Add C API functions for working with lists:

- wrenEnsureSlots()
  Lets you go the foreign slot stack to make room for a temporary work
  area.

- wrenSetSlotNewList()
  Creates a new empty list and stores it in a slot.

- wrenInsertInList()
  Takes a value from one slot and inserts it into the list in another.

Still need more functions like getting elements from a list, removing,
etc. but this at least lets you create, populate, and return lists from
foreign methods.

src/include/wren.h

@@ -240,6 +240,14 @@ void* wrenAllocateForeign(WrenVM* vm, size_t size);
 // Returns the number of slots available to the current foreign method.
 int wrenGetSlotCount(WrenVM* vm);
 
+// Ensures that the foreign method stack has at least [numSlots] available for
+// use, growing the stack if needed.
+//
+// Does not shrink the stack if it has more than enough slots.
+//
+// It is an error to call this from a finalizer.
+void wrenEnsureSlots(WrenVM* vm, int numSlots);
+
 // TODO: Update docs.
 
 // The following functions read one of the arguments passed to a foreign call.
@@ -327,6 +335,9 @@ void wrenSetSlotBytes(WrenVM* vm, int slot, const char* bytes, int length);
 // Stores the numeric [value] in [slot].
 void wrenSetSlotDouble(WrenVM* vm, int slot, double value);
 
+// Stores a new empty list in [slot].
+void wrenSetSlotNewList(WrenVM* vm, int slot);
+
 // Stores null in [slot].
 void wrenSetSlotNull(WrenVM* vm, int slot);
 
@@ -343,4 +354,11 @@ void wrenSetSlotString(WrenVM* vm, int slot, const char* text);
 // This does not release the handle for the value.
 void wrenSetSlotValue(WrenVM* vm, int slot, WrenValue* value);
 
+// Takes the value stored at [elementSlot] and inserts it into the list stored
+// at [listSlot] at [index].
+//
+// As in Wren, negative indexes can be used to insert from the end. To append
+// an element, use `-1` for the index.
+void wrenInsertInList(WrenVM* vm, int listSlot, int index, int elementSlot);
+
 #endif

src/vm/wren_vm.c

@@ -420,6 +420,48 @@ static bool checkArity(WrenVM* vm, Value value, int numArgs)
   return false;
 }
 
+// Ensures [fiber]'s stack has at least [needed] slots.
+static void ensureStack(WrenVM* vm, ObjFiber* fiber, int needed)
+{
+  if (fiber->stackCapacity >= needed) return;
+
+  int capacity = wrenPowerOf2Ceil(needed);
+    
+  Value* oldStack = fiber->stack;
+  fiber->stack = (Value*)wrenReallocate(vm, fiber->stack,
+                                        sizeof(Value) * fiber->stackCapacity,
+                                        sizeof(Value) * capacity);
+  fiber->stackCapacity = capacity;
+  
+  // If the reallocation moves the stack, then we need to shift every pointer
+  // into the stack to point to its new location.
+  if (fiber->stack != oldStack)
+  {
+    // Top of the stack.
+    long offset = fiber->stack - oldStack;
+    fiber->stackTop += offset;
+    
+    // Stack pointer for each call frame.
+    for (int i = 0; i < fiber->numFrames; i++)
+    {
+      fiber->frames[i].stackStart += offset;
+    }
+    
+    // Open upvalues.
+    for (ObjUpvalue* upvalue = fiber->openUpvalues;
+         upvalue != NULL;
+         upvalue = upvalue->next)
+    {
+      upvalue->value += offset;
+    }
+    
+    if (vm->foreignStackStart != NULL)
+    {
+      vm->foreignStackStart += offset;
+    }
+  }
+}
+
 // Pushes [function] onto [fiber]'s callstack and invokes it. Expects [numArgs]
 // arguments (including the receiver) to be on the top of the stack already.
 // [function] can be an `ObjFn` or `ObjClosure`.
@@ -435,50 +477,12 @@ static inline void callFunction(
         sizeof(CallFrame) * max);
     fiber->frameCapacity = max;
   }
-
+  
   // Grow the stack if needed.
   int stackSize = (int)(fiber->stackTop - fiber->stack);
   int needed = stackSize + wrenUpwrapClosure(function)->maxSlots;
-
+  ensureStack(vm, fiber, needed);
-  if (fiber->stackCapacity < needed)
+  
-  {
-    int capacity = wrenPowerOf2Ceil(needed);
-
-    Value* oldStack = fiber->stack;
-    fiber->stack = (Value*)wrenReallocate(vm, fiber->stack,
-        sizeof(Value) * fiber->stackCapacity,
-        sizeof(Value) * capacity);
-    fiber->stackCapacity = capacity;
-
-    // If the reallocation moves the stack, then we need to shift every pointer
-    // into the stack to point to its new location.
-    if (fiber->stack != oldStack)
-    {
-      // Top of the stack.
-      long offset = fiber->stack - oldStack;
-      fiber->stackTop += offset;
-
-      // Stack pointer for each call frame.
-      for (int i = 0; i < fiber->numFrames; i++)
-      {
-        fiber->frames[i].stackStart += offset;
-      }
-
-      // Open upvalues.
-      for (ObjUpvalue* upvalue = fiber->openUpvalues;
-           upvalue != NULL;
-           upvalue = upvalue->next)
-      {
-        upvalue->value += offset;
-      }
-      
-      if (vm->foreignStackStart != NULL)
-      {
-        vm->foreignStackStart += offset;
-      }
-    }
-  }
-
   wrenAppendCallFrame(vm, fiber, function, fiber->stackTop - numArgs);
 }
 
