NextAction refactoring to eliminate sentinel arrays and pointers (mod-playerbots#1923)

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# Description

This PR aims to refactor the NextAction declaration to achieve two
goals:

## Eliminate C-style sentinel arrays

Currently, a double pointer (`NextAction**`) approach is being used.
This an old pre-C++11 (< 2011) trick before `std::vector<>` became a
thing.
This approach is painful for developers because they constantly need to
declare their `NextAction` arrays as:
```cpp
NextAction::array(0, new NextAction("foo", 1.0f), nullptr)
```
Instead of:
```cpp
{ new NextAction("foo", 1.0f) }
```
The first argument of `NextAction::array` is actually a hack. It is used
to have a named argument so `va_args` can find the remaining arguments.
It is set to 0 everywhere but in fact does nothing. This is very
confusing to people unfamiliar with this antiquated syntax.
The last argument `nullptr` is what we call a sentinel. It's a `nullptr`
because `va_args` is looking for a `nullptr` to stop iterating. It's
also a hack and also leads to confusion.

## Eliminate unnecessary pointers for `NextAction`

Pointers can be used for several reasons, to cite a few:
- Indicate strong, absolute identity.
- Provide strong but transferable ownership (unlike references).
- When a null value is acceptable (`nullptr`).
- When copy is expensive.

`NextAction` meets none of these criteria:
- It has no identity because it is purely behavioural.
- It is never owned by anything as it is passed around and never fetched
from a registry.
- The only situations where it can be `nullptr` are errors that should
in fact throw an `std::invalid_argument` instead.
- They are extremely small objects that embark a single `std::string`
and a single `float`.

Pointers should be avoided when not strictly necessary because they can
quickly lead to undefined behaviour due to unhandled `nullptr`
situations. They also make the syntax heavier due to the necessity to
constantly check for `nullptr`. Finally, they aren't even good for
performance in that situation because shifting a pointer so many times
is likely more expensive than copying such a trivial object.

# End goal

The end goal is to declare `NextAction` arrays this way:
```cpp
{ NextAction("foo", 1.0f) }
```

> [!NOTE]
> Additional note: `NextAction` is nothing but a hacky proxy to an
`Action` constructor. This should eventually be reworked to use handles
instead of strings. This would make copying `NextAction` even cheaper
and remove the need for the extremely heavy stringly typed current
approach. Stringly typed entities are a known anti-pattern so we need to
move on from those.

Author: SmashingQuasar

src/TravelMgr.cpp

@@ -3409,13 +3409,14 @@ void TravelMgr::LoadQuestTravelTable()
                                     {
                                         Strategy* strat = con->GetStrategy(stratName);
 
-                                        if (strat->getDefaultActions())
-                                            for (uint32 i = 0; i < NextAction::size(strat->getDefaultActions()); i++)
-                                            {
-                                                NextAction* nextAction = strat->getDefaultActions()[i];
+                                        const std::vector<NextAction> defaultActions  = strat->getDefaultActions();
 
+                                        if (defaultActions.size() > 0)
+                                        {
+                                            for (NextAction nextAction : defaultActions)
+                                            {
                                                 std::ostringstream aout;
-                                                aout << nextAction->getRelevance() << "," << nextAction->getName()
+                                                aout << nextAction.getRelevance() << "," << nextAction.getName()
                                                      << ",,S:" << stratName;
 
                                                 if (actions.find(aout.str().c_str()) != actions.end())
@@ -3427,27 +3428,24 @@ void TravelMgr::LoadQuestTravelTable()
 
                                                 actions.insert_or_assign(aout.str().c_str(), classSpecLevel);
                                             }
+                                        }
 
                                         std::vector<TriggerNode*> triggers;
                                         strat->InitTriggers(triggers);
-                                        for (auto& triggerNode : triggers)
-                                        {
-                                            // out << " TN:" << triggerNode->getName();
 
+                                        for (TriggerNode*& triggerNode : triggers)
+                                        {
                                             if (Trigger* trigger = con->GetTrigger(triggerNode->getName()))
                                             {
                                                 triggerNode->setTrigger(trigger);
 
-                                                NextAction** nextActions = triggerNode->getHandlers();
+                                                std::vector<NextAction> nextActions = triggerNode->getHandlers();
 
