// Package memory implements the Cyrene v2 memory subsystem described in
// DevDocs/design/03-记忆.md.
//
// It exposes three layers:
//
//   - Store: a SQLite-backed long-term memory store. It persists memory items
//     with persona/role isolation (design §0 layer ②, §4) and offers
//     Add/Get/ListByPersona/Delete plus a manual "记住" path (design §1).
//   - WorkingMemory: a per-session sliding window of messages plus a summary
//     (design §0 layer ①).
//   - Retriever: relevance + recency retrieval and context injection
//     (design §2).
//
// Extraction (design §1) is exposed through the Extractor interface; the
// concrete M2 implementation is the rule-based ManualExtractor, which records
// what the user explicitly asks to remember. LLM-driven automatic extraction is
// deferred to milestone M4 and is not implemented here.
//
// Milestone M4 additions that this package deliberately leaves out but keeps
// extension points for: persistent vector storage, decay scheduling, and the
// project-index hierarchy (design §3). The schema already carries decay_weight
// and the retriever already plugs an optional embedding.Embedder, so those
// features land without a schema migration.
package memory

import (
	"context"
	"crypto/rand"
	"database/sql"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"strings"
	"time"
)

// Scope classifies who a long-term memory belongs to. It is orthogonal to Kind
// and both are stored on every item (design §3 note).
type Scope string

const (
	// ScopeUser is a memory about a user's habits, shared across personas.
	ScopeUser Scope = "user"
	// ScopePersona is a persona's everyday memory; other personas are read-only.
	ScopePersona Scope = "persona"
	// ScopeProject is a project-scoped memory, readable and writable by all.
	ScopeProject Scope = "project"
)

// valid reports whether s is one of the defined scopes.
func (s Scope) valid() bool {
	switch s {
	case ScopeUser, ScopePersona, ScopeProject:
		return true
	default:
		return false
	}
}

// Kind classifies the content category of a long-term memory item.
type Kind string

const (
	// KindRelationship is a memory about "you and me": preferences, habits,
	// agreements (design §0 "关系记忆").
	KindRelationship Kind = "relationship"
	// KindWorldEvent is a memory about objective facts and events in the world
	// (design §0 "世界/事件记忆").
	KindWorldEvent Kind = "world_event"
)

// valid reports whether k is one of the defined kinds.
func (k Kind) valid() bool {
	switch k {
	case KindRelationship, KindWorldEvent:
		return true
	default:
		return false
	}
}

// MemoryItem is a single long-term memory record. The zero value is not a valid
// item: PersonaID and Content are required, Scope and Kind are normalized to
// their defaults by Add.
type MemoryItem struct {
	ID                 string
	PersonaID          string
	RoleID             string
	Scope              Scope
	Kind               Kind
	Content            string
	Tags               []string
	Confidence         float64
	SourceConversation string
	CreatedAt          time.Time
	UpdatedAt          time.Time
	DecayWeight        float64
}

// Sentinels distinguishing memory package failures. Each is wrapped with %w at
// the call site so callers can use errors.Is.
var (
	// ErrMissingPersona is returned when an item has no PersonaID.
	ErrMissingPersona = errors.New("memory: persona_id is required")

	// ErrMissingContent is returned when an item has no content.
	ErrMissingContent = errors.New("memory: content is required")

	// ErrInvalidScope is returned when an item's Scope is not a defined scope.
	ErrInvalidScope = errors.New("memory: invalid scope")

	// ErrInvalidKind is returned when an item's Kind is not a defined kind.
	ErrInvalidKind = errors.New("memory: invalid kind")

	// ErrNotFound is returned when an item id does not exist.
	ErrNotFound = errors.New("memory: item not found")

	// ErrNilDB is returned when NewStore receives a nil *sql.DB.
	ErrNilDB = errors.New("memory: nil db")
)

// Schema defaults and time-window constants.
const (
	defaultConfidence  = 1.0
	defaultDecayWeight = 1.0
)

