A-M baby

Task/Hailstone-sequence/D/hailstone-sequence-1.d

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+import std.stdio, std.algorithm, std.range, std.typecons;
+
+auto hailstone(int n) pure nothrow {
+  auto result = [n];
+  while (n != 1) {
+    n = n & 1 ? n*3 + 1 : n/2;
+    result ~= n;
+  }
+  return result;
+}
+
+void main() {
+  enum M = 27;
+  auto h = hailstone(M);
+  writeln("hailstone(", M, ")= ", h[0 .. 4], " ... " , h[$-4 .. $]);
+  writeln("length hailstone(", M, ")= ", h.length);
+
+  enum N = 100_000;
+  auto s = iota(1, N).map!(i => tuple(hailstone(i).length, i))();
+  auto p = reduce!max(s);
+  writeln("Longest sequence in [1,", N, "]= ",p[1]," with len ",p[0]);
+}

Task/Hailstone-sequence/D/hailstone-sequence-2.d

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+import std.stdio, std.algorithm, std.range, std.typecons;
+
+struct Hail {
+  int n;
+  bool empty() { return n == 0; }
+  int front() { return n; }
+  void popFront() { n = n == 1 ? 0 : (n & 1 ? n*3 + 1 : n/2); }
+}
+
+void main() {
+  enum M = 27;
+  auto h = array(Hail(M));
+  writeln("hailstone(", M, ")= ", h[0 .. 4], " ... " , h[$-4 .. $]);
+  writeln("length hailstone(", M, ")= ", h.length);
+
+  enum N = 100_000;
+  auto s = map!(i => tuple(walkLength(Hail(i)), i))(iota(1, N));
+  auto p = reduce!max(s);
+  writeln("Longest sequence in [1,", N, "]= ",p[1]," with len ",p[0]);
+}