// Package ledger implements GL's transport-independent use cases.
package ledger

import (
	"context"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"sort"
	"strconv"
	"time"

	domain "gl/domain/ledger"
)

var ErrInvalidArgument = errors.New("invalid ledger request")

type Repository interface {
	Append(context.Context, domain.Journal) (domain.AppendResult, error)
	AppendEvent(context.Context, domain.TransactionEvent) (domain.TransactionEvent, bool, error)
	GetByID(context.Context, string) (domain.Journal, error)
	GetByIdempotencyKey(context.Context, string) (domain.Journal, error)
	List(context.Context, domain.JournalFilter) ([]domain.Journal, error)
	Balance(context.Context, domain.AccountReference, time.Time) (domain.Amount, error)
}

type IDGenerator func() (string, error)

type Service struct {
	repository Repository
	newID      IDGenerator
}

func NewService(repository Repository, newID IDGenerator) *Service {
	if newID == nil {
		newID = newUUID
	}
	return &Service{repository: repository, newID: newID}
}

type EntryCommand struct {
	LineNumber  uint32
	Account     domain.AccountReference
	Amount      string
	Description string
}

type AppendJournalCommand struct {
	SourceService       string
	IdempotencyKey      string
	SourceTransactionID string
	TrackingCode        string
	EffectKind          string
	EventVersion        uint32
	Entries             []EntryCommand
	ReversalOfJournalID string
	OccurredAt          time.Time
	CorrelationID       string
	ActorID             string
	Blockchain          domain.BlockchainReference
	Metadata            map[string]string
}

type AppendJournalResult struct {
	Journal        domain.Journal
	AlreadyExisted bool
}

func (s *Service) AppendJournal(ctx context.Context, command AppendJournalCommand) (AppendJournalResult, error) {
	journalID, err := s.newID()
	if err != nil {
		return AppendJournalResult{}, fmt.Errorf("generate journal id: %w", err)
	}
	journal := domain.Journal{
		ID:                  journalID,
		SourceService:       command.SourceService,
		IdempotencyKey:      command.IdempotencyKey,
		SourceTransactionID: command.SourceTransactionID,
		TrackingCode:        command.TrackingCode,
		EffectKind:          command.EffectKind,
		EventVersion:        command.EventVersion,
		ReversalOfJournalID: command.ReversalOfJournalID,
		OccurredAt:          command.OccurredAt,
		CorrelationID:       command.CorrelationID,
		ActorID:             command.ActorID,
		Blockchain:          command.Blockchain,
		Metadata:            cloneMetadata(command.Metadata),
		Entries:             make([]domain.Entry, 0, len(command.Entries)),
	}
	for _, input := range command.Entries {
		amount, parseErr := domain.ParseAmount(input.Amount)
		if parseErr != nil {
			return AppendJournalResult{}, invalid("entry %d amount: %v", input.LineNumber, parseErr)
		}
		journal.Entries = append(journal.Entries, domain.Entry{
			LineNumber:  input.LineNumber,
			Account:     input.Account,
			Amount:      amount,
			Description: input.Description,
		})
	}
	sort.Slice(journal.Entries, func(i, j int) bool {
		return journal.Entries[i].LineNumber < journal.Entries[j].LineNumber
	})

	journal.PayloadHash, err = journalHash(journal)
	if err != nil {
		return AppendJournalResult{}, fmt.Errorf("hash journal: %w", err)
	}
	if err := journal.Validate(); err != nil {
		return AppendJournalResult{}, invalid("%v", err)
	}
	if journal.ReversalOfJournalID != "" {
		original, getErr := s.repository.GetByID(ctx, journal.ReversalOfJournalID)
		if getErr != nil {
			return AppendJournalResult{}, fmt.Errorf("read reversal target: %w", getErr)
		}
		if err := validateFullReversal(original, journal); err != nil {
			return AppendJournalResult{}, invalid("%v", err)
		}
	}

	appendResult, err := s.repository.Append(ctx, journal)
	if err != nil {
		return AppendJournalResult{}, err
	}
	stored, err := s.repository.GetByID(ctx, appendResult.JournalID)
	if err != nil {
		return AppendJournalResult{}, fmt.Errorf("read appended journal: %w", err)
	}
	return AppendJournalResult{Journal: stored, AlreadyExisted: appendResult.AlreadyExists}, nil
}

