package backup

import (
	"context"
	"encoding/json"
	"fmt"
	"io"
	"os"
	"sort"
	"time"

	"go.mongodb.org/mongo-driver/v2/bson"
	"go.mongodb.org/mongo-driver/v2/mongo"
)

// DumpOptions configures one backup.
type DumpOptions struct {
	Client   *mongo.Client
	Database string
	Exclude  []string

	// KeyHex is KEY_ENCRYPTION_KEY. It is fingerprinted and discarded; it is
	// never written to the archive.
	KeyHex string

	// AllowNoKey permits a backup of a deployment that stores no encrypted
	// material. The manifest then records a null fingerprint, which restore
	// reports rather than treating as a match.
	AllowNoKey bool

	VantageVersion string
	Out            io.Writer
}

// Dump writes a complete archive of one database to opt.Out.
//
// Collections are enumerated live rather than read from a list. A backup tool
// has no equivalent of AssertNoScopedCollectionMissed to catch a hardcoded list
// drifting, and the first symptom of that drift would be a restore silently
// missing a collection added since the list was written.
func Dump(ctx context.Context, opt DumpOptions) (Manifest, error) {
	fingerprint, err := dumpFingerprint(opt)
	if err != nil {
		return Manifest{}, err
	}

	db := opt.Client.Database(opt.Database)
	names, err := db.ListCollectionNames(ctx, bson.M{})
	if err != nil {
		return Manifest{}, fmt.Errorf("list collections: %w", err)
	}
	sort.Strings(names)

	excluded := map[string]bool{}
	for _, e := range opt.Exclude {
		excluded[e] = true
	}

	serverVersion, err := mongoServerVersion(ctx, opt.Client)
	if err != nil {
		return Manifest{}, err
	}
	hostname, err := os.Hostname()
	if err != nil {
		hostname = "unknown"
	}

	w := NewWriter(opt.Out)
	entries := make([]CollectionEntry, 0, len(names))
	for _, name := range names {
		if excluded[name] {
			continue
		}
		entry, err := dumpCollection(ctx, w, db, name)
		if err != nil {
			return Manifest{}, err
		}
		entries = append(entries, entry)
	}

	m := Manifest{
		FormatVersion:      FormatVersion,
		CreatedAt:          time.Now().UTC(),
		VantageVersion:     opt.VantageVersion,
		Hostname:           hostname,
		MongoDB:            opt.Database,
		MongoServerVersion: serverVersion,
		KeyFingerprint:     fingerprint,
		Collections:        entries,
		Excluded:           append([]string{}, opt.Exclude...),
	}
	if err := w.Close(m); err != nil {
		return Manifest{}, err
	}
	return m, nil
}

// dumpFingerprint applies the key policy before any output is produced. An
// archive of ciphertext whose key was never recorded is worse than no archive,
// because it looks like a backup.
func dumpFingerprint(opt DumpOptions) (*string, error) {
	if opt.KeyHex == "" {
		if opt.AllowNoKey {
			return nil, nil
		}
		return nil, fmt.Errorf("%w: pass --allow-no-key only if this deployment stores no encrypted data", ErrNoKey)
	}
	fp, err := FingerprintHex(opt.KeyHex)
	if err != nil {
		return nil, err
	}
	return &fp, nil
}

func dumpCollection(ctx context.Context, w *Writer, db *mongo.Database, name string) (CollectionEntry, error) {
	cur, err := db.Collection(name).Find(ctx, bson.M{})
	if err != nil {
		return CollectionEntry{}, fmt.Errorf("find %s: %w", name, err)
	}
	defer cur.Close(ctx)

	var docs [][]byte
	for cur.Next(ctx) {
		// cur.Current is only valid until the next Next, and it is written to
		// the archive verbatim rather than through a map, so every BSON type
		// survives exactly as the server stored it.
		docs = append(docs, append([]byte(nil), cur.Current...))
	}
	if err := cur.Err(); err != nil {
		return CollectionEntry{}, fmt.Errorf("iterate %s: %w", name, err)
	}

	entry, err := w.WriteCollection(name, docs)
	if err != nil {
		return CollectionEntry{}, err
	}
	if err := dumpIndexes(ctx, w, db, name); err != nil {
		return CollectionEntry{}, err
	}
	return entry, nil
}

func dumpIndexes(ctx context.Context, w *Writer, db *mongo.Database, name string) error {
	cur, err := db.Collection(name).Indexes().List(ctx)
	if err != nil {
		return fmt.Errorf("list indexes on %s: %w", name, err)
	}
	defer cur.Close(ctx)

	// The specs are read as raw BSON and re-encoded as extended JSON, one
	// element per index, so key order and every option the server reported —
	// partialFilterExpression, collation, weights and the rest — survive
	// verbatim. Decoding into bson.M would lose compound key order, and
	// reconstructing an index from a hand-picked set of options would drop
	// whatever was not picked.
	var specs []bson.Raw
	if err := cur.All(ctx, &specs); err != nil {
		return fmt.Errorf("read indexes on %s: %w", name, err)
	}
	encoded := make([]json.RawMessage, 0, len(specs))
	for _, spec := range specs {
		ej, err := bson.MarshalExtJSON(spec, false, false)
		if err != nil {
			return fmt.Errorf("encode indexes on %s: %w", name, err)
		}
		encoded = append(encoded, ej)
	}
	raw, err := json.Marshal(encoded)
	if err != nil {
		return fmt.Errorf("encode indexes on %s: %w", name, err)
	}
	return w.WriteIndexes(name, raw)
}

func mongoServerVersion(ctx context.Context, client *mongo.Client) (string, error) {
	var res struct {
		Version string `bson:"version"`
	}
	err := client.Database("admin").RunCommand(ctx, bson.D{{Key: "buildInfo", Value: 1}}).Decode(&res)
	if err != nil {
		return "", fmt.Errorf("buildInfo: %w", err)
	}
	return res.Version, nil
}