Lesson 11 · Named days of the week - Build a Cron Expression Parser and Scheduler

Named days of the week

The day-of-week field accepts three-letter names too - `SUN` is 0, `MON` is 1, up to `SAT` at 6. Today you translate day names the same way you translated months.

The goal

Accept SUN through SAT (any case) in the day-of-week field as aliases for 0 through 6.

Start here - the target

TO DO

Scenario: Day names translate to their numbers

Giventhe day-of-week field

WhenparseField compiles 'MON-FRI'

Thenthe set is {1, 2, 3, 4, 5}

AndparseField compiles 'SUN' to {0} and 'SAT' to {6}, case-insensitively

Background

Days of the week get the same name treatment as months, and it is the same technique: before the field is split into parts, upper-case the text and swap each three-letter day name for its number. Sunday is 0, Monday 1, and so on through Saturday at 6, matching the day-of-week bounds you set back in lesson 2. Once the substitution is done the field is numeric again, so MON-FRI becomes 1-5 and expands to the working week.

Keep the month and day tables separate even though the mechanism is shared - SUN is only meaningful in the day-of-week field, and JAN only in the month field. Apply each field’s name table only to that field. This sets up the one genuinely surprising day-of-week rule, which is the next lesson: the number 7 is also Sunday.

Make it work

var days = map[string]string{
  "SUN": "0", "MON": "1", "TUE": "2", "WED": "3",
  "THU": "4", "FRI": "5", "SAT": "6",
}
// for the day-of-week field, upper-case and replace each 3-letter
// name with its number BEFORE splitting into parts.

CheckpointDONE

The day-of-week field accepts SUN through SAT as aliases for 0 through 6. Commit and stop here.