Just another update

Task/Gray-code/D/gray-code-1.d

@@ -1,8 +1,8 @@
-uint grayEncode(in uint n) pure nothrow {
+uint grayEncode(in uint n) pure nothrow @nogc {
     return n ^ (n >> 1);
 }
 
-uint grayDecode(uint n) pure nothrow {
+uint grayDecode(uint n) pure nothrow @nogc {
     auto p = n;
     while (n >>= 1)
         p ^= n;

Task/Gray-code/D/gray-code-2.d

@@ -1,35 +1,36 @@
-import std.stdio, std.conv, std.algorithm;
+import std.stdio, std.algorithm;
 
-T[] gray(int N : 1, T)() { return [to!T(0), 1]; }
+T[] gray(int N : 1, T)() pure nothrow {
+    return [T(0), 1];
+}
 
-/// recursively generate gray encoding mapping table
-T[] gray(int N, T)() pure nothrow {
-    assert(N <= T.sizeof * 8, "N exceed number of bit of T");
-    enum T M = to!T(2) ^^ (N - 1);
+/// Recursively generate gray encoding mapping table.
+T[] gray(int N, T)() pure nothrow if (N <= T.sizeof * 8) {
+    enum T M = T(2) ^^ (N - 1);
     T[] g = gray!(N - 1, T)();
-    foreach (i; 0 .. M)
+    foreach (immutable i; 0 .. M)
         g ~= M + g[M - i - 1];
     return g;
 }
 
-T[][] grayDict(int N, T)() {
+T[][] grayDict(int N, T)() pure nothrow {
     T[][] dict = [gray!(N, T)(), [0]];
-    // append inversed gray encoding mapping
-    foreach (i; 1 .. dict[0].length)
+    // Append inversed gray encoding mapping.
+    foreach (immutable i; 1 .. dict[0].length)
         dict[1] ~= countUntil(dict[0], i);
     return dict;
 }
 
-enum M { Encode = 0, Decode = 1 };
+enum M { Encode = 0, Decode = 1 }
 
-T gray(int N, T)(in T n, in int mode=M.Encode) {
-    // generated at compile time
+T gray(int N, T)(in T n, in int mode=M.Encode) pure nothrow {
+    // Generated at compile time.
     enum dict = grayDict!(N, T)();
     return dict[mode][n];
 }
 
 void main() {
-    foreach (i; 0 .. 32) {
+    foreach (immutable i; 0 .. 32) {
         immutable encoded = gray!(5)(i, M.Encode);
         immutable decoded = gray!(5)(encoded, M.Decode);
         writefln("%2d: %5b => %5b : %2d", i, i, encoded, decoded);