package bstore

import (
	"encoding"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"os"
	"reflect"
	"sync"
	"time"

	bolt "go.etcd.io/bbolt"
)

var (
	ErrAbsent       = errors.New("absent") // If a function can return an ErrAbsent, it can be compared directly, without errors.Is.
	ErrZero         = errors.New("must be nonzero")
	ErrUnique       = errors.New("not unique")
	ErrReference    = errors.New("referential inconsistency")
	ErrMultiple     = errors.New("multiple results")
	ErrSeq          = errors.New("highest autoincrement sequence value reached")
	ErrType         = errors.New("unknown/bad type")
	ErrIncompatible = errors.New("incompatible types")
	ErrFinished     = errors.New("query finished")
	ErrStore        = errors.New("internal/storage error") // E.g. when buckets disappear, possibly by external users of the underlying BoltDB database.
	ErrParam        = errors.New("bad parameters")

	errTxClosed    = errors.New("transaction is closed")
	errNestedIndex = errors.New("struct tags index/unique only allowed at top-level structs")
)

var sanityChecks bool // Only enabled during tests.

// DB is a database storing Go struct values in an underlying bolt database.
// DB is safe for concurrent use, unlike a Tx or a Query.
type DB struct {
	bdb *bolt.DB

	// Read transaction take an rlock on types. Register can make changes and
	// needs a wlock.
	typesMutex sync.RWMutex
	types      map[reflect.Type]storeType
	typeNames  map[string]storeType // Go type name to store type, for checking duplicates.

	statsMutex sync.Mutex
	stats      Stats
}

// Tx is a transaction on DB.
//
// A Tx is not safe for concurrent use.
type Tx struct {
	db  *DB // If nil, this transaction is closed.
	btx *bolt.Tx

	bucketCache map[bucketKey]*bolt.Bucket

	stats Stats
}

// bucketKey represents a subbucket for a type.
type bucketKey struct {
	typeName string
	sub      string // Empty for top-level type bucket, otherwise "records", "types" or starting with "index.".
}

type index struct {
	Unique bool
	Name   string // Normally named after the field. But user can specify alternative name with "index" or "unique" struct tag with parameter.
	Fields []field

	tv *typeVersion
}

type storeType struct {
	Name    string       // Name of type as stored in database. Different from the current Go type name if the uses the "typename" struct tag.
	Type    reflect.Type // Type we parse into for new values.
	Current *typeVersion

	// Earlier schema versions. Older type versions can still be stored. We
	// prepare them for parsing into the reflect.Type. Some stored fields in
	// old versions may be ignored: when a later schema has removed the field,
	// that old stored field is considered deleted and will be ignored when
	// parsing.
	Versions map[uint32]*typeVersion
}

// note: when changing, possibly update func equal as well.
type typeVersion struct {
	Version       uint32              // First uvarint of a stored record references this version.
	OndiskVersion uint32              // Version of on-disk format. Currently always 1.
	Noauto        bool                // If true, the primary key is an int but opted out of autoincrement.
	Fields        []field             // Fields that we store. Embed/anonymous fields are kept separately in embedFields, and are not stored.
	Indices       map[string]*index   // By name of index.
	ReferencedBy  map[string]struct{} // Type names that reference this type. We require they are registered at the same time to maintain referential integrity.

	name         string
	referencedBy []*index            // Indexes (from other types) that reference this type.
	references   map[string]struct{} // Keys are the type names referenced. This is a summary for the references from Fields.
	embedFields  []embed             // Embed/anonymous fields, their values are stored through Fields, we keep them for setting values.

	fillPercent float64 // For "records" bucket. Set to 1 for append-only/mostly use as set with HintAppend, 0.5 otherwise.
}

// note: when changing, possibly update func equal as well.
// embed/anonymous fields are represented as type embed. The fields inside the embed type are of this type field.
type field struct {
	Name       string
	Type       fieldType
	Nonzero    bool
	References []string // Referenced fields. Only for the top-level struct fields, not for nested structs.
	Default    string   // As specified in struct tag. Processed version is defaultValue.

