//! This crate provides foldhash, a fast, non-cryptographic, minimally

//! DoS-resistant hashing algorithm designed for computational uses such as

//! hashmaps, bloom filters, count sketching, etc.

//!

//! When should you **not** use foldhash:

//!

//! - You are afraid of people studying your long-running program's behavior

//!   to reverse engineer its internal random state and using this knowledge to

//!   create many colliding inputs for computational complexity attacks.

//!

//! - You expect foldhash to have a consistent output across versions or

//!   platforms, such as for persistent file formats or communication protocols.

//!   

//! - You are relying on foldhash's properties for any kind of security.

//!   Foldhash is **not appropriate for any cryptographic purpose**.

//!

//! Foldhash has two variants, one optimized for speed which is ideal for data

//! structures such as hash maps and bloom filters, and one optimized for

//! statistical quality which is ideal for algorithms such as

//! [HyperLogLog](https://en.wikipedia.org/wiki/HyperLogLog) and

//! [MinHash](https://en.wikipedia.org/wiki/MinHash).

//!

//! Foldhash can be used in a `#![no_std]` environment by disabling its default

//! `"std"` feature.

//!

//! # Usage

//!

//! The easiest way to use this crate with the standard library [`HashMap`] or

//! [`HashSet`] is to import them from `foldhash` instead, along with the

//! extension traits to make [`HashMap::new`] and [`HashMap::with_capacity`]

//! work out-of-the-box:

//!

//! ```rust

//! use foldhash::{HashMap, HashMapExt};

//!

//! let mut hm = HashMap::new();

//! hm.insert(42, "hello");

//! ```

//!

//! You can also avoid the convenience types and do it manually by initializing

//! a [`RandomState`](fast::RandomState), for example if you are using a different hash map

//! implementation like [`hashbrown`](https://docs.rs/hashbrown/):

//!

//! ```rust

//! use hashbrown::HashMap;

//! use foldhash::fast::RandomState;

//!

//! let mut hm = HashMap::with_hasher(RandomState::default());

//! hm.insert("foo", "bar");

//! ```

//!

//! The above methods are the recommended way to use foldhash, which will

//! automatically generate a randomly generated hasher instance for you. If you

//! absolutely must have determinism you can use [`FixedState`](fast::FixedState)

//! instead, but note that this makes you trivially vulnerable to HashDoS

//! attacks and might lead to quadratic runtime when moving data from one

//! hashmap/set into another:

//!

//! ```rust

//! use std::collections::HashSet;

//! use foldhash::fast::FixedState;

//!

//! let mut hm = HashSet::with_hasher(FixedState::with_seed(42));

//! hm.insert([1, 10, 100]);

//! ```

//!

//! If you rely on statistical properties of the hash for the correctness of

//! your algorithm, such as in [HyperLogLog](https://en.wikipedia.org/wiki/HyperLogLog),

//! it is suggested to use the [`RandomState`](quality::RandomState)

//! or [`FixedState`](quality::FixedState) from the [`quality`] module instead

//! of the [`fast`] module. The latter is optimized purely for speed in hash

//! tables and has known statistical imperfections.

//!

//! Finally, you can also directly use the [`RandomState`](quality::RandomState)

//! or [`FixedState`](quality::FixedState) to manually hash items using the

//! [`BuildHasher`](std::hash::BuildHasher) trait:

//! ```rust

//! use std::hash::BuildHasher;

//! use foldhash::quality::RandomState;

//!

//! let random_state = RandomState::default();

//! let hash = random_state.hash_one("hello world");

//! ```

//!

//! ## Seeding

//!

//! Foldhash relies on a single 8-byte per-hasher seed which should be ideally

//! be different from each instance to instance, and also a larger

//! [`SharedSeed`] which may be shared by many different instances.

//!

//! To reduce overhead, this [`SharedSeed`] is typically initialized once and

//! stored. To prevent each hashmap unnecessarily containing a reference to this

//! value there are three kinds of [`BuildHasher`](core::hash::BuildHasher)s

//! foldhash provides (both for [`fast`] and [`quality`]):

//!

