package api

import (
	"bufio"
	"crypto/sha1"
	"encoding/hex"
	"errors"
	"fmt"
	"io"
	"strings"
	"time"
)

// ICSEvent is a single VEVENT, reduced to the fields doot shows.
type ICSEvent struct {
	UID         string
	Summary     string
	Description string
	Location    string
	Start       time.Time
	End         time.Time
	AllDay      bool
}

// StableID returns an id that stays the same across refreshes, scoped to the
// feed it came from.
//
// Scoping matters: two feeds can legitimately contain the same UID (the same
// fixture published by two aggregators), and an unscoped id would make one
// silently overwrite the other. Feeds that omit UID entirely fall back to a
// hash of the event's own content, which is stable as long as the event is.
func (e ICSEvent) StableID(feedID string) string {
	key := e.UID
	if key == "" {
		key = fmt.Sprintf("%s|%s|%s", e.Summary, e.Start.UTC().Format(time.RFC3339), e.Location)
	}
	sum := sha1.Sum([]byte(feedID + "|" + key))
	return "ics-" + hex.EncodeToString(sum[:])[:20]
}

// ParseICS reads an iCalendar stream and returns its VEVENTs.
//
// Deliberately not supported: RRULE. Recurring events are expanded by the
// publisher in every schedule feed this targets — each fixture is its own
// VEVENT — and a correct RRULE expansion (COUNT/UNTIL/BYDAY/EXDATE, across DST
// boundaries) is a large amount of subtle code. A recurring event will appear
// once, at its first occurrence, rather than silently wrongly.
func ParseICS(r io.Reader) ([]ICSEvent, error) {
	lines, err := unfold(r)
	if err != nil {
		return nil, err
	}

	sawCalendar := false
	for _, l := range lines {
		if strings.EqualFold(strings.TrimSpace(l), "BEGIN:VCALENDAR") {
			sawCalendar = true
			break
		}
	}
	if !sawCalendar {
		// Usually an HTML error page from a feed URL that has moved or died.
		// Returning zero events would look like "the team has no games".
		return nil, errors.New("not an iCalendar document (no BEGIN:VCALENDAR)")
	}

	var events []ICSEvent
	var cur *ICSEvent
	for _, line := range lines {
		trimmed := strings.TrimSpace(line)
		switch {
		case strings.EqualFold(trimmed, "BEGIN:VEVENT"):
			cur = &ICSEvent{}
			continue
		case strings.EqualFold(trimmed, "END:VEVENT"):
			if cur != nil {
				if !cur.Start.IsZero() {
					if cur.End.IsZero() {
						cur.End = defaultEnd(*cur)
					}
					events = append(events, *cur)
				}
				cur = nil
			}
			continue
		}
		if cur == nil {
			continue // outside a VEVENT (VCALENDAR headers, VTIMEZONE, VALARM)
		}

		name, params, value := splitProperty(line)
		switch strings.ToUpper(name) {
		case "UID":
			cur.UID = value
		case "SUMMARY":
			cur.Summary = unescapeText(value)
		case "DESCRIPTION":
			cur.Description = unescapeText(value)
		case "LOCATION":
			cur.Location = unescapeText(value)
		case "DTSTART":
			if t, allDay, ok := parseICSTime(value, params); ok {
				cur.Start, cur.AllDay = t, allDay
			}
		case "DTEND":
			if t, _, ok := parseICSTime(value, params); ok {
				cur.End = t
			}
		}
	}
	return events, nil
}

// defaultEnd supplies an end for an event that gave none. A zero end would
// render as an event running from its start to year zero.
func defaultEnd(e ICSEvent) time.Time {
	if e.AllDay {
		return e.Start.AddDate(0, 0, 1)
	}
	return e.Start.Add(time.Hour)
}

// unfold reads the stream and joins continuation lines. Per RFC 5545 a line
// beginning with a space or tab continues the previous one; splitting a long
// SUMMARY is routine, so not unfolding truncates titles mid-word.
func unfold(r io.Reader) ([]string, error) {
	scanner := bufio.NewScanner(r)
	// Feeds with long DESCRIPTIONs exceed bufio's default 64KB line limit.
	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)

	var out []string
	for scanner.Scan() {
		line := strings.TrimRight(scanner.Text(), "\r")
		if len(line) > 0 && (line[0] == ' ' || line[0] == '\t') && len(out) > 0 {
			out[len(out)-1] += line[1:]
			continue
		}
		out = append(out, line)
	}
	if err := scanner.Err(); err != nil {
		return nil, err
	}
	return out, nil
}

// splitProperty divides "NAME;PARAM=VAL;P2=V2:value" into its parts. The colon
// inside a value (a URL, a time) must not be treated as the separator, so only
// the first colon outside a quoted parameter counts.
func splitProperty(line string) (name string, params map[string]string, value string) {
	params = map[string]string{}

	inQuotes := false
	colon := -1
	for i, r := range line {
		if r == '"' {
			inQuotes = !inQuotes
		}
		if r == ':' && !inQuotes {
			colon = i
			break
		}
	}
	if colon == -1 {
		return line, params, ""
	}

	head := line[:colon]
	value = line[colon+1:]

	parts := strings.Split(head, ";")
	name = parts[0]
	for _, p := range parts[1:] {
		if eq := strings.Index(p, "="); eq > 0 {
			k := strings.ToUpper(strings.TrimSpace(p[:eq]))
			v := strings.Trim(strings.TrimSpace(p[eq+1:]), `"`)
			params[k] = v
		}
	}
	return name, params, value
}

// parseICSTime handles the three forms that appear in practice:
//
//	20260913T201500Z                     UTC
//	20260920T131500 with TZID=Zone       wall-clock time in that zone
//	20260913 with VALUE=DATE             all-day
//
// The TZID case is the one worth care: those digits are local to the named
// zone, and reading them as UTC shifts the event by the zone's offset — wrong
// by hours, but plausible-looking, so easy to miss.
func parseICSTime(value string, params map[string]string) (t time.Time, allDay bool, ok bool) {
	value = strings.TrimSpace(value)
	if value == "" {
		return time.Time{}, false, false
	}

	if strings.EqualFold(params["VALUE"], "DATE") || len(value) == 8 {
		parsed, err := time.ParseInLocation("20060102", value, time.UTC)
		if err != nil {
			return time.Time{}, false, false
		}
		return parsed, true, true
	}

	if strings.HasSuffix(value, "Z") {
		parsed, err := time.ParseInLocation("20060102T150405Z", value, time.UTC)
		if err != nil {
			return time.Time{}, false, false
		}
		return parsed, false, true
	}

	loc := time.UTC
	if tzid := params["TZID"]; tzid != "" {
		if l, err := time.LoadLocation(tzid); err == nil {
			loc = l
		}
		// An unknown TZID falls back to UTC rather than dropping the event;
		// a visible event at the wrong hour beats a missing one.
	}
	parsed, err := time.ParseInLocation("20060102T150405", value, loc)
	if err != nil {
		return time.Time{}, false, false
	}
	return parsed, false, true
}

// unescapeText reverses RFC 5545 text escaping.
func unescapeText(s string) string {
	var b strings.Builder
	for i := 0; i < len(s); i++ {
		if s[i] != '\\' || i+1 >= len(s) {
			b.WriteByte(s[i])
			continue
		}
		i++
		switch s[i] {
		case 'n', 'N':
			b.WriteByte('\n')
		case '\\', ',', ';':
			b.WriteByte(s[i])
		default:
			// Unknown escape: keep both characters rather than eat them.
			b.WriteByte('\\')
			b.WriteByte(s[i])
		}
	}
	return b.String()
}