month.go

package command

import "bytes"

// months maps the GNU -M three-letter month abbreviations to calendar order.
// An unknown month sorts as 0, before January, matching GNU sort.
var months = map[string]int{
	"JAN": 1, "FEB": 2, "MAR": 3, "APR": 4, "MAY": 5, "JUN": 6,
	"JUL": 7, "AUG": 8, "SEP": 9, "OCT": 10, "NOV": 11, "DEC": 12,
}

// monthLess compares keys by calendar month (-M), e.g. Jan < Feb. Keys that are
// not month names share rank 0 and fall back to lexical order.
func monthLess(a, b []byte) bool {
	ma := monthRank(a)
	mb := monthRank(b)
	if ma != mb {
		return ma < mb
	}
	return lexicalLess(a, b)
}

// monthRank returns the calendar rank of a month abbreviation, or 0 if unknown.
func monthRank(key []byte) int {
	abbr := bytes.ToUpper(bytes.TrimSpace(key))
	if len(abbr) > 3 {
		abbr = abbr[:3]
	}
	return months[string(abbr)]
}