generate the crc32 lookup table at const time

The real goal here is that we can figure out where the numbers came from.

libbz2-rs-sys/src/crctable.rs

@@ -1,34 +1,51 @@
-pub(crate) static BZ2_CRC32TABLE: [u32; 256] = [
-    0, 0x4c11db7, 0x9823b6e, 0xd4326d9, 0x130476dc, 0x17c56b6b, 0x1a864db2, 0x1e475005, 0x2608edb8,
-    0x22c9f00f, 0x2f8ad6d6, 0x2b4bcb61, 0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd, 0x4c11db70,
-    0x48d0c6c7, 0x4593e01e, 0x4152fda9, 0x5f15adac, 0x5bd4b01b, 0x569796c2, 0x52568b75, 0x6a1936c8,
-    0x6ed82b7f, 0x639b0da6, 0x675a1011, 0x791d4014, 0x7ddc5da3, 0x709f7b7a, 0x745e66cd, 0x9823b6e0,
-    0x9ce2ab57, 0x91a18d8e, 0x95609039, 0x8b27c03c, 0x8fe6dd8b, 0x82a5fb52, 0x8664e6e5, 0xbe2b5b58,
-    0xbaea46ef, 0xb7a96036, 0xb3687d81, 0xad2f2d84, 0xa9ee3033, 0xa4ad16ea, 0xa06c0b5d, 0xd4326d90,
-    0xd0f37027, 0xddb056fe, 0xd9714b49, 0xc7361b4c, 0xc3f706fb, 0xceb42022, 0xca753d95, 0xf23a8028,
-    0xf6fb9d9f, 0xfbb8bb46, 0xff79a6f1, 0xe13ef6f4, 0xe5ffeb43, 0xe8bccd9a, 0xec7dd02d, 0x34867077,
-    0x30476dc0, 0x3d044b19, 0x39c556ae, 0x278206ab, 0x23431b1c, 0x2e003dc5, 0x2ac12072, 0x128e9dcf,
-    0x164f8078, 0x1b0ca6a1, 0x1fcdbb16, 0x18aeb13, 0x54bf6a4, 0x808d07d, 0xcc9cdca, 0x7897ab07,
-    0x7c56b6b0, 0x71159069, 0x75d48dde, 0x6b93dddb, 0x6f52c06c, 0x6211e6b5, 0x66d0fb02, 0x5e9f46bf,
-    0x5a5e5b08, 0x571d7dd1, 0x53dc6066, 0x4d9b3063, 0x495a2dd4, 0x44190b0d, 0x40d816ba, 0xaca5c697,
-    0xa864db20, 0xa527fdf9, 0xa1e6e04e, 0xbfa1b04b, 0xbb60adfc, 0xb6238b25, 0xb2e29692, 0x8aad2b2f,
-    0x8e6c3698, 0x832f1041, 0x87ee0df6, 0x99a95df3, 0x9d684044, 0x902b669d, 0x94ea7b2a, 0xe0b41de7,
-    0xe4750050, 0xe9362689, 0xedf73b3e, 0xf3b06b3b, 0xf771768c, 0xfa325055, 0xfef34de2, 0xc6bcf05f,
-    0xc27dede8, 0xcf3ecb31, 0xcbffd686, 0xd5b88683, 0xd1799b34, 0xdc3abded, 0xd8fba05a, 0x690ce0ee,
-    0x6dcdfd59, 0x608edb80, 0x644fc637, 0x7a089632, 0x7ec98b85, 0x738aad5c, 0x774bb0eb, 0x4f040d56,
-    0x4bc510e1, 0x46863638, 0x42472b8f, 0x5c007b8a, 0x58c1663d, 0x558240e4, 0x51435d53, 0x251d3b9e,
-    0x21dc2629, 0x2c9f00f0, 0x285e1d47, 0x36194d42, 0x32d850f5, 0x3f9b762c, 0x3b5a6b9b, 0x315d626,
-    0x7d4cb91, 0xa97ed48, 0xe56f0ff, 0x1011a0fa, 0x14d0bd4d, 0x19939b94, 0x1d528623, 0xf12f560e,
-    0xf5ee4bb9, 0xf8ad6d60, 0xfc6c70d7, 0xe22b20d2, 0xe6ea3d65, 0xeba91bbc, 0xef68060b, 0xd727bbb6,
-    0xd3e6a601, 0xdea580d8, 0xda649d6f, 0xc423cd6a, 0xc0e2d0dd, 0xcda1f604, 0xc960ebb3, 0xbd3e8d7e,
-    0xb9ff90c9, 0xb4bcb610, 0xb07daba7, 0xae3afba2, 0xaafbe615, 0xa7b8c0cc, 0xa379dd7b, 0x9b3660c6,
-    0x9ff77d71, 0x92b45ba8, 0x9675461f, 0x8832161a, 0x8cf30bad, 0x81b02d74, 0x857130c3, 0x5d8a9099,
-    0x594b8d2e, 0x5408abf7, 0x50c9b640, 0x4e8ee645, 0x4a4ffbf2, 0x470cdd2b, 0x43cdc09c, 0x7b827d21,
-    0x7f436096, 0x7200464f, 0x76c15bf8, 0x68860bfd, 0x6c47164a, 0x61043093, 0x65c52d24, 0x119b4be9,
-    0x155a565e, 0x18197087, 0x1cd86d30, 0x29f3d35, 0x65e2082, 0xb1d065b, 0xfdc1bec, 0x3793a651,
-    0x3352bbe6, 0x3e119d3f, 0x3ad08088, 0x2497d08d, 0x2056cd3a, 0x2d15ebe3, 0x29d4f654, 0xc5a92679,
-    0xc1683bce, 0xcc2b1d17, 0xc8ea00a0, 0xd6ad50a5, 0xd26c4d12, 0xdf2f6bcb, 0xdbee767c, 0xe3a1cbc1,
-    0xe760d676, 0xea23f0af, 0xeee2ed18, 0xf0a5bd1d, 0xf464a0aa, 0xf9278673, 0xfde69bc4, 0x89b8fd09,
-    0x8d79e0be, 0x803ac667, 0x84fbdbd0, 0x9abc8bd5, 0x9e7d9662, 0x933eb0bb, 0x97ffad0c, 0xafb010b1,
-    0xab710d06, 0xa6322bdf, 0xa2f33668, 0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4,
-];
+/// The polynomial used for the crc32 lookup table.
+///
+/// See also https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Polynomial_representations
+const POLYNOMIAL: u32 = 0x04C11DB7;
+
+/// Most implementations (ethernet, zlib) use the reflected version of this polynomial.
+const _: () = assert!(POLYNOMIAL.reverse_bits() == 0xEDB88320);
+
+/// Lookup table to speed up crc32 checksum calculation.
+///
+/// The original C implementation notes:
+///
+/// > I think this is an implementation of the AUTODIN-II,
+/// > Ethernet & FDDI 32-bit CRC standard.  Vaguely derived
+/// > from code by Rob Warnock, in Section 51 of the
+/// > comp.compression FAQ.
+pub(crate) static BZ2_CRC32TABLE: [u32; 256] = generate_crc32_table(POLYNOMIAL);
+
+/// Generate the crc32 lookup table.
+///
+/// Note that contrary to most material you'll find on the internet, we're using the non-reflected
+/// polynomial, which impacts some of the logic (e.g. we bitwise and with 0x80000000 instead of 0x1).
+///
+/// This [article] has some excellent additional detail on how crc works, and how to make it fast.
+///
+/// [article]: https://create.stephan-brumme.com/crc32/
+const fn generate_crc32_table(polynomial: u32) -> [u32; 256] {
+    let mut table = [0u32; 256];
+
+    let mut i = 0;
+    while i < 256 {
+        let mut crc = (i as u32) << 24;
+
+        let mut j = 0;
+        while j < 8 {
+            if (crc & 0x80000000) != 0 {
+                crc = (crc << 1) ^ polynomial;
+            } else {
+                crc <<= 1;
+            }
+
+            j += 1;
+        }
+
+        table[i] = crc;
+
+        i += 1;
+    }
+
+    table
+}