package store

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"strings"
	"time"
	"unicode"
)

// Tasks — the capture store (Vikunja #130). One row per piece of work, with a
// lifecycle instead of a valid-time: captured, then either done or dropped.
//
// Why not facts: a fact is a claim about the world and a correction supersedes
// it (append-only, voids_id). A task is not a claim — it is work, it has a
// state that moves forward once, and the read the prioritiser needs is "every
// live task right now", which over an append-only log would mean replaying
// history on every question.
//
// Nothing in this file schedules, fires or announces anything. Capture is a
// store, not a trigger: a task exists to be answered when asked about, and the
// owner's reminders remain the only thing that speaks unprompted.
const (
	// TaskCandidate — Maven derived this task from something she read (mail,
	// once the email reader exists) and the owner has not confirmed it. A
	// candidate is inert: it is listed as a candidate and never counted as work
	// he agreed to.
	TaskCandidate = "candidate"
	// TaskOpen — work the owner stated himself, or a candidate he confirmed.
	TaskOpen = "open"
	// TaskDone — finished.
	TaskDone = "done"
	// TaskDropped — declined, or a candidate rejected. Kept for provenance, so
	// the same mail cannot resurrect it silently; nothing re-proposes a
	// dropped task.
	TaskDropped = "dropped"
)

// Task — one captured piece of work.
//
// Source is provenance in the same vocabulary facts use: "tap:voice" for
// something he said, "tap:web" for the review page, "email:<account>" for a
// mail-derived candidate. Evidence is the free-text trail a derived task came
// from (a subject line), empty for anything he stated himself — it is what
// makes a candidate reviewable instead of mysterious.
//
// Due is optional. Weight is an explicit importance hint (0 = none), which the
// prioritiser reads; capture never invents one.
type Task struct {
	ID         int64
	CreatedTs  time.Time
	Text       string
	Source     string
	Evidence   string
	Status     string
	Due        *time.Time
	Weight     int
	ResolvedTs *time.Time
}

var (
	ErrTaskNotFound = errors.New("store: task not found")
	ErrTaskEmpty    = errors.New("store: task text is empty")
	ErrTaskStatus   = errors.New("store: invalid task status")
)

// liveTaskStatuses — the two statuses that count as outstanding work.
var liveTaskStatuses = []string{TaskCandidate, TaskOpen}

// CaptureTask inserts a task, or returns the existing live task when the same
// work is already outstanding. created reports which happened, so a caller can
// tell the owner "уже в списке" instead of pretending it wrote something.
//
// Dedupe is on the normalised text among LIVE rows only (see the partial unique
// index in migration #14): a weekly errand can be captured again once the last
// one is done, but a mail that gets re-read produces no second row. This is the
// property the email intake depends on — it may call CaptureTask for every
// message it extracts from, as often as it likes, without growing the list.
func (s *Store) CaptureTask(ctx context.Context, t Task) (id int64, created bool, err error) {
	text := strings.TrimSpace(t.Text)
	if text == "" {
		return 0, false, ErrTaskEmpty
	}
	status := t.Status
	if status == "" {
		status = TaskOpen
	}
	if status != TaskCandidate && status != TaskOpen {
		// Capturing straight into a resolved state is meaningless — a task is
		// captured live and moved later.
		return 0, false, fmt.Errorf("%w: capture status %q", ErrTaskStatus, status)
	}
	norm := NormalizeTaskText(text)
	created2 := t.CreatedTs
	if created2.IsZero() {
		created2 = time.Now()
	}
	var due sql.NullInt64
	if t.Due != nil {
		due = sql.NullInt64{Int64: t.Due.UnixMilli(), Valid: true}
	}

	res, err := s.db.ExecContext(ctx,
		`INSERT INTO tasks (created_ts, text, norm, source, evidence, status, due_ts, weight)
		 VALUES (?,?,?,?,?,?,?,?)
		 ON CONFLICT (norm) WHERE status IN ('candidate','open') DO NOTHING`,
		created2.UnixMilli(), text, norm, t.Source, t.Evidence, status, due, t.Weight)
	if err != nil {
		return 0, false, fmt.Errorf("capture task: %w", err)
	}
	if n, err := res.RowsAffected(); err != nil {
		return 0, false, fmt.Errorf("capture task: rows affected: %w", err)
	} else if n > 0 {
		id, err := res.LastInsertId()
		if err != nil {
			return 0, false, fmt.Errorf("capture task: last insert id: %w", err)
		}
		return id, true, nil
	}

	// Already live — hand back the row that won.
	existing, err := s.lookupLiveTaskByNorm(ctx, norm)
	if err != nil {
		return 0, false, err
	}
	return existing.ID, false, nil
}

// lookupLiveTaskByNorm finds the outstanding task with this normalised text.
func (s *Store) lookupLiveTaskByNorm(ctx context.Context, norm string) (Task, error) {
	row := s.db.QueryRowContext(ctx, taskSelect+`
		 WHERE norm = ? AND status IN ('candidate','open')`, norm)
	t, err := scanTask(row)
	if errors.Is(err, sql.ErrNoRows) {
		return Task{}, ErrTaskNotFound
	}
	if err != nil {
		return Task{}, fmt.Errorf("lookup live task: %w", err)
	}
	return t, nil
}

const taskSelect = `SELECT id, created_ts, text, source, evidence, status, due_ts, weight, resolved_ts FROM tasks`

// LookupTask returns one task by id.
func (s *Store) LookupTask(ctx context.Context, id int64) (Task, error) {
	row := s.db.QueryRowContext(ctx, taskSelect+` WHERE id = ?`, id)
	t, err := scanTask(row)
	if errors.Is(err, sql.ErrNoRows) {
		return Task{}, fmt.Errorf("%w: id=%d", ErrTaskNotFound, id)
	}
	if err != nil {
		return Task{}, fmt.Errorf("lookup task: %w", err)
	}
	return t, nil
}

// ListTasks returns tasks in one status, newest first. An empty status returns
// every row; "live" returns candidate + open, which is what every read path
// that means "outstanding work" wants.
func (s *Store) ListTasks(ctx context.Context, status string) ([]Task, error) {
	q := taskSelect
	var args []any
	switch status {
	case "":
	case "live":
		q += ` WHERE status IN (?,?)`
		args = append(args, liveTaskStatuses[0], liveTaskStatuses[1])
	default:
		q += ` WHERE status = ?`
		args = append(args, status)
	}
	q += ` ORDER BY created_ts DESC, id DESC`

	rows, err := s.db.QueryContext(ctx, q, args...)
	if err != nil {
		return nil, fmt.Errorf("list tasks: %w", err)
	}
	defer rows.Close()
	var out []Task
	for rows.Next() {
		t, err := scanTask(rows)
		if err != nil {
			return nil, fmt.Errorf("list tasks: %w", err)
		}
		out = append(out, t)
	}
	return out, rows.Err()
}

// SetTaskStatus moves a task once, forward. The legal moves are:
//
//	candidate → open     (the owner confirms a derived task)
//	candidate → dropped  (he rejects it)
//	open      → done     (finished)
//	open      → dropped  (abandoned)
//
// Anything else — including re-resolving a resolved task — is refused with
// ErrTaskNotFound-wrapped detail, the same one-way shape proposed_routines and
// tools use: an answered question is not answered twice.
//
// Resolving frees the dedupe key, which is the point: the work can recur.
func (s *Store) SetTaskStatus(ctx context.Context, id int64, status string, ts time.Time) error {
	var from []string
	switch status {
	case TaskOpen:
		from = []string{TaskCandidate}
	case TaskDone:
		from = []string{TaskOpen}
	case TaskDropped:
		from = []string{TaskCandidate, TaskOpen}
	default:
		return fmt.Errorf("%w: %q", ErrTaskStatus, status)
	}

	// resolved_ts is only meaningful for a terminal state; confirming a
	// candidate leaves it null (the task is still live).
	var resolved sql.NullInt64
	if status == TaskDone || status == TaskDropped {
		resolved = sql.NullInt64{Int64: ts.UnixMilli(), Valid: true}
	}

	q := `UPDATE tasks SET status = ?, resolved_ts = ? WHERE id = ? AND status IN (?` +
		strings.Repeat(",?", len(from)-1) + `)`
	args := []any{status, resolved, id}
	for _, f := range from {
		args = append(args, f)
	}
	res, err := s.db.ExecContext(ctx, q, args...)
	if err != nil {
		return fmt.Errorf("set task status: %w", err)
	}
	n, err := res.RowsAffected()
	if err != nil {
		return fmt.Errorf("set task status: rows affected: %w", err)
	}
	if n == 0 {
		return fmt.Errorf("%w: id=%d not in %v", ErrTaskNotFound, id, from)
	}
	return nil
}

// NormalizeTaskText is the dedupe key: lowercased, punctuation dropped,
// whitespace collapsed. Exported because the intake seam (and its tests) needs
// to reason about what will and will not be treated as the same task.
//
// Deliberately shallow — no stemming, no synonyms. Russian morphology would
// need a real lemmatiser to do better, and a normaliser that guesses would
// silently swallow two different tasks. This only catches the case that
// actually happens: the same sentence arriving twice with different casing or
// punctuation.
func NormalizeTaskText(s string) string {
	var b strings.Builder
	space := true // leading space collapses to nothing
	for _, r := range strings.ToLower(s) {
		switch {
		case unicode.IsLetter(r) || unicode.IsDigit(r):
			b.WriteRune(r)
			space = false
		case !space:
			b.WriteRune(' ')
			space = true
		}
	}
	return strings.TrimSpace(b.String())
}

func scanTask(sc scanner) (Task, error) {
	var t Task
	var created int64
	var due, resolved sql.NullInt64
	if err := sc.Scan(&t.ID, &created, &t.Text, &t.Source, &t.Evidence, &t.Status, &due, &t.Weight, &resolved); err != nil {
		return Task{}, err
	}
	t.CreatedTs = time.UnixMilli(created).UTC()
	t.Due = millisToTime(due)
	t.ResolvedTs = millisToTime(resolved)
	return t, nil
}