bump version to v1.3.0

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "const-crc32"
-version = "1.1.0"
+version = "1.3.0"
 edition = "2021"
 authors = ["Jonathan Strong <jstrong@shipyard.rs>"]
 license = "MIT"

README.md

@@ -18,7 +18,10 @@ if used on dynamic data at runtime. Usage should generally be restricted to decl
 
 ## `#[const_eval_limit]`
 
-This crate sets `#[const_eval_limit]` to 1,000,000,000 to avoid hitting the limit when
-executing the `const fn`, which requires `#![feature(const_eval_limit)]`.
+You may need to increase the crate-wide `const_eval_limit` setting to use `const_crc32` for larger byte slices.
+
+Increating `const_eval_limit` requires the nightly-only `#![feature(const_eval_limit)]`.
+
+Previously, this crate set the limit itself, however, as of the 2022-10-30 nightly, the value set in `const_crc32` does not increase the limit for crates which use the library.
 
 Compile time for `const` data around 100k is less than 1s.

src/lib.rs

@@ -8,12 +8,9 @@
 //! assert_eq!(CKSUM, 0x414fa339_u32);
 //! ```
 
-#![feature(const_eval_limit)]
-
-#![const_eval_limit = "1000000000"]
-
 /// used to generate up a [u32; 256] lookup table in `crc32`. this computes
 /// the table on demand for a given "index" `i`
+#[rustfmt::skip]
 const fn table_fn(i: u32) -> u32 {
     let mut out = i;
 
@@ -49,7 +46,46 @@ const TABLE: [u32; 256] = get_table();
 /// if used on dynamic data at runtime. Usage should generally be restricted to declaring
 /// `const` variables based on `static` or `const` data available at build time.
 pub const fn crc32(buf: &[u8]) -> u32 {
-    let mut out = !0u32;
+    crc32_seed(buf, 0)
+}
+
+/// Calculate crc32 checksum, using provided `seed` as the initial state, instead of the
+/// default initial state of `0u32`.
+///
+/// # Examples
+///
+/// Calculating the checksum from several parts of a larger input:
+///
+/// ```
+/// const BYTES: &[u8] = "The quick brown fox jumps over the lazy dog".as_bytes();
+///
+/// let mut cksum = 0u32;
+///
+/// cksum = const_crc32::crc32_seed(&BYTES[0..10], cksum);
+/// cksum = const_crc32::crc32_seed(&BYTES[10..15], cksum);
+/// cksum = const_crc32::crc32_seed(&BYTES[15..], cksum);
+///
+/// assert_eq!(cksum, const_crc32::crc32(BYTES));
+/// ```
+///
+/// Using separate seeds for different kinds of data, to produce different checksums depending
+/// on what kind of data the bytes represent:
+///
+/// ```
+/// const THING_ONE_SEED: u32 = 0xbaaaaaad_u32;
+/// const THING_TWO_SEED: u32 = 0x2bad2bad_u32;
+///
+/// let thing_one_bytes = "bump! thump!".as_bytes();
+/// let thing_two_bytes = "thump! bump!".as_bytes();
+///
+/// let thing_one_cksum = const_crc32::crc32_seed(thing_one_bytes, THING_ONE_SEED);
+/// let thing_two_cksum = const_crc32::crc32_seed(thing_two_bytes, THING_TWO_SEED);
+///
+/// assert_ne!(thing_one_cksum, thing_two_cksum);
+/// ```
+#[inline]
+pub const fn crc32_seed(buf: &[u8], seed: u32) -> u32 {
+    let mut out = !seed;
     let mut i = 0usize;
     while i < buf.len() {
         out = (out >> 8) ^ TABLE[((out & 0xff) ^ (buf[i] as u32)) as usize];
@@ -66,12 +102,10 @@ mod tests {
     fn crc32_compute_table() -> [u32; 256] {
         let mut crc32_table = [0; 256];
 
-        for n in 0..256 {
-            crc32_table[n as usize] = (0..8).fold(n as u32, |acc, _| {
-                match acc & 1 {
-                    1 => 0xedb88320 ^ (acc >> 1),
-                    _ => acc >> 1,
-                }
+        for n in 0..256_u32 {
+            crc32_table[n as usize] = (0..8).fold(n, |acc, _| match acc & 1 {
+                1 => 0xedb88320 ^ (acc >> 1),
+                _ => acc >> 1,
             });
         }
 
@@ -81,7 +115,7 @@ mod tests {
     #[test]
     fn check_table_fn_against_example_code() {
         let table = crc32_compute_table();
-        for i in 0..256{
+        for i in 0..256 {
             assert_eq!(table[i], table_fn(i as u32));
         }
     }
@@ -93,6 +127,28 @@ mod tests {
         assert_eq!(crc32(BYTES), crc32fast::hash(BYTES));
     }
 
+    #[test]
+    fn use_seed_to_checksum_from_partial_inputs() {
+        const BYTES: &[u8] = "The quick brown fox jumps over the lazy dog".as_bytes();
+        let mut cksum = crc32(&BYTES[0..10]);
+        cksum = crc32_seed(&BYTES[10..], cksum);
+        assert_eq!(cksum, crc32(BYTES));
+    }
+
+    #[test]
+    fn use_seed_to_checksum_from_many_chunks() {
+        let mut buf = [0u8; 1024];
+        let mut rng = thread_rng();
+        rng.fill(&mut buf[..]);
+
+        let mut cksum = 0;
+        for chunk in buf[..].chunks(7) {
+            cksum = crc32_seed(chunk, cksum);
+        }
+
+        assert_eq!(cksum, crc32(&buf[..]));
+    }
+
     #[test]
     fn check_random_inputs_against_crc32_fast() {
         const N_ITER: usize = 100;