1 files changed, 52 insertions, 0 deletions
diff --git a/internal/calendar/datespec.go b/internal/calendar/datespec.go
new file mode 100644
index 0000000..d0040cc
--- /dev/null
+++ b/internal/calendar/datespec.go
@@ -0,0 +1,52 @@
+package calendar
+
+import (
+	"strconv"
+	"strings"
+	"time"
+)
+
+// resolveDate returns the date (UTC midnight) that spec names in year, or
+// ok=false if the spec is unparseable.
+func resolveDate(spec DateSpec, year int, easter time.Time) (time.Time, bool) {
+	s := strings.TrimSpace(string(spec))
+	switch {
+	case len(s) == 5 && s[2] == '-': // MM-DD
+		mo, err1 := strconv.Atoi(s[0:2])
+		da, err2 := strconv.Atoi(s[3:5])
+		if err1 != nil || err2 != nil {
+			return time.Time{}, false
+		}
+		return time.Date(year, time.Month(mo), da, 0, 0, 0, 0, time.UTC), true
+	case strings.HasPrefix(s, "easter"):
+		return offset(easter, s[len("easter"):])
+	case strings.HasPrefix(s, "christmas"):
+		xmas := time.Date(year, 12, 25, 0, 0, 0, 0, time.UTC)
+		return offset(xmas, s[len("christmas"):])
+	case strings.HasPrefix(s, "sunday-after "):
+		base, ok := resolveDate(DateSpec(strings.TrimPrefix(s, "sunday-after ")), year, easter)
+		if !ok {
+			return time.Time{}, false
+		}
+		return nextWeekday(base.AddDate(0, 0, 1), time.Sunday), true
+	}
+	return time.Time{}, false
+}
+
+// offset parses "±N" and adds N days to base.
+func offset(base time.Time, pm string) (time.Time, bool) {
+	if pm == "" {
+		return base, true
+	}
+	n, err := strconv.Atoi(pm) // strconv.Atoi handles a leading '+' and '-'
+	if err != nil {
+		return time.Time{}, false
+	}
+	return base.AddDate(0, 0, n), true
+}
+
+// nextWeekday returns the first day >= from whose weekday is wd.
+func nextWeekday(from time.Time, wd time.Weekday) time.Time {
+	delta := (int(wd) - int(from.Weekday()) + 7) % 7
+	return from.AddDate(0, 0, delta)
+}