First part of issue trifectatechfoundation#14

Add a naive implementation of CRC-32 based
on the C code from the PNG specification.

zlib-rs/Cargo.toml

@@ -18,3 +18,5 @@ libc.workspace = true
 [dev-dependencies]
 libloading = "0.8.1"
 libz-ng-sys = "1.1.12"
+crc32fast = "1.3.2"
+quickcheck = "1.0.3"

zlib-rs/src/crc32.rs

@@ -0,0 +1,46 @@
+use std::sync::Once;
+
+const LSB_POLY: u32 = 0xedb8_8320u32;
+const START_CHECKSUM: u32 = 0xffff_ffffu32;
+const FINAL_XOR: u32 = 0xffff_ffffu32;
+
+// This could be auto-generated
+static mut CRC_TABLE: [u32; 256] = [0; 256];
+static CRC_TABLE_ONCE: Once = Once::new();
+
+fn get_crc_table() -> &'static [u32] {
+    CRC_TABLE_ONCE.call_once(|| {
+        for i in 0..256u16 {
+            let mut c = u32::from(i);
+            for k in 0..8 {
+                if c & 1 != 0 {
+                    c = LSB_POLY ^ (c >> 1);
+                } else {
+                    c >>= 1;
+                }
+            }
+            unsafe { CRC_TABLE[usize::from(i)] = c; }
+        }
+    });
+    unsafe { &CRC_TABLE[..] }
+}
+
+fn naive_crc32(data: &[u8]) -> u32 {
+    let crc_table = get_crc_table();
+    let crc = data.iter().fold(START_CHECKSUM, |crc, val| {
+        let i = (crc as u8) ^ *val;
+        crc_table[usize::from(i)] ^ (crc >> 8)
+    });
+    crc ^ FINAL_XOR
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    quickcheck::quickcheck! {
+        fn naive_is_crc32fast(v: Vec<u8>) -> bool {
+            naive_crc32(&v[..]) == crc32fast::hash(&v[..])
+        }
+    }
+}

zlib-rs/src/lib.rs

@@ -1,4 +1,5 @@
 mod adler32;
+mod crc32;
 pub mod allocate;
 pub mod c_api;
 pub mod deflate;