	// If not the zero reflect.Value, set this value instead of a zero value on insert.
	// This is always a non-pointer value. Only set for the current typeVersion
	// linked to a Go type.
	defaultValue reflect.Value

	// Only set if this typeVersion will parse this field. We check
	// structField.Type for non-nil before parsing this field. We don't parse it
	// if this field is no longer in the type, or if it has been removed and
	// added again in later schema versions.
	structField reflect.StructField

	indices map[string]*index
}

// embed is for embed/anonymous fields. the fields inside are represented as a type field.
type embed struct {
	Name        string
	Type        fieldType
	structField reflect.StructField
}

type kind int

const (
	kindInvalid kind = iota
	kindBytes
	kindBool
	kindInt
	kindInt8
	kindInt16
	kindInt32
	kindInt64
	kindUint
	kindUint8
	kindUint16
	kindUint32
	kindUint64
	kindFloat32
	kindFloat64
	kindMap
	kindSlice
	kindString
	kindTime
	kindBinaryMarshal
	kindStruct
)

var kindStrings = []string{
	"(invalid)",
	"bytes",
	"bool",
	"int",
	"int8",
	"int16",
	"int32",
	"int64",
	"uint",
	"uint8",
	"uint16",
	"uint32",
	"uint64",
	"float32",
	"float64",
	"map",
	"slice",
	"string",
	"time",
	"binarymarshal",
	"struct",
}

func (k kind) String() string {
	return kindStrings[k]
}

type fieldType struct {
	Ptr              bool       // If type is a pointer.
	Kind             kind       // Type with possible Ptr deferenced.
	Fields           []field    // For kindStruct.
	MapKey, MapValue *fieldType // For kindMap.
	List             *fieldType // For kindSlice.
}

func (ft fieldType) String() string {
	s := ft.Kind.String()
	if ft.Ptr {
		return s + "ptr"
	}
	return s
}

// Options configure how a database should be opened or initialized.
type Options struct {
	Timeout   time.Duration // Abort if opening DB takes longer than Timeout.
	Perm      fs.FileMode   // Permissions for new file if created. If zero, 0600 is used.
	MustExist bool          // Before opening, check that file exists. If not, io/fs.ErrNotExist is returned.
}

// Open opens a bstore database and registers types by calling Register.
//
// If the file does not exist, a new database file is created, unless opts has
// MustExist set. Files are created with permission 0600, or with Perm from
// Options if nonzero.
//
// Only one DB instance can be open for a file at a time. Use opts.Timeout to
// specify a timeout during open to prevent indefinite blocking.
func Open(path string, opts *Options, typeValues ...any) (*DB, error) {
	var bopts *bolt.Options
	if opts != nil && opts.Timeout > 0 {
		bopts = &bolt.Options{Timeout: opts.Timeout}
	}
	var mode fs.FileMode = 0600
	if opts != nil && opts.Perm != 0 {
		mode = opts.Perm
	}
	if opts != nil && opts.MustExist {
		if _, err := os.Stat(path); err != nil {
			return nil, err
		}
	}
	bdb, err := bolt.Open(path, mode, bopts)
	if err != nil {
		return nil, err
	}

	typeNames := map[string]storeType{}
	types := map[reflect.Type]storeType{}
	db := &DB{bdb: bdb, typeNames: typeNames, types: types}
	if err := db.Register(typeValues...); err != nil {
		bdb.Close()
		return nil, err
	}
	return db, nil
}

// Close closes the underlying database.
func (db *DB) Close() error {
	return db.bdb.Close()
}

// Stats returns usage statistics for the lifetime of DB.  Stats are tracked
// first in a Query or a Tx. Stats from a Query are propagated to its Tx when
// the Query finishes. Stats from a Tx are propagated to its DB when the
// transaction ends.
func (db *DB) Stats() Stats {
	db.statsMutex.Lock()
	defer db.statsMutex.Unlock()
	return db.stats
}

// Stats returns usage statistics for this transaction.
// When a transaction is rolled back or committed, its statistics are copied
// into its DB.
func (tx *Tx) Stats() Stats {
	return tx.stats
}

// WriteTo writes the entire database to w, not including changes made during this transaction.
func (tx *Tx) WriteTo(w io.Writer) (n int64, err error) {
	return tx.btx.WriteTo(w)
}

// return a bucket through cache.
func (tx *Tx) bucket(bk bucketKey) (*bolt.Bucket, error) {
	if tx.bucketCache == nil {
		tx.bucketCache = map[bucketKey]*bolt.Bucket{}
	}
	b := tx.bucketCache[bk]
	if b != nil {
		return b, nil
	}
	top := tx.bucketCache[bucketKey{bk.typeName, ""}]
	if top == nil {
		tx.stats.Bucket.Get++
		top = tx.btx.Bucket([]byte(bk.typeName))
		if top == nil {
			return nil, fmt.Errorf("%w: missing bucket for type %q", ErrStore, bk.typeName)
		}
		tx.bucketCache[bucketKey{bk.typeName, ""}] = top
	}
	if bk.sub == "" {
		return top, nil
	}

