path: root/cpp/024brace

//: Structured programming
//:
//: Our jump operators are quite inconvenient to use, so mu provides a
//: lightweight tool called 'transform_braces' to work in a slightly more
//: convenient format with nested braces:
//:
//:   {
//:     some instructions
//:     {
//:       more instructions
//:     }
//:   }
//:
//: Braces are just labels, they require no special parsing. The operations
//: 'loop' and 'break' jump to just after the enclosing '{' and '}'
//: respectively.
//:
//: Conditional and unconditional 'loop' and 'break' should give us 80% of the
//: benefits of the control-flow primitives we're used to in other languages,
//: like 'if', 'while', 'for', etc.

:(scenario "brace_conversion")
recipe main [
  {
    break
    1:integer <- copy 0:literal
  }
]
+after-brace: recipe main
+after-brace: jump 1:offset
+after-brace: copy ...

//: one-time setup
:(after "int main")
Transform.push_back(transform_braces);

:(code)
void transform_braces(const recipe_number r) {
  const int OPEN = 0, CLOSE = 1;
  list<pair<int/*OPEN/CLOSE*/, size_t/*step index*/> > braces;
  for (size_t index = 0; index < Recipe[r].steps.size(); ++index) {
    const instruction& inst = Recipe[r].steps[index];
    if (inst.label == "{") {
      trace("brace") << r << ": push (open, " << index << ")";
      braces.push_back(pair<int,size_t>(OPEN, index));
    }
    if (inst.label == "}") {
      trace("brace") << "push (close, " << index << ")";
      braces.push_back(pair<int,size_t>(CLOSE, index));
    }
  }
  stack<size_t/*step index*/> open_braces;
  trace("after-brace") << "recipe " << Recipe[r].name;
  for (size_t index = 0; index < Recipe[r].steps.size(); ++index) {
    instruction& inst = Recipe[r].steps[index];
    if (inst.label == "{") open_braces.push(index);
    else if (inst.label == "}") open_braces.pop();
    else if (inst.operation == Recipe_number["loop"]) {
      inst.operation = Recipe_number["jump"];
      if (inst.ingredients.size() > 0 && inst.ingredients[0].types[0] == 0) {
        // explicit target; a later phase will handle it
        ;
      }
      else {
        reagent ing(0);  // literal
        ing.value = open_braces.top()-index;
        inst.ingredients.push_back(ing);
        trace("after-brace") << "jump " << ing.value << ":offset";
      }
    }
    else if (inst.operation == Recipe_number["break"]) {
      inst.operation = Recipe_number["jump"];
      if (inst.ingredients.size() > 0 && inst.ingredients[0].types[0] == 0) {
        // explicit target; a later phase will handle it
        ;
      }
      else {
        reagent ing(0);  // literal
        ing.value = index - matching_brace(open_braces.top(), braces) - 1;
        inst.ingredients.push_back(ing);
        trace("after-brace") << "jump " << ing.value << ":offset";
      }
    }
    else {
      trace("after-brace") << inst.name << " ...";
    }
  }
}

size_t matching_brace(size_t index, const list<pair<int, size_t> >& braces) {
  int stacksize;
  for (list<pair<int, size_t> >::const_iterator p = braces.begin(); p != braces.end(); ++p) {
    if (p->second <= index) continue;
    stacksize += (p->first ? 1 : -1);
    if (stacksize == -1) return p->second;
  }
  return -1;
}

:(scenario "loop")
recipe main [
  1:integer <- copy 0:literal
  2:integer <- copy 0:literal
  {
    3:integer <- copy 0:literal
    loop
  }
]
+after-brace: recipe main
+after-brace: copy ...
+after-brace: copy ...
+after-brace: copy ...
+after-brace: jump -2:offset