@@ -1625,23 +1629,39 @@ void wrenPopRoot(WrenVM* vm)
   vm->numTempRoots--;
 }
 
+// Returns true if the VM is in a foreign method that's a finalizer.
+//
+// Finalizers don't run in the context of a fiber and have a single magic stack
+// slot, so need to be handled a little specially.
+static bool isInFinalizer(WrenVM* vm)
+{
+  return vm->fiber == NULL ||
+         vm->foreignStackStart < vm->fiber->stack ||
+         vm->foreignStackStart > vm->fiber->stackTop;
+}
+
 int wrenGetSlotCount(WrenVM* vm)
 {
   ASSERT(vm->foreignStackStart != NULL, "Must be in foreign call.");
 
-  // If no fiber is executing or the foreign stack is not in it, we must be in
+  if (isInFinalizer(vm)) return 1;
-  // a finalizer, in which case the "stack" just has one object, the object
-  // being finalized.
-  if (vm->fiber == NULL ||
-      vm->foreignStackStart < vm->fiber->stack ||
-      vm->foreignStackStart > vm->fiber->stackTop)
-  {
-    return 1;
-  }
-
   return (int)(vm->fiber->stackTop - vm->foreignStackStart);
 }
 
+void wrenEnsureSlots(WrenVM* vm, int numSlots)
+{
+  ASSERT(!isInFinalizer(vm), "Cannot grow the stack in a finalizer.");
+  
+  int currentSize = (int)(vm->fiber->stackTop - vm->foreignStackStart);
+  if (currentSize >= numSlots) return;
+  
+  // Grow the stack if needed.
+  int needed = (int)(vm->foreignStackStart - vm->fiber->stack) + numSlots;
+  ensureStack(vm, vm->fiber, needed);
+  
+  vm->fiber->stackTop = vm->foreignStackStart + numSlots;
+}
+
 // Ensures that [slot] is a valid index into a foreign method's stack of slots.
 static void validateForeignSlot(WrenVM* vm, int slot)
 {
@@ -1723,6 +1743,11 @@ void wrenSetSlotDouble(WrenVM* vm, int slot, double value)
   setSlot(vm, slot, NUM_VAL(value));
 }
 
+void wrenSetSlotNewList(WrenVM* vm, int slot)
+{
+  setSlot(vm, slot, OBJ_VAL(wrenNewList(vm, 0)));
+}
+
 void wrenSetSlotNull(WrenVM* vm, int slot)
 {
   setSlot(vm, slot, NULL_VAL);
@@ -1740,3 +1765,19 @@ void wrenSetSlotValue(WrenVM* vm, int slot, WrenValue* value)
 
   setSlot(vm, slot, value->value);
 }
+
+void wrenInsertInList(WrenVM* vm, int listSlot, int index, int elementSlot)
+{
+  validateForeignSlot(vm, listSlot);
+  validateForeignSlot(vm, elementSlot);
+  ASSERT(IS_LIST(vm->foreignStackStart[listSlot]), "Must insert into a list.");
+  
+  ObjList* list = AS_LIST(vm->foreignStackStart[listSlot]);
+  
+  // Negative indices count from the end.
+  if (index < 0) index = list->elements.count + 1 + index;
+  
+  ASSERT(index <= list->elements.count, "Index out of bounds.");
+  
+  wrenListInsert(vm, list, vm->foreignStackStart[elementSlot], index);
+}

