//! This crate offers rust implementations of simple and twisted tabulation hashing for 32-bit integer values.
//!
//! Instatiating `Tab32Simple` or `Tab32Twisted` will initialize a table and
//! create a random hash function from the respective hash family.
//! The hash value of a 32-bit integer can be computed by calling its `hash` method.
//!
//! # Example:
//!
//! ```rust
//! use tab_hash::Tab32Simple;
//!
//! fn main() {
//!     let keys = vec![0, 8, 15, 47, 11];
//!     let simple = Tab32Simple::new();
//!     for k in keys {
//!         println!("{}", simple.hash(k));
//!     }
//! }
//! ```
//!
//! To reprocude hashes, save the table used by the hash function and save it.
//! The function can be recreated using the `with_table` constructor.
//!
//! ```rust
//! use tab_hash::Tab32Twisted;
//!
//! fn main() {
//!     let key = 42;
//!     let twisted_1 = Tab32Twisted::new();
//!     let twisted_2 = Tab32Twisted::with_table(twisted_1.get_table());
//!     let twisted_3 = Tab32Twisted::new();
//!     assert_eq!(twisted_1.hash(key), twisted_2.hash(key));
//!     assert_ne!(twisted_1.hash(key), twisted_3.hash(key));
//! }
//! ```
//!
//! # Note:
//! These hash functions do not implement the `std::hash::Hasher` trait,
//! since they do not work on arbitrary length byte streams.
//!
//! # Literature:
//! This implementation is based on the articles of Mihai Patrascu and Mikkel Thorup:
//! - [Simple Tabulation Hashing](http://dx.doi.org/10.1145/1993636.1993638)
//! - [Twisted Tabulation Hashing](https://doi.org/10.1137/1.9781611973105.16)
use rand;

/// Split up a 32bit number into 8bit chunks
fn byte_chunks(x: u32) -> [u8; 4] {
    [
        (x & 0x0000_00FF) as u8,
        ((x & 0x0000_FF00) >> 8) as u8,
        ((x & 0x00FF_0000) >> 16) as u8,
        ((x & 0xFF00_0000) >> 24) as u8,
    ]
}

/// A universal hash function for 32-bit integers using simple tabulation.
///
/// Usage:
/// ```rust
/// use tab_hash::Tab32Simple;
///
/// fn main() {
///     let keys = vec![0, 8, 15, 47, 11];
///     let simple = Tab32Simple::new();
///     for k in keys {
///         println!("{}", simple.hash(k));
///     }
/// }
/// ```
pub struct Tab32Simple {
    table: [[u32; 256]; 4],
}

impl Tab32Simple {
    /// Create a new simple tabulation hash function with a random table.
    pub fn new() -> Self {
        Tab32Simple {
            table: Tab32Simple::initialize_table(),
        }
    }

    /// Create a new simple tabulation hash function with a given table.
    pub fn with_table(table: [[u32; 256]; 4]) -> Self {
        Tab32Simple { table }
    }

    /// Generate a table of 32bit uints for simple tabulation hashing
    fn initialize_table() -> [[u32; 256]; 4] {
        let table: [[u32; 256]; 4] =
            array_init::array_init(|_| array_init::array_init(|_| rand::random()));
        table
    }

    /// Get the table used by this hash function.
    pub fn get_table(&self) -> [[u32; 256]; 4] {
        self.table
    }

    /// Compute simple tabulation hash value for a 32bit integer number.
    pub fn hash(&self, x: u32) -> u32 {
        let mut h: u32 = 0; // initialize hash values as 0

        for (i, c) in byte_chunks(x).iter().enumerate() {
            h ^= self.table[i as usize][*c as usize];
        }
        h
    }
}

/// A universal hash function for 32-bit integers using twisted tabulation.
///
/// Usage:
/// ```rust
/// use tab_hash::Tab32Twisted;
///
/// fn main() {
///     let keys = vec![0, 8, 15, 47, 11];
///     let twisted = Tab32Twisted::new();
///     for k in keys {
///         println!("{}", twisted.hash(k));
///     }
/// }
/// ```
pub struct Tab32Twisted {
    table: [[u64; 256]; 4],
}

impl Tab32Twisted {
    /// Create a new twisted tabulation hash function with a random table.
    pub fn new() -> Self {
        Tab32Twisted {
            table: Tab32Twisted::initialize_table(),
        }
    }

    /// Create a new twisted tabulation hash function with a given table.
    pub fn with_table(table: [[u64; 256]; 4]) -> Self {
        Tab32Twisted { table }
    }

    /// Generate a table of 64bit uints for twisted tabulation hashing
    fn initialize_table() -> [[u64; 256]; 4] {
        let table: [[u64; 256]; 4] =
            array_init::array_init(|_| array_init::array_init(|_| rand::random()));
        table
    }

    /// Get the table used by this hash function.
    pub fn get_table(&self) -> [[u64; 256]; 4] {
        self.table
    }

    /// Compute twisted tabulation hash value for a 32bit integer number.
    pub fn hash(&self, x: u32) -> u32 {
        let mut h: u64 = 0; // initialize hash values as 0
        let chunks = byte_chunks(x);
        for (i, c) in chunks[0..3].iter().enumerate() {
            h ^= self.table[i as usize][*c as usize];
        }
        // compute address for last chunk by XOring the lowest byte of the
        // current hash value with the content of the last chunk of the key
        let c = chunks[3] ^ (h & 0xFF) as u8;
        h ^= self.table[3][c as usize];
        // shift out the 32 low bits of the resulting hash
        h = h.overflowing_shr(32).0;

        h as u32
    }
}