Building static Gf256 table at compile-time.

Cargo.toml

@@ -1,9 +1,11 @@
 [package]
 name = "rusty_secrets"
-version = "0.0.1"
-description = "Implementation of threshold Shamir secret sharing in the Rust programming language." 
+version = "0.0.2"
+description = "Implementation of threshold Shamir secret sharing in the Rust programming language."
 license = "GPLv3"
 readme = "README.md"
+build = "build.rs"
+
 
 [dependencies]
 getopts = "^0.2.14"

build.rs

@@ -0,0 +1,98 @@
+use std::env;
+use std::fs::File;
+use std::io::Write;
+use std::path::Path;
+use std::fmt;
+use std::num::Wrapping;
+
+const POLY: u8 = 0x1D;
+
+/// replicates the least significant bit to every other bit
+#[inline]
+fn mask(bit: u8) -> u8 {
+    (Wrapping(0u8) - Wrapping(bit & 1)).0
+}
+
+/// multiplies a polynomial with x and returns the residual
+/// of the polynomial division with POLY as divisor
+#[inline]
+fn xtimes(poly: u8) -> u8 {
+	(poly << 1) ^ (mask(poly >> 7) & POLY)
+}
+
+struct Tables {
+	exp: [u8; 256],
+	log: [u8; 256],
+	inv: [u8; 256]
+}
+
+fn generate_tables(mut file: &File) {
+    let mut tabs = Tables {
+    	exp: [0; 256],
+    	log: [0; 256],
+    	inv: [0; 256]
+    };
+
+    let mut tmp = 1;
+    for power in 0..255usize {
+    	tabs.exp[power] = tmp;
+    	tabs.log[tmp as usize] = power as u8;
+    	tmp = xtimes(tmp);
+    }
+    tabs.exp[255] = 1;
+    for x in 1..256usize {
+    	let l = tabs.log[x];
+    	let nl = if l==0 { 0 } else { 255 - l };
+    	let i = tabs.exp[nl as usize];
+    	tabs.inv[x] = i;
+    }
+
+    match write!(file, "{}", tabs) {
+        Ok(()) => {}
+        Err(_) => panic!("Could not format the table. Aborting build.")
+    };
+}
+
+fn farray(array: [u8; 256], f: &mut fmt::Formatter) -> fmt::Result {
+    for (index, value) in array.into_iter().enumerate() {
+        try!(write!(f, "{}", value));
+        if index != array.len()-1 {
+            try!(write!(f, ","));
+        }
+    }
+    Ok(())
+}
+
+impl fmt::Display for Tables {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        try!(write!(f, "Tables {{\n"));
+        try!(write!(f, "    exp: ["));
+        try!(farray(self.exp, f));
+        try!(write!(f, "],\n"));
+        try!(write!(f, "    log: ["));
+        try!(farray(self.log, f));
+        try!(write!(f, "],\n"));
+        try!(write!(f, "    inv: ["));
+        try!(farray(self.inv, f));
+        try!(write!(f, "]\n"));
+        write!(f, "}};")
+    }
+}
+
+#[allow(unused_must_use)]
+fn main() {
+    let out_dir = env::var("OUT_DIR").unwrap();
+    let dest = Path::new(&out_dir).join("nothinghardcoded.rs");
+
+    let mut f = File::create(&dest).unwrap();
+
+    write!(f, "pub struct Tables {{
+    pub exp: [u8; 256],
+    pub log: [u8; 256],
+    pub inv: [u8; 256]
+}}
+
+pub static TABLES: Tables = ");
+
+    generate_tables(&f);
+}

src/lib/gf256.rs

@@ -1,59 +1,12 @@
 //! This module provides the Gf256 type which is used to represent
 //! elements of a finite field wich 256 elements.
 
-use std::num::Wrapping;
 use std::ops::{ Add, Sub, Mul, Div };
-use std::sync::{ Once, ONCE_INIT };
 
-const POLY: u8 = 0x1D; // represents x^8 + x^4 + x^3 + x^2 + 1
-
-/// replicates the least significant bit to every other bit
-#[inline]
-fn mask(bit: u8) -> u8 {
-    (Wrapping(0u8) - Wrapping(bit & 1)).0
-}
-
-/// multiplies a polynomial with x and returns the residual
-/// of the polynomial division with POLY as divisor
-#[inline]
-fn xtimes(poly: u8) -> u8 {
-	(poly << 1) ^ (mask(poly >> 7) & POLY)
-}
-
-/// Tables used for multiplication and division
-struct Tables {
-	exp: [u8; 256],
-	log: [u8; 256],
-	inv: [u8; 256]
-}
-
-static INIT: Once = ONCE_INIT;
-static mut TABLES: Tables = Tables {
-	exp: [0; 256],
-	log: [0; 256],
-	inv: [0; 256]
-};
+include!(concat!(env!("OUT_DIR"), "/nothinghardcoded.rs"));
 
 fn get_tables() -> &'static Tables {
-	INIT.call_once(|| {
-		// mutable access is fine because of synchronization via INIT
-		let tabs = unsafe { &mut TABLES };
-		let mut tmp = 1;
-		for power in 0..255usize {
-			tabs.exp[power] = tmp;
-			tabs.log[tmp as usize] = power as u8;
-			tmp = xtimes(tmp);
-		}
-		tabs.exp[255] = 1;
-		for x in 1..256usize {
-			let l = tabs.log[x];
-			let nl = if l==0 { 0 } else { 255 - l };
-			let i = tabs.exp[nl as usize];
-			tabs.inv[x] = i;
-		}
-	});
-	// We're guaranteed to have TABLES initialized by now
-	return unsafe { &TABLES };
+	return &TABLES;
 }
 
 /// Type for elements of a finite field with 256 elements
@@ -93,11 +46,6 @@ impl Gf256 {
 		let tabs = get_tables();
 		Gf256 { poly: tabs.exp[power as usize] }
 	}
-/*
-	pub fn inv(&self) -> Option<Gf256> {
-		self.log().map(|l| Gf256::exp(255 - l))
-	}
-*/
 }
 
 impl Add<Gf256> for Gf256 {