package caldav

import (
	"crypto/rand"
	"fmt"
	"strconv"
	"strings"
	"time"
)

// parseVTodos extracts every VTODO block from a calendar-data string. Hand-
// rolled because importing a full iCalendar parser for the half-dozen fields
// projax cares about is overkill. Tolerates folded lines per RFC 5545 §3.1.
func parseVTodos(ics string) []Todo {
	ics = unfold(ics)
	lines := strings.Split(ics, "\n")
	var out []Todo
	var inTodo bool
	var cur Todo
	for _, ln := range lines {
		ln = strings.TrimRight(ln, "\r")
		if ln == "BEGIN:VTODO" {
			inTodo = true
			cur = Todo{Status: "NEEDS-ACTION"}
			continue
		}
		if ln == "END:VTODO" {
			if cur.UID != "" {
				out = append(out, cur)
			}
			inTodo = false
			continue
		}
		if !inTodo {
			continue
		}
		key, val := splitLine(ln)
		switch key {
		case "UID":
			cur.UID = val
		case "SUMMARY":
			cur.Summary = unescapeText(val)
		case "STATUS":
			cur.Status = strings.ToUpper(val)
		case "PRIORITY":
			if n, err := strconv.Atoi(val); err == nil {
				cur.Priority = n
			}
		case "DUE":
			if t, ok := parseICalTime(val); ok {
				cur.Due = &t
			}
		case "LAST-MODIFIED":
			if t, ok := parseICalTime(val); ok {
				cur.LastModified = &t
			}
		case "CATEGORIES":
			// CATEGORIES is comma-separated per RFC 5545. Some clients emit
			// multiple CATEGORIES lines; we merge by appending. The unescape
			// is per-entry because commas inside a category value MUST be
			// escaped (`\,`), so we split on bare commas only after unescape.
			for _, raw := range strings.Split(val, ",") {
				t := strings.TrimSpace(unescapeText(raw))
				if t == "" {
					continue
				}
				cur.Categories = append(cur.Categories, t)
			}
		}
	}
	return out
}

// ProjaxCategoryFor returns the projax-namespaced CATEGORIES entry for
// the given primary-path (e.g. "projax:admin.vacations.greece"). Used by
// both the write side (tag-on-create) and the read side (per-item filter).
func ProjaxCategoryFor(primaryPath string) string {
	return "projax:" + primaryPath
}

// HasProjaxTag reports whether the VTODO carries any `projax:` category.
// Used to decide whether the per-item filter kicks in: a list with at
// least one projax: tag is "managed" by projax and the detail page only
// shows todos matching THIS item's path; a list with zero projax: tags
// is a legacy/unmanaged list and the detail page shows everything.
func HasProjaxTag(t Todo) bool {
	for _, c := range t.Categories {
		if strings.HasPrefix(c, "projax:") {
			return true
		}
	}
	return false
}

// HasProjaxTagFor reports whether the VTODO carries the specific
// `projax:<primaryPath>` category. A todo can carry multiple projax: tags
// (when it belongs to multiple projax items) — any match returns true.
func HasProjaxTagFor(t Todo, primaryPath string) bool {
	want := ProjaxCategoryFor(primaryPath)
	for _, c := range t.Categories {
		if c == want {
			return true
		}
	}
	return false
}

// AnyTodoHasProjaxTag reports whether the slice contains at least one
// projax-tagged VTODO. The detail page uses this to decide between the
// projax-managed filter (show only matching) and the legacy unmanaged
// path (show all).
func AnyTodoHasProjaxTag(todos []Todo) bool {
	for _, t := range todos {
		if HasProjaxTag(t) {
			return true
		}
	}
	return false
}

// parseVEvents extracts every VEVENT block from a calendar-data string.
// Mirrors parseVTodos but for read-only event listing (no writeback). DTSTART
// with VALUE=DATE marks the event all-day; the parser inspects the raw line
// before splitLine drops params. RRULE presence flips Recurring=true; the
// rule itself is intentionally NOT parsed — projax surfaces the literal
// DTSTART occurrence and a recurring badge.
func parseVEvents(ics string) []Event {
	ics = unfold(ics)
	lines := strings.Split(ics, "\n")
	var out []Event
	var inEvent bool
	var cur Event
	for _, ln := range lines {
		ln = strings.TrimRight(ln, "\r")
		if ln == "BEGIN:VEVENT" {
			inEvent = true
			cur = Event{}
			continue
		}
		if ln == "END:VEVENT" {
			if cur.UID != "" {
				out = append(out, cur)
			}
			inEvent = false
			continue
		}
		if !inEvent {
			continue
		}
		key, val := splitLine(ln)
		switch key {
		case "UID":
			cur.UID = val
		case "SUMMARY":
			cur.Summary = unescapeText(val)
		case "DESCRIPTION":
			cur.Description = unescapeText(val)
		case "LOCATION":
			cur.Location = unescapeText(val)
		case "DTSTART":
			if t, ok := parseICalTime(val); ok {
				cur.Start = t
			}
			if hasDateOnlyParam(ln) {
				cur.AllDay = true
			}
		case "DTEND":
			if t, ok := parseICalTime(val); ok {
				cur.End = t
			}
		case "RRULE":
			cur.Recurring = true
		}
	}
	return out
}

