Add support for retrieving list values from a slot

This PR adds support for retrieving list information from a slot. Rather
than take the whole list, two different methods are provided for
retrieving a) the length of the list, and b) a specific value in the
list.

AUTHORS

@@ -16,3 +16,4 @@ Evan Hahn <me@evanhahn.com>
 Starbeamrainbowlabs <contact@starbeamrainbowlabs.com>
 Alexander Roper <minirop@gmail.com>
 Will Speak <will@willspeak.me>
+Damien Radtke <damienradtke@gmail.com>

src/include/wren.h

@@ -345,6 +345,12 @@ double wrenGetSlotDouble(WrenVM* vm, int slot);
 // foreign class.
 void* wrenGetSlotForeign(WrenVM* vm, int slot);
 
+// Returns the size of the list stored in [slot].
+int wrenGetSlotListSize(WrenVM* vm, int slot);
+
+// Reads a value from the list in [slot].
+WrenValue* wrenGetSlotListValue(WrenVM* vm, int slot, int index);
+
 // Reads a string from [slot].
 //
 // The memory for the returned string is owned by Wren. You can inspect it

src/vm/wren_vm.c

@@ -1547,6 +1547,26 @@ void* wrenGetSlotForeign(WrenVM* vm, int slot)
   return AS_FOREIGN(vm->apiStack[slot])->data;
 }
 
+int wrenGetSlotListSize(WrenVM* vm, int slot)
+{
+  validateApiSlot(vm, slot);
+  ASSERT(IS_LIST(vm->apiStack[slot]),
+         "Slot must hold a list instance.");
+
+  ValueBuffer elements = AS_LIST(vm->apiStack[slot])->elements;
+  return elements.count;
+}
+
+WrenValue* wrenGetSlotListValue(WrenVM* vm, int slot, int index)
+{
+  validateApiSlot(vm, slot);
+  ASSERT(IS_LIST(vm->apiStack[slot]),
+         "Slot must hold a list instance.");
+
+  ValueBuffer elements = AS_LIST(vm->apiStack[slot])->elements;
+  return wrenCaptureValue(vm, elements.data[index]);
+}
+
 const char* wrenGetSlotString(WrenVM* vm, int slot)
 {
   validateApiSlot(vm, slot);

test/api/slots.c

@@ -153,6 +153,19 @@ static void foreignClassAllocate(WrenVM* vm)
   wrenSetSlotNewForeign(vm, 0, 0, 4);
 }
 
+static void getListSize(WrenVM* vm)
+{
+  wrenSetSlotDouble(vm, 0, wrenGetSlotListSize(vm, 1));
+}
+
+static void getListValue(WrenVM* vm)
+{
+  int index = (int)wrenGetSlotDouble(vm, 2);
+  WrenValue* value = wrenGetSlotListValue(vm, 1, index);
+  wrenSetSlotValue(vm, 0, value);
+  wrenReleaseValue(vm, value);
+}
+
 WrenForeignMethodFn slotsBindMethod(const char* signature)
 {
   if (strcmp(signature, "static Slots.noSet") == 0) return noSet;
@@ -161,6 +174,8 @@ WrenForeignMethodFn slotsBindMethod(const char* signature)
   if (strcmp(signature, "static Slots.slotTypes(_,_,_,_,_,_,_)") == 0) return slotTypes;
   if (strcmp(signature, "static Slots.ensure()") == 0) return ensure;
   if (strcmp(signature, "static Slots.ensureOutsideForeign()") == 0) return ensureOutsideForeign;
+  if (strcmp(signature, "static Slots.getListSize(_)") == 0) return getListSize;
+  if (strcmp(signature, "static Slots.getListValue(_,_)") == 0) return getListValue;
 
   return NULL;
 }
@@ -168,4 +183,4 @@ WrenForeignMethodFn slotsBindMethod(const char* signature)
 void slotsBindClass(const char* className, WrenForeignClassMethods* methods)
 {
   methods->allocate = foreignClassAllocate;
-}
+}

test/api/slots.wren

@@ -5,6 +5,8 @@ class Slots {
   foreign static slotTypes(bool, foreignObj, list, nullObj, num, string, unknown)
   foreign static ensure()
   foreign static ensureOutsideForeign()
+  foreign static getListSize(list)
+  foreign static getListValue(list, index)
 }
 
 foreign class ForeignType {
@@ -30,3 +32,7 @@ System.print(Slots.ensure())
 
 System.print(Slots.ensureOutsideForeign())
 // expect: 0 -> 20 (190)
+
+var ducks = ["Huey", "Dewey", "Louie"]
+System.print(Slots.getListSize(ducks))     // expect: 3
+System.print(Slots.getListValue(ducks, 1)) // expect: Dewey