clang/test/SemaCXX/constexpr-turing.cpp

   1 // RUN: %clang_cc1 -verify -std=c++11 %s
   2 // expected-no-diagnostics
   3
   4 // A direct proof that constexpr is Turing-complete, once DR1454 is implemented.
   5
   6 const unsigned halt = (unsigned)-1;
   7
   8 enum Dir { L, R };
   9 struct Action {
  10   bool tape;
  11   Dir dir;
  12   unsigned next;
  13 };
  14 using State = Action[2];
  15
  16 // An infinite tape!
  17 struct Tape {
  18   constexpr Tape() : l(0), val(false), r(0) {}
  19   constexpr Tape(const Tape &old, bool write) :
  20     l(old.l), val(write), r(old.r) {}
  21   constexpr Tape(const Tape &old, Dir dir) :
  22     l(dir == L ? old.l ? old.l->l : 0 : &old),
  23     val(dir == L ? old.l ? old.l->val : false
  24                  : old.r ? old.r->val : false),
  25     r(dir == R ? old.r ? old.r->r : 0 : &old) {}
  26   const Tape *l;
  27   bool val;
  28   const Tape *r;
  29 };
  30 constexpr Tape update(const Tape &old, bool write) { return Tape(old, write); }
  31 constexpr Tape move(const Tape &old, Dir dir) { return Tape(old, dir); }
  32
  33 // Run turing machine 'tm' on tape 'tape' from state 'state'. Return number of
  34 // steps taken until halt.
  35 constexpr unsigned run(const State *tm, const Tape &tape, unsigned state) {
  36   return state == halt ? 0 :
  37          run(tm, move(update(tape, tm[state][tape.val].tape),
  38                       tm[state][tape.val].dir),
  39              tm[state][tape.val].next) + 1;
  40 }
  41
  42 // 3-state busy beaver. S(bb3) = 21.
  43 constexpr State bb3[] = {
  44   { { true, R, 1 }, { true, R, halt } },
  45   { { true, L, 1 }, { false, R, 2 } },
  46   { { true, L, 2 }, { true, L, 0 } }
  47 };
  48 static_assert(run(bb3, Tape(), 0) == 21, "");
  49
  50 // 4-state busy beaver. S(bb4) = 107.
  51 constexpr State bb4[] = {
  52   { { true, R, 1 }, { true, L, 1 } },
  53   { { true, L, 0 }, { false, L, 2 } },
  54   { { true, R, halt }, { true, L, 3 } },
  55   { { true, R, 3 }, { false, R, 0 } } };
  56 static_assert(run(bb4, Tape(), 0) == 107, "");