// Store is a long-term memory store backed by a *sql.DB.
//
// The store owns its schema: NewStore creates the memory_items table and its
// indexes idempotently. Concurrency is delegated to *sql.DB, which is safe for
// concurrent use by contract, so Store holds no further lock. Every query is
// parameterized (?) and lists columns explicitly (no SELECT *), and reads take
// a context.Cancel.
type Store struct {
	db  *sql.DB
	log *slog.Logger
}

// rowScanner is satisfied by both *sql.Row and *sql.Rows, so a single scanItem
// helper serves Get and ListByPersona.
type rowScanner interface {
	Scan(dest ...any) error
}

// NewStore builds a Store on db, ensuring the memory_items schema exists.
//
// db must be a live, migrated connection (the migration framework itself is
// owned by internal/db). A nil db returns ErrNilDB. log may be nil, in which
// case a discard logger is used. ctx controls schema creation and must be
// honored by the caller.
func NewStore(ctx context.Context, db *sql.DB, log *slog.Logger) (*Store, error) {
	if db == nil {
		return nil, ErrNilDB
	}
	log = discardLogger(log)
	for _, ddl := range []string{
		createMemoryItemsTable,
		createMemoryItemsPersonaIndex,
		createMemoryItemsPersonaCreatedIndex,
	} {
		if _, err := db.ExecContext(ctx, ddl); err != nil {
			return nil, fmt.Errorf("memory: apply schema: %w", err)
		}
	}
	return &Store{db: db, log: log}, nil
}

// Add persists item, assigning an id and missing timestamps, and normalizing
// Scope/Kind/Confidence/DecayWeight to their defaults. It returns the stored
// item with the id and timestamps filled in.
func (s *Store) Add(ctx context.Context, item MemoryItem) (MemoryItem, error) {
	if err := item.normalize(); err != nil {
		return MemoryItem{}, err
	}
	var err error
	if item, err = applyTimestamps(item); err != nil {
		return MemoryItem{}, err
	}
	tagsJSON, err := json.Marshal(item.Tags)
	if err != nil {
		return MemoryItem{}, fmt.Errorf("memory: encode tags: %w", err)
	}

	_, err = s.db.ExecContext(ctx, `INSERT INTO memory_items
		(id, persona_id, role_id, scope, kind, content, tags, confidence,
		 source_conversation, created_at, updated_at, decay_weight)
		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
		item.ID, item.PersonaID, item.RoleID, string(item.Scope), string(item.Kind),
		item.Content, string(tagsJSON), item.Confidence, item.SourceConversation,
		formatTime(item.CreatedAt), formatTime(item.UpdatedAt), item.DecayWeight)
	if err != nil {
		return MemoryItem{}, fmt.Errorf("memory: insert item: %w", err)
	}
	return item, nil
}

// Get returns the item with the given id. It returns ErrNotFound when the id is
// empty or does not exist.
func (s *Store) Get(ctx context.Context, id string) (MemoryItem, error) {
	if strings.TrimSpace(id) == "" {
		return MemoryItem{}, ErrNotFound
	}
	row := s.db.QueryRowContext(ctx, `SELECT id, persona_id, role_id, scope, kind,
		content, tags, confidence, source_conversation, created_at, updated_at,
		decay_weight FROM memory_items WHERE id = ?`, id)
	item, err := scanItem(row)
	if errors.Is(err, sql.ErrNoRows) {
		return MemoryItem{}, ErrNotFound
	}
	if err != nil {
		return MemoryItem{}, fmt.Errorf("memory: get item: %w", err)
	}
	return item, nil
}

