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179 lines
5.3 KiB
179 lines
5.3 KiB
//! 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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//! const CKSUM: u32 = const_crc32::crc32(BYTES); |
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//! assert_eq!(CKSUM, 0x414fa339_u32); |
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//! ``` |
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/// used to generate up a [u32; 256] lookup table in `crc32`. this computes |
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/// the table on demand for a given "index" `i` |
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#[rustfmt::skip] |
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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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const fn get_table() -> [u32; 256] { |
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let mut table: [u32; 256] = [0u32; 256]; |
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let mut i = 0; |
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while i < 256 { |
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table[i] = table_fn(i as u32); |
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i += 1; |
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} |
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table |
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} |
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const TABLE: [u32; 256] = get_table(); |
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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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crc32_seed(buf, 0) |
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} |
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/// Calculate crc32 checksum, using provided `seed` as the initial state, instead of the |
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/// default initial state of `0u32`. |
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/// |
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/// # Examples |
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/// |
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/// Calculating the checksum from several parts of a larger input: |
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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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/// |
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/// let mut cksum = 0u32; |
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/// |
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/// cksum = const_crc32::crc32_seed(&BYTES[0..10], cksum); |
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/// cksum = const_crc32::crc32_seed(&BYTES[10..15], cksum); |
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/// cksum = const_crc32::crc32_seed(&BYTES[15..], cksum); |
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/// |
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/// assert_eq!(cksum, const_crc32::crc32(BYTES)); |
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/// ``` |
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/// |
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/// Using separate seeds for different kinds of data, to produce different checksums depending |
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/// on what kind of data the bytes represent: |
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/// |
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/// ``` |
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/// const THING_ONE_SEED: u32 = 0xbaaaaaad_u32; |
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/// const THING_TWO_SEED: u32 = 0x2bad2bad_u32; |
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/// |
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/// let thing_one_bytes = "bump! thump!".as_bytes(); |
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/// let thing_two_bytes = "thump! bump!".as_bytes(); |
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/// |
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/// let thing_one_cksum = const_crc32::crc32_seed(thing_one_bytes, THING_ONE_SEED); |
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/// let thing_two_cksum = const_crc32::crc32_seed(thing_two_bytes, THING_TWO_SEED); |
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/// |
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/// assert_ne!(thing_one_cksum, thing_two_cksum); |
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/// ``` |
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#[inline] |
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pub const fn crc32_seed(buf: &[u8], seed: u32) -> u32 { |
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let mut out = !seed; |
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let mut i = 0usize; |
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while i < buf.len() { |
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out = (out >> 8) ^ TABLE[((out & 0xff) ^ (buf[i] as u32)) as usize]; |
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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_u32 { |
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crc32_table[n as usize] = (0..8).fold(n, |acc, _| 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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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 use_seed_to_checksum_from_partial_inputs() { |
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const BYTES: &[u8] = "The quick brown fox jumps over the lazy dog".as_bytes(); |
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let mut cksum = crc32(&BYTES[0..10]); |
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cksum = crc32_seed(&BYTES[10..], cksum); |
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assert_eq!(cksum, crc32(BYTES)); |
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} |
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#[test] |
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fn use_seed_to_checksum_from_many_chunks() { |
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let mut buf = [0u8; 1024]; |
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let mut rng = thread_rng(); |
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rng.fill(&mut buf[..]); |
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let mut cksum = 0; |
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for chunk in buf[..].chunks(7) { |
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cksum = crc32_seed(chunk, cksum); |
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} |
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assert_eq!(cksum, crc32(&buf[..])); |
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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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#[test] |
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fn check_const_eval_limit_not_reached_on_100k_data() { |
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const BYTES: &[u8] = &[42u8; 1024 * 100]; |
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const CKSUM: u32 = crc32(BYTES); |
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assert_eq!(CKSUM, crc32fast::hash(&BYTES[..])); |
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} |
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// #[test] |
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// fn check_const_eval_limit_not_reached_on_1mb_data() { |
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// const BYTES: &[u8] = &[42u8; 1024 * 1024]; |
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// const CKSUM: u32 = crc32(BYTES); |
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// assert_eq!(CKSUM, crc32fast::hash(&BYTES[..])); |
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// } |
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}
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