beehive.lua

A functional behavior tree implementation in lua.

Install

Copy the beehive folder somewhere and start behaving.

Example

Here is a high-level example of using these functions with custom behavior tree functions.

-- Ask some other system if this entity can see the player.
local function canSeePlayer(entity, dt)
  if lineOfSightSystem:canSeePlayer(entity) then
    return 'success'
  else
    return 'failure'
  end
end

-- Wait a bit of time before succeeding.
local function waitRandom(low, hi)
  local elapsed = 0
  local span = love.math.random(lo, hi)

  return function(entity, dt)
    elapsed = elapsed + dt
    if elapsed >= span then
      elapsed = 0
      span = love.math.random(lo, hi)
      return 'success'
    end
    return 'running'
  end
end

local function wanderOnPlatform(entity, dt)
  -- Wandering on platform left as exercise to the reader.
  -- Imagine it returns 'running' until the entity gets to
  -- the edge of the platform. Then it returns 'success'.
  return 'success'
end

local function shootBlaster(entity, dt)
  -- Makes entity shoot, then always returns 'success'.
  return 'success'
end

-- Walk to the edge of the platform, then wait a bit.
local function walkAround()
  return sequence({
    wanderOnPlatform,
    waitRandom(1, 3)
  })
end

-- If the player is visible, then shoot them 3 times in rapid succession.
local function shootPlayer()
  return sequence({
    canSeePlayer,
    repeat(3, sequence({
      shootBlaster,
      waitRandom(.3, .3)
    }))
  })
end

-- Mostly spend your time walking around. If the player pops
-- up, shoot them.
return function()
  return selector({
    shootPlayer(),
    walkAround()
  })
end

The returned function (at the end there) is a behavior tree function: it expects two arguments, entity and dt (because the "leaf" functions expect entity and dt). Traverse this tree every frame to behave.

Usage

Every node in the behavior tree is a function that is invoked every time through the tree. This library provides the basic utility functions for behavior trees and some extra ones just to be nice.

NOTE: This library expects each brain to receive its own instance of the tree of functions. You can't create the tree of functions once and then re-use it across seven different entities. The provided functions each maintain some state across executions and their results can't be re-used.

The main three are:

• selector
• sequence
• parallel

The nice-to-have ones are:

• fail
• repeat
• invert

Each of these will return one of 'success', 'failure', or 'running' as strings after its execution.

In beehive, a "behavior tree function" is a function that takes a variable number of arguments. When invoked, that function will return one of 'success', 'failure', or 'running'.

Each of the functions takes variadic arguments, i.e. pass in anything you want and your child functions will receive them. Most of the examples stick with entity and dt, but you can use whatever you want.

Each of the functions below, when run, returns a behavior tree function.

selector

This function takes one argument, a list of behavior tree functions.

This function will run each of its children until it finds one that returns 'success'. It will start from the beginning of its list of children the next time it is executed if it succeeded last time.

If one of its children returns 'running' it will return 'running'. On its next invocation, it will run only that child until it either succeeds or fails.

sequence

This function takes one argument, a list of behavior tree functions.

This function will run each of its children until it finds one that returns 'failure'. It will start from the beginning of its list of children the next time it is executed if it failed last time.

If one of its children returns 'running' it will return 'running'. On its next invocation, it will run only that child until it either succeeds or fails.

parallel

This function takes one argument, a list of behavior tree functions.

This function will run all of its children when invoked. If it receives any result other than 'success' it will return that value. If all its children succeed, then it will return 'success'.

NB: parallel will return the last non-'success' value that it receives from its children, i.e. if the second-to-last returns 'failure' but the last returns 'running' then this function will return 'running'.

fail

Returns 'failure' unconditionally.

repeat

This function takes two arguments: a number (times) and a behavior tree function.

On every invocation is will run its child function and increment a counter. If the counter is less than the times argument, this function will return 'running'. Once the counter exceeds the times argument it will return 'success'.

If its child returns 'failure' it will reset its counter.

invert

This function takes one argument, a behavior tree function.

It will invoke its child function. If its result is 'success', this will return 'failure'. If the result is 'failure' this will return 'success'. Otherwise, it returns the result received from its child function (hopefully the last case is 'running').

LÖVE example

Using the most excellent LÖVE framework, you might do something like this:

-- This is the example function from above.
-- It shoots the player when they're seen, but spends
-- most of its time walking around.
function enemyBrain()
  return selector({
    shootPlayer(),
    walkAround()
  })
end

function Enemy(x, y)
  -- Each enemy gets its own copy of the tree because we call
  -- the enemyBrain function here whenever we make a new one.
  local brain = enemyBrain()
  return {
    pos = {
      x = x,
      y = y,
    },

    update = function(self, dt)
      -- The instance of the entity is passed to the tree.
      -- Any nodes have access to entity methods and properties.
      brain(self, dt)
    end,

    draw = function(self)
      -- Cool drawing code here...
    end,
  }
end

function love.load()
  enemy1 = Enemy(3, 4)
  enemy2 = Enemy(8, 11)
end

function love.update(dt)
  enemy1:update(dt)
  enemy2:update(dt)
end

function love.draw()
  enemy1:draw()
  enemy2:draw()
end

Dev

The unit tests require busted. The Makefile just calls busted.