Decision Modeling and Optimization in Game Design, Part 8: Graph Search

This article is the eighth in a continuing series on the use of decision modeling and optimization techniques for game design.  The full list of articles includes:

• Part 1: Introduction (Blogspot) (Gamasutra)
• Part 2: Optimization Basics and Unrolling a Simulation (Blogspot) (Gamasutra)
• Part 3: Allocation and Facility Location Problems (Blogspot) (Gamasutra)
• Part 4: Competitive Balancing Problems (Blogspot) (Gamasutra)
• Part 5: Class Assignment Problems (Blogspot) (Gamasutra)
• Part 6: Parametric Design Techniques (Blogspot) (Gamasutra)
• Part 7: Production Queues (Blogspot) (Gamasutra)
• Part 8: Graph Search (Blogspot) (Gamasutra)
• Part 9: Modular Level Design (Blogspot) (Gamasutra)

The spreadsheet for this article can be downloaded here: link

Metroid Prime World Design

If you’ve ever played any of the games in the Metroid Prime trilogy (or for that matter, any of the Metroid games), then you’re familiar with the way a typical Metroid world is set up.  Each map is a complex, interlocking set of rooms, with many parts of the map sealed off by requiring access to different suits, weapons, and power-ups.  As the player gains access to new technologies such as "morph ball mode," "spider ball mode," and different suits and weapons, different parts of the map structure gradually unlock, forcing a large amount of re-traversal for the player to visit each new unlocked area.

As a result of this complexity, handling all of the traversal and re-traversal opportunities available to the player at each point in time is one of the major game design challenges on a Metroid style of game.  If we move any rooms, change the connections between rooms, or change the location where the player gets a certain suit or weapon or power-up, that has the potential to completely change the player's flow and quite possibly break the game.

The in-game map looks something like this:

If you’re a programmer, you’d probably look at a map structure like this and immediately realize the potential for all sorts of design tools you could use to build and manage the map structure.  Most likely, you'd make a system where designers could:

• chart out a map's structure graphically,
• get instantaneous feedback on where the player could move at each stage of the game,
• view the combination of rooms the player could access with any combination of power-ups, and
• instantly determine the fastest route from any room to any other room with any possible combination of power-ups (or indicate that no such path exists).

Under the hood, you'd probably use an A* search or Dijkstra's search along the graph, taking the player’s movement capabilities into account while crossing each graph edge.

A tool like that would be relatively easy to create; it would probably only take a talented programmer a few days to write, and it would likely save designers several weeks over the life of the project.

Surprisingly, though, the creators of the Metroid Prime series never had a tool like that.  When I worked at Retro Studios / Nintendo between 2003 and 2008, we did it the old-fashioned way, and our then-Assistant Producer meticulously maintained large printed maps of all the worlds of each Metroid Prime game, hanging all of them around the company meeting room.  Each time one of the worlds changed, he would edit the associated map graph and re-print all the affected world maps.  These maps looked a lot like this:

As much as we'd like to actually create a "map design tool" like this, we promised in Part 1 that we wouldn't write any code in this series.  So that kind of a full-featured map design tool is outside the scope of this article.

What we'll do instead is show how we can handle a problem like this directly inside Excel.  So even if we're designers who can't get any programmer support to build this kind of a tool, we can still create something similar without having to rely on outside help.  And once we're done, we'll have a good template in place that programmers can take and run with to build exactly that kind of a map design tool.

The Metroid World Example

To illustrate this problem, we will build an example "Metroid world" and show how to solve it in Excel.  In order to prove that you can actually handle a problem like this from right inside Excel regardless of complexity, we’re going to make this a very complex problem, with 17 rooms and non-obvious connections between them.

Our example world will look like this:

Each bubble shows a different room, and the arrows indicate the connections between pairs of rooms.  The number next to each connection indicates the cost of traversing that connection in either direction.  We'll assume these costs fully account for the traversal time to get between any pair of rooms, and there are no costs associated with the rooms themselves (i.e. the traversal costs are to some arbitrary point inside each room).

We'll explain the coloring of the different rooms later.

Once we've built the model, we'll be able to ask it questions such as "what's the shortest path for the player to get from the Library to the Sanctuary?"

