Add fn from_radix() for radix > 36.

bigint/src/bigint.rs

@@ -1011,7 +1011,7 @@ impl BigInt {
     }
 
     /// Returns the integer formatted as a string in the given radix.
-    /// `radix` must be in the range `[2, 36]`.
+    /// `radix` must be in the range `2...36`.
     ///
     /// # Examples
     ///
@@ -1037,7 +1037,7 @@ impl BigInt {
     /// number. Conversion to an alphabet has to be performed afterwards.
     /// In contrast to the usual arabic style of written numbers as returned by
     /// `to_str_radix`, the most significant digit comes last.
-    /// `radix` must be in the range `[2, 256]`.
+    /// `radix` must be in the range `2...256`.
     ///
     /// # Examples
     ///

bigint/src/biguint.rs

@@ -1023,7 +1023,7 @@ impl BigUint {
     }
 
     /// Returns the integer formatted as a string in the given radix.
-    /// `radix` must be in the range `[2, 36]`.
+    /// `radix` must be in the range `2...36`.
     ///
     /// # Examples
     ///
@@ -1041,10 +1041,10 @@ impl BigUint {
     }
 
     /// Returns the integer in a given base. Each digit is given as an u8
-    /// number. Conversion to an alphabet has to be performed afterwards.
-    /// In contrast to the usual arabic style of written numbers as returned by
+    /// number. (The output is not given in a human readable alphabet.)
+    /// In contrast to the usual arabic ordering of written digits as returned by
     /// `to_str_radix`, the most significant digit comes last.
-    /// `radix` must be in the range `[2, 256]`.
+    /// `radix` must be in the range `2...256`.
     ///
     /// # Examples
     ///
@@ -1060,7 +1060,9 @@ impl BigUint {
         to_radix_reversed(self, radix)
     }
 
-    /// Creates and initializes a `BigUint`.
+    /// Creates and initializes a `BigUint`. The input slice must contrain
+    /// ascii/utf8 characters in [0-9a-zA-Z].
+    /// `radix` must be in the range `2...36`.
     ///
     /// # Examples
     ///
@@ -1076,6 +1078,45 @@ impl BigUint {
         str::from_utf8(buf).ok().and_then(|s| BigUint::from_str_radix(s, radix).ok())
     }
 
+    /// Creates and initializes a `BigUint`. Each u8 of the input slice is
+    /// interpreted as one digit of the number and must therefore be less than `radix`.
+    /// In contrast to the usual arabic ordering of written digits as required by
+    /// `from_str_radix`, the most significant digit comes last.
+    /// `radix` must be in the range `2...256`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use num_bigint::{BigUint};
+    ///
+    /// let inbase190 = &[15, 33, 125, 12, 14];
+    /// let a = BigUint::from_radix(inbase190, 190).unwrap();
+    /// assert_eq!(a.to_radix(190), inbase190);
+    /// ```
+    pub fn from_radix(buf: &[u8], radix: u32) -> Option<BigUint> {
+        assert!(2 <= radix && radix <= 256, "The radix must be within 2...256");
+
+        if radix != 256 && buf.iter().any(|&b| b >= radix as u8) {
+            return None;
+        }
+
+        let res = if radix.is_power_of_two() {
+            // Powers of two can use bitwise masks and shifting instead of multiplication
+            let bits = ilog2(radix);
+            if big_digit::BITS % bits == 0 {
+                from_bitwise_digits_le(buf, bits)
+            } else {
+                from_inexact_bitwise_digits_le(buf, bits)
+            }
+        } else {
+            let mut v = Vec::from(buf);
+            v.reverse();
+            from_radix_digits_be(&v, radix)
+        };
+
+        Some(res)
+    }
+
     /// Determines the fewest bits necessary to express the `BigUint`.
     #[inline]
     pub fn bits(&self) -> usize {

bigint/src/tests/biguint.rs

@@ -1021,8 +1021,8 @@ fn test_to_str_radix() {
 }
 
 #[test]
-fn test_to_radix() {
-    const GROUND_TRUTH : &[(&[u8], u32, &[u8])] = &[
+fn test_from_and_to_radix() {
+    const GROUND_TRUTH : &'static[(&'static[u8], u32, &'static[u8])] = &[
         (b"0",          42, &[0]),
         (b"ffffeeffbb", 2, &[1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1,
                              1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1,
@@ -1286,10 +1286,15 @@ fn test_to_radix() {
         (b"ffffeeffbb", 256, &[187, 255, 238, 255, 255]),
     ];
 
-    for &(bigint, radix, solution) in GROUND_TRUTH.iter() {
+    for &(bigint, radix, inbaseradix) in GROUND_TRUTH.iter() {
         let bigint = BigUint::parse_bytes(bigint, 16).unwrap();
-        assert_eq!(bigint.to_radix(radix), solution);
+        // to_radix
+        assert_eq!(bigint.to_radix(radix), inbaseradix);
+        // from_radix
+        assert_eq!(BigUint::from_radix(inbaseradix, radix).unwrap(), bigint);
     }
+
+    assert!(BigUint::from_radix(&[10,100,10], 50).is_none());
 }
 
 #[test]