A-M baby

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

@@ -0,0 +1,37 @@
+import std.stdio, std.conv, std.algorithm;
+
+T[] gray(int N : 1, T)() { return [to!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);
+    T[] g = gray!(N - 1, T)();
+    foreach (i; 0 .. M)
+        g ~= M + g[M - i - 1];
+    return g;
+}
+
+T[][] grayDict(int N, T)() {
+    T[][] dict = [gray!(N, T)(), [0]];
+    // append inversed gray encoding mapping
+    foreach (i; 1 .. dict[0].length)
+        dict[1] ~= countUntil(dict[0], i);
+    return dict;
+}
+
+enum M { Encode = 0, Decode = 1 };
+
+T gray(int N, T)(in T n, in int mode=M.Encode) {
+    // generated at compile time
+    enum dict = grayDict!(N, T)();
+    return dict[mode][n];
+}
+
+void main() {
+    foreach (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);
+    }
+}