//! 1. [`RandomState`](fast::RandomState), which always generates a

//!    random per-hasher seed and implicitly stores a reference to [`SharedSeed::global_random`].

//! 2. [`FixedState`](fast::FixedState), which by default uses a fixed

//!    per-hasher seed and implicitly stores a reference to [`SharedSeed::global_fixed`].

//! 3. [`SeedableRandomState`](fast::SeedableRandomState), which works like

//!    [`RandomState`](fast::RandomState) by default but can be seeded in any manner.

//!    This state must include an explicit reference to a [`SharedSeed`], and thus

//!    this struct is 16 bytes as opposed to just 8 bytes for the previous two.

#![cfg_attr(all(not(test), not(feature = "std")), no_std)]

#![warn(missing_docs)]

pub mod fast;

pub mod quality;

mod seed;

pub use seed::SharedSeed;

#[cfg(feature = "std")]

mod convenience;

#[cfg(feature = "std")]

pub use convenience::*;

// Arbitrary constants with high entropy. Hexadecimal digits of pi were used.

const ARBITRARY0: u64 = 0x243f6a8885a308d3;

const ARBITRARY1: u64 = 0x13198a2e03707344;

const ARBITRARY2: u64 = 0xa4093822299f31d0;

const ARBITRARY3: u64 = 0x082efa98ec4e6c89;

const ARBITRARY4: u64 = 0x452821e638d01377;

const ARBITRARY5: u64 = 0xbe5466cf34e90c6c;

const ARBITRARY6: u64 = 0xc0ac29b7c97c50dd;

const ARBITRARY7: u64 = 0x3f84d5b5b5470917;

const ARBITRARY8: u64 = 0x9216d5d98979fb1b;

const ARBITRARY9: u64 = 0xd1310ba698dfb5ac;

#[inline(always)]

const fn folded_multiply(x: u64, y: u64) -> u64 {

    // The following code path is only fast if 64-bit to 128-bit widening

    // multiplication is supported by the architecture. Most 64-bit

    // architectures except SPARC64 and Wasm64 support it. However, the target

    // pointer width doesn't always indicate that we are dealing with a 64-bit

    // architecture, as there are ABIs that reduce the pointer width, especially

    // on AArch64 and x86-64. WebAssembly (regardless of pointer width) supports

    // 64-bit to 128-bit widening multiplication with the `wide-arithmetic`

    // proposal.

    #[cfg(any(

        all(

            target_pointer_width = "64",

            not(any(target_arch = "sparc64", target_arch = "wasm64")),

        ),

        target_arch = "aarch64",

        target_arch = "x86_64",

        all(target_family = "wasm", target_feature = "wide-arithmetic"),

    ))]

    {

        // We compute the full u64 x u64 -> u128 product, this is a single mul

        // instruction on x86-64, one mul plus one mulhi on ARM64.

        let full = (x as u128).wrapping_mul(y as u128);

        let lo = full as u64;

        let hi = (full >> 64) as u64;

        // The middle bits of the full product fluctuate the most with small

        // changes in the input. This is the top bits of lo and the bottom bits

        // of hi. We can thus make the entire output fluctuate with small

        // changes to the input by XOR'ing these two halves.

        lo ^ hi

    }

    #[cfg(not(any(

        all(

            target_pointer_width = "64",

            not(any(target_arch = "sparc64", target_arch = "wasm64")),

        ),

        target_arch = "aarch64",

        target_arch = "x86_64",

        all(target_family = "wasm", target_feature = "wide-arithmetic"),

    )))]

    {

        // u64 x u64 -> u128 product is quite expensive on 32-bit.

        // We approximate it by expanding the multiplication and eliminating

        // carries by replacing additions with XORs:

        //    (2^32 hx + lx)*(2^32 hy + ly) =

        //    2^64 hx*hy + 2^32 (hx*ly + lx*hy) + lx*ly ~=

        //    2^64 hx*hy ^ 2^32 (hx*ly ^ lx*hy) ^ lx*ly

        // Which when folded becomes:

        //    (hx*hy ^ lx*ly) ^ (hx*ly ^ lx*hy).rotate_right(32)

        let lx = x as u32;

        let ly = y as u32;

        let hx = (x >> 32) as u32;

        let hy = (y >> 32) as u32;

        let ll = (lx as u64).wrapping_mul(ly as u64);

        let lh = (lx as u64).wrapping_mul(hy as u64);

        let hl = (hx as u64).wrapping_mul(ly as u64);

        let hh = (hx as u64).wrapping_mul(hy as u64);

        (hh ^ ll) ^ (hl ^ lh).rotate_right(32)

    }

}

#[inline(always)]

const fn rotate_right(x: u64, r: u32) -> u64 {

    #[cfg(any(

        target_pointer_width = "64",

        target_arch = "aarch64",

        target_arch = "x86_64",

        target_family = "wasm",

    ))]

    {

        x.rotate_right(r)

    }

    #[cfg(not(any(

        target_pointer_width = "64",

        target_arch = "aarch64",

        target_arch = "x86_64",

        target_family = "wasm",

    )))]

    {

        // On platforms without 64-bit arithmetic rotation can be slow, rotate

        // each 32-bit half independently.

        let lo = (x as u32).rotate_right(r);

        let hi = ((x >> 32) as u32).rotate_right(r);

        ((hi as u64) << 32) | lo as u64

    }

}

/// Hashes strings >= 16 bytes, has unspecified behavior when bytes.len() < 16.

fn hash_bytes_medium(bytes: &[u8], mut s0: u64, mut s1: u64, fold_seed: u64) -> u64 {

    // Process 32 bytes per iteration, 16 bytes from the start, 16 bytes from

    // the end. On the last iteration these two chunks can overlap, but that is

    // perfectly fine.

    let left_to_right = bytes.chunks_exact(16);

    let mut right_to_left = bytes.rchunks_exact(16);

    for lo in left_to_right {

        let hi = right_to_left.next().unwrap();

        let unconsumed_start = lo.as_ptr();

        let unconsumed_end = hi.as_ptr_range().end;

        if unconsumed_start >= unconsumed_end {

            break;

        }

        let a = u64::from_ne_bytes(lo[0..8].try_into().unwrap());

        let b = u64::from_ne_bytes(lo[8..16].try_into().unwrap());

        let c = u64::from_ne_bytes(hi[0..8].try_into().unwrap());

        let d = u64::from_ne_bytes(hi[8..16].try_into().unwrap());

        s0 = folded_multiply(a ^ s0, c ^ fold_seed);

        s1 = folded_multiply(b ^ s1, d ^ fold_seed);

    }

    s0 ^ s1

}

/// Hashes strings >= 16 bytes, has unspecified behavior when bytes.len() < 16.

#[cold]

#[inline(never)]

fn hash_bytes_long(

    bytes: &[u8],

    mut s0: u64,

    mut s1: u64,

    mut s2: u64,

    mut s3: u64,

    fold_seed: u64,

) -> u64 {

    let chunks = bytes.chunks_exact(64);

    let remainder = chunks.remainder().len();

    for chunk in chunks {

        let a = u64::from_ne_bytes(chunk[0..8].try_into().unwrap());

        let b = u64::from_ne_bytes(chunk[8..16].try_into().unwrap());

        let c = u64::from_ne_bytes(chunk[16..24].try_into().unwrap());

        let d = u64::from_ne_bytes(chunk[24..32].try_into().unwrap());

        let e = u64::from_ne_bytes(chunk[32..40].try_into().unwrap());

        let f = u64::from_ne_bytes(chunk[40..48].try_into().unwrap());

        let g = u64::from_ne_bytes(chunk[48..56].try_into().unwrap());

        let h = u64::from_ne_bytes(chunk[56..64].try_into().unwrap());

        s0 = folded_multiply(a ^ s0, e ^ fold_seed);

        s1 = folded_multiply(b ^ s1, f ^ fold_seed);

        s2 = folded_multiply(c ^ s2, g ^ fold_seed);

        s3 = folded_multiply(d ^ s3, h ^ fold_seed);

    }

    s0 ^= s2;

    s1 ^= s3;

    if remainder > 0 {

        hash_bytes_medium(&bytes[bytes.len() - remainder.max(16)..], s0, s1, fold_seed)

    } else {

        s0 ^ s1

    }

}