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Copyright (c) 2018-2019 Drew DeVault

Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
of the Software, and to permit persons to whom the Software is furnished to do
so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
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//: Continuations are a powerful primitive for constructing advanced kinds of
//: control *policies* like back-tracking.
//:
//: In Mu, continuations are first-class and delimited, and are constructed
//: out of two primitives:
//:
//:  * 'call-with-continuation-mark' marks the top of the call stack and then
//:    calls the provided function.
//:  * 'return-continuation-until-mark' copies the top of the stack
//:    until the mark, and returns it as the result of
//:    call-with-continuation-mark (which might be a distant ancestor on the call
//:    stack; intervening calls don't return)
//:
//: The resulting slice of the stack can now be called just like a regular
//: function.

:(before "End Mu Types Initialization")
type_ordinal continuation = Type_ordinal["continuation"] = Next_type_ordinal++;
Type[continuation].name = "continuation";

//: A continuation can be called like a recipe.
:(before "End is_mu_recipe Atom Cases(r)")
if (r.type->name == "continuation") return true;

//: However, it can't be type-checked like most recipes. Pretend it's like a
//: header-less recipe.
:(after "Begin Reagent->Recipe(r, recipe_header)")
if (r.type->atom && r.type->name == "continuation") {
  result_header.has_header = false;
  return result_header;
}

:(scenario delimited_continuation)
recipe main [
  1:continuation <- call-with-continuation-mark f, 77  # 77 is an argument to f
  2:number <- copy 5
  {
    2:number <- call 1:continuation, 2:number  # 2 is an argument to g, the 'top' of the continuation
    3:boolean <- greater-or-equal 2:number, 8
    break-if 3:boolean
    loop
  }
]
recipe f [
  11:number <- next-ingredient
  12:number <- g 11:number
  return 12:number
]
recipe g [
  21:number <- next-ingredient
  rewind-ingredients
  return-continuation-until-mark
  # calls of the continuation start from here
  22:number <- next-ingredient
  23:number <- add 22:number, 1
  return 23:number
]
# first call of 'g' executes the part before return-continuation-until-mark
+mem: storing 77 in location 21
+run: {2: "number"} <- copy {5: "literal"}
+mem: storing 5 in location 2
# calls of the continuation execute the part after return-continuation-until-mark
+run: {2: "number"} <- call {1: "continuation"}, {2: "number"}
+mem: storing 5 in location 22
+mem: storing 6 in location 2
+run: {2: "number"} <- call {1: "continuation"}, {2: "number"}
+mem: storing 6 in location 22
+mem: storing 7 in location 2
+run: {2: "number"} <- call {1: "continuation"}, {2: "number"}
+mem: storing 7 in location 22
+mem: storing 8 in location 2
# first call of 'g' does not execute the part after return-continuation-until-mark
-mem: storing 77 in location 22
# calls of the continuation don't execute the part before return-continuation-until-mark
-mem: storing 5 in location 21
-mem: storing 6 in location 21
-mem: storing 7 in location 21
# termination
-mem: storing 9 in location 2

:(before "End call Fields")
bool is_base_of_continuation;
:(before "End call Constructor")
is_base_of_continuation = false;

:(before "End Primitive Recipe Declarations")
CALL_WITH_CONTINUATION_MARK,
:(before "End Primitive Recipe Numbers")
Recipe_ordinal["call-with-continuation-mark"] = CALL_WITH_CONTINUATION_MARK;
:(before "End Primitive Recipe Checks")
case CALL_WITH_CONTINUATION_MARK: {
  break;
}
:(before "End Primitive Recipe Implementations")
case CALL_WITH_CONTINUATION_MARK: {
  // like call, but mark the current call as a 'base of continuation' call
  // before pushing the next one on it
  if (Trace_stream) {
    ++Trace_stream->callstack_depth;
    trace("trace") << "delimited continuation; incrementing callstack depth to " << Trace_stream->callstack_depth << end();
    assert(Trace_stream->callstack_depth < 9000);  // 9998-101 plus cushion
  }
  const instruction& caller_instruction = current_instruction();
  Current_routine->calls.front().is_base_of_continuation = true;
  Current_routine->calls.push_front(call(Recipe_ordinal[current_instruction().ingredients.at(0).name]));
  ingredients.erase(ingredients.begin());  // drop the callee
  finish_call_housekeeping(caller_instruction, ingredients);
  continue;
}