A Simpler Problem

Let's start by working our way up to that with a simpler problem.  Assume we have 6 rooms, 'A' through 'F', with connections as shown in the following directed graph.

We want to know: what's the fastest way from A to F?  Is it ABEF, ABCEF, ABCDF, ADF  ...?

You can quickly figure it out yourself by adding up the weights in all four potential paths, but let's see how we'd encode it in Excel Solver.

First, we make a column with all the possible arcs: A->B, A->D, B->C, etc.  For each arc, we list the total distance of that arc (its traversal cost) in the "Total Distance" column.  To the right of that, we have columns for each room, and for each arc, we put a '1' in the room where the arc originates and a '-1' in the room where the arc terminates (so, for example, "C->D" has a '1' in the 'C' column and a '-1' in the 'D' column).

We also have a "Decision Variables" column in yellow -- this indicates which arcs the decision model will actually use.  Right now, we'll set all of those to 0 to indicate that they're not currently used (i.e. the problem hasn't yet been solved).

Because we're following the formatting conventions listed in Part 2, the decision variables are yellow and the predefined values stated as part of the problem description are in blue.

Now, we add two rows beneath the right part of the matrix to sum up the current target values: this is the "Current" row shown below.

The "Target" row indicates what we're attempting to achieve.  We simply put in a '1' for the origin, a '-1' for the destination, and '0' for all the other values.  We will use these when we set up the constraints later in the exercise in order to make sure that any potential solution begins and ends where we want it to.

Finally, we set up the objective function.  This is simply the decision variables multiplied by the respective values in the "Total Distance" column (implemented with the Excel SUMPRODUCT() formula).  In other words, if any arc is used, its decision variable will be 1, and we will multiply it by the respective length of that arc in the "Total Distance" column; otherwise, its decision variable will be 0, and we will get a 0.  This will ensure that the objective function accurately measures the total distance of all the arcs that are actually used in whatever solution we find.

Now, we solve (see Part 2 if you need a refresher on how to find and use Excel Solver).

In our objective ("Set Objective"), we specify the orange objective cell.  Since the objective represents the total distance, and we want to minimize the total distance, we select the "Min" radio button below that.  For our decision cells ("By Changing Variable Cells"), we specify the yellow "Decision Variables" column.  In the constraints section, we specify that the yellow decision cells have to be integers that equal 0 or 1.  We also specify that the "Current" row has to equal the "Target" row, to ensure that the path begins and ends at the correct rooms.

Before we solve, note that "Select a Solving Method" at the bottom has "GRG Nonlinear" specified instead of the usual "Evolutionary" solver.  We'll have a more detailed discussion of the different solver types in a future episode, but for now, suffice it to say that the GRG Nonlinear solver ("GRG" stands for Generalized Reduced Gradient) sometimes handles problems of this type much better than the Evolutionary solver does.

Now, we hit Solve, and Solver fills in '1' values for the decision variables associated with A->B, B->C, C->D, and D->F, for a total path cost of 8 (note that Solver may take a few minutes to find this solution).  You can verify by hand that this is the correct answer by adding up all of the path costs of the other possible paths, which are all greater than 8.

You can find this solution in the "Sample solution" worksheet of the attached spreadsheet.

Searching in the Metroid World

With that framework in hand, let's generalize it to the large Metroid sample world above.

One big change in this version is that this example includes bidirectional links instead of directed arcs between rooms.  This means our decision variables will no longer simply be '0' or '1' to indicate whether an arc forms part of the shortest path; instead, we'll also use '-1' to indicate that the path is traversed backwards.

[Note that this approach only works because all of the arcs in the Metroid example are bidirectional.  If we had any directional arcs in our example, we would have to use the previous approach, and specify any bidirectional arcs as two separate rows in the spreadsheet (for example, we'd have to specify "Lab -> Oculory" and "Oculory -> Lab" on two separate rows to indicate that the player can move between the two rooms in either direction).]

Our spreadsheet looks similar to the previous one, except that we now add two columns, "Traversal direction" and "Absolute value of decision variables."  "Traversal direction" is just a visual assistant to indicate which direction we'll traverse a link, with a decision variable value of '1' translated into "Forward" and a value of '-1' translated to "Backward."

