A-M baby

Task/Find-the-missing-permutation/D/find-the-missing-permutation-1.d

@@ -0,0 +1,13 @@
+import std.stdio, std.string, std.algorithm, std.array, std.range;
+
+void main() {
+    const givenSet = "ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC
+        BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA BACD BADC BDAC CBDA
+        DBCA DCAB"d.split().zip(repeat(true)).assocArray();
+
+    auto p = "ABCD"d.dup;
+    do {
+        if (p !in givenSet)
+            writeln("Missing permutation: ", p);
+    } while (p.nextPermutation());
+}

Task/Find-the-missing-permutation/D/find-the-missing-permutation-2.d

@@ -0,0 +1,40 @@
+import std.stdio, std.string, std.algorithm, std.conv;
+
+void main() {
+    const perms = "ABCD CABD ACDB DACB BCDA ACBD ADCB CDAB DABC
+                   BCAD CADB CDBA CBAD ABDC ADBC BDCA DCBA BACD
+                   BADC BDAC CBDA DBCA DCAB".split();
+
+    // Version 1: XOR all the ASCII values, the uneven one gets
+    // flushed out; based on Perl 6 (via Go)
+    ubyte[4] b;
+    foreach (perm; perms)
+        foreach (i, c; perm)
+            b[i] ^= c;
+    writeln(cast(char[])b);
+
+    // Version 2 : Sum ASCII values
+    auto sumr = perms[0].reduce!q{a + b}(); // sum row
+    foreach (i; 0 .. 4) {
+        // sum columns
+        const sumc = reduce!((a,b)=> text(to!int(a) + b[i]))("0",perms);
+        // see how much it falls short
+        write(cast(char)(sumr - to!int(sumc) % sumr));
+    }
+    writeln();
+
+    // Version 3: some sort of checksum, don't ask
+    // me: translation of Java
+    enum int len = 4;
+    int maxCode = len - 1;
+    foreach_reverse (i; 3 .. len + 1)
+        maxCode *= i; // maxCode will be 36
+    foreach (i; 0 .. len) {
+        int code = 0;
+        foreach (p; perms)
+            code += perms[0].countUntil(p[i]);
+
+        // code will come up 3, 1, 0, 2 short of 36
+        write(cast(char)perms[0][maxCode - code]);
+    }
+}