//: save the slice of current call stack until the 'call-with-continuation-mark'
//: call, and return it as the result.
//: todo: implement delimited continuations in Mu's memory
:(before "End Globals")
map<long long int, call_stack> Delimited_continuation;
long long int Next_delimited_continuation_id = 0;
:(before "End Reset")
Delimited_continuation.clear();
Next_delimited_continuation_id = 0;

:(before "End Primitive Recipe Declarations")
RETURN_CONTINUATION_UNTIL_MARK,
:(before "End Primitive Recipe Numbers")
Recipe_ordinal["return-continuation-until-mark"] = RETURN_CONTINUATION_UNTIL_MARK;
:(before "End Primitive Recipe Checks")
case RETURN_CONTINUATION_UNTIL_MARK: {
  break;
}
:(before "End Primitive Recipe Implementations")
case RETURN_CONTINUATION_UNTIL_MARK: {
  // first clear any existing ingredients, to isolate the creation of the
  // continuation from its calls
  Current_routine->calls.front().ingredient_atoms.clear();
  Current_routine->calls.front().next_ingredient_to_process = 0;
  // copy the current call stack until the most recent marked call
  call_stack::iterator find_base_of_continuation(call_stack& c);  // manual prototype containing '::'
  call_stack::iterator base = find_base_of_continuation(Current_routine->calls);
  if (base == Current_routine->calls.end()) {
    raise << maybe(current_recipe_name()) << "couldn't find a 'call-with-continuation-mark' to return to\n" << end();
    raise << maybe(current_recipe_name()) << "call stack:\n" << end();
    for (call_stack::iterator p = Current_routine->calls.begin();  p != Current_routine->calls.end();  ++p)
      raise << maybe(current_recipe_name()) << "  " << get(Recipe, p->running_recipe).name << '\n' << end();
    break;
  }
  Delimited_continuation[Next_delimited_continuation_id] = call_stack(Current_routine->calls.begin(), base);
  while (Current_routine->calls.begin() != base) {
    if (Trace_stream) {
      --Trace_stream->callstack_depth;
      assert(Trace_stream->callstack_depth >= 0);
    }
    Current_routine->calls.pop_front();
  }
  // return it as the result of the marked call
  products.resize(1);
  products.at(0).push_back(Next_delimited_continuation_id);
  ++Next_delimited_continuation_id;
  break;  // continue to process rest of marked call
}

:(code)
call_stack::iterator find_base_of_continuation(call_stack& c) {
  for (call_stack::iterator p = c.begin(); p != c.end(); ++p)
    if (p->is_base_of_continuation) return p;
  return c.end();
}

//: overload 'call' for continuations
:(after "Begin Call")
if (current_instruction().ingredients.at(0).type->atom
    && current_instruction().ingredients.at(0).type->name == "continuation") {
  // copy multiple calls on to current call stack
  assert(scalar(ingredients.at(0)));
  if (Delimited_continuation.find(ingredients.at(0).at(0)) == Delimited_continuation.end())
    raise << maybe(current_recipe_name()) << "no such delimited continuation " << current_instruction().ingredients.at(0).original_string << '\n' << end();
  const call_stack& new_calls = Delimited_continuation[ingredients.at(0).at(0)];
  const call& caller = (SIZE(new_calls) > 1) ? *++new_calls.begin() : Current_routine->calls.front();
  for (call_stack::const_reverse_iterator p = new_calls.rbegin(); p != new_calls.rend(); ++p)
    Current_routine->calls.push_front(*p);
  if (Trace_stream) {
    Trace_stream->callstack_depth += SIZE(new_calls);
    trace("trace") << "calling delimited continuation; growing callstack depth to " << Trace_stream->callstack_depth << end();
    assert(Trace_stream->callstack_depth < 9000);  // 9998-101 plus cushion
  }
  ++current_step_index();  // skip past the return-continuation-until-mark
  ingredients.erase(ingredients.begin());  // drop the callee
  finish_call_housekeeping(to_instruction(caller), ingredients);
  continue;
}