We also change the objective function so that it multiplies the total distance column by the "Absolute value of decision variables" column rather than the decision variables themselves (since traversing a link backwards gives us a '-1' in the "Decision Variables" column, and multiplying this by the distance value would give us a negative distance).

We also need to change our constraints so that it accepts integers between -1 and 1 in the "Decision Variables" column instead of 0 to 1.

In the "Target" row below the room source/destination matrix, we enter a '1' for the Library and a '-1' for the Sanctuary to ask Solver to figure out the shortest path from the Library to the Sanctuary.

After these changes, we solve again.  Note that because this is a much more challenging problem, we now have to tweak the Solver options a bit.  In the Solver dialog, we hit Options, select the GRG Nonlinear tab, and then check "Use Multistart" and set "Population Size" to 20 (see this link for more info).  This will make Solver do a much more exhaustive search and will make it much more likely to find the global optimum, and the expense of taking a bit more time to solve the problem.  With this option set, Solver may take 3-5 minutes to find the correct solution.

After Solver finds a solution, we scan the "Traversal direction" column to make sense of the solution that Solver came up with.  We get the following as the shortest path:

Library -> Sanctum -> Plaza -> Starport -> Chapel -> Sanctuary

...  for a total cost of 21.

You can find this setup in the "Metroid world solution" worksheet of the attached spreadsheet.

Adding Movement Constraints

Of course, this still doesn't quite give us what we want.  The actual Metroid worlds have complex movement requirements that limit the player's access at any point depending on the exact combination of equipment he has acquired.  What we really want is a way to limit the search to only those rooms that are valid with a given combination of equipment.

There are a lot of different ways to do this, but for the sake of simplicity, we'll just assume each room in the Metroid world graph above requires one particular type of equipment.  Then we'll solve that problem and leave any more sophisticated implementations as an exercise for the reader.

We will assume the following colors in the graph have the following meanings:

• Pink rooms (Library, Ruins, etc) don't require any special equipment.
• Green rooms (Sanctum, Plaza, etc) require the player to have the morphball technology
• Blue rooms (Courtyard, Throne Room, etc) require the player to have the Varia suit upgrade
• Red rooms (Chapel, Furnace, etc) require the player to have access to missiles

The goal is to use Solver to help us figure out how the optimal path for the player changes depending on whether or not he has the morphball, Varia suit, and/or missiles.

We'll start by adding an "equipment required" table near the top of the spreadsheet:

This table uses a '1' for each cell to indicate that the player currently has access to that technology, or a '0' if he doesn't.  So far, we've been assuming that the player can travel anywhere, so we've filled in a '1' for each technology.

Now, we add two rows directly below the arc matrix, "Room category" and "Multiplier."

Room category specifies the equipment type of the room listed in the same column -- None ('0'), Morphball ('1'), Varia suit ('2'), or Missiles ('3').

The Multiplier row contains a formula that checks the room category in each column against the "Equipment required" table above.  It produces a '1' if the player possesses the appropriate equipment, and a '0' otherwise.

The grey cells below that are a table that combines the arc reachability table above with the "Multiplier" row.  It produces a '1' if the room in the same column is included in the path for the corresponding row, and the multiplier in the same column is 1 (in other words, it multiplies the absolute value from the arc reachability table by the multiplier in the same row).

We also add an additional column to the table just to the right of the "Total distance" column: "Traversable with current equipment."  This column will produce a '1' if both rooms in the arc are reachable with the player's current equipment.  If not, it will produce a value of '999' in order to make the arc prohibitively expensive.

Finally, we change our objective cell (the orange cell called "minimum total distance") to also include this column in its SUMPRODUCT() formula.

Now, we run Solver exactly as before.  Make sure you have the Options set as specified above (Use Multistart = TRUE with a Population Size of at least 20), or you may not get the desired results.

First, we'll set the value for "Morphball" in the "Equipment required" table at the top of the spreadsheet to 0 and re-solve (note that this may take 3-5 minutes to solve).  This is the optimal path with no morphball:

Library -> Ruins -> Survey Room -> Courtyard -> Lab -> Oculory -> Chapel -> Sanctuary

...  for a total cost of 25.