	tx.stats.Bucket.Get++
	b = top.Bucket([]byte(bk.sub))
	if b == nil {
		return nil, fmt.Errorf("%w: missing bucket %q for type %q", ErrStore, bk.sub, bk.typeName)
	}
	tx.bucketCache[bk] = b
	return b, nil
}

func (tx *Tx) typeBucket(typeName string) (*bolt.Bucket, error) {
	return tx.bucket(bucketKey{typeName, ""})
}

func (tx *Tx) recordsBucket(typeName string, fillPercent float64) (*bolt.Bucket, error) {
	b, err := tx.bucket(bucketKey{typeName, "records"})
	if err != nil {
		return nil, err
	}
	b.FillPercent = fillPercent
	return b, nil
}

func (tx *Tx) indexBucket(idx *index) (*bolt.Bucket, error) {
	return tx.bucket(bucketKey{idx.tv.name, "index." + idx.Name})
}

// Drop removes a type and its data from the database.
// If the type is currently registered, it is unregistered and no longer available.
// If a type is still referenced by another type, eg through a "ref" struct tag,
// ErrReference is returned.
// If the type does not exist, ErrAbsent is returned.
func (db *DB) Drop(name string) error {
	return db.Write(func(tx *Tx) error {
		tx.stats.Bucket.Get++
		if tx.btx.Bucket([]byte(name)) == nil {
			return ErrAbsent
		}

		if st, ok := db.typeNames[name]; ok && len(st.Current.referencedBy) > 0 {
			return fmt.Errorf("%w: type is still referenced", ErrReference)
		} else if ok {
			for ref := range st.Current.references {
				var n []*index
				for _, idx := range db.typeNames[ref].Current.referencedBy {
					if idx.tv != st.Current {
						n = append(n, idx)
					}
				}
				db.typeNames[ref].Current.referencedBy = n
			}
			delete(db.typeNames, name)
			delete(db.types, st.Type)
		}

		tx.stats.Bucket.Delete++
		return tx.btx.DeleteBucket([]byte(name))
	})
}

// Delete calls Delete on a new writable Tx.
func (db *DB) Delete(values ...any) error {
	return db.Write(func(tx *Tx) error {
		return tx.Delete(values...)
	})
}

// Get calls Get on a new read-only Tx.
func (db *DB) Get(values ...any) error {
	return db.Read(func(tx *Tx) error {
		return tx.Get(values...)
	})
}

// Insert calls Insert on a new writable Tx.
func (db *DB) Insert(values ...any) error {
	return db.Write(func(tx *Tx) error {
		return tx.Insert(values...)
	})
}

// Update calls Update on a new writable Tx.
func (db *DB) Update(values ...any) error {
	return db.Write(func(tx *Tx) error {
		return tx.Update(values...)
	})
}

var typeKinds = map[reflect.Kind]kind{
	reflect.Bool:    kindBool,
	reflect.Int:     kindInt,
	reflect.Int8:    kindInt8,
	reflect.Int16:   kindInt16,
	reflect.Int32:   kindInt32,
	reflect.Int64:   kindInt64,
	reflect.Uint:    kindUint,
	reflect.Uint8:   kindUint8,
	reflect.Uint16:  kindUint16,
	reflect.Uint32:  kindUint32,
	reflect.Uint64:  kindUint64,
	reflect.Float32: kindFloat32,
	reflect.Float64: kindFloat64,
	reflect.Map:     kindMap,
	reflect.Slice:   kindSlice,
	reflect.String:  kindString,
}

func typeKind(t reflect.Type) (kind, error) {
	if t.Kind() == reflect.Slice && t.Elem().Kind() == reflect.Uint8 {
		return kindBytes, nil
	}

	k, ok := typeKinds[t.Kind()]
	if ok {
		return k, nil
	}

	if t == reflect.TypeOf(zerotime) {
		return kindTime, nil
	}

	if reflect.PointerTo(t).AssignableTo(reflect.TypeOf((*encoding.BinaryMarshaler)(nil)).Elem()) {
		return kindBinaryMarshal, nil
	}

	if t.Kind() == reflect.Struct {
		return kindStruct, nil
	}
	return kind(0), fmt.Errorf("%w: unsupported type %v", ErrType, t)
}

func typeName(t reflect.Type) (string, error) {
	tags, err := newStoreTags(t.Field(0).Tag.Get("bstore"), true)
	if err != nil {
		return "", err
	}
	if name, err := tags.Get("typename"); err != nil {
		return "", err
	} else if name != "" {
		return name, nil
	}
	return t.Name(), nil
}

