Struct chalk_engine::Minimums

``````pub(crate) struct Minimums {
pub(crate) positive: TimeStamp,
pub(crate) negative: TimeStamp,
}``````
Expand description

The `Minimums` structure is used to track the dependencies between some item E on the evaluation stack. In particular, it tracks cases where the success of E depends (or may depend) on items deeper in the stack than E (i.e., with lower DFNs).

`positive` tracks the lowest index on the stack to which we had a POSITIVE dependency (e.g. `foo(X) :- bar(X)`) – meaning that in order for E to succeed, the dependency must succeed. It is initialized with the index of the predicate on the stack. So imagine we have a stack like this:

``````    // 0 foo(X)   <-- bottom of stack
// 1 bar(X)
// 2 baz(X)   <-- top of stack``````

In this case, `positive` would be initially 0, 1, and 2 for `foo`, `bar`, and `baz` respectively. This reflects the fact that the answers for `foo(X)` depend on the answers for `foo(X)`. =)

Now imagine that we had a clause `baz(X) :- foo(X)`, inducing a cycle. In this case, we would update `positive` for `baz(X)` to be 0, reflecting the fact that its answers depend on the answers for `foo(X)`. Similarly, the minimum for `bar` would (eventually) be updated, since it too transitively depends on `foo`. `foo` is unaffected.

`negative` tracks the lowest index on the stack to which we had a NEGATIVE dependency (e.g., `foo(X) :- not { bar(X) }`) – meaning that for E to succeed, the dependency must fail. This is initially `usize::MAX`, reflecting the fact that the answers for `foo(X)` do not depend on `not(foo(X))`. When negative cycles are encountered, however, this value must be updated.

Fields

`positive: TimeStamp``negative: TimeStamp`

Implementations

Update our fields to be the minimum of our current value and the values from other.

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