Next, we set "Morphall" back to 1 and set the cell below it to 0 to find out the optimal path when the player doesn't have the Varia suit:

Library -> Sanctum -> Plaza -> Starport -> Chapel -> Sanctuary

You'll notice that this is exactly the same as the solution we found in the previous worksheet, before we began taking the player's equipment into account.

What happens when we set both of these to 0, so the player doesn't have the morphball or the Varia suit?  Re-solving, we get:

Library -> Ruins -> Foyer -> Mausoleum -> Sanctuary

for a total cost of 41.  As you can see from the graph above, going down the very expensive Foyer -> Mausoleum arc (with a cost of 27) is the only option available when both the morphball (green) and Varia suit (blue) are disabled  ... and this is, in fact, the shortest path that includes that link.

Problem solved!

You can find this spreadsheet in the "With equipment requirements" worksheet of the attached spreadsheet.

Conclusion

In this example, we've shown how you can use decision modeling and optimization techniques as a powerful tool to solve graph search and reachability problems.  This particular Metroid game world example is of course only one application of this technique; there are many more.  You could just as easily use it to find the shortest path through a star system in a space game with direct links between stars, for example, or the shortest path along a flight path network in an MMO such as World of WarCraft.

Tune in next time, when we'll show how you can use decision modeling and optimization techniques to not only search through game world configurations, but to help design those configurations in the first place.


-Paul Tozour

This article was edited by Professor Christopher Thomas Ryan, Assistant Professor of Operations Management at the University of Chicago Booth School of Business.

Decision Modeling and Optimization in Game Design, Part 7: Production Queues

This article is the seventh in a continuing series on the use of decision modeling and optimization techniques for game design.  The full list of articles includes:

• Part 1: Introduction (Blogspot) (Gamasutra)
• Part 2: Optimization Basics and Unrolling a Simulation (Blogspot) (Gamasutra)
• Part 3: Allocation and Facility Location Problems (Blogspot) (Gamasutra)
• Part 4: Competitive Balancing Problems (Blogspot) (Gamasutra)
• Part 5: Class Assignment Problems (Blogspot) (Gamasutra)
• Part 6: Parametric Design Techniques (Blogspot) (Gamasutra)
• Part 7: Production Queues (Blogspot) (Gamasutra)
• Part 8: Graph Search (Blogspot) (Gamasutra)
• Part 9: Modular Level Design (Blogspot) (Gamasutra)

The spreadsheet for this article can be downloaded here: link

Introduction

In the previous articles in this series, we introduced the concepts behind decision modeling and optimization and showed how they can be useful in framing and solving a surprising number of design decisions.  We also discussed many of the limitations of these techniques.

In this post, we'll show how these techniques can be used to optimize an undeniably hard design problem.  So, hang on to your hats: this example is going to be a beast.

Once we're done, though, it should be very clear how this approach can be useful for tackling some very difficult design problems that can't reasonably be solved any other way.  Although it's a bit of work to set it up, the spreadsheet we end up with will serve as indisputable proof that decision models can answer some questions that are difficult or impossible to solve any other way.

Our example is based on a production queue for a colony in a classic "4X" strategy game.  As designers, we want to know: what's the best order to build things in our colony?

This will give us all sorts of insights into whether there is a "dominant strategy" that allows players to most easily win the game, or whether there are problems with the balancing of our game's buildings or the game rules.

If we were to have some sort of system that allowed us to automatically see the best production queue for a given city or colony, that would be fantastically useful, wouldn't it?  Then we could tinker with the costs and benefits of each building, or experiment with our game rules, and see how the optimal production queue changed.

In this installment, we're going to show you how to build exactly that kind of system.

Although this example is limited by a simplified game model and the limitations of Excel and Excel Solver, it should be clear to anyone with a programming background how you could easily extrapolate from this example to your own model in a high-level language such as C++, C#, or Java, and potentially integrate it with your own game as well.

