1 files changed, 57 insertions, 0 deletions
diff --git a/cpp/039wait.cc b/cpp/039wait.cc
new file mode 100644
index 00000000..eba3a437
--- /dev/null
+++ b/cpp/039wait.cc
@@ -0,0 +1,57 @@
+//: Routines can be put in a 'waiting' state, from which it will be ready to
+//: run again when a specific memory location changes its value. This is mu's
+//: basic technique for orchestrating the order in which different routines
+//: operate.
+
+:(scenario "stalled_routine")
+recipe f1 [
+  1:integer <- copy 0:literal
+  run f2:recipe
+  wait-for-location 1:integer
+  # now wait for f2 to run and modify location 1 before using its value
+  2:integer <- copy 1:integer
+]
+recipe f2 [
+  1:integer <- copy 34:literal
+]
+# if we got the synchronization wrong we'd be storing 0 in location 2
++mem: storing 34 in location 2
+
+//: define the new state that all routines can be in
+
+:(before "End routine States")
+WAITING,
+:(before "End routine Fields")
+// only if state == WAITING
+size_t waiting_on_location;
+int old_value_of_wating_location;
+:(before "End routine Constructor")
+waiting_on_location = old_value_of_wating_location = 0;
+
+//: primitive recipe to put routines in that state
+
+:(before "End Primitive Recipe Declarations")
+WAIT_FOR_LOCATION,
+:(before "End Primitive Recipe Numbers")
+Recipe_number["wait-for-location"] = WAIT_FOR_LOCATION;
+:(before "End Primitive Recipe Implementations")
+case WAIT_FOR_LOCATION: {
+  reagent loc = canonize(current_instruction().ingredients[0]);
+  Current_routine->state = WAITING;
+  Current_routine->waiting_on_location = loc.value;
+  Current_routine->old_value_of_wating_location = Memory[loc.value];
+  trace("run") << "waiting for " << loc.value << " to change from " << Memory[loc.value];
+  break;
+}
+
+//: scheduler tweak to get routines out of that state
+
+:(before "End Scheduler State Transitions")
+for (size_t i = 0; i < Routines.size(); ++i) {
+  if (Routines[i]->state != WAITING) continue;
+  if (Memory[Routines[i]->waiting_on_location] != Routines[i]->old_value_of_wating_location) {
+    trace("schedule") << "waking up routine\n";
+    Routines[i]->state = RUNNING;
+    Routines[i]->waiting_on_location = Routines[i]->old_value_of_wating_location = 0;
+  }
+}