-                                                for (uint32 i = 0; i < NextAction::size(nextActions); i++)
+                                                // for (uint32_t i = 0; i < nextActions.size(); ++i)
+                                                for (NextAction nextAction : nextActions)
                                                 {
-                                                    NextAction* nextAction = nextActions[i];
-                                                    // out << " A:" << nextAction->getName() << "(" <<
-                                                    // nextAction->getRelevance() << ")";
-
                                                     std::ostringstream aout;
-                                                    aout << nextAction->getRelevance() << "," << nextAction->getName()
+                                                    aout << nextAction.getRelevance() << "," << nextAction.getName()
                                                          << "," << triggerNode->getName() << "," << stratName;
 
                                                     if (actions.find(aout.str().c_str()) != actions.end())

src/strategy/Action.cpp

@@ -8,90 +8,6 @@
 #include "Playerbots.h"
 #include "Timer.h"
 
-uint32 NextAction::size(NextAction** actions)
-{
-    if (!actions)
-        return 0;
-
-    uint32 size = 0;
-    for (size = 0; actions[size];)
-        ++size;
-
-    return size;
-}
-
-NextAction** NextAction::clone(NextAction** actions)
-{
-    if (!actions)
-        return nullptr;
-
-    uint32 size = NextAction::size(actions);
-
-    NextAction** res = new NextAction*[size + 1];
-    for (uint32 i = 0; i < size; i++)
-        res[i] = new NextAction(*actions[i]);
-
-    res[size] = nullptr;
-
-    return res;
-}
-
-NextAction** NextAction::merge(NextAction** left, NextAction** right)
-{
-    uint32 leftSize = NextAction::size(left);
-    uint32 rightSize = NextAction::size(right);
-
-    NextAction** res = new NextAction*[leftSize + rightSize + 1];
-
-    for (uint32 i = 0; i < leftSize; i++)
-        res[i] = new NextAction(*left[i]);
-
-    for (uint32 i = 0; i < rightSize; i++)
-        res[leftSize + i] = new NextAction(*right[i]);
-
-    res[leftSize + rightSize] = nullptr;
-
-    NextAction::destroy(left);
-    NextAction::destroy(right);
-
-    return res;
-}
-
-NextAction** NextAction::array(uint32 nil, ...)
-{
-    va_list vl;
-    va_start(vl, nil);
-
-    uint32 size = 0;
-    NextAction* cur = nullptr;
-    do
-    {
-        cur = va_arg(vl, NextAction*);
-        ++size;
-    } while (cur);
-
-    va_end(vl);
-
-    NextAction** res = new NextAction*[size];
-    va_start(vl, nil);
-    for (uint32 i = 0; i < size; i++)
-        res[i] = va_arg(vl, NextAction*);
-    va_end(vl);
-
-    return res;
-}
-
-void NextAction::destroy(NextAction** actions)
-{
-    if (!actions)
-        return;
-
-    for (uint32 i = 0; actions[i]; i++)
-        delete actions[i];
-
-    delete[] actions;
-}
-
 Value<Unit*>* Action::GetTargetValue() { return context->GetValue<Unit*>(GetTargetName()); }
 
 Unit* Action::GetTarget() { return GetTargetValue()->Get(); }
@@ -101,4 +17,4 @@ ActionBasket::ActionBasket(ActionNode* action, float relevance, bool skipPrerequ
 {
 }
 
-bool ActionBasket::isExpired(uint32 msecs) { return getMSTime() - created >= msecs; }
+bool ActionBasket::isExpired(uint32_t msecs) { return getMSTime() - created >= msecs; }

src/strategy/Action.h

@@ -3,8 +3,7 @@
  * and/or modify it under version 3 of the License, or (at your option), any later version.
  */
 
-#ifndef _PLAYERBOT_ACTION_H
-#define _PLAYERBOT_ACTION_H
+#pragma once
 
 #include "AiObject.h"
 #include "Common.h"
@@ -24,15 +23,26 @@ class NextAction
     std::string const getName() { return name; }
     float getRelevance() { return relevance; }
 
-    static uint32 size(NextAction** actions);
-    static NextAction** clone(NextAction** actions);
-    static NextAction** merge(NextAction** what, NextAction** with);
-    static NextAction** array(uint32 nil, ...);
-    static void destroy(NextAction** actions);
+    static std::vector<NextAction> merge(std::vector<NextAction> const& what, std::vector<NextAction> const& with)
+    {
+        std::vector<NextAction> result = {};
+
+        for (NextAction const& action : what)
+        {
+            result.push_back(action);
+        }
+
+        for (NextAction const& action : with)
+        {
+            result.push_back(action);
+        }
+
+        return result;
+    };
 
 private:
     float relevance;
-    std::string const name;
+    std::string name;
 };
 
 class Action : public AiNamedObject
@@ -52,9 +62,9 @@ class Action : public AiNamedObject
     virtual bool Execute([[maybe_unused]] Event event) { return true; }
     virtual bool isPossible() { return true; }
     virtual bool isUseful() { return true; }
-    virtual NextAction** getPrerequisites() { return nullptr; }
-    virtual NextAction** getAlternatives() { return nullptr; }
-    virtual NextAction** getContinuers() { return nullptr; }
+    virtual std::vector<NextAction> getPrerequisites() { return {}; }
+    virtual std::vector<NextAction> getAlternatives() { return {}; }
+    virtual std::vector<NextAction> getContinuers() { return {}; }
     virtual ActionThreatType getThreatType() { return ActionThreatType::None; }
     void Update() {}
     void Reset() {}
@@ -73,39 +83,44 @@ class Action : public AiNamedObject
 class ActionNode
 {
 public:
-    ActionNode(std::string const name, NextAction** prerequisites = nullptr, NextAction** alternatives = nullptr,
-               NextAction** continuers = nullptr)
-        : name(name), action(nullptr), continuers(continuers), alternatives(alternatives), prerequisites(prerequisites)
-    {
-    }  // reorder arguments - whipowill
-
-    virtual ~ActionNode()
-    {
-        NextAction::destroy(prerequisites);
-        NextAction::destroy(alternatives);
-        NextAction::destroy(continuers);
-    }
+    ActionNode(
+        std::string name,
+        std::vector<NextAction> prerequisites = {},
+        std::vector<NextAction> alternatives = {},
+        std::vector<NextAction> continuers = {}
+    ) :
+    name(std::move(name)),
+    action(nullptr),
+    continuers(continuers),
+    alternatives(alternatives),
+    prerequisites(prerequisites)
+    {}
+
+    virtual ~ActionNode() = default;
 
     Action* getAction() { return action; }
     void setAction(Action* action) { this->action = action; }
-    std::string const getName() { return name; }
+    const std::string getName() { return name; }
 
-    NextAction** getContinuers() { return NextAction::merge(NextAction::clone(continuers), action->getContinuers()); }
-    NextAction** getAlternatives()
+    std::vector<NextAction> getContinuers()
     {
-        return NextAction::merge(NextAction::clone(alternatives), action->getAlternatives());
+        return NextAction::merge(this->continuers, action->getContinuers());
     }
-    NextAction** getPrerequisites()
+    std::vector<NextAction> getAlternatives()
     {
-        return NextAction::merge(NextAction::clone(prerequisites), action->getPrerequisites());
+        return NextAction::merge(this->alternatives, action->getAlternatives());
+    }
+    std::vector<NextAction> getPrerequisites()
+    {
+        return NextAction::merge(this->prerequisites, action->getPrerequisites());
     }
 
 private:
-    std::string const name;
+    const std::string name;
     Action* action;
-    NextAction** continuers;
-    NextAction** alternatives;
-    NextAction** prerequisites;
+    std::vector<NextAction> continuers;
+    std::vector<NextAction> alternatives;
+    std::vector<NextAction> prerequisites;
 };
 
 class ActionBasket
@@ -121,14 +136,12 @@ class ActionBasket
     bool isSkipPrerequisites() { return skipPrerequisites; }
     void AmendRelevance(float k) { relevance *= k; }
     void setRelevance(float relevance) { this->relevance = relevance; }
-    bool isExpired(uint32 msecs);
+    bool isExpired(uint32_t msecs);
 
 private:
     ActionNode* action;
     float relevance;
     bool skipPrerequisites;
     Event event;
-    uint32 created;
+    uint32_t created;
 };
-
-#endif