package web

import (
	"strconv"
	"strings"
	"time"

	"github.com/m/projax/store"
)

// parsePER strips a trailing YYMMDD segment off the path and returns the
// shorter path + the parsed date. If the last segment doesn't match the
// 6-digit form, the input is returned unchanged with a nil date. We
// deliberately ignore collision-tag suffixes (`.a`/`.b`) at v0.1 — collision
// is a display concern only per the PER standard, so a URL ending in `.a`
// just won't strip and will 404 on its own.
func parsePER(path string) (basePath string, eventDate *time.Time) {
	idx := strings.LastIndex(path, ".")
	if idx < 0 || idx == len(path)-1 {
		return path, nil
	}
	last := path[idx+1:]
	if len(last) != 6 {
		return path, nil
	}
	for _, c := range last {
		if c < '0' || c > '9' {
			return path, nil
		}
	}
	yy, _ := strconv.Atoi(last[0:2])
	mm, _ := strconv.Atoi(last[2:4])
	dd, _ := strconv.Atoi(last[4:6])
	// YY → 20YY (PER spec applies to m's century).
	t := time.Date(2000+yy, time.Month(mm), dd, 0, 0, 0, 0, time.UTC)
	// Reject impossible dates: time.Date normalises (e.g. Feb 30 → Mar 2),
	// so a round-trip mismatch signals "not a real date".
	if t.Year() != 2000+yy || int(t.Month()) != mm || t.Day() != dd {
		return path, nil
	}
	return path[:idx], &t
}

// formatPERDate is the inverse of the YYMMDD slice of parsePER. Used for
// rendering computed PERs in the Documents section.
func formatPERDate(t time.Time) string {
	return t.UTC().Format("060102")
}

// computePERs annotates each dated link with the canonical PER under which
// it should display, including a collision tag (`.a`/`.b`/…) when multiple
// links share the same `event_date`. Inputs must already be ordered by
// (event_date DESC, created_at ASC) — matches store.DatedLinks output. The
// tag is render-time only per the PER v0.1 spec; we never store it.
type DocumentRow struct {
	Link *store.ItemLink
	PER  string // basePath + . + YYMMDD + optional .a/.b
	Tag  string // "" | "a" | "b" | …
}

func computePERs(basePath string, links []*store.ItemLink) []DocumentRow {
	// Group by date so we can assign collision tags inside each group.
	type group struct {
		date  time.Time
		links []*store.ItemLink
	}
	groups := []group{}
	for _, l := range links {
		if l.EventDate == nil {
			continue
		}
		d := *l.EventDate
		// New group when date changes (input is already sorted by event_date DESC).
		if len(groups) == 0 || !groups[len(groups)-1].date.Equal(d) {
			groups = append(groups, group{date: d})
		}
		groups[len(groups)-1].links = append(groups[len(groups)-1].links, l)
	}
	out := make([]DocumentRow, 0, len(links))
	for _, g := range groups {
		// Within a date, the first link is bare; the rest get .a, .b, …
		// (input is sorted by created_at ASC within the same date).
		for i, l := range g.links {
			row := DocumentRow{Link: l, PER: basePath + "." + formatPERDate(g.date)}
			if i > 0 {
				tag := collisionTag(i)
				row.Tag = tag
				row.PER = row.PER + "." + tag
			}
			out = append(out, row)
		}
	}
	return out
}

// collisionTag returns the alpha-only suffix for the n-th colliding link
// (1-indexed: 1→"a", 2→"b", …, 26→"z", 27→"aa", 28→"ab", …). Matches the
// rule documented in docs/standards/per.md §"Collision handling".
func collisionTag(n int) string {
	if n <= 0 {
		return ""
	}
	out := ""
	for n > 0 {
		n--
		out = string(rune('a'+(n%26))) + out
		n /= 26
	}
	return out
}