test/api/lists.c

@@ -0,0 +1,46 @@
+#include <string.h>
+
+#include "lists.h"
+
+static void newList(WrenVM* vm)
+{
+  wrenSetSlotNewList(vm, 0);
+}
+
+// Helper function to store a double in a slot then insert it into the list at
+// slot zero.
+static void insertNumber(WrenVM* vm, int index, double value)
+{
+  wrenSetSlotDouble(vm, 1, value);
+  wrenInsertInList(vm, 0, index, 1);
+}
+
+static void insert(WrenVM* vm)
+{
+  wrenSetSlotNewList(vm, 0);
+  
+  wrenEnsureSlots(vm, 2);
+  
+  // Appending.
+  insertNumber(vm, 0, 1.0);
+  insertNumber(vm, 1, 2.0);
+  insertNumber(vm, 2, 3.0);
+  
+  // Inserting.
+  insertNumber(vm, 0, 4.0);
+  insertNumber(vm, 1, 5.0);
+  insertNumber(vm, 2, 6.0);
+  
+  // Negative indexes.
+  insertNumber(vm, -1, 7.0);
+  insertNumber(vm, -2, 8.0);
+  insertNumber(vm, -3, 9.0);
+}
+
+WrenForeignMethodFn listsBindMethod(const char* signature)
+{
+  if (strcmp(signature, "static Lists.newList()") == 0) return newList;
+  if (strcmp(signature, "static Lists.insert()") == 0) return insert;
+
+  return NULL;
+}

test/api/lists.h

@@ -0,0 +1,3 @@
+#include "wren.h"
+
+WrenForeignMethodFn listsBindMethod(const char* signature);

test/api/lists.wren

@@ -0,0 +1,10 @@
+class Lists {
+  foreign static newList()
+  foreign static insert()
+}
+
+var list = Lists.newList()
+System.print(list is List) // expect: true
+System.print(list.count) // expect: 0
+
+System.print(Lists.insert()) // expect: [4, 5, 6, 1, 2, 3, 9, 8, 7]

test/api/main.c

@@ -7,6 +7,7 @@
 #include "benchmark.h"
 #include "call.h"
 #include "foreign_class.h"
+#include "lists.h"
 #include "slots.h"
 #include "value.h"
 
@@ -36,6 +37,9 @@ static WrenForeignMethodFn bindForeignMethod(
   method = foreignClassBindMethod(fullName);
   if (method != NULL) return method;
   
+  method = listsBindMethod(fullName);
+  if (method != NULL) return method;
+  
   method = slotsBindMethod(fullName);
   if (method != NULL) return method;
   

test/api/slots.c

@@ -1,3 +1,4 @@
+#include <stdio.h>
 #include <string.h>
 
 #include "slots.h"
@@ -73,11 +74,38 @@ static void setSlots(WrenVM* vm)
   }
 }
 
+static void ensure(WrenVM* vm)
+{
+  int before = wrenGetSlotCount(vm);
+  
+  wrenEnsureSlots(vm, 20);
+  
+  int after = wrenGetSlotCount(vm);
+  
+  // Use the slots to make sure they're available.
+  for (int i = 0; i < 20; i++)
+  {
+    wrenSetSlotDouble(vm, i, i);
+  }
+  
+  int sum = 0;
+
+  for (int i = 0; i < 20; i++)
+  {
+    sum += (int)wrenGetSlotDouble(vm, i);
+  }
+  
+  char result[100];
+  sprintf(result, "%d -> %d (%d)", before, after, sum);
+  wrenSetSlotString(vm, 0, result);
+}
+
 WrenForeignMethodFn slotsBindMethod(const char* signature)
 {
   if (strcmp(signature, "static Slots.noSet") == 0) return noSet;
   if (strcmp(signature, "static Slots.getSlots(_,_,_,_,_)") == 0) return getSlots;
   if (strcmp(signature, "static Slots.setSlots(_,_,_,_)") == 0) return setSlots;
+  if (strcmp(signature, "static Slots.ensure()") == 0) return ensure;
 
   return NULL;
 }

test/api/slots.wren

@@ -1,9 +1,8 @@
 class Slots {
   foreign static noSet
-
   foreign static getSlots(bool, num, string, bytes, value)
-
   foreign static setSlots(a, b, c, d)
+  foreign static ensure()
 }
 
