package report

import (
	"fmt"
	"time"
)

// Period is the window a report covers: an inclusive range of ISO dates, with
// either side empty for unbounded. Dates are stored as YYYY-MM-DD, so comparing
// them as strings compares them as dates and nothing has to be parsed to filter
// on one.
type Period struct {
	// Label is what to call the window: "last month", "all time", "2026-06".
	Label string
	// Span names the dates Label stands for, because "last month" says nothing
	// on its own about which month it was. Empty when the label already is the
	// span, as it is for a single month and for all time.
	Span string
	From string
	To   string
}

// String names the window, and what it covers when the label does not.
func (p Period) String() string {
	if p.Span == "" {
		return p.Label
	}
	return p.Label + " · " + p.Span
}

// LastMonth is the label of the window a report opens on. The month that has
// just ended is the last one a statement can be complete for: this month is
// still filling up, so its total is not comparable with any other.
const LastMonth = "last month"

// Periods is the axis a report steps along, widest window first: the named
// ranges, then every month the index holds that is older than last month, most
// recent of those first. months is expected newest first, as store.Months
// returns it; anything in it that a named range already covers is left out of
// the tail rather than offered twice.
//
// The named ranges are relative to today, not to the newest statement. "Last
// month" means the month that has just ended, so if no statement covers it yet
// the report for it is empty — which is the honest answer, and one the user can
// act on, where silently reporting an older month they did not ask for is not.
func Periods(today time.Time, months []string) []Period {
	thisMonth := time.Date(today.Year(), today.Month(), 1, 0, 0, 0, 0, time.UTC)
	lastMonth := thisMonth.AddDate(0, -1, 0)

	// A range ends with this month rather than with the last complete one:
	// "last 3 months" is asked in order to see what is happening now, and
	// leaving out the days since the 1st would answer a different question.
	rolling := func(back int, label string) Period {
		from := thisMonth.AddDate(0, -back, 0)
		return Period{
			Label: label,
			Span:  from.Format("2006-01") + " … " + thisMonth.Format("2006-01"),
			From:  isoDate(from),
			To:    isoDate(endOfMonth(thisMonth)),
		}
	}
	month := func(start time.Time, label string) Period {
		p := Period{Label: label, From: isoDate(start), To: isoDate(endOfMonth(start))}
		if label != start.Format("2006-01") {
			p.Span = start.Format("2006-01")
		}
		return p
	}
	year := time.Date(today.Year(), time.January, 1, 0, 0, 0, 0, time.UTC)

	out := []Period{
		{Label: "all time"},
		{
			Label: "this year",
			Span:  fmt.Sprintf("%d", today.Year()),
			From:  isoDate(year),
			To:    isoDate(year.AddDate(1, 0, -1)),
		},
		rolling(11, "last 12 months"),
		rolling(2, "last 3 months"),
		month(thisMonth, "this month"),
		month(lastMonth, LastMonth),
	}

	cutoff := lastMonth.Format("2006-01")
	for _, ym := range months {
		// Months at or after last month are already on the axis above, and one
		// dated in the future is a parsing bug rather than a window to offer.
		if ym >= cutoff {
			continue
		}
		start, err := time.Parse("2006-01", ym)
		if err != nil {
			continue
		}
		out = append(out, month(start, ym))
	}
	return out
}

// DefaultIndex is where on the axis a report opens.
func DefaultIndex(ps []Period) int {
	for i, p := range ps {
		if p.Label == LastMonth {
			return i
		}
	}
	return 0
}

func isoDate(t time.Time) string { return t.Format("2006-01-02") }

// endOfMonth takes the first of a month to its last day, whatever its length.
func endOfMonth(first time.Time) time.Time { return first.AddDate(0, 1, -1) }