package practice

import (
	"context"
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"strings"
	"time"
	"unicode/utf8"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgxpool"
)

const NameLimit = 40
const NoteLimit = 2000
const MinuteLimit = 2147483647 // PostgreSQL integer storage limit; minutes remain exact.

var (
	ErrInvalid      = errors.New("输入无效，请检查后重试")
	ErrUnavailable  = errors.New("项目或记录不可用，请刷新后重试")
	ErrUnauthorized = errors.New("登录已失效，请重新登录")
	ErrArchived     = errors.New("该项目已移出常用列表，请选择其他项目")
	ErrConflict     = errors.New("这次提交已保存了不同内容，请刷新记录后更正")
	ErrDeleted      = errors.New("这次提交的记录已被删除，不会重复创建")
)

var presets = []string{"零基础", "基础提升", "软开素质", "足髋训练", "核心臀腿", "小球核心", "天鹅臂颈", "呼吸训练"}

type Store struct {
	db  *pgxpool.Pool
	now func() time.Time
}

// New keeps all ownership, dates, archival and persistence rules behind business operations.
// now is injectable so business-day boundaries can be verified without changing the host clock.
func New(db *pgxpool.Pool, now func() time.Time) *Store {
	if now == nil {
		now = time.Now
	}
	return &Store{db: db, now: now}
}

func newID() string          { return rand.Text() }
func digest(s string) string { sum := sha256.Sum256([]byte(s)); return hex.EncodeToString(sum[:]) }

// Today is a calendar date in the fixed first-release business timezone, not the device timezone.
func (s *Store) Today() string {
	return s.now().In(time.FixedZone("Asia/Shanghai", 8*60*60)).Format(time.DateOnly)
}

type Session struct {
	Token     string    `json:"token"`
	ExpiresAt time.Time `json:"expiresAt"`
}

// Login accepts only a server-verified, app-scoped WeChat identity. User creation,
// presets and the session commit together; deleted presets never reappear on login.
func (s *Store) Login(ctx context.Context, identity string) (Session, error) {
	if identity == "" {
		return Session{}, ErrUnauthorized
	}
	tx, err := s.db.Begin(ctx)
	if err != nil {
		return Session{}, err
	}
	defer tx.Rollback(ctx)
	id := newID()
	result, err := tx.Exec(ctx, "INSERT INTO users(id,wechat_identity) VALUES($1,$2) ON CONFLICT(wechat_identity) DO NOTHING", id, identity)
	if err != nil {
		return Session{}, err
	}
	if result.RowsAffected() == 1 {
		for _, name := range presets {
			if _, err = tx.Exec(ctx, "INSERT INTO projects(id,user_id,name) VALUES($1,$2,$3)", newID(), id, name); err != nil {
				return Session{}, err
			}
		}
	} else if err = tx.QueryRow(ctx, "SELECT id FROM users WHERE wechat_identity=$1", identity).Scan(&id); err != nil {
		return Session{}, err
	}
	// Serialize session maintenance for this user as well as business writes.
	if _, err = tx.Exec(ctx, "SELECT id FROM users WHERE id=$1 FOR UPDATE", id); err != nil {
		return Session{}, err
	}
	if _, err = tx.Exec(ctx, "DELETE FROM sessions WHERE user_id=$1 AND expires_at <= $2", id, s.now()); err != nil {
		return Session{}, err
	}
	session := Session{Token: newID() + newID(), ExpiresAt: s.now().Add(30 * 24 * time.Hour)}
	if _, err = tx.Exec(ctx, "INSERT INTO sessions(token_hash,user_id,expires_at) VALUES($1,$2,$3)", digest(session.Token), id, session.ExpiresAt); err != nil {
		return Session{}, err
	}
	if err = tx.Commit(ctx); err != nil {
		return Session{}, err
	}
	return session, nil
}

