increase #[const_eval_limit] to avoid compilation errors for larger byte slices
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@ -1,6 +1,6 @@
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[package]
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name = "const-crc32"
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version = "1.0.1"
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version = "1.1.0"
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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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@ -15,3 +15,10 @@ assert_eq!(CKSUM, 0x414fa339_u32);
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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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## `#[const_eval_limit]`
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This crate sets `#[const_eval_limit]` to 1,000,000,000 to avoid hitting the limit when
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executing the `const fn`, which requires `#![feature(const_eval_limit)]`.
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Compile time for `const` data around 100k is less than 1s.
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43
src/lib.rs
43
src/lib.rs
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@ -8,7 +8,11 @@
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//! assert_eq!(CKSUM, 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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#![feature(const_eval_limit)]
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#![const_eval_limit = "1000000000"]
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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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const fn table_fn(i: u32) -> u32 {
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let mut out = i;
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@ -25,6 +29,20 @@ const fn table_fn(i: u32) -> u32 {
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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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@ -32,12 +50,9 @@ const fn table_fn(i: u32) -> u32 {
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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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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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@ -91,4 +106,18 @@ mod tests {
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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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