package calendar

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"sort"
	"strings"
	"time"

	"gitea.lab.audasmedia.com.au/sam/compass_teams_nhs/internal/store"
)

// DefaultDuration is how long a school event is blocked out when Compass gives
// only a start time. The week grid stores a start, not an end.
const DefaultDuration = time.Hour

// Marker builds the idempotency marker stored in an event's description. It
// carries the SQLite item id, so a re-run can recognise its own event even when
// Home Assistant reports no event id.
func Marker(itemID int64) string {
	return fmt.Sprintf("[compass-nhs:%d]", itemID)
}

// ItemMarker is the full description tag written to the calendar.
func ItemMarker(itemID int64) string { return "source: " + Marker(itemID) }

// Options narrows one sync run.
type Options struct {
	// ChildID limits the run to one child (nil means all children).
	ChildID *int64
	// ItemID pushes exactly one item (used by the item screen).
	ItemID *int64
	// Limit caps how many events are written in one run (0 means no cap).
	Limit int
	// DryRun selects the candidates and reports them without writing.
	DryRun bool
	// Now is the clock, so tests are deterministic.
	Now time.Time
	// Duration overrides DefaultDuration.
	Duration time.Duration
}

// Created is one event that the run wrote (or would write on a dry run).
type Created struct {
	ItemID  int64  `json:"item_id"`
	Child   string `json:"child"`
	Title   string `json:"title"`
	Start   string `json:"start"`
	UID     string `json:"uid,omitempty"`
	DryRun  bool   `json:"dry_run,omitempty"`
	Summary string `json:"summary"`
}

// Result is the outcome of one sync run. Failed is non-zero when a write
// failed; the caller then reports loudly (FR-12).
type Result struct {
	Entity     string    `json:"entity"`
	Candidates int       `json:"candidates"`
	Created    int       `json:"created"`
	Skipped    int       `json:"skipped"`
	Failed     int       `json:"failed"`
	Events     []Created `json:"events,omitempty"`
	Errors     []string  `json:"errors,omitempty"`
	DryRun     bool      `json:"dry_run,omitempty"`
}

// Sync pushes every important school event that is not in the calendar yet.
//
// It is idempotent in two ways: an item that already carries a Home Assistant
// event id is not a candidate, and an event whose description carries this
// item's marker is adopted instead of written again. Every attempt is recorded
// in the append-only calendar_pushes table, so a failure is visible and loud
// rather than swallowed (FR-12).
func Sync(ctx context.Context, st *store.Store, c *Client, opts Options, log *slog.Logger) (Result, error) {
	if st == nil {
		return Result{}, errors.New("calendar: store is required")
	}
	if c == nil {
		return Result{}, errors.New("calendar: client is required")
	}
	if log == nil {
		log = slog.Default()
	}
	now := opts.Now
	if now.IsZero() {
		now = time.Now()
	}
	duration := opts.Duration
	if duration <= 0 {
		duration = DefaultDuration
	}

	res := Result{Entity: c.Entity(), DryRun: opts.DryRun}

	candidates, err := st.CalendarCandidates(ctx, opts.ChildID, opts.ItemID, now, opts.Limit)
	if err != nil {
		return res, err
	}
	res.Candidates = len(candidates)
	if len(candidates) == 0 {
		return res, nil
	}

	// One window covers every candidate, so the marker scan is a single call.
	windowEnd := now.Add(24 * time.Hour)
	for _, it := range candidates {
		if it.DueAt != nil && it.DueAt.After(windowEnd) {
			windowEnd = it.DueAt.Add(24 * time.Hour)
		}
	}
	existing, err := c.Events(ctx, now.Add(-24*time.Hour), windowEnd)
	if err != nil {
		// A failed read is not a licence to write blind: it would risk
		// duplicates, so record one loud failure and stop.
		msg := fmt.Sprintf("calendar: reading existing events failed: %v", err)
		res.Failed = len(candidates)
		res.Errors = append(res.Errors, msg)
		for _, it := range candidates {
			if _, rerr := st.RecordCalendarPush(ctx, it.ID, c.Entity(), "", false, msg); rerr != nil {
				log.Error("record calendar push", "item", it.ID, "err", rerr)
			}
		}
		log.Error("calendar sync aborted", "entity", c.Entity(), "err", err)
		return res, nil
	}
	known := indexByMarker(existing)

	for _, it := range candidates {
		if it.DueAt == nil {
			continue
		}
		marker := Marker(it.ID)

