package transfer

import (
	"errors"
	"sort"
	"sync"
	"time"
)

var (
	ErrTransferIDEmpty      = errors.New("transfer id is empty")
	ErrTransferExists       = errors.New("transfer already exists")
	ErrTransferNotFound     = errors.New("transfer not found")
	ErrTransferBytesInvalid = errors.New("transfer bytes must be non-negative")
)

type Manager struct {
	mu        sync.Mutex
	now       func() time.Time
	transfers map[string]*managedTransfer
}

type managedTransfer struct {
	snapshot Snapshot
}

func NewManager() *Manager {
	return NewManagerWithClock(time.Now)
}

func NewManagerWithClock(now func() time.Time) *Manager {
	if now == nil {
		now = time.Now
	}
	return &Manager{
		now:       now,
		transfers: make(map[string]*managedTransfer),
	}
}

func (m *Manager) StartOutgoing(desc Descriptor) (Snapshot, error) {
	return m.start(desc, DirectionSend, StateNegotiating)
}

func (m *Manager) StartIncoming(desc Descriptor) (Snapshot, error) {
	return m.start(desc, DirectionReceive, StatePrepared)
}

func (m *Manager) Activate(id string) (Snapshot, error) {
	return m.update(id, func(snapshot *Snapshot) error {
		snapshot.State = StateActive
		return nil
	})
}

func (m *Manager) Pause(id string) (Snapshot, error) {
	return m.update(id, func(snapshot *Snapshot) error {
		if snapshot.State.Terminal() {
			return nil
		}
		snapshot.State = StatePaused
		return nil
	})
}

func (m *Manager) Resume(id string, confirmedBytes int64) (Snapshot, error) {
	if confirmedBytes < 0 {
		return Snapshot{}, ErrTransferBytesInvalid
	}
	return m.update(id, func(snapshot *Snapshot) error {
		switch snapshot.Direction {
		case DirectionSend:
			if confirmedBytes > snapshot.SentBytes {
				snapshot.SentBytes = confirmedBytes
			}
			snapshot.AckedBytes = confirmedBytes
			if snapshot.Size > 0 && snapshot.AckedBytes > snapshot.Size {
				snapshot.AckedBytes = snapshot.Size
			}
		case DirectionReceive:
			if confirmedBytes > snapshot.ReceivedBytes {
				snapshot.ReceivedBytes = confirmedBytes
			}
			if snapshot.Size > 0 && snapshot.ReceivedBytes > snapshot.Size {
				snapshot.ReceivedBytes = snapshot.Size
			}
		}
		snapshot.State = StateActive
		snapshot.InflightBytes = inflightBytes(*snapshot)
		return nil
	})
}

func (m *Manager) RecordSend(id string, sentBytes int64) (Snapshot, error) {
	if sentBytes < 0 {
		return Snapshot{}, ErrTransferBytesInvalid
	}
	return m.update(id, func(snapshot *Snapshot) error {
		snapshot.SentBytes += sentBytes
		if snapshot.Size > 0 && snapshot.SentBytes > snapshot.Size {
			snapshot.SentBytes = snapshot.Size
		}
		snapshot.InflightBytes = inflightBytes(*snapshot)
		if !snapshot.State.Terminal() {
			snapshot.State = StateActive
		}
		return nil
	})
}

func (m *Manager) RecordReceive(id string, recvBytes int64) (Snapshot, error) {
	if recvBytes < 0 {
		return Snapshot{}, ErrTransferBytesInvalid
	}
	return m.update(id, func(snapshot *Snapshot) error {
		snapshot.ReceivedBytes += recvBytes
		if snapshot.Size > 0 && snapshot.ReceivedBytes > snapshot.Size {
			snapshot.ReceivedBytes = snapshot.Size
		}
		if !snapshot.State.Terminal() {
			snapshot.State = StateActive
		}
		return nil
	})
}

