Undum 2 documentation

New Choice System

New in Version 2

In version one of Undum, all links from one situation to another had to be created manually using HTML. If you wanted a set of choices to offer, you had to create the links yourself, put them in the appropriate HTML container. If you wanted those choices to depend on the current state of the game (say, a choice to look behind a secret panel that was only visible when the panel was discovered), you had to code that logic yourself in HTML. This meant that, in practice, most Undum games were simpler, and rarely used branches that depended on context.

In version 2 both of these issues were addressed: the ability to quickly build the links to new situations in the standard HTML format (using the System.writeChoices method), and the ability to generate a list of situations that are available, given the current state of the game.

Generating Choice HTML

Thew new System.writeChoices method allows you to give a list of situation id's and have Undum generate a standard looking block of choices.

It does this by asking each choice how it prefers to be displayed, by calling its optionText method. This allows situtions to change how they are displayed depending on the current state of the character.

Also called is the situations canChoose method. This will normally return true, but if it returns false, the option will still be displayed, but not as a link, and will not be clickable. This allows you to show the player that an option would be available, if they did something else first, such as increase a quality.

On its own System.writeChoices is still mostly manual: it finds the link text for you, and builds the HTML, but you still have to give it a set of situation ids that you want for your choices.

Generating Choices

Undum now also provides the System.getSituationIdChoices method which automatically compiles a list of situation ids, which can then be passed to System.writeChoices for display. This method is powerful and complex, so we'll explore its use in increasing depth.

Generating Choices by Tags

Situations now can have one or more tags associated with them. You can ask System.getSituationIdChoices to return the ids of any situtions that match a tag. This allows you to easily build decisions that you can extend later. You might have a 'chapter' tag, and you mark each situation which begins a chapter using this tag, you can then do.


to return all chapter choices.

The way tags are processed tries to be intelligent. You can match on more than one tag, and any situation matching either tag will be returned, but each situation will be returned no more than once. You can even mix tags and explicit situation ids:

system.getSituationIdChoices(['#chapter', 'introduction', '#endmatter'])

Ordering Choices

Choices returned by System.getSituationIdChoices will be ordered based on a new displayOrder numeric property of situations. This allows you to make sure situations appear in a logical order, regardless of whether they were selected by id or by tag.

Conditional Appearance

So far, any matched situation will always be featured in the list of choices. A situation can't be visible some times and not others, depending on the current state of the game. (As we saw with the canChoose method, we can have it be clickable only in some states).

To allow situations to be totally absent in some cases, there is a new canView method on situations. This is only used by system.getSituationIdChoices, and allows a situation to opt out of being included, if its conditions for appearance are not met. This allows us to implement the secret panel from the introduction.

We could have a 'go-to-the-basement' situation with tag 'from-the-hallway' available always, while 'go-to-the-secret-room' has the tag 'from-the-hallway', but a canView method:

secretRoomSituation.canView = function(character, system, host) {
    return character.sandbox.has_found_secret_panel;

We can then call system.getSituationIdChoices(['#from-the-hallway']) and have the correct choices displayed, depending on the current state of the game.


One common requirement is to have a set of choices which can be 'overrulled' if some condition is true. So we might want the player to choose where on the island to go to, but if the character is injured, we might want to only allow the character to go to the hospital.

By default, all situations have a priority of 1. If you give a situation a higher priority, then it will be considered first. If its canView method returns true, then the high priority situation will be the only one displayed.

So the injury example might have

  • Docks tag:location canView:always priority:1
  • Market tag:location canView:always priority:1
  • Hospital tag:location canView:when hits < 10 priority:2
  • Doctor tag:location canView:when hits < 10 priority:2

Then if the hits quality = 10, we'd see

  • Docks
  • Market

but if the hits quality dropped to 9, we'd see just

  • Hospital
  • Doctor

Note that any number of results can be displayed in both cases, but the higher priorities mask the lower ones.

If you are working with choices that all have a canView function, it is a good idea to have a 'fallback' situation that can always be viewed, but has a priority of zero. This will only be visible if none of the others are available, and will prevent the game from ending at that point.

Maximum Choices

When you ask for a list of choices, you can specify a maximum number to return. If more than this number of choices are available, then the system will select a random subset to return, to make sure you get the number you asked for.

By default, all matching situations are equally likely to be returned, but you can make some situations rarer or more common by setting their frequency value. By default, this value is 1 for a situation.

A situation with a frequency of 100 will be chosen 100 times more often than a situation with a frequency of 1, if there is one situation that needs selecting. In cases where more than one situation needs chosing, the frequencies are a little less intuitive.

Consider three situations with frequencies of 1, 1, 100, competing for two spaces. The 100-frequency situation will be chosen almost every time, but for the other space, one of the 1-frequency situations must be chosen. So the actual probabilities will be roughly 50%, 50%, 100%, even though the frequencies were 1, 1, 100. When selecting more than one result, frequencies can only be a guide.

Minimum Choices

Although rarely used, you can also ask for a minimum number of choices. This changes the way priority values work. It starts from the highest priority situations, as normal, but rather than returning only those at the highest priority level, it checks to see if that set is enough to meet its minimum. If not, then the next priority level down will be considered as well, and so on, until the minimum is reached.

If a minimum and a maximum is given (the maximum being at least as large as the minimum), then only the lowest priority situations will be randomly selected for any remaining spaces, higher priority situations will be guaranteed to be chosen.

The most common use of this parameter is to set a very high value, such as Number.INT_MAX, with no maximum limit. This ensures that all valid situations are returned, regardless of their priority level.

Choices and SimpleSituation

SimpleSituation provides built-in support for ending its content with a block of choices, using System.getSituationIdChoices to generate the list and System.writeChoices to generate the HTML. To use this, simply pass in the list of ids and tags as the choices option. You can additionally specify minChoices and maxChoices if you need them.