// Get value for a key. For insert a next sequence may be generated for the
// primary key.
func (tv typeVersion) keyValue(tx *Tx, rv reflect.Value, insert bool, rb *bolt.Bucket) ([]byte, reflect.Value, bool, error) {
	f := tv.Fields[0]
	krv := rv.FieldByIndex(f.structField.Index)
	var seq bool
	if krv.IsZero() {
		if !insert {
			return nil, reflect.Value{}, seq, fmt.Errorf("%w: primary key can not be zero value", ErrParam)
		}
		if tv.Noauto {
			return nil, reflect.Value{}, seq, fmt.Errorf("%w: primary key cannot be zero value without autoincrement", ErrParam)
		}
		id, err := rb.NextSequence()
		if err != nil {
			return nil, reflect.Value{}, seq, fmt.Errorf("next primary key: %w", err)
		}
		switch f.Type.Kind {
		case kindInt, kindInt8, kindInt16, kindInt32, kindInt64:
			if krv.OverflowInt(int64(id)) {
				return nil, reflect.Value{}, seq, fmt.Errorf("%w: next primary key sequence does not fit in type", ErrSeq)
			}
			krv.SetInt(int64(id))
		case kindUint, kindUint8, kindUint16, kindUint32, kindUint64:
			if krv.OverflowUint(id) {
				return nil, reflect.Value{}, seq, fmt.Errorf("%w: next primary key sequence does not fit in type", ErrSeq)
			}
			krv.SetUint(id)
		default:
			// todo: should check this during register.
			return nil, reflect.Value{}, seq, fmt.Errorf("%w: unsupported autoincrement primary key type %v", ErrZero, f.Type.Kind)
		}
		seq = true
	} else if !tv.Noauto && insert {
		// We let user insert their own ID for our own autoincrement
		// PK. But we update the internal next sequence if the users's
		// PK is highest yet, so a future autoincrement insert will succeed.
		switch f.Type.Kind {
		case kindInt, kindInt8, kindInt16, kindInt32, kindInt64:
			v := krv.Int()
			if v > 0 && uint64(v) > rb.Sequence() {
				if err := rb.SetSequence(uint64(v)); err != nil {
					return nil, reflect.Value{}, seq, fmt.Errorf("%w: updating sequence: %s", ErrStore, err)
				}
			}
		case kindUint, kindUint8, kindUint16, kindUint32, kindUint64:
			v := krv.Uint()
			if v > rb.Sequence() {
				if err := rb.SetSequence(v); err != nil {
					return nil, reflect.Value{}, seq, fmt.Errorf("%w: updating sequence: %s", ErrStore, err)
				}
			}
		}
	}

	k, err := packPK(krv)
	if err != nil {
		return nil, reflect.Value{}, seq, err
	}
	if seq {
		tx.stats.Records.Get++
		if rb.Get(k) != nil {
			return nil, reflect.Value{}, seq, fmt.Errorf("%w: internal error: next sequence value is already present", ErrUnique)
		}
	}
	return k, krv, seq, err
}

// Read calls function fn with a new read-only transaction, ensuring transaction rollback.
func (db *DB) Read(fn func(*Tx) error) error {
	db.typesMutex.RLock()
	defer db.typesMutex.RUnlock()
	return db.bdb.View(func(btx *bolt.Tx) error {
		tx := &Tx{db: db, btx: btx}
		tx.stats.Reads++
		defer tx.addStats()
		return fn(tx)
	})
}

// Write calls function fn with a new read-write transaction. If fn returns
// nil, the transaction is committed. Otherwise the transaction is rolled back.
func (db *DB) Write(fn func(*Tx) error) error {
	db.typesMutex.RLock()
	defer db.typesMutex.RUnlock()
	return db.bdb.Update(func(btx *bolt.Tx) error {
		tx := &Tx{db: db, btx: btx}
		tx.stats.Writes++
		defer tx.addStats()
		return fn(tx)
	})
}

// lookup storeType based on name of rt.
func (db *DB) storeType(rt reflect.Type) (storeType, error) {
	st, ok := db.types[rt]
	if !ok {
		return storeType{}, fmt.Errorf("%w: %v", ErrType, rt)
	}
	return st, nil
}

// HintAppend sets a hint whether changes to the types indicated by each struct
// from values is (mostly) append-only.
//
// This currently sets the BoltDB bucket FillPercentage to 1 for efficient use
// of storage space.
func (db *DB) HintAppend(append bool, values ...any) error {
	db.typesMutex.Lock()
	defer db.typesMutex.Unlock()
	for _, v := range values {
		t := reflect.TypeOf(v)
		st, err := db.storeType(t)
		if err != nil {
			return err
		}
		if append {
			st.Current.fillPercent = 1.0
		} else {
			st.Current.fillPercent = 0.5
		}
	}
	return nil
}