domains.md

Domains

Domains are a special type. A domain stores integers which typically represent potential choices. For example, a domain may represent the hours a shift might start or the destinations a traveler could arrive at.

Structurally, a domain is an ordered, compact, set of integers. Domains maintain a minimal representation of ranges as we apply operators to them to create new ones. They are also, conveniently, immutable.

Nextmv's SDK provides two domain types: model.Domain and store.Domain. model.Domain is just an integer domain, unattached to a store. store.Domain has many of the same methods but with similar mechanics to store.Slice and store.Map. Thus, model.Domain is the underlying type for store.Domain, which must be associated with a store.

Let's create a few domains to see how they work.

s1 := store.New()
d1 := store.NewDomain(s1)
d2 := store.NewDomain(s1, model.NewRange(1, 10))
d3 := store.NewDomain(s1, model.NewRange(-5, 5), model.NewRange(15, 25))

We can pass as many integer ranges as we want to store.NewDomain. If we pass none, the domain is empty, like d1. d2 contains the integers 1 through 10, while d3 contains two disjoint ranges of integers, -5 though 5 and 15 through 25.

Let's apply a change set to these domains and encode the store into JSON.

s2 := s1.Apply(
    d1.Add(42, 43, 44),
    d2.Remove(2, 4, 6, 8, 10),
    d3.AtLeast(10),
)

enc := json.NewEncoder(os.Stdout)
enc.Encode(s2)

In s2, d1 contains 42 through 44, d2 contains only odd numbers, and d3 contains the range 15 through 25. Note that each of these domains is encoded in a compact JSON representation. This is similar to its internal data.

[[[42,44]],[1,3,5,7,9],[[15,25]]]

Frequently, we want to create and operate on multiple domains at once. We can use functions like store.NewDomain or store.Repeat to create a slice of related domains. In the code below, we create five domains, each containing the values 1 through 10.

s3 := store.New()
d := store.Repeat(s3, 5, model.NewDomain(model.NewRange(1, 10)))

The store.Domains type has many of the same methods as the store.Domain type. Change methods require a domain index to modify as their first argument.

s4 := s3.Apply(
    d.Add(0, 42),
    d.Assign(2, 5),
    d.Remove(4, 2, 3, 4),
)
enc.Encode(s4)

The change set above results in the following domains.

[[[[1,10],42],[[1,10]],5,[[1,10]],[1,[5,10]]]]

There are many methods available on domains. Some allow us to modify them, while others help us select an individual domain from a collection of them. Take a look at the Go package documentation to see what domains have to offer. Run the source to get the outputs described here.

Exercises

• Create a domain on a store and a domain unattached to a store. Modify both of these domains in various ways.
• Create multiple distinct domains on a store. Use a selector method, like Smallest or Largest to select an individual domain by index. Assign that domain a value.
• Create a collection of domains each containing more than one value. Remove values from the domains until calling Singleton returns true.

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