Master of Orion

In 1993, a small studio in Texas released a brilliant classic "4X" strategy game called Master of Orion.  Three years later, they followed it up with another classic, Master of Orion II: Battle at Antares.  These games were two of the initial forerunners in the space-based "4X" strategy genre, and laid the groundwork for later franchises such as Stardock's successful Galactic Civilizations series.  Our example in this case will use colony in a game broadly similar to Master of Orion II.

A colony in MoO 2 is essentially identical to a city in a typical '4X' strategy game such as Civilization.  It serves as a growing population center, and the player can build one building at a time in the colony to increase its capabilities.

Let's say you're designing a new game in the style of MoO 2.  You want to know what the optimal initial build order is in the general case -- that is, all things being equal, what's the best sequence of actions for a player to build up a colony?

Being able to answer this question will be enormously useful to us in helping to balance the game and design all of the different building types.

Your population begins at 1 million people.  It has a defined "maximum population" of 10 million people.  It grows at a rate that depends on the ratio of the current population to the maximum population:

• At 10% of maximum population or less, it grows at 2.5% of the current population each turn
• At 20% of maximum population size, it grows at 3% of the current population each turn
• At 30% of maximum population size, it grows at 3.5% of the current population each turn
• At 40% of maximum population size, it grows at 4% of the current population each turn
• At 50% of maximum population size, it grows at 4.5% of the current population each turn
• At 60% of maximum population size, it grows at 4% of the current population each turn
• At 70% of maximum population size, it grows at 3.5% of the current population each turn
• At 80% of maximum population size, it grows at 3% of the current population each turn
• At 90% of maximum population size, it grows at 2.5% of the current population each turn

Also assume that the population is divided into "citizens," with each "citizen" representing 1 million people (rounded down).  Each citizen requires 1 unit of "food."  Each citizen can be dedicated to either farming or production.  Each citizen designated as a farmer will grow 3 units of food, and the colony will automatically allocate as many farmers as needed to growing enough food to ensure all citizens are fed, with the remaining citizens dedicated to production (i.e. they are "workers").

Also assume that a colony produces 1 unit of "production," plus 2 additional units for each citizen designated as a "worker."

There are 5 initial building types in a colony:

• A Hydroponic Farm generates 2 additional food every turn.  It costs 32 production units to build.
• The Biospheres improvement increases maximum population size by 3.  It costs 56 production units to build.
• The Cloning Center improvement adds 100,000 additional population every turn.  It costs 36 production units to build.
• The Factory increases the production of each worker citizen by 1.  It costs 44 production units to build.
• The Robotic Factory adds a flat 5 production units plus an additional 1 production per worker.  It costs 56 production units to build.

Additionally, there is a sixth thing we can build: Housing.  We can build "Housing" whenever we like; unlike the others, it is not a discrete "building" but a target for us to allocate resources toward.  Every turn that we build Housing, we generate an additional 10,000 population per unit of production.

This setup is a bit simplified compared to Master of Orion 2, but it's rich enough to prove the point we need to make here.

Our question is: given the above, what should the player build, and when should he build it, in order to get to the best possible colony as quickly as possible?

This is an even trickier problem than it first appears.  If it were simply a matter of finding the best ordering of a Hydroponic Farm, Biospheres, Cloning Center, etc, to build then we could just look through all 5!=120 combinations.

But in fact, the player can build Housing on any turn, and this has a huge effect on the colony's population.

So we're left with a much more complicated question: which of the 6 things (5 building types or Housing) should the player build for each of the N turns until all the buildings have been completed?

The Simulation

To solve this, we'll build a model conceptually similar to the one we used to solve for the tax rate in Part 2 of this series: we'll put the starting stats for the colony at the top of the attached spreadsheet and then show each new turn of the game on a subsequent row.  You can think of this as "unrolling" the game simulation onto the spreadsheet, with the vertical axis representing time.

We'll start by making a table of "Production ID numbers."  These are fixed numerical IDs we'll use to refer to each of the five building types, plus Housing.  For example, '2' means "Biospheres."  We'll show why this is useful in a moment.

Since we're carefully following the formatting standards we laid out in Part 2, these cells are all blue, indicating that they're fixed values that are part of the problem definition.

Next, we'll set up a table for the population growth rates we described earlier:

We'll also put in a table with some of the constants we described for the colony in the previous section.

