package cli

import (
	"errors"
	"fmt"
	"time"
)

var (
	errDateExclusive  = errors.New("--date and --from/--to are mutually exclusive")
	errFromRequiresTo = errors.New("--from requires --to")
	errToRequiresFrom = errors.New("--to requires --from")
	errFromAfterTo    = errors.New("is after --to")
)

// ParseTargetDates turns the --date, --from and --to flags into the days
// to extract. It returns nil when none of them is set, which bsdaily.Run
// takes to mean the snapshot date minus one day.
func ParseTargetDates(dateFlag, fromFlag, toFlag string) ([]time.Time, error) {
	hasDate := dateFlag != ""
	hasFrom := fromFlag != ""
	hasTo := toFlag != ""

	// Validate mutual exclusivity
	if hasDate && (hasFrom || hasTo) {
		return nil, errDateExclusive
	}

	if hasFrom != hasTo {
		if hasFrom {
			return nil, errFromRequiresTo
		}

		return nil, errToRequiresFrom
	}

	if hasDate {
		t, err := time.Parse("2006-01-02", dateFlag)
		if err != nil {
			return nil, fmt.Errorf(
				"invalid --date %q (expected YYYY-MM-DD): %w", dateFlag, err)
		}

		return []time.Time{t}, nil
	}

	if !hasFrom {
		return nil, nil
	}

	from, err := time.Parse("2006-01-02", fromFlag)
	if err != nil {
		return nil, fmt.Errorf(
			"invalid --from %q (expected YYYY-MM-DD): %w", fromFlag, err)
	}

	to, err := time.Parse("2006-01-02", toFlag)
	if err != nil {
		return nil, fmt.Errorf(
			"invalid --to %q (expected YYYY-MM-DD): %w", toFlag, err)
	}

	if from.After(to) {
		return nil, fmt.Errorf("--from %s %w %s", fromFlag, errFromAfterTo, toFlag)
	}

	var targetDates []time.Time

	for d := from; !d.After(to); d = d.AddDate(0, 0, 1) {
		targetDates = append(targetDates, d)
	}

	return targetDates, nil
}