func (m *Manager) SetAckedBytes(id string, ackedBytes int64) (Snapshot, error) {
	if ackedBytes < 0 {
		return Snapshot{}, ErrTransferBytesInvalid
	}
	return m.update(id, func(snapshot *Snapshot) error {
		snapshot.AckedBytes = ackedBytes
		if snapshot.Size > 0 && snapshot.AckedBytes > snapshot.Size {
			snapshot.AckedBytes = snapshot.Size
		}
		if snapshot.AckedBytes > snapshot.SentBytes {
			snapshot.SentBytes = snapshot.AckedBytes
		}
		snapshot.InflightBytes = inflightBytes(*snapshot)
		return nil
	})
}

func (m *Manager) BeginCommit(id string) (Snapshot, error) {
	return m.update(id, func(snapshot *Snapshot) error {
		snapshot.State = StateCommitting
		return nil
	})
}

func (m *Manager) BeginVerify(id string) (Snapshot, error) {
	return m.update(id, func(snapshot *Snapshot) error {
		snapshot.State = StateVerifying
		return nil
	})
}

func (m *Manager) Complete(id string) (Snapshot, error) {
	now := m.currentTime()
	return m.updateWithTime(id, now, func(snapshot *Snapshot, now time.Time) error {
		snapshot.State = StateDone
		snapshot.CompletedAt = now.UnixNano()
		snapshot.InflightBytes = inflightBytes(*snapshot)
		return nil
	})
}

func (m *Manager) Abort(id string, err error) (Snapshot, error) {
	return m.finishWithError(id, StateAborted, err)
}

func (m *Manager) Fail(id string, err error) (Snapshot, error) {
	return m.finishWithError(id, StateFailed, err)
}

func (m *Manager) RecordRetry(id string) (Snapshot, error) {
	return m.update(id, func(snapshot *Snapshot) error {
		snapshot.RetryCount++
		return nil
	})
}

func (m *Manager) RecordTimeout(id string) (Snapshot, error) {
	return m.update(id, func(snapshot *Snapshot) error {
		snapshot.TimeoutCount++
		return nil
	})
}

func (m *Manager) SetStage(id string, stage string) (Snapshot, error) {
	return m.update(id, func(snapshot *Snapshot) error {
		snapshot.Stage = stage
		return nil
	})
}

func (m *Manager) SetFailureStage(id string, stage string) (Snapshot, error) {
	return m.update(id, func(snapshot *Snapshot) error {
		snapshot.LastFailureStage = stage
		if stage != "" {
			snapshot.Stage = stage
		}
		return nil
	})
}

func (m *Manager) MergeMetadata(id string, metadata Metadata) (Snapshot, error) {
	return m.update(id, func(snapshot *Snapshot) error {
		if len(metadata) == 0 {
			return nil
		}
		if snapshot.Metadata == nil {
			snapshot.Metadata = make(Metadata, len(metadata))
		}
		for key, value := range metadata {
			if value == "" {
				delete(snapshot.Metadata, key)
				continue
			}
			snapshot.Metadata[key] = value
		}
		return nil
	})
}

func (m *Manager) RecordTelemetry(id string, delta TelemetryDelta) (Snapshot, error) {
	return m.update(id, func(snapshot *Snapshot) error {
		if delta.SourceReadDuration > 0 {
			snapshot.SourceReadDuration += delta.SourceReadDuration
		}
		if delta.StreamWriteDuration > 0 {
			snapshot.StreamWriteDuration += delta.StreamWriteDuration
		}
		if delta.SinkWriteDuration > 0 {
			snapshot.SinkWriteDuration += delta.SinkWriteDuration
		}
		if delta.SyncDuration > 0 {
			snapshot.SyncDuration += delta.SyncDuration
		}
		if delta.VerifyDuration > 0 {
			snapshot.VerifyDuration += delta.VerifyDuration
		}
		if delta.CommitDuration > 0 {
			snapshot.CommitDuration += delta.CommitDuration
		}
		if delta.CommitWaitDuration > 0 {
			snapshot.CommitWaitDuration += delta.CommitWaitDuration
		}
		if delta.SourceReadCount > 0 {
			snapshot.SourceReadCount += delta.SourceReadCount
		}
		if delta.StreamWriteCount > 0 {
			snapshot.StreamWriteCount += delta.StreamWriteCount
		}
		if delta.SinkWriteCount > 0 {
			snapshot.SinkWriteCount += delta.SinkWriteCount
		}
		return nil
	})
}