		if ev, ok := known[marker]; ok {
			// Adopt an event this run (or an earlier one) already wrote.
			res.Skipped++
			uid := ev.UID
			if uid == "" {
				// No id from Home Assistant: keep the item out of the next
				// candidate set by adopting the marker as its reference.
				uid = marker
			}
			if !opts.DryRun {
				if err := st.MarkItemCalendarSynced(ctx, it.ID, uid, now); err != nil {
					log.Error("mark calendar item", "item", it.ID, "err", err)
				}
				if _, err := st.RecordCalendarPush(ctx, it.ID, c.Entity(), uid, true, ""); err != nil {
					log.Error("record calendar push", "item", it.ID, "err", err)
				}
			}
			res.Events = append(res.Events, Created{
				ItemID: it.ID, Child: it.Child, Title: it.Title,
				Start: it.DueAt.In(time.Local).Format(time.RFC3339), UID: uid,
				Summary: "already in the calendar",
			})
			continue
		}

		created := Created{
			ItemID: it.ID, Child: it.Child, Title: it.Title,
			Start:  it.DueAt.In(time.Local).Format(time.RFC3339),
			DryRun: opts.DryRun,
		}
		if opts.DryRun {
			res.Created++
			res.Events = append(res.Events, created)
			continue
		}

		uid, err := c.CreateEvent(ctx, Event{
			Summary:     eventSummary(it),
			Description: eventDescription(it),
			Start:       it.DueAt,
			End:         ptrTime(it.DueAt.Add(duration)),
		})
		if err != nil {
			res.Failed++
			msg := fmt.Sprintf("item %d (%s %q): %v", it.ID, it.Child, it.Title, err)
			res.Errors = append(res.Errors, msg)
			log.Error("calendar write failed", "item", it.ID, "child", it.Child, "title", it.Title, "entity", c.Entity(), "err", err)
			if _, rerr := st.RecordCalendarPush(ctx, it.ID, c.Entity(), "", false, err.Error()); rerr != nil {
				log.Error("record calendar push", "item", it.ID, "err", rerr)
			}
			continue
		}
		if uid == "" {
			// Home Assistant did not report an event id. The marker in the
			// description still makes the next run idempotent, but say so.
			log.Warn("home assistant returned no event id; relying on the description marker",
				"item", it.ID, "child", it.Child, "title", it.Title, "entity", c.Entity())
		}
		if err := st.MarkItemCalendarSynced(ctx, it.ID, uid, now); err != nil {
			log.Error("mark calendar item", "item", it.ID, "err", err)
		}
		if _, err := st.RecordCalendarPush(ctx, it.ID, c.Entity(), uid, true, ""); err != nil {
			log.Error("record calendar push", "item", it.ID, "err", err)
		}
		res.Created++
		created.UID = uid
		res.Events = append(res.Events, created)
	}

	sort.Slice(res.Events, func(i, j int) bool { return res.Events[i].Start < res.Events[j].Start })
	return res, nil
}

// eventSummary is the calendar entry title. The child's name is prefixed, so a
// shared family calendar says who the event belongs to.
func eventSummary(it store.CalendarItem) string {
	title := strings.TrimSpace(it.Title)
	if it.Child != "" {
		return it.Child + " — " + title
	}
	return title
}

// eventDescription carries the school detail, the Compass link and the
// idempotency marker. The marker is what makes a re-run safe.
func eventDescription(it store.CalendarItem) string {
	var b strings.Builder
	body := strings.TrimSpace(it.Body)
	if body != "" && body != strings.TrimSpace(it.Title) {
		b.WriteString(body)
		b.WriteString("\n")
	}
	if it.URL != "" {
		b.WriteString(it.URL)
		b.WriteString("\n")
	}
	if it.Subject != "" {
		b.WriteString("Subject: " + it.Subject + "\n")
	}
	b.WriteString("\n" + ItemMarker(it.ID))
	return b.String()
}

// indexByMarker maps a marker to the event that carries it.
func indexByMarker(events []Event) map[string]Event {
	out := make(map[string]Event, len(events))
	for _, ev := range events {
		if m := findMarker(ev.Description); m != "" {
			out[m] = ev
		}
	}
	return out
}

// ptrTime returns a pointer to t, so an event end can hold a computed time.
func ptrTime(t time.Time) *time.Time { return &t }

// findMarker returns the first compass-nhs marker in a text, or "".
func findMarker(text string) string {
	const open, closeTok = "[compass-nhs:", "]"
	i := strings.Index(text, open)
	if i < 0 {
		return ""
	}
	start := i + len(open)
	j := strings.Index(text[start:], closeTok)
	if j < 0 {
		return ""
	}
	value := text[start : start+j]
	if value == "" {
		return ""
	}
	for _, r := range value {
		if r < '0' || r > '9' {
			return ""
		}
	}
	return open + value + closeTok
}