// ListByPersona returns every item for personaID, newest first. It is the
// persona-isolation boundary: items belonging to other personas are never
// returned, so personas never share memory (design §4).
func (s *Store) ListByPersona(ctx context.Context, personaID string) ([]MemoryItem, error) {
	if strings.TrimSpace(personaID) == "" {
		return nil, ErrMissingPersona
	}
	rows, err := s.db.QueryContext(ctx, `SELECT id, persona_id, role_id, scope, kind,
		content, tags, confidence, source_conversation, created_at, updated_at,
		decay_weight FROM memory_items WHERE persona_id = ?
		ORDER BY created_at DESC`, personaID)
	if err != nil {
		return nil, fmt.Errorf("memory: list persona items: %w", err)
	}
	defer func() { _ = rows.Close() }()

	items := make([]MemoryItem, 0)
	for rows.Next() {
		item, err := scanItem(rows)
		if err != nil {
			return nil, fmt.Errorf("memory: scan persona item: %w", err)
		}
		items = append(items, item)
	}
	if err := rows.Err(); err != nil {
		return nil, fmt.Errorf("memory: iterate persona items: %w", err)
	}
	return items, nil
}

// Delete removes the item with the given id and returns ErrNotFound when the id
// is empty or no row was removed. Callers should only pass ids they obtained
// through a persona-scoped read (ListByPersona/Get) so deletion stays within
// the caller's persona boundary.
func (s *Store) Delete(ctx context.Context, id string) error {
	if strings.TrimSpace(id) == "" {
		return ErrNotFound
	}
	res, err := s.db.ExecContext(ctx, `DELETE FROM memory_items WHERE id = ?`, id)
	if err != nil {
		return fmt.Errorf("memory: delete item: %w", err)
	}
	n, err := res.RowsAffected()
	if err != nil {
		return fmt.Errorf("memory: delete row count: %w", err)
	}
	if n == 0 {
		return ErrNotFound
	}
	return nil
}

// RememberManual writes a single long-term memory item for content text — the
// "记住 X" path from design §1. The item is scoped to the persona by default and
// recorded as a relationship memory at full confidence. Returns the stored item.
func (s *Store) RememberManual(ctx context.Context, personaID, roleID, text string) (MemoryItem, error) {
	item := MemoryItem{
		PersonaID: personaID,
		RoleID:    roleID,
		Scope:     ScopePersona,
		Kind:      KindRelationship,
		Content:   strings.TrimSpace(text),
	}
	if item.Content == "" {
		return MemoryItem{}, ErrMissingContent
	}
	return s.Add(ctx, item)
}

// normalize fills default Scope/Kind/Confidence/DecayWeight then validates.
// It is called by Add so callers may omit values while still catching
// explicitly invalid ones.
func (it *MemoryItem) normalize() error {
	if it.Scope == "" {
		it.Scope = ScopePersona
	}
	if it.Kind == "" {
		it.Kind = KindRelationship
	}
	if it.Confidence == 0 {
		it.Confidence = defaultConfidence
	}
	if it.DecayWeight == 0 {
		it.DecayWeight = defaultDecayWeight
	}
	return it.validate()
}

// validate checks required and enumerated fields. It is pure: it does not apply
// defaults, so it can also be used to reject a caller-provided value.
func (it *MemoryItem) validate() error {
	if strings.TrimSpace(it.PersonaID) == "" {
		return ErrMissingPersona
	}
	if strings.TrimSpace(it.Content) == "" {
		return ErrMissingContent
	}
	if !it.Scope.valid() {
		return ErrInvalidScope
	}
	if !it.Kind.valid() {
		return ErrInvalidKind
	}
	return nil
}

// applyTimestamps assigns an id and missing CreatedAt/UpdatedAt values. It
// returns an error only when a fresh id cannot be generated.
func applyTimestamps(item MemoryItem) (MemoryItem, error) {
	now := time.Now().UTC()
	if item.ID == "" {
		id, err := newID()
		if err != nil {
			return MemoryItem{}, err
		}
		item.ID = id
	}
	if item.CreatedAt.IsZero() {
		item.CreatedAt = now
	}
	if item.UpdatedAt.IsZero() {
		item.UpdatedAt = now
	}
	return item, nil
}