 // If nothing is set in the return slot, it retains its previous value, the
@@ -14,3 +13,5 @@ var value = ["value"]
 System.print(Slots.getSlots(true, "by\0te", 12.34, "str", value) == value) // expect: true
 
 System.print(Slots.setSlots(value, 0, 0, 0) == value) // expect: true
+
+System.print(Slots.ensure()) // expect: 1 -> 20 (190)

util/xcode/wren.xcodeproj/project.pbxproj

@@ -26,6 +26,7 @@
 		29729F331BA70A620099CA20 /* io.wren.inc in Sources */ = {isa = PBXBuildFile; fileRef = 29729F301BA70A620099CA20 /* io.wren.inc */; };
 		2986F6D71ACF93BA00BCE26C /* wren_primitive.c in Sources */ = {isa = PBXBuildFile; fileRef = 2986F6D51ACF93BA00BCE26C /* wren_primitive.c */; };
 		29932D511C20D8C900099DEE /* benchmark.c in Sources */ = {isa = PBXBuildFile; fileRef = 29932D4F1C20D8C900099DEE /* benchmark.c */; };
+		29932D541C210F8D00099DEE /* lists.c in Sources */ = {isa = PBXBuildFile; fileRef = 29932D521C210F8D00099DEE /* lists.c */; };
 		29A427341BDBE435001E6E22 /* wren_opt_meta.c in Sources */ = {isa = PBXBuildFile; fileRef = 29A4272E1BDBE435001E6E22 /* wren_opt_meta.c */; };
 		29A427351BDBE435001E6E22 /* wren_opt_meta.c in Sources */ = {isa = PBXBuildFile; fileRef = 29A4272E1BDBE435001E6E22 /* wren_opt_meta.c */; };
 		29A427361BDBE435001E6E22 /* wren_opt_meta.wren.inc in Sources */ = {isa = PBXBuildFile; fileRef = 29A427301BDBE435001E6E22 /* wren_opt_meta.wren.inc */; };
@@ -106,6 +107,8 @@
 		2986F6D61ACF93BA00BCE26C /* wren_primitive.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = wren_primitive.h; path = ../../src/vm/wren_primitive.h; sourceTree = "<group>"; };
 		29932D4F1C20D8C900099DEE /* benchmark.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = benchmark.c; path = ../../test/api/benchmark.c; sourceTree = "<group>"; };
 		29932D501C20D8C900099DEE /* benchmark.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = benchmark.h; path = ../../test/api/benchmark.h; sourceTree = "<group>"; };
+		29932D521C210F8D00099DEE /* lists.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = lists.c; path = ../../test/api/lists.c; sourceTree = "<group>"; };
+		29932D531C210F8D00099DEE /* lists.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = lists.h; path = ../../test/api/lists.h; sourceTree = "<group>"; };
 		29A4272E1BDBE435001E6E22 /* wren_opt_meta.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = wren_opt_meta.c; path = ../../src/optional/wren_opt_meta.c; sourceTree = "<group>"; };
 		29A4272F1BDBE435001E6E22 /* wren_opt_meta.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = wren_opt_meta.h; path = ../../src/optional/wren_opt_meta.h; sourceTree = "<group>"; };
 		29A427301BDBE435001E6E22 /* wren_opt_meta.wren.inc */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.pascal; name = wren_opt_meta.wren.inc; path = ../../src/optional/wren_opt_meta.wren.inc; sourceTree = "<group>"; };
@@ -249,6 +252,8 @@
 				293D46951BB43F9900200083 /* call.h */,
 				29D009A81B7E39A8000CE58C /* foreign_class.c */,
 				29D009A91B7E39A8000CE58C /* foreign_class.h */,
+				29932D521C210F8D00099DEE /* lists.c */,
+				29932D531C210F8D00099DEE /* lists.h */,
 				29D009AA1B7E39A8000CE58C /* slots.c */,
 				29D009AB1B7E39A8000CE58C /* slots.h */,
 				29D009AC1B7E39A8000CE58C /* value.c */,
@@ -358,6 +363,7 @@
 			files = (
 				29A427371BDBE435001E6E22 /* wren_opt_meta.wren.inc in Sources */,
 				29729F321BA70A620099CA20 /* io.c in Sources */,
+				29932D541C210F8D00099DEE /* lists.c in Sources */,
 				291647C81BA5EC5E006142EE /* modules.c in Sources */,
 				29DC14A11BBA2FEC008A8274 /* scheduler.c in Sources */,
 				29A427391BDBE435001E6E22 /* wren_opt_random.c in Sources */,