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// Copyright (c) 2020 Stu Small
//
// Licensed under the Apache License, Version 2.0
// <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT
// license <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. All files in the project carrying such notice may not be copied,
// modified, or distributed except according to those terms.

use std::io::{Read, Result};

const C1: u32 = 0x85eb_ca6b;
const C2: u32 = 0xc2b2_ae35;
const R1: u32 = 16;
const R2: u32 = 13;
const M: u32 = 5;
const N: u32 = 0xe654_6b64;

/// Use the 32 bit variant of murmur3 to hash some [Read] implementation.
///
/// # Example
/// ```
/// use std::io::Cursor;
/// use murmur3::murmur3_32;
/// let hash_result = murmur3_32(&mut Cursor::new("hello world"), 0);
/// ```
pub fn murmur3_32<T: Read>(source: &mut T, seed: u32) -> Result<u32> {
    let mut buffer: [u8; 4] = [0; 4];
    let mut processed = 0;
    let mut state = seed;
    loop {
        match source.read(&mut buffer)? {
            4 => {
                processed += 4;
                let k = u32::from_le_bytes(buffer);
                state ^= calc_k(k);
                state = state.rotate_left(R2);
                state = (state.wrapping_mul(M)).wrapping_add(N);
            }
            3 => {
                processed += 3;
                let k: u32 = ((buffer[2] as u32) << 16) | ((buffer[1] as u32) << 8) | (buffer[0] as u32);
                state ^= calc_k(k);
            }
            2 => {
                processed += 2;
                let k: u32 = ((buffer[1] as u32) << 8) | (buffer[0] as u32);
                state ^= calc_k(k);
            }
            1 => {
                processed += 1;
                let k: u32 = buffer[0] as u32;
                state ^= calc_k(k);
            }
            0 => return Ok(finish(state, processed)),
            _ => panic!("Internal buffer state failure"),
        }
    }
}

fn finish(state: u32, processed: u32) -> u32 {
    let mut hash = state;
    hash ^= processed as u32;
    hash ^= hash.wrapping_shr(R1);
    hash = hash.wrapping_mul(C1);
    hash ^= hash.wrapping_shr(R2);
    hash = hash.wrapping_mul(C2);
    hash ^= hash.wrapping_shr(R1);
    hash
}

fn calc_k(k: u32) -> u32 {
    const C1: u32 = 0xcc9e_2d51;
    const C2: u32 = 0x1b87_3593;
    const R1: u32 = 15;
    k.wrapping_mul(C1).rotate_left(R1).wrapping_mul(C2)
}