// scanItem reads one memory_items row. tags is stored as a JSON array string;
// timestamps are stored as RFC3339 strings.
func scanItem(sc rowScanner) (MemoryItem, error) {
	var (
		it                   MemoryItem
		scope, kind          string
		tagsJSON             string
		createdAt, updatedAt string
	)
	if err := sc.Scan(&it.ID, &it.PersonaID, &it.RoleID, &scope, &kind,
		&it.Content, &tagsJSON, &it.Confidence, &it.SourceConversation,
		&createdAt, &updatedAt, &it.DecayWeight); err != nil {
		return MemoryItem{}, err
	}
	it.Scope = Scope(scope)
	it.Kind = Kind(kind)
	if err := json.Unmarshal([]byte(tagsJSON), &it.Tags); err != nil {
		return MemoryItem{}, fmt.Errorf("memory: decode tags: %w", err)
	}
	if it.Tags == nil {
		it.Tags = []string{}
	}

	var err error
	if it.CreatedAt, err = parseTime(createdAt); err != nil {
		return MemoryItem{}, err
	}
	if it.UpdatedAt, err = parseTime(updatedAt); err != nil {
		return MemoryItem{}, err
	}
	return it, nil
}

// newID returns a 128-bit cryptographically random id, hex-encoded. It avoids a
// UUID dependency so the package stays self-contained. rand.Read fails only if
// the platform entropy source is broken, so the error is propagated.
func newID() (string, error) {
	b := make([]byte, 16)
	if _, err := rand.Read(b); err != nil {
		return "", fmt.Errorf("memory: generate id: %w", err)
	}
	return hex.EncodeToString(b), nil
}

// formatTime encodes t in UTC as an RFC3339 string. The value is always written
// as UTC so ordering and round-trips are stable across processes.
func formatTime(t time.Time) string {
	return t.UTC().Format(time.RFC3339Nano)
}

// parseTime decodes a stored RFC3339 string. Blank input yields a zero time.
func parseTime(s string) (time.Time, error) {
	if strings.TrimSpace(s) == "" {
		return time.Time{}, nil
	}
	t, err := time.Parse(time.RFC3339Nano, s)
	if err != nil {
		return time.Time{}, fmt.Errorf("memory: parse time %q: %w", s, err)
	}
	return t, nil
}

// discardLogger returns log if non-nil, otherwise a logger that writes nowhere.
func discardLogger(log *slog.Logger) *slog.Logger {
	if log != nil {
		return log
	}
	return slog.New(slog.NewTextHandler(io.Discard, nil))
}

// Schema DDL. The migration framework in internal/db is the authoritative
// mechanism for domain tables, but it is intentionally not used here: the db
// package's migration tests assert an exact migration count, and the memory
// store must be self-contained and usable from any *sql.DB. The table and
// indexes are therefore created idempotently by NewStore.
const createMemoryItemsTable = `
CREATE TABLE IF NOT EXISTS memory_items (
    id                  TEXT PRIMARY KEY,
    persona_id          TEXT NOT NULL,
    role_id             TEXT NOT NULL DEFAULT '',
    scope               TEXT NOT NULL,
    kind                TEXT NOT NULL,
    content             TEXT NOT NULL,
    tags                TEXT NOT NULL DEFAULT '[]',
    confidence          REAL NOT NULL DEFAULT 1.0,
    source_conversation TEXT NOT NULL DEFAULT '',
    created_at          TEXT NOT NULL,
    updated_at          TEXT NOT NULL,
    decay_weight        REAL NOT NULL DEFAULT 1.0
)`

const createMemoryItemsPersonaIndex = `
CREATE INDEX IF NOT EXISTS idx_memory_items_persona_id
    ON memory_items (persona_id)`

const createMemoryItemsPersonaCreatedIndex = `
CREATE INDEX IF NOT EXISTS idx_memory_items_persona_id_created_at
    ON memory_items (persona_id, created_at)`