func (m *Manager) Snapshot(id string) (Snapshot, bool) {
	m.mu.Lock()
	defer m.mu.Unlock()
	transfer, ok := m.transfers[id]
	if !ok {
		return Snapshot{}, false
	}
	return cloneSnapshot(transfer.snapshot), true
}

func (m *Manager) Snapshots() []Snapshot {
	m.mu.Lock()
	defer m.mu.Unlock()
	out := make([]Snapshot, 0, len(m.transfers))
	for _, transfer := range m.transfers {
		out = append(out, cloneSnapshot(transfer.snapshot))
	}
	sort.Slice(out, func(i int, j int) bool {
		return out[i].ID < out[j].ID
	})
	return out
}

func (m *Manager) Restore(snapshot Snapshot) (Snapshot, error) {
	if snapshot.ID == "" {
		return Snapshot{}, ErrTransferIDEmpty
	}
	m.mu.Lock()
	defer m.mu.Unlock()
	m.transfers[snapshot.ID] = &managedTransfer{snapshot: cloneSnapshot(snapshot)}
	return cloneSnapshot(snapshot), nil
}

func (m *Manager) start(desc Descriptor, direction Direction, state State) (Snapshot, error) {
	if desc.ID == "" {
		return Snapshot{}, ErrTransferIDEmpty
	}
	now := m.currentTime()
	m.mu.Lock()
	defer m.mu.Unlock()
	if _, exists := m.transfers[desc.ID]; exists {
		return Snapshot{}, ErrTransferExists
	}
	snapshot := Snapshot{
		ID:        desc.ID,
		Direction: direction,
		Channel:   normalizeChannel(desc.Channel),
		State:     state,
		Size:      desc.Size,
		Checksum:  desc.Checksum,
		Metadata:  cloneMetadata(desc.Metadata),
		StartedAt: now.UnixNano(),
		UpdatedAt: now.UnixNano(),
	}
	m.transfers[desc.ID] = &managedTransfer{snapshot: snapshot}
	return cloneSnapshot(snapshot), nil
}

func (m *Manager) finishWithError(id string, state State, err error) (Snapshot, error) {
	now := m.currentTime()
	return m.updateWithTime(id, now, func(snapshot *Snapshot, now time.Time) error {
		snapshot.State = state
		snapshot.CompletedAt = now.UnixNano()
		if err != nil {
			snapshot.LastError = err.Error()
		}
		snapshot.InflightBytes = inflightBytes(*snapshot)
		return nil
	})
}

func (m *Manager) update(id string, fn func(*Snapshot) error) (Snapshot, error) {
	return m.updateWithTime(id, m.currentTime(), func(snapshot *Snapshot, _ time.Time) error {
		return fn(snapshot)
	})
}

func (m *Manager) updateWithTime(id string, now time.Time, fn func(*Snapshot, time.Time) error) (Snapshot, error) {
	m.mu.Lock()
	defer m.mu.Unlock()
	transfer, ok := m.transfers[id]
	if !ok {
		return Snapshot{}, ErrTransferNotFound
	}
	snapshot := &transfer.snapshot
	if err := fn(snapshot, now); err != nil {
		return Snapshot{}, err
	}
	snapshot.UpdatedAt = now.UnixNano()
	return cloneSnapshot(*snapshot), nil
}

func (m *Manager) currentTime() time.Time {
	return m.now()
}

func inflightBytes(snapshot Snapshot) int64 {
	if snapshot.Direction != DirectionSend {
		return 0
	}
	if snapshot.SentBytes <= snapshot.AckedBytes {
		return 0
	}
	return snapshot.SentBytes - snapshot.AckedBytes
}