// hasDateOnlyParam reports whether the property line carried VALUE=DATE
// (rather than DATE-TIME) before the value separator. This matters because
// splitLine throws params away, so the caller has to inspect the raw line
// to know if the date is all-day or has a clock component.
func hasDateOnlyParam(ln string) bool {
	colon := strings.Index(ln, ":")
	if colon < 0 {
		return false
	}
	head := ln[:colon]
	semi := strings.Index(head, ";")
	if semi < 0 {
		return false
	}
	params := strings.ToUpper(head[semi+1:])
	for _, p := range strings.Split(params, ";") {
		if p == "VALUE=DATE" {
			return true
		}
	}
	return false
}

// unfold collapses RFC 5545 line continuations (a CRLF followed by a single
// SP or HT continues the previous line).
func unfold(s string) string {
	s = strings.ReplaceAll(s, "\r\n", "\n")
	var b strings.Builder
	lines := strings.Split(s, "\n")
	for i, ln := range lines {
		if i > 0 && len(ln) > 0 && (ln[0] == ' ' || ln[0] == '\t') {
			b.WriteString(ln[1:])
			continue
		}
		if i > 0 {
			b.WriteByte('\n')
		}
		b.WriteString(ln)
	}
	return b.String()
}

// splitLine separates "KEY;PARAMS:VALUE" into ("KEY", "VALUE"). Params dropped
// — we don't need TZID etc. for v1.
func splitLine(ln string) (string, string) {
	colon := strings.Index(ln, ":")
	if colon < 0 {
		return "", ""
	}
	head := ln[:colon]
	val := ln[colon+1:]
	if semi := strings.Index(head, ";"); semi >= 0 {
		head = head[:semi]
	}
	return head, val
}

// parseICalTime recognises both `YYYYMMDDTHHMMSSZ` (UTC) and bare `YYYYMMDD`.
// Floating local-time forms are coerced to UTC for ranking — single user, no
// tz acrobatics needed at v1.
func parseICalTime(v string) (time.Time, bool) {
	v = strings.TrimSpace(v)
	if len(v) == 8 {
		if t, err := time.Parse("20060102", v); err == nil {
			return t, true
		}
	}
	if len(v) >= 15 {
		layouts := []string{"20060102T150405Z", "20060102T150405"}
		for _, l := range layouts {
			if t, err := time.Parse(l, v); err == nil {
				return t, true
			}
		}
	}
	return time.Time{}, false
}

// unescapeText reverses RFC 5545 §3.3.11 text encoding.
func unescapeText(s string) string {
	s = strings.ReplaceAll(s, `\n`, "\n")
	s = strings.ReplaceAll(s, `\N`, "\n")
	s = strings.ReplaceAll(s, `\,`, ",")
	s = strings.ReplaceAll(s, `\;`, ";")
	s = strings.ReplaceAll(s, `\\`, `\`)
	return s
}

// escapeText applies RFC 5545 §3.3.11 escaping. CR/LF become \n; backslash,
// comma, and semicolon are backslash-escaped. Everything else passes through.
func escapeText(s string) string {
	s = strings.ReplaceAll(s, `\`, `\\`)
	s = strings.ReplaceAll(s, "\r\n", `\n`)
	s = strings.ReplaceAll(s, "\n", `\n`)
	s = strings.ReplaceAll(s, "\r", `\n`)
	s = strings.ReplaceAll(s, `,`, `\,`)
	s = strings.ReplaceAll(s, `;`, `\;`)
	return s
}

// foldLine wraps a single logical iCal line so no physical line exceeds 75
// octets, prepending a single space to each continuation line as per RFC 5545
// §3.1. Folding is octet-based, not rune-based — but we keep care not to split
// in the middle of a UTF-8 sequence.
func foldLine(line string) string {
	const limit = 75
	if len(line) <= limit {
		return line
	}
	var b strings.Builder
	for i := 0; i < len(line); {
		end := i + limit
		if i > 0 {
			// Continuation lines reserve one octet for the leading space.
			end = i + (limit - 1)
		}
		if end > len(line) {
			end = len(line)
		}
		// Back off so we don't split inside a multi-byte UTF-8 sequence.
		for end < len(line) && end > i && (line[end]&0xC0) == 0x80 {
			end--
		}
		chunk := line[i:end]
		if i > 0 {
			b.WriteString("\r\n ")
		}
		b.WriteString(chunk)
		i = end
	}
	return b.String()
}