type AppendEventCommand struct {
	SourceService       string
	IdempotencyKey      string
	SourceTransactionID string
	TrackingCode        string
	EventVersion        uint32
	State               domain.TransactionState
	ErrorCode           string
	ErrorMessage        string
	OccurredAt          time.Time
	CorrelationID       string
	ActorID             string
	Blockchain          domain.BlockchainReference
	Metadata            map[string]string
}

func (s *Service) AppendEvent(ctx context.Context, command AppendEventCommand) (domain.TransactionEvent, bool, error) {
	eventID, err := s.newID()
	if err != nil {
		return domain.TransactionEvent{}, false, fmt.Errorf("generate event id: %w", err)
	}
	event := domain.TransactionEvent{
		ID:                  eventID,
		SourceService:       command.SourceService,
		IdempotencyKey:      command.IdempotencyKey,
		SourceTransactionID: command.SourceTransactionID,
		TrackingCode:        command.TrackingCode,
		EventVersion:        command.EventVersion,
		State:               command.State,
		ErrorCode:           command.ErrorCode,
		ErrorMessage:        command.ErrorMessage,
		OccurredAt:          command.OccurredAt,
		CorrelationID:       command.CorrelationID,
		ActorID:             command.ActorID,
		Blockchain:          command.Blockchain,
		Metadata:            cloneMetadata(command.Metadata),
	}
	event.PayloadHash, err = eventHash(event)
	if err != nil {
		return domain.TransactionEvent{}, false, fmt.Errorf("hash transaction event: %w", err)
	}
	if err := event.Validate(); err != nil {
		return domain.TransactionEvent{}, false, invalid("%v", err)
	}
	return s.repository.AppendEvent(ctx, event)
}

func (s *Service) GetJournal(ctx context.Context, journalID, idempotencyKey string) (domain.Journal, error) {
	if (journalID == "") == (idempotencyKey == "") {
		return domain.Journal{}, invalid("exactly one journal lookup is required")
	}
	if journalID != "" {
		return s.repository.GetByID(ctx, journalID)
	}
	return s.repository.GetByIdempotencyKey(ctx, idempotencyKey)
}

type ListCommand struct {
	Account      *domain.AccountReference
	AssetID      *int64
	RecordedFrom *time.Time
	RecordedTo   *time.Time
	PageSize     uint32
	PageToken    string
}

func (s *Service) List(ctx context.Context, command ListCommand) ([]domain.Journal, string, error) {
	if command.Account != nil {
		if err := command.Account.Validate(); err != nil {
			return nil, "", invalid("account: %v", err)
		}
	}
	if command.AssetID != nil && *command.AssetID <= 0 {
		return nil, "", invalid("asset id must be positive")
	}
	pageSize := int(command.PageSize)
	if pageSize == 0 {
		pageSize = 50
	}
	if pageSize > 200 {
		return nil, "", invalid("page size must not exceed 200")
	}
	offset, err := decodePageToken(command.PageToken)
	if err != nil {
		return nil, "", invalid("page token: %v", err)
	}
	journals, err := s.repository.List(ctx, domain.JournalFilter{
		Account:      command.Account,
		AssetID:      command.AssetID,
		RecordedFrom: command.RecordedFrom,
		RecordedTo:   command.RecordedTo,
		Limit:        pageSize + 1,
		Offset:       offset,
	})
	if err != nil {
		return nil, "", err
	}
	next := ""
	if len(journals) > pageSize {
		journals = journals[:pageSize]
		next = encodePageToken(offset + pageSize)
	}
	return journals, next, nil
}

func (s *Service) Balance(ctx context.Context, account domain.AccountReference, asOf time.Time) (domain.Amount, error) {
	if err := account.Validate(); err != nil {
		return domain.Amount{}, invalid("account: %v", err)
	}
	return s.repository.Balance(ctx, account, asOf)
}

func (s *Service) Replay(ctx context.Context, commands []AppendJournalCommand) ([]AppendJournalResult, error) {
	if len(commands) == 0 || len(commands) > 200 {
		return nil, invalid("replay batch must contain between 1 and 200 journals")
	}
	results := make([]AppendJournalResult, 0, len(commands))
	for _, command := range commands {
		result, err := s.AppendJournal(ctx, command)
		if err != nil {
			return nil, err
		}
		results = append(results, result)
	}
	return results, nil
}