Now that we have these tables in place, let's build the simulation.

We start by modeling population growth.  This part of the spreadsheet lists the turns along the leftmost column, and in the subsequent columns, it lists the current population (in thousands), the number of "citizens" (population in millions), the current population maximum, the ratio of current to maximum population.  Then, the "Growth Rate Next Turn" column looks up the current growth rate value in the "Growth table" above, while the "Growth Next Turn" multiplies this factor by the current population (since the growth rate is in terms of a percentage of current population).

You'll notice, however, that this is actually 35 instead of 25 for the first two turns.  This is because we're actually building Housing for now, which adds another 10k population a turn.  We'll get to that part of the spreadsheet in a moment.

"Population (in thousands)" grows every turn by adding the previous population to the "Growth Next Turn."  So, the 1000 population on turn 0 becomes 1000+35=1035 on turn 1, and so on.

Next, we'll model the citizens' job roles:

We know that each farmer produces 1 "unit" of food.  In this part of the spreadsheet, we determine the number of farmers and workers each turn.  Each new "citizen" added to the city will become a worker by default.  Any time the colony's food demand (= number of "citizens") exceeds the current food production (=3 x number of farmers), we'll reallocate one of these workers to a farming role instead.  The production column lists current production according to the formulas stated at the top (1 unit of production + 2 per worker, plus any modifiers based on whether we've built things like a Factory or a Robotic Factory).

Now, we model the 5 different buildings.  For each building, we simply list the "production units" required to construct it in the top row.  In each subsequent turn, the production units will decrease if we're working toward that building.  If the value in any of these columns becomes 0 in later turns, it indicates that the building has been completed successfully.

This gives us an easy way to track both the status of any building (whether it's been created or not), and our progress toward completing each of them.

Finally, in the rightmost column, we put in our decision cells.  These cells will be integers that express our production decision for what to build each turn.  Because we're following the formatting conventions laid out in Part 2, these cells are yellow, because they're the ones we're going to tweak with Solver.

The formulas in the spreadsheet are a bit too complex to go into in detail here; the reader is invited to download the attached spreadsheet.  However, the important thing to note is that as we change the cells in this column to any value from 1 through 5 (the Production ID Numbers we listed in the table above), the spreadsheet allocates all our production resources that turn toward any of the five different buildings in the "Production Remaining" column (download the spreadsheet and tinker with the values in this column to see for yourself).  If we keep them at 0, it represents Housing, and it adds to the colony's population instead.

Optimizing

There's still one thing we're missing: an objective function to help guide Excel Solver to the right answer.

Our objective function should be something that expresses how powerful our city is -- its total population and the sum of all its capabilities.  A highly skilled, "min-maxing" player will want to build the colony to its maximum capability level -- that is, maximum population, food production, and production output -- so that it can make the greatest possible contribution to the player's space empire.

Moreover, such a player will also want to achieve the maximum level of food and production output as early as possible, so that if the colony is interrupted by the need to do something else before this initial production queue is completed -- for example, it's attacked by an alien race and needs to retarget to military production -- it's likely to have a higher production capability at such a point to help it best address that challenge.

In other words, even though the blue curve and the red curve in the illustration below both get to the same endpoint of total production capability over time, we should prefer the red curve because it gets us to a higher level of total production capability earlier than the blue curve does.  So we care about the total area under the curve, not just the endpoint of the curve.

We'll build this as a customized function by adding three different values together:

• The total food production throughout all turns of the simulation.  It might be enough for us to simply take the total value of food production on the last turn, but that might not differentiate between scenarios where we ramped up to maximum food production earlier in the simulation and those where we only ramped up later.  All things being equal, we want to get to maximum food production as early as possible, so we'll take the sum of the food production over all turns.
• The total production over all turns.  Just as with food production, it makes the most sense to add the sum of production over all turns to reward earlier production gains.
• An additional penalty for any buildings left unfinished.  We want to ensure that all 5 building types are completed by the end, so we'll take the sum of all the remaining production units required on all 5 buildings and subtract it from the total.  This should also help Solver find the best solutions more easily, since buildings don't have any effect on food or production until they're completed, and this factor helps reward Solver for partially completing buildings, whereas the previous two elements did not.

We then add those three values together, and we now have the following at the bottom of the spreadsheet.

Now that we've set up the spreadsheet, running Solver is surprisingly easy (again, if you're unfamiliar with Solver or can't find it in your copy of Excel, see Part 2).  This simply sets the orange "total score" cell above as the optimization objective, tells Solver to try to maximize its value, and sets the decision cells to the entire row of yellow cells in the "Work on what?" column.  It also tells Solver that these need to be integers between 0 and 5 (matching the 6 different values listed as our Production IDs in the table above).

Now, we hit Solve.

Not So Good  ...

When you run Solver, you'll notice that it takes quite a long time to solve.  Since it's using the Evolutionary solving method (the drop-down box at the bottom of the dialog shown above), it can take a very long time to experiment with lots of different possible solutions and try to evolve the best solution.

In our first run, we get a total of 593 for our objective function after 64 turns of game time.  This isn't very good; only the Hydroponic Farm and the Robotic Factory have been built.  Repeated attempts to re-optimize don't seem to improve the solution at all.

The real problem here is that there are just too many parameters to optimize.  We've asked Solver to solve 64 different parameters; there are a massive number of possible solutions.  We've given Solver too big a haystack and too small a needle.

If you wanted to throw money at the problem, you could probably fix it by licensing a more powerful Solver engine from the folks who make Excel Solver.

But of course, that's not the approach I would recommend!  There are inefficiencies in our decision model setup, and it would be far better for us to fix those first and see if fixing them won't let Solver find a solution.

As anyone who's done AI pathfinding knows, the best way to optimize your pathfinding algorithm usually isn't to tweak the algorithm itself, it's to simplify the space that it needs to search on.  The simpler and coarser the pathfinding representation, the more quickly any search algorithm will run.

So we need to do the same thing: we need to take our 64 individual "What should I build this turn?" decisions and transform them into a much smaller set of decisions that Solver can handle using some kind of dimensionality reduction.



It Only *Looks* More Complicated ...

Let's rephrase our question.  We can take advantage of the fact that any time we start building a particular building, we'll want to keep building it until we're finished.  It seems safe to assume that the optimal production queue will build each of the five buildings in a focused stretch until they are done -- that is, there's nothing to be gained from stopping production on a building before it's finished to work on something else.

This assumption allows us to rephrase the question from "What should I build every single turn?" to a much simpler series of questions:

• How many turns should I build Housing before I create the first building?  (0 or more)
• Which building should I build first?  (Production ID 1 through 5)
• How many turns should I build Housing between the first and second buildings?  (0 or more)
• Which building should I build second?  (Production ID 1 through 5)
• How many turns should I build Housing between the second and third buildings?  (0 or more)
• Which building should I build third?  (Production ID 1 through 5)
• Etc  ...

Then we continue that pattern until all 5 buildings have been built.

In order to do this, we first implement a table above the spreadsheet that looks like this:

The yellow decision cells on the left are the various "How many turns of housing?" questions we will ask Solver.

The yellow decision cells on the right are the five production IDs of the five different buildings.

In the center, we have our production table, which lists the sequence of events -- in this example, 18 turns of Housing, followed by building #3 (Cloning Center), then 1 more turn of Housing, then building #5 (Robotic Factory), then 0 turns of Housing, then building #1 (Hydroponic Farm), etc.

Finally, in order to implement this change, we take the entire spreadsheet and copy it to a new worksheet in Excel.  Then, we replace all the cells in the "Work on what?" column with formulas that essentially express the build order as explained in the previous paragraph.  This will cause the Production table above to essentially "program" the build order.  We also change these cells from yellow to grey since they are no longer the decision cells.

The exact formula is very complicated, and I don't want to derail the article by going into all the minutiae of how it's expressed in Excel, but as always, you're welcome to download the spreadsheet and check it out for yourself if you're curious.  (In short, we break it out into two separate columns, "Turns of housing in a row" that lists how many turns remain to build housing, and "Index in production table" that increments as we go further down the production table.  The "Work on what?" column returns 0 (Housing) for even-numbered rows in the Production Table, and returns the corresponding Production ID for odd-numbered rows.)

We also have to change all of our Solver settings.  The dialog now looks like this:

Here, the decision cells ("By Changing Variable Cells") are the yellow cells at the left and right sides of the new Production Table we showed above ("Turns of Housing" and "Production ordering").  The constraints simply state that the "Turns of Housing" cells need to be integers between 0 and 40, and the "Production ordering" values need to be integers between 1 and 5.

[If you've been reading this series diligently, you may be surprised at what's missing in this step.  After all, the Production ordering table on the right side lists cells for the 5 buildings to create, each specified by a Production ID value from 1 to 5.  Don't we need to add some sort of special constraint here to ensure that they end up with unique values, and that we don't cause Solver to try to build the same buildings twice?

The answer is no  ...  or at least, probably not.  The objective function we've specified above should already reward Solver for building all the building types, and it won't benefit from trying to build a building more than once, so that should already naturally ensure that they're unique.  So we'll leave off implementing this constraint until we actually need it (and as you'll see, we never will, so we can forget about it).]


Conclusion

Now, we can run Solver again, and in just a few minutes, Solver should give us our final answer, with our objective function having a value of 884 (if Solver doesn't reach this value for you, try re-running the Solver; you may also need to mess with the Solver's evolutionary optimization settings to help it attain this value).

This solution represents the following:

• Build Housing for 10 turns.
• Then build a Cloning Center.
• Then build a Robotic Factory.
• Then build a Factory.
• Then build a Hydroponic Farm
• Finally, build Biospheres.

That's it!

In a relatively short time frame, we've solved a problem that it would be extremely difficult to solve with guesswork or with any number of testers, and we've gained useful design insights in the process.

Not only that, we now have a tool that can allow us to quickly see how future design changes will affect the optimal build order.  If we decide at some point down the road to lower the cost of Biospheres from 56 to 40, or change Hydroponic Farms to generate +3 food instead of +2, we can make that change and re-optimize the model to see how it changes the optimal initial build order.

From here, it's well worth looking at whether Cloning Centers and Robotic Factories are too powerful, too cheap, or too useful, or whether Hydroponic Farms and Biospheres might be too weak or too expensive.  It might be a good idea to play around with the benefits and the production costs of each of these buildings and find the exact point at which Solver changes its answer.  If we could find the point where Solver changed its mind about the ordering of two buildings, that would mean that we'd found the crossover point of those two buildings along a cost/benefit curve.

Ideally, we'd like to build a tool that would give us an instant sensitivity analysis illustrating how the value of each building changed as its cost (or any other parameter) changed -- though that's a topic for another day.

It's also worth noting that the buildings in this example lend themselves to a fairly well-defined objective function.  This won't necessarily be the case in general, as many buildings could have more ambiguous or context-sensitive contributions to a colony's success.  For example, the Marine Barracks in the example above generates marines to help with ground defense in case the planet is invaded.  The value of a Marine Barracks will scale depending on the planet's likelihood of getting invaded, and it will naturally be much more useful closer to the empire's borders than it will be in a backwater planet in the farthest sectors of the map where enemies can't reach -- there's no point wasting resources on building defenses for a planet that will never be invaded.

It also depends to a certain extent on how many turns have elapsed in the game itself, since there is unlikely to be much planetary invasion in the early game, while space empires are just developing, whereas later in the game, enemies become much more aggressive and their technologies allow them to invade more distant planets.

In these cases, we're left to improvise when determining how such a building would contribute to the production queue's objective function.  In the case of a Marine Barracks, we might have a contribution to the objective function that scales over time, or we might model multiple classes of colonies with different defensive needs (say, Low, Medium, and High), and give Marine Barracks and other such defensive buildings fixed weightings in each case, and see how the optimal production queue changes in each scenario.

Stay tuned! In future articles, we'll build decision models to help us design game levels, analyze game world layouts in a Metroid Prime style of game, and much more.


-Paul Tozour


This article was edited by Rob Zubek of SomaSim Games and Professor Christopher Thomas Ryan, Assistant Professor of Operations Management at the University of Chicago Booth School of Business.