// joinICS folds each logical line and joins them with CRLF terminators
// (RFC 5545 §3.1 — content lines MUST be terminated with CRLF).
func joinICS(lines []string) string {
	var b strings.Builder
	for _, ln := range lines {
		b.WriteString(foldLine(ln))
		b.WriteString("\r\n")
	}
	return b.String()
}

// formatICalUTC formats t in `YYYYMMDDTHHMMSSZ` form (RFC 5545 UTC date-time).
func formatICalUTC(t time.Time) string { return t.UTC().Format("20060102T150405Z") }

// formatICalDate formats t in `YYYYMMDD` form (RFC 5545 DATE).
func formatICalDate(t time.Time) string { return t.UTC().Format("20060102") }

// NewUID generates an RFC 4122 v4 UUID rendered as a hyphenated lowercase
// string. The "@projax" suffix that some clients append is intentionally
// omitted — the UID is opaque to projax and we treat it as such.
func NewUID() string {
	var b [16]byte
	if _, err := rand.Read(b[:]); err != nil {
		// crypto/rand failure on Linux is extraordinary; fall back to time-based
		// so projax keeps functioning, with the trade-off that the new UID is
		// less random.
		now := time.Now().UnixNano()
		for i := 0; i < 16; i++ {
			b[i] = byte(now >> (i * 4))
		}
	}
	b[6] = (b[6] & 0x0F) | 0x40 // version 4
	b[8] = (b[8] & 0x3F) | 0x80 // variant RFC 4122
	return fmt.Sprintf("%x-%x-%x-%x-%x", b[0:4], b[4:6], b[6:8], b[8:10], b[10:16])
}

// VTodoEdit describes a partial update to an existing VTODO. Fields left nil
// are not changed in the stored ICS. To clear a field, supply a pointer to an
// empty value where allowed; the writer will emit the bare key with no value.
// Use ClearDue to clear DUE explicitly.
type VTodoEdit struct {
	Summary   *string
	Status    *string
	Completed *time.Time // sets COMPLETED; pass time.Time{} via ClearCompleted to remove the line
	Due       *time.Time
	ClearDue  bool
	Priority  *int
	// Categories: optional CATEGORIES list. BuildVTodoICS writes them
	// directly on a fresh VTODO. ApplyVTodoEdit intentionally ignores
	// this field — existing categories pass through unchanged via the
	// unknown-property preserve path, which is what every edit/complete/
	// delete flow wants. Tag-on-create is the only write path that
	// uses it.
	Categories []string
}

// BuildVTodoICS serialises a fresh VTODO as a complete VCALENDAR document,
// suitable for PUT to a CalDAV server. UID is the only required input; the
// other VTodoEdit fields populate optional properties. DTSTAMP is set to now.
func BuildVTodoICS(uid string, e VTodoEdit) string {
	now := time.Now().UTC()
	lines := []string{
		"BEGIN:VCALENDAR",
		"VERSION:2.0",
		"PRODID:-//projax//caldav writeback//EN",
		"CALSCALE:GREGORIAN",
		"BEGIN:VTODO",
		"UID:" + uid,
		"DTSTAMP:" + formatICalUTC(now),
		"CREATED:" + formatICalUTC(now),
		"LAST-MODIFIED:" + formatICalUTC(now),
	}
	if e.Summary != nil {
		lines = append(lines, "SUMMARY:"+escapeText(*e.Summary))
	}
	status := "NEEDS-ACTION"
	if e.Status != nil && *e.Status != "" {
		status = strings.ToUpper(*e.Status)
	}
	lines = append(lines, "STATUS:"+status)
	if status == "COMPLETED" {
		ct := now
		if e.Completed != nil && !e.Completed.IsZero() {
			ct = *e.Completed
		}
		lines = append(lines, "COMPLETED:"+formatICalUTC(ct))
		lines = append(lines, "PERCENT-COMPLETE:100")
	}
	if e.Due != nil && !e.Due.IsZero() {
		lines = append(lines, dueLine(*e.Due))
	}
	if e.Priority != nil {
		lines = append(lines, fmt.Sprintf("PRIORITY:%d", *e.Priority))
	}
	if len(e.Categories) > 0 {
		// RFC 5545 CATEGORIES — comma-separated, single line. Escape commas
		// inside individual entries so the round-trip survives parseVTodos.
		escaped := make([]string, 0, len(e.Categories))
		for _, c := range e.Categories {
			escaped = append(escaped, escapeText(c))
		}
		lines = append(lines, "CATEGORIES:"+strings.Join(escaped, ","))
	}
	lines = append(lines, "END:VTODO", "END:VCALENDAR")
	return joinICS(lines)
}