func validateFullReversal(original, reversal domain.Journal) error {
	if len(original.Entries) != len(reversal.Entries) {
		return fmt.Errorf("reversal must contain every original entry")
	}
	expected := make(map[string]int, len(original.Entries))
	for _, entry := range original.Entries {
		expected[entryKey(entry.Account, entry.Amount.Negate())]++
	}
	for _, entry := range reversal.Entries {
		key := entryKey(entry.Account, entry.Amount)
		if expected[key] == 0 {
			return fmt.Errorf("reversal entries must exactly negate the original journal")
		}
		expected[key]--
	}
	return nil
}

func entryKey(account domain.AccountReference, amount domain.Amount) string {
	return fmt.Sprintf("%s\x00%s\x00%s\x00%d\x00%s", account.Class, account.OwnerType, account.OwnerID, account.AssetID, amount.String())
}

func journalHash(journal domain.Journal) (string, error) {
	entries := make([]map[string]any, 0, len(journal.Entries))
	for _, entry := range journal.Entries {
		entries = append(entries, map[string]any{
			"line_number": entry.LineNumber,
			"class":       entry.Account.Class,
			"owner_type":  entry.Account.OwnerType,
			"owner_id":    entry.Account.OwnerID,
			"asset_id":    entry.Account.AssetID,
			"amount":      entry.Amount.String(),
			"description": entry.Description,
		})
	}
	return hashPayload(map[string]any{
		"source_service":        journal.SourceService,
		"idempotency_key":       journal.IdempotencyKey,
		"source_transaction_id": journal.SourceTransactionID,
		"tracking_code":         journal.TrackingCode,
		"effect_kind":           journal.EffectKind,
		"event_version":         journal.EventVersion,
		"entries":               entries,
		"reversal_of":           journal.ReversalOfJournalID,
		"occurred_at":           journal.OccurredAt.UTC().Format(time.RFC3339Nano),
		"correlation_id":        journal.CorrelationID,
		"actor_id":              journal.ActorID,
		"blockchain":            journal.Blockchain,
		"metadata":              journal.Metadata,
	})
}

func eventHash(event domain.TransactionEvent) (string, error) {
	return hashPayload(map[string]any{
		"source_service":        event.SourceService,
		"idempotency_key":       event.IdempotencyKey,
		"source_transaction_id": event.SourceTransactionID,
		"tracking_code":         event.TrackingCode,
		"event_version":         event.EventVersion,
		"state":                 event.State,
		"error_code":            event.ErrorCode,
		"error_message":         event.ErrorMessage,
		"occurred_at":           event.OccurredAt.UTC().Format(time.RFC3339Nano),
		"correlation_id":        event.CorrelationID,
		"actor_id":              event.ActorID,
		"blockchain":            event.Blockchain,
		"metadata":              event.Metadata,
	})
}

func hashPayload(payload any) (string, error) {
	encoded, err := json.Marshal(payload)
	if err != nil {
		return "", err
	}
	hash := sha256.Sum256(encoded)
	return hex.EncodeToString(hash[:]), nil
}

func newUUID() (string, error) {
	value := make([]byte, 16)
	if _, err := rand.Read(value); err != nil {
		return "", err
	}
	value[6] = (value[6] & 0x0f) | 0x40
	value[8] = (value[8] & 0x3f) | 0x80
	encoded := hex.EncodeToString(value)
	return encoded[0:8] + "-" + encoded[8:12] + "-" + encoded[12:16] + "-" + encoded[16:20] + "-" + encoded[20:32], nil
}

func cloneMetadata(metadata map[string]string) map[string]string {
	clone := make(map[string]string, len(metadata))
	for key, value := range metadata {
		clone[key] = value
	}
	return clone
}

func invalid(format string, args ...any) error {
	return fmt.Errorf("%w: %s", ErrInvalidArgument, fmt.Sprintf(format, args...))
}

func encodePageToken(offset int) string {
	return base64.RawURLEncoding.EncodeToString([]byte(strconv.Itoa(offset)))
}

func decodePageToken(token string) (int, error) {
	if token == "" {
		return 0, nil
	}
	decoded, err := base64.RawURLEncoding.DecodeString(token)
	if err != nil {
		return 0, err
	}
	offset, err := strconv.Atoi(string(decoded))
	if err != nil || offset < 0 {
		return 0, fmt.Errorf("invalid offset")
	}
	return offset, nil
}