Ai Overview

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Ai Overview

This article is about helping to understand how the Ai in Shards Online works.

Some Basics:

The Artificial Intelligence for NPCs follows a general Ai Principle which is the cycle of

• Sense
• Think
• Act

The State Machine

The model of implementation is a Finite State Machine(FSM). [Wikipedia Article Finite State Machine]

All AI files used by mobs require directly or indirectly the FSM and extend or alter it or its parameters to their needs.

To understand how the FSM would transition from one state (e.g. "Idle") to another state (e.g. "Combat") in Shards Online (SO) it is important to understand the first step: "Sense".

Sense

How does an NPC sense the world?

There are several ways:

Messages

Messages using the Sendmessage function tell the NPC someone or something sent it a message. To understand Messages the NPC needs to have the appropriate EventHandlers.

Views

Views are added to objects by using the AddView function. A view is basically a search function performed on a configurably regular basis which triggers an event if something matching that search function enters its configured range. Mobiles can carry around views with them.

FindObjects

The FindObjects function can be executed whenever an NPC needs to find something.

Each of these sensing mechanisms can call functions provided by the core Ai which lives in base_ai_state_machine.lua.

SchedulePulse()
AI.StateMachine.Init(initialState)
AI.StateMachine.Shutdown()
AI.StateMachine.ChangeState(newState)
AI.StateMachine.ChangeSubState(newState)
AI.StateMachine.EndSubStates(success,reason)
HandleAiPulse()

Especially important is function AI.StateMachine.ChangeState(newState) which could be called when something happens our NPC needs to react to, e.g. a wolf entering the perception range of a turkey.

In base_ai_mob.lua we find the following line of code in the initialization part:

AddView("chaseRange",SearchMobileInRange(chaseViewRange,false,false,true))

And at the end of the file we find the Event Handler in case some Mobile enters that range and triggered the view:

--On the target coming into range
RegisterEventHandler(EventType.EnterView, "chaseRange", 
    function (objRef)
        HandleMobEnterView(objRef,false)
    end)

Think

So it seems that HandleMobEnterView is our canditate doing the real stuff.
Below is a shortened version cleared from comments and DebugMessages, with added comments to better understand it.
Nothing of the core logic of the function is changed

function HandleMobEnterView(mobileObj,ignoreVision,forceAttack)
    -- Sanity Checks
    if (ignoreVision == nil) then ignoreVision = false end
    if not(AI.IsActive()) then return end
    -- This check makes mobs ignore admins as possible targets
    if (not AI.IsValidTarget(mobileObj)) then return end
    -- Is what we see frined of foe?
    local isFriend = IsFriend(mobileObj)
    if (forceAttack) then isFriend = false  end
    -- if it is foe either attack or be alert
    if (not isFriend) then
        AI.AddToAggroList(mobileObj,2)
        if (forceAttack) then
            this:SendMessage("AttackEnemy",mobileObj,true)  
        else
            AI.StateMachine.ChangeState("Alert")   
        end         
    end
end

In the last part there are two ways to react, either by a manual state change to the "Alert" state of the FSM or an indirect way by sending a message to attack, which in turn will find the appropriate combat state.

So - it seems we have at least half-way understood the sensing, and the function above is already "thinking", but what about the acting and what are these dreaded "states" which seem to be so important?

Act

A state in the SO statemachine is implemented as a collection of stuff inside a table. So - lets have a look at that table:

--Essential states of any mob 
AI.StateMachine.AllStates = {
    Disabled =           {-- More Stuff in here },
    DecidingIdle =       {-- More Stuff in here },
    Idle =               {-- More Stuff in here },
    GoToLocation =       {-- More Stuff in here },
    Pursue =             {-- More Stuff in here },
    GoHome =             {-- More Stuff in here },
    Alert =              {-- More Stuff in here },
    WhereDidHeGo =       {-- More Stuff in here },
    Wander =             {-- More Stuff in here },
    DecidingCombat =     {-- More Stuff in here },
    AttackSpecial =      {-- More Stuff in here },
    AttackAbility =      {-- More Stuff in here },
    Chase =              {-- More Stuff in here },
    FleeToSafeLocation = {-- More Stuff in here },
    Flee =               {-- More Stuff in here },
    Dead =               {-- More Stuff in here },
    Melee =              {-- More Stuff in here },
}

So - we see a big table with a bunch of states, each with a descriptive name which gives us an idea about the state, but how do states work?

Anatomy of a statemachine state

Basically they are mini-programs interpreted by the statemachine logic - so lets see how such a state functions:
This is a version of the GoToLocation state above, again - I removed DebugMessages an comments and added my own comments for better understanding

GoToLocation = {
        -- This defines how often AiPulse() will be called in ms (500 ms in this example)
        -- It has a big influence on how costly this script will be in terms of CPU cycles
        GetPulseFrequencyMS = function() return 500 end,

        -- This function always gets called first when entering a state
        OnEnterState = function()
            if CheckPathLocation(AI.Destination) then DecideIdleState() return end
            this:PathTo(AI.Destination,AI.GetSetting("ChargeSpeed"),"GoToLocation")
        end,

        -- The heartbeat of the state. 
        -- It gets called regularly as defined by GetPulseFrequencyMS()
        AiPulse = function ()
            if CheckPathLocation(AI.Destination) then DecideIdleState() return end
            this:PathTo(AI.Destination,AI.GetSetting("ChargeSpeed"),"GoToLocation")
        end,

       -- Gets called last, before we exit the state
        OnExitState = function()
            AI.Destination = nil
        end,
    }