proxmox-backup/src/backup/chunker.rs

//! Slinding window chunker (Buzhash)
//!
//! This is a rewrite of *casync* chunker (cachunker.h) in rust.
//!
//! Hashing by cyclic polynomial (also called Buzhash) has the benefit
//! of avoiding multiplications, using barrel shifts instead. For more
//! information please take a look at the [Rolling
//! Hash](https://en.wikipedia.org/wiki/Rolling_hash) artikel from
//! wikipedia.

use std::io::Write;

const CA_CHUNKER_WINDOW_SIZE: usize = 48;

pub struct Chunker {
    h: u32,
    window_size: usize,
    chunk_size: usize,

    chunk_size_min: usize,
    chunk_size_max: usize,
    chunk_size_avg: usize,

    discriminator: u32,

    window: [u8; CA_CHUNKER_WINDOW_SIZE],
}

const BUZHASH_TABLE: [u32; 256] = [
    0x458be752, 0xc10748cc, 0xfbbcdbb8, 0x6ded5b68,
    0xb10a82b5, 0x20d75648, 0xdfc5665f, 0xa8428801,
    0x7ebf5191, 0x841135c7, 0x65cc53b3, 0x280a597c,
    0x16f60255, 0xc78cbc3e, 0x294415f5, 0xb938d494,
    0xec85c4e6, 0xb7d33edc, 0xe549b544, 0xfdeda5aa,
    0x882bf287, 0x3116737c, 0x05569956, 0xe8cc1f68,
    0x0806ac5e, 0x22a14443, 0x15297e10, 0x50d090e7,
    0x4ba60f6f, 0xefd9f1a7, 0x5c5c885c, 0x82482f93,
    0x9bfd7c64, 0x0b3e7276, 0xf2688e77, 0x8fad8abc,
    0xb0509568, 0xf1ada29f, 0xa53efdfe, 0xcb2b1d00,
    0xf2a9e986, 0x6463432b, 0x95094051, 0x5a223ad2,
    0x9be8401b, 0x61e579cb, 0x1a556a14, 0x5840fdc2,
    0x9261ddf6, 0xcde002bb, 0x52432bb0, 0xbf17373e,
    0x7b7c222f, 0x2955ed16, 0x9f10ca59, 0xe840c4c9,
    0xccabd806, 0x14543f34, 0x1462417a, 0x0d4a1f9c,
    0x087ed925, 0xd7f8f24c, 0x7338c425, 0xcf86c8f5,
    0xb19165cd, 0x9891c393, 0x325384ac, 0x0308459d,
    0x86141d7e, 0xc922116a, 0xe2ffa6b6, 0x53f52aed,
    0x2cd86197, 0xf5b9f498, 0xbf319c8f, 0xe0411fae,
    0x977eb18c, 0xd8770976, 0x9833466a, 0xc674df7f,
    0x8c297d45, 0x8ca48d26, 0xc49ed8e2, 0x7344f874,
    0x556f79c7, 0x6b25eaed, 0xa03e2b42, 0xf68f66a4,
    0x8e8b09a2, 0xf2e0e62a, 0x0d3a9806, 0x9729e493,
    0x8c72b0fc, 0x160b94f6, 0x450e4d3d, 0x7a320e85,
    0xbef8f0e1, 0x21d73653, 0x4e3d977a, 0x1e7b3929,
    0x1cc6c719, 0xbe478d53, 0x8d752809, 0xe6d8c2c6,
    0x275f0892, 0xc8acc273, 0x4cc21580, 0xecc4a617,
    0xf5f7be70, 0xe795248a, 0x375a2fe9, 0x425570b6,
    0x8898dcf8, 0xdc2d97c4, 0x0106114b, 0x364dc22f,
    0x1e0cad1f, 0xbe63803c, 0x5f69fac2, 0x4d5afa6f,
    0x1bc0dfb5, 0xfb273589, 0x0ea47f7b, 0x3c1c2b50,
    0x21b2a932, 0x6b1223fd, 0x2fe706a8, 0xf9bd6ce2,
    0xa268e64e, 0xe987f486, 0x3eacf563, 0x1ca2018c,
    0x65e18228, 0x2207360a, 0x57cf1715, 0x34c37d2b,
    0x1f8f3cde, 0x93b657cf, 0x31a019fd, 0xe69eb729,
    0x8bca7b9b, 0x4c9d5bed, 0x277ebeaf, 0xe0d8f8ae,
    0xd150821c, 0x31381871, 0xafc3f1b0, 0x927db328,
    0xe95effac, 0x305a47bd, 0x426ba35b, 0x1233af3f,
    0x686a5b83, 0x50e072e5, 0xd9d3bb2a, 0x8befc475,
    0x487f0de6, 0xc88dff89, 0xbd664d5e, 0x971b5d18,
    0x63b14847, 0xd7d3c1ce, 0x7f583cf3, 0x72cbcb09,
    0xc0d0a81c, 0x7fa3429b, 0xe9158a1b, 0x225ea19a,
    0xd8ca9ea3, 0xc763b282, 0xbb0c6341, 0x020b8293,
    0xd4cd299d, 0x58cfa7f8, 0x91b4ee53, 0x37e4d140,
    0x95ec764c, 0x30f76b06, 0x5ee68d24, 0x679c8661,
    0xa41979c2, 0xf2b61284, 0x4fac1475, 0x0adb49f9,
    0x19727a23, 0x15a7e374, 0xc43a18d5, 0x3fb1aa73,
    0x342fc615, 0x924c0793, 0xbee2d7f0, 0x8a279de9,
    0x4aa2d70c, 0xe24dd37f, 0xbe862c0b, 0x177c22c2,
    0x5388e5ee, 0xcd8a7510, 0xf901b4fd, 0xdbc13dbc,
    0x6c0bae5b, 0x64efe8c7, 0x48b02079, 0x80331a49,
    0xca3d8ae6, 0xf3546190, 0xfed7108b, 0xc49b941b,
    0x32baf4a9, 0xeb833a4a, 0x88a3f1a5, 0x3a91ce0a,
    0x3cc27da1, 0x7112e684, 0x4a3096b1, 0x3794574c,
    0xa3c8b6f3, 0x1d213941, 0x6e0a2e00, 0x233479f1,
    0x0f4cd82f, 0x6093edd2, 0x5d7d209e, 0x464fe319,
    0xd4dcac9e, 0x0db845cb, 0xfb5e4bc3, 0xe0256ce1,
    0x09fb4ed1, 0x0914be1e, 0xa5bdb2c3, 0xc6eb57bb,
    0x30320350, 0x3f397e91, 0xa67791bc, 0x86bc0e2c,
    0xefa0a7e2, 0xe9ff7543, 0xe733612c, 0xd185897b,
    0x329e5388, 0x91dd236b, 0x2ecb0d93, 0xf4d82a3d,
    0x35b5c03f, 0xe4e606f0, 0x05b21843, 0x37b45964,
    0x5eff22f4, 0x6027f4cc, 0x77178b3c, 0xae507131,
    0x7bf7cabc, 0xf9c18d66, 0x593ade65, 0xd95ddf11,
];

impl Chunker {

    /// Create a new Chunker instance, which produces and average chunk
    /// size of `chunk_size_avg`. We allow variation from
    /// `chunk_size_avg/4` up to a maximum of `chunk_size_avg*4`.
    pub fn new(chunk_size_avg: usize) -> Self {
        // The chunk cut discriminator. In order to get an average
        // chunk size of avg, we cut whenever for a hash value "h" at
        // byte "i" given the descriminator "d(avg)": h(i) mod d(avg)
        // == d(avg) - 1. Note that the discriminator calculated like
        // this only yields correct results as long as the minimal
        // chunk size is picked as avg/4, and the maximum chunk size
        // as avg*4. If they are picked differently the result might
        // be skewed into either direction.
        let avg = chunk_size_avg as f64;
        let discriminator = (avg / (-1.42888852e-7 * avg + 1.33237515)) as u32;

        Self {
            h: 0,
            window_size: 0,
            chunk_size: 0,
            chunk_size_min: chunk_size_avg>>2,
            chunk_size_max: chunk_size_avg<<2,
            chunk_size_avg: chunk_size_avg,
            discriminator:  discriminator,
            window: [0u8; CA_CHUNKER_WINDOW_SIZE],
        }
    }

    /// Scans the specified data for a chunk border. Returns 0 if none
    /// was found (and the function should be called with more data
    /// later on), or another value indicating the position of a
    /// border.
    pub fn scan(&mut self, data: &[u8]) -> usize {

        let window_len = self.window.len();
        let data_len = data.len();

        let mut pos = 0;

        if self.window_size <  window_len {
            let need =  window_len - self.window_size;
            let copy_len = if need < data_len { need } else { data_len };
            unsafe {
                let src_ptr = data.as_ptr().add(pos);
                let dest_ptr = self.window.as_mut_ptr().add(self.window_size);
                std::ptr::copy_nonoverlapping(src_ptr, dest_ptr, copy_len);
            }
            pos += copy_len;
            self.window_size += copy_len;
            self.chunk_size += copy_len;

            // return if window is still not full
            if self.window_size <  window_len {
                return 0;
            }

            self.start();
        }

        let idx = self.chunk_size % CA_CHUNKER_WINDOW_SIZE;

        while pos < data_len {
            // roll window
            let enter = data[pos];
            let leave = self.window[idx];
            self.h = self.h.rotate_left(1) ^
                BUZHASH_TABLE[leave as usize].rotate_left(CA_CHUNKER_WINDOW_SIZE as u32) ^
                BUZHASH_TABLE[enter as usize];

            self.chunk_size += 1;

            pos += 1;
            if self.shall_break() {
                self.h = 0;
                self.chunk_size = 0;
                self.window_size = 0;
                return pos;
            }
        }

        0
    }

    #[inline(always)]
    fn shall_break(&self) -> bool {

        if self.chunk_size >= self.chunk_size_max { return true; }

        if self.chunk_size < self.chunk_size_min { return false; }

        (self.h % self.discriminator) == (self.discriminator - 1)
    }

    fn start(&mut self) {

        let window_len = self.window.len();

        for i in 0..window_len-1 {
            let byte = self.window[i];
            self.h ^= BUZHASH_TABLE[(byte as usize)].rotate_left((window_len - (i + 1)) as u32);
        }

        let byte = self.window[window_len-1];
        self.h ^= BUZHASH_TABLE[(byte as usize)];
    }
}