package domain

import (
	"crypto/sha256"
	"errors"
	"fmt"

	rvboxv1 "github.com/rvbox/rvbox/gen/go/rvbox/v1"
)

var (
	ErrInvalidEventSequence   = errors.New("event sequence must be nonzero")
	ErrEventSequenceGap       = errors.New("event sequence gap")
	ErrConflictingDuplicate   = errors.New("conflicting duplicate event")
	ErrMissingDuplicate       = errors.New("duplicate event has no durable fingerprint")
	ErrInvalidTruncationRange = errors.New("invalid output truncation range")
)

type EventFingerprint [sha256.Size]byte

func FingerprintBytes(value []byte) EventFingerprint { return sha256.Sum256(value) }

type EventSequenceDecision uint8

const (
	EventSequenceInvalid EventSequenceDecision = iota
	EventSequenceAppend
	EventSequenceDuplicate
)

// ClassifyEventSequence admits exactly the next event, or an immutable replay
// whose stored fingerprint is supplied. Wire event-sequence gaps are never
// repaired by inventing events.
func ClassifyEventSequence(last, incoming uint64, stored *EventFingerprint, candidate EventFingerprint) (EventSequenceDecision, error) {
	if incoming == 0 {
		return EventSequenceