//! Fast, SIMD-accelerated CRC32 (IEEE) checksum computation.

//!

//! ## Usage

//!

//! ### Simple usage

//!

//! For simple use-cases, you can call the [`hash()`] convenience function to

//! directly compute the CRC32 checksum for a given byte slice:

//!

//! ```rust

//! let checksum = crc32fast::hash(b"foo bar baz");

//! ```

//!

//! ### Advanced usage

//!

//! For use-cases that require more flexibility or performance, for example when

//! processing large amounts of data, you can create and manipulate a [`Hasher`]:

//!

//! ```rust

//! use crc32fast::Hasher;

//!

//! let mut hasher = Hasher::new();

//! hasher.update(b"foo bar baz");

//! let checksum = hasher.finalize();

//! ```

//!

//! ## Performance

//!

//! This crate contains multiple CRC32 implementations:

//!

//! - A fast baseline implementation which processes up to 16 bytes per iteration

//! - An optimized implementation for modern `x86` using `sse` and `pclmulqdq` instructions

//!

//! Calling the [`Hasher::new`] constructor at runtime will perform a feature detection to select the most

//! optimal implementation for the current CPU feature set.

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

#![deny(missing_docs)]

use core::fmt;

use core::hash;

mod baseline;

mod combine;

mod specialized;

mod table;

/// Computes the CRC32 hash of a byte slice.

///

/// Check out [`Hasher`] for more advanced use-cases.

pub fn hash(buf: &[u8]) -> u32 {

    let mut h = Hasher::new();

    h.update(buf);

    h.finalize()

}

#[derive(Clone)]

enum State {

    Baseline(baseline::State),

    Specialized(specialized::State),

}

#[derive(Clone)]

/// Represents an in-progress CRC32 computation.

pub struct Hasher {

    amount: u64,

    state: State,

}

const DEFAULT_INIT_STATE: u32 = 0;

impl Hasher {

    /// Create a new `Hasher`.

    ///

    /// This will perform a CPU feature detection at runtime to select the most

    /// optimal implementation for the current processor architecture.

    pub fn new() -> Self {

        Self::new_with_initial(DEFAULT_INIT_STATE)

    }

    /// Create a new `Hasher` with an initial CRC32 state.

    ///

    /// This works just like `Hasher::new`, except that it allows for an initial

    /// CRC32 state to be passed in.

    pub fn new_with_initial(init: u32) -> Self {

        Self::new_with_initial_len(init, 0)

    }

    /// Create a new `Hasher` with an initial CRC32 state.

    ///

    /// As `new_with_initial`, but also accepts a length (in bytes). The

    /// resulting object can then be used with `combine` to compute `crc(a ||

    /// b)` from `crc(a)`, `crc(b)`, and `len(b)`.

    pub fn new_with_initial_len(init: u32, amount: u64) -> Self {

        Self::internal_new_specialized(init, amount)

            .unwrap_or_else(|| Self::internal_new_baseline(init, amount))

    }

    #[doc(hidden)]

    // Internal-only API. Don't use.

    pub fn internal_new_baseline(init: u32, amount: u64) -> Self {

        Hasher {

            amount,

            state: State::Baseline(baseline::State::new(init)),

        }

    }

    #[doc(hidden)]

    // Internal-only API. Don't use.

    pub fn internal_new_specialized(init: u32, amount: u64) -> Option<Self> {

        {

            if let Some(state) = specialized::State::new(init) {

                return Some(Hasher {

                    amount,

                    state: State::Specialized(state),

                });

            }

        }

        None

    }

    /// Process the given byte slice and update the hash state.

    pub fn update(&mut self, buf: &[u8]) {

        self.amount += buf.len() as u64;

        match self.state {

            State::Baseline(ref mut state) => state.update(buf),

            State::Specialized(ref mut state) => state.update(buf),

        }

    }

    /// Finalize the hash state and return the computed CRC32 value.

    pub fn finalize(self) -> u32 {

        match self.state {

            State::Baseline(state) => state.finalize(),

            State::Specialized(state) => state.finalize(),

        }

    }

    /// Reset the hash state.

    pub fn reset(&mut self) {

        self.amount = 0;

        match self.state {

            State::Baseline(ref mut state) => state.reset(),

            State::Specialized(ref mut state) => state.reset(),

        }

    }

    /// Combine the hash state with the hash state for the subsequent block of bytes.

    pub fn combine(&mut self, other: &Self) {

        self.amount += other.amount;

        let other_crc = other.clone().finalize();

        match self.state {

            State::Baseline(ref mut state) => state.combine(other_crc, other.amount),

            State::Specialized(ref mut state) => state.combine(other_crc, other.amount),

        }

    }

}

impl fmt::Debug for Hasher {

    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

        f.debug_struct("crc32fast::Hasher").finish()

    }

}

impl Default for Hasher {

    fn default() -> Self {

        Self::new()

    }

}

impl hash::Hasher for Hasher {

    fn write(&mut self, bytes: &[u8]) {

        self.update(bytes)

    }

    fn finish(&self) -> u64 {

        u64::from(self.clone().finalize())

    }

}

#[cfg(test)]

mod test {

    use super::Hasher;

    quickcheck::quickcheck! {

        fn combine(bytes_1: Vec<u8>, bytes_2: Vec<u8>) -> bool {

            let mut hash_a = Hasher::new();

            hash_a.update(&bytes_1);

            hash_a.update(&bytes_2);

            let mut hash_b = Hasher::new();

            hash_b.update(&bytes_2);

            let mut hash_c = Hasher::new();

            hash_c.update(&bytes_1);

            hash_c.combine(&hash_b);

            hash_a.finalize() == hash_c.finalize()

        }

        fn combine_from_len(bytes_1: Vec<u8>, bytes_2: Vec<u8>) -> bool {

            let mut hash_a = Hasher::new();

            hash_a.update(&bytes_1);

            let mut hash_b = Hasher::new();

            hash_b.update(&bytes_2);

            let mut hash_ab = Hasher::new();

            hash_ab.update(&bytes_1);

            hash_ab.update(&bytes_2);

            let ab = hash_ab.finalize();

            hash_a.combine(&hash_b);

            hash_a.finalize() == ab

        }

    }

}