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CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

Project Overview

A Go library for parsing cron expressions and computing the next matching time(s). Fork of github.com/gorhill/cronexpr. Module path is github.com/toba/cronexpr, requires Go 1.25+. Supports 5-field (standard), 6-field (with seconds), and 7-field (with seconds and year) cron expressions, plus predefined aliases (@yearly, @monthly, @weekly, @daily, @hourly).

Build & Test Commands

go test ./...              # run all tests
go test -run TestZero      # run a single test by name
go test -bench .           # run benchmarks
go test -v                 # verbose test output

Tests are in-package (package cronexpr), so they have access to unexported types.

Architecture

Three source files, one exported type (Expression), two entry points (Parse/MustParse):

  • cronexpr.go — Public API. Defines the Expression struct and the Parse/MustParse constructors, plus Next/NextN methods. Next walks fields from year down to second using slices.BinarySearch against pre-computed sorted value lists; on mismatch it delegates to the appropriate next* function in cronexpr_next.go.
  • cronexpr_parse.go — Parsing engine. Tokenizes each cron field using regex patterns, handles special characters (L, W, #), and populates the Expression fields. Uses fieldDescriptor structs to parameterize parsing per field type. Regex patterns are lazily compiled and cached in a sync.Map.
  • cronexpr_next.go — Time advancement. Contains nextYear through nextSecond functions that cascade when a field overflows. calculateActualDaysOfMonth merges day-of-month and day-of-week constraints per the crontab spec (if both are restricted, either match triggers).

Cron Expression Semantics

  • Day-of-week 7 is normalized to 0 (both mean Sunday)
  • 5 fields → seconds default to 0, year defaults to wildcard
  • 6 fields → seconds default to 0
  • When both day-of-month and day-of-week are restricted (not *), a day matches if either field matches (union, per crontab spec)
  • W (nearest weekday) does not cross month boundaries
  • Year range is 1970–2099