// dueLine emits a DUE property. If the time has no clock component (00:00:00),
// it is encoded as DATE (`DUE;VALUE=DATE:YYYYMMDD`); otherwise as UTC
// date-time. Single-user, single-timezone — no VTIMEZONE acrobatics required.
func dueLine(t time.Time) string {
	if t.Hour() == 0 && t.Minute() == 0 && t.Second() == 0 {
		return "DUE;VALUE=DATE:" + formatICalDate(t)
	}
	return "DUE:" + formatICalUTC(t)
}

// ApplyVTodoEdit returns a new ICS document derived from the existing one with
// the supplied edits applied. Unknown properties (DESCRIPTION, CATEGORIES,
// ATTENDEE, X-*) are preserved — only the changed keys are rewritten. Folded
// lines are normalised on read. LAST-MODIFIED is always bumped to now.
func ApplyVTodoEdit(ics string, e VTodoEdit) string {
	now := time.Now().UTC()
	lines := strings.Split(unfold(ics), "\n")
	// Helper to locate the VTODO segment so we don't touch VCALENDAR-level keys.
	vtStart, vtEnd := -1, -1
	for i, raw := range lines {
		ln := strings.TrimRight(raw, "\r")
		if ln == "BEGIN:VTODO" {
			vtStart = i
		}
		if ln == "END:VTODO" {
			vtEnd = i
			break
		}
	}
	if vtStart < 0 || vtEnd < 0 {
		// Malformed; fall back to a from-scratch build with a fresh UID so the
		// caller gets *something* well-formed back rather than silent garbage.
		return BuildVTodoICS(NewUID(), e)
	}

	// Build a set of keys we plan to overwrite. We also drop COMPLETED when
	// status flips away from COMPLETED.
	overwrite := map[string]string{}
	if e.Summary != nil {
		overwrite["SUMMARY"] = "SUMMARY:" + escapeText(*e.Summary)
	}
	if e.Status != nil && *e.Status != "" {
		s := strings.ToUpper(*e.Status)
		overwrite["STATUS"] = "STATUS:" + s
		switch s {
		case "COMPLETED":
			ct := now
			if e.Completed != nil && !e.Completed.IsZero() {
				ct = *e.Completed
			}
			overwrite["COMPLETED"] = "COMPLETED:" + formatICalUTC(ct)
			overwrite["PERCENT-COMPLETE"] = "PERCENT-COMPLETE:100"
		default:
			// reopen / cancel: clear COMPLETED and PERCENT-COMPLETE if present.
			overwrite["COMPLETED"] = ""
			overwrite["PERCENT-COMPLETE"] = ""
		}
	}
	if e.Due != nil && !e.Due.IsZero() {
		overwrite["DUE"] = dueLine(*e.Due)
	}
	if e.ClearDue {
		overwrite["DUE"] = ""
	}
	if e.Priority != nil {
		overwrite["PRIORITY"] = fmt.Sprintf("PRIORITY:%d", *e.Priority)
	}
	// LAST-MODIFIED always bumps.
	overwrite["LAST-MODIFIED"] = "LAST-MODIFIED:" + formatICalUTC(now)
	// DTSTAMP per RFC 5545 also reflects last sync; safe to bump.
	overwrite["DTSTAMP"] = "DTSTAMP:" + formatICalUTC(now)

	seen := map[string]bool{}
	out := make([]string, 0, len(lines))
	for i, raw := range lines {
		ln := strings.TrimRight(raw, "\r")
		// Pass everything outside the VTODO block through verbatim.
		if i <= vtStart || i >= vtEnd {
			out = append(out, ln)
			continue
		}
		key, _ := splitLine(ln)
		key = strings.ToUpper(key)
		if repl, ok := overwrite[key]; ok {
			seen[key] = true
			if repl == "" {
				continue // explicit clear
			}
			out = append(out, repl)
			continue
		}
		out = append(out, ln)
	}
	// Insert any overwrite keys that weren't present in the source ICS just
	// before END:VTODO. Skip explicit clears (repl == "") so we don't append
	// empty lines.
	endIdx := -1
	for i, ln := range out {
		if strings.TrimRight(ln, "\r") == "END:VTODO" {
			endIdx = i
			break
		}
	}
	if endIdx >= 0 {
		extras := []string{}
		for key, repl := range overwrite {
			if seen[key] || repl == "" {
				continue
			}
			extras = append(extras, repl)
		}
		if len(extras) > 0 {
			before := append([]string{}, out[:endIdx]...)
			before = append(before, extras...)
			before = append(before, out[endIdx:]...)
			out = before
		}
	}
	// joinICS handles fold + CRLF on each logical line.
	return joinICS(out)
}