// Authenticate never accepts a client-provided user ID. Only opaque, unexpired
// bearer tokens are usable; their hashes, not tokens, are kept in PostgreSQL.
func (s *Store) Authenticate(ctx context.Context, token string) (string, error) {
	if len(token) != 52 {
		return "", ErrUnauthorized
	}
	var user string
	err := s.db.QueryRow(ctx, "SELECT user_id FROM sessions WHERE token_hash=$1 AND expires_at>$2", digest(token), s.now()).Scan(&user)
	if errors.Is(err, pgx.ErrNoRows) {
		return "", ErrUnauthorized
	}
	return user, err
}

// write serializes a user's mutations. Project removal, record writes and
// idempotency receipts therefore see a consistent state even across devices.
func (s *Store) write(ctx context.Context, user string, fn func(pgx.Tx) error) error {
	tx, err := s.db.Begin(ctx)
	if err != nil {
		return err
	}
	defer tx.Rollback(ctx)
	var id string
	if err = tx.QueryRow(ctx, "SELECT id FROM users WHERE id=$1 FOR UPDATE", user).Scan(&id); err != nil {
		return err
	}
	if err = fn(tx); err != nil {
		return err
	}
	return tx.Commit(ctx)
}

type Project struct {
	ID         string `json:"id"`
	Name       string `json:"name"`
	Archived   bool   `json:"archived"`
	HasRecords bool   `json:"hasRecords"`
}

func (s *Store) Projects(ctx context.Context, user string, archived bool) ([]Project, error) {
	rows, err := s.db.Query(ctx, `SELECT p.id,p.name,p.archived,EXISTS(SELECT 1 FROM records r WHERE r.user_id=p.user_id AND r.project_id=p.id)
		FROM projects p WHERE p.user_id=$1 AND ($2 OR NOT p.archived) ORDER BY p.position`, user, archived)
	if err != nil {
		return nil, err
	}
	defer rows.Close()
	projects := []Project{}
	for rows.Next() {
		var p Project
		if err := rows.Scan(&p.ID, &p.Name, &p.Archived, &p.HasRecords); err != nil {
			return nil, err
		}
		projects = append(projects, p)
	}
	return projects, rows.Err()
}

func (s *Store) SaveProject(ctx context.Context, user, id, name string) (Project, error) {
	name = strings.TrimSpace(name)
	if name == "" || utf8.RuneCountInString(name) > NameLimit || strings.ContainsRune(name, 0) {
		return Project{}, ErrInvalid
	}
	p := Project{ID: id, Name: name}
	err := s.write(ctx, user, func(tx pgx.Tx) error {
		if id == "" {
			p.ID = newID()
			_, err := tx.Exec(ctx, "INSERT INTO projects(id,user_id,name) VALUES($1,$2,$3)", p.ID, user, name)
			return err
		}
		err := tx.QueryRow(ctx, `UPDATE projects SET name=$3 WHERE user_id=$1 AND id=$2 RETURNING archived,
			EXISTS(SELECT 1 FROM records WHERE user_id=$1 AND project_id=$2)`, user, id, name).Scan(&p.Archived, &p.HasRecords)
		if errors.Is(err, pgx.ErrNoRows) {
			return ErrUnavailable
		}
		return err
	})
	return p, err
}

func (s *Store) RemoveProject(ctx context.Context, user, id string) (string, error) {
	mode := "deleted"
	err := s.write(ctx, user, func(tx pgx.Tx) error {
		var used bool
		err := tx.QueryRow(ctx, `SELECT EXISTS(SELECT 1 FROM records WHERE user_id=$1 AND project_id=$2)
			FROM projects WHERE user_id=$1 AND id=$2`, user, id).Scan(&used)
		if errors.Is(err, pgx.ErrNoRows) {
			return ErrUnavailable
		}
		if err != nil {
			return err
		}
		if used {
			mode = "archived"
			_, err = tx.Exec(ctx, "UPDATE projects SET archived=true WHERE user_id=$1 AND id=$2", user, id)
		} else {
			_, err = tx.Exec(ctx, "DELETE FROM projects WHERE user_id=$1 AND id=$2", user, id)
		}
		return err
	})
	return mode, err
}