Added table-driven inverse for GF(2^8)

build.rs

@@ -0,0 +1,29 @@
+use std::path::Path;
+use crate::gf256::GF256;
+
+#[path="src/gf.rs"]
+mod gf;
+#[path="src/gf256.rs"]
+mod gf256;
+
+fn write_tbl(path: impl AsRef<Path>, name: &str, tbl: &[u8; 256]) {
+    let mut res = format!("const INV_TBL: [{name}; {tbl_len}] = [\n", tbl_len=tbl.len());
+    for v in tbl {
+        res += &format!("\t{name}({v}),\n");
+    }
+    res += "];\n";
+
+    std::fs::write(path, res).unwrap();
+}
+
+pub fn main() {
+    let mut gf256_inv_tbl = [0;256];
+    for i in 0..256 {
+        gf256_inv_tbl[i] = u8::from(GF256::from(i as u8).minv_slow());
+    }
+    let mut out_file = std::path::PathBuf::from(std::env::var_os("OUT_DIR").unwrap());
+    out_file.push("gf256_inv.rs");
+    write_tbl(&out_file, "GF256", &gf256_inv_tbl);
+
+    println!("cargo::rustc-cfg=rssss_have_codegen")
+}