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/target
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/Cargo.lock
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*.swp
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[package]
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name = "const-crc32"
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version = "1.0.0"
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edition = "2021"
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authors = ["Jonathan Strong <jstrong@shipyard.rs>"]
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license = "MIT"
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description = "A `const fn` implementation of crc32 checksum algorithm"
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repository = "https://git.shipyard.rs/jstrong/const-crc32"
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keywords = ["checksum", "crc", "crc32", "const"]
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[dev-dependencies]
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crc32fast = "1.2"
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rand = "0.8"
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MIT License
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Copyright (c) 2022 Jonathan Strong
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Permission is hereby granted, free of charge, to any person obtaining a copy
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of this software and associated documentation files (the "Software"), to deal
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in the Software without restriction, including without limitation the rights
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to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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copies of the Software, and to permit persons to whom the Software is
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furnished to do so, subject to the following conditions:
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The above copyright notice and this permission notice shall be included in all
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copies or substantial portions of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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SOFTWARE.
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//! A `const fn` crc32 checksum implementation.
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//!
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//! # Examples
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//!
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//! ```
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//! const BYTES: &[u8] = "The quick brown fox jumps over the lazy dog".as_bytes();
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//! assert_eq!(const_crc32::crc32(BYTES), 0x414fa339_u32);
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//! ```
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/// typically crc32 implementations set up a [u32; 256] lookup table. this computes
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/// the table on demand for a given "index" `i`
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const fn table_fn(i: u32) -> u32 {
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let mut out = i;
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out = if out & 1 == 1 { 0xedb88320 ^ (out >> 1) } else { out >> 1 };
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out = if out & 1 == 1 { 0xedb88320 ^ (out >> 1) } else { out >> 1 };
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out = if out & 1 == 1 { 0xedb88320 ^ (out >> 1) } else { out >> 1 };
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out = if out & 1 == 1 { 0xedb88320 ^ (out >> 1) } else { out >> 1 };
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out = if out & 1 == 1 { 0xedb88320 ^ (out >> 1) } else { out >> 1 };
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out = if out & 1 == 1 { 0xedb88320 ^ (out >> 1) } else { out >> 1 };
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out = if out & 1 == 1 { 0xedb88320 ^ (out >> 1) } else { out >> 1 };
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out = if out & 1 == 1 { 0xedb88320 ^ (out >> 1) } else { out >> 1 };
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out
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}
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/// A `const fn` crc32 checksum implementation.
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///
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/// Note: this is a naive implementation that should be expected to have poor performance
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/// if used on dynamic data at runtime. Usage should generally be restricted to declaring
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/// `const` variables based on `static` or `const` data available at build time.
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pub const fn crc32(buf: &[u8]) -> u32 {
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let mut out = !0u32;
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let mut i = 0;
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loop {
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if i >= buf.len() { break }
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out = (out >> 8) ^ table_fn((out & 0xff) ^ (buf[i] as u32));
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i += 1;
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}
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!out
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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use rand::prelude::*;
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fn crc32_compute_table() -> [u32; 256] {
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let mut crc32_table = [0; 256];
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for n in 0..256 {
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crc32_table[n as usize] = (0..8).fold(n as u32, |acc, _| {
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match acc & 1 {
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1 => 0xedb88320 ^ (acc >> 1),
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_ => acc >> 1,
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}
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});
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}
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crc32_table
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}
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#[test]
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fn check_table_fn_against_example_code() {
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let table = crc32_compute_table();
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for i in 0..256{
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assert_eq!(table[i], table_fn(i as u32));
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}
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}
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#[test]
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fn simple_test() {
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const BYTES: &[u8] = "The quick brown fox jumps over the lazy dog".as_bytes();
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assert_eq!(crc32(BYTES), 0x414fa339_u32);
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assert_eq!(crc32(BYTES), crc32fast::hash(BYTES));
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}
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#[test]
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fn check_random_inputs_against_crc32_fast() {
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const N_ITER: usize = 100;
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const BUFSIZE: usize = 4096;
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let mut buf = [0u8; BUFSIZE];
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let mut rng = thread_rng();
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for _ in 0..N_ITER {
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rng.fill(&mut buf[..]);
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assert_eq!(crc32(&buf[..]), crc32fast::hash(&buf[..]));
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}
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}
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}
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