Lesson 04 · The wildcard - Build a Cron Expression Parser and Scheduler

The wildcard

The asterisk is cron's most common token - it means every value the field allows. Today parseField expands `*` to the field's full range, and Parse records that a `*` day field is unrestricted.

The goal

Expand `*` to the full set of a field's values and mark `*` day fields unrestricted.

Start here - the target

TO DO

Scenario: A wildcard expands to every value in range

Giventhe minute field bounds 0-59 and the month field bounds 1-12

WhenparseField compiles '*' for each

Thenthe minute set is every value 0 through 59 (60 values) and the month set is 1 through 12 (12 values)

AndParse('* * * * *') sets DomRestricted false and DowRestricted false

Background

A * in a cron field is a wildcard: it places no constraint, so it stands for every value the field can take. For minutes that is the whole set 0 through 59; for months, 1 through 12. Because a field compiles to a set of allowed values, the wildcard is just the special case where the set is the field’s entire range - no new machinery, only the widest possible set.

The wildcard is also what the day-restriction flags key off. A day-of-month or day-of-week of * means “any day,” so it is not a restriction, and Parse records DomRestricted or DowRestricted as false. That distinction looks idle today, but it is the switch that decides, several lessons from now, whether the two day fields are combined with AND or with OR. Pin both the full expansion and the cleared flags.

Make it work

// in parseField, before the number path:
if text == "*" {
  set := map[int]bool{}
  for v := spec.Min; v <= spec.Max; v++ { set[v] = true }
  return set, nil
}

CheckpointDONE

`*` expands to a field's full range and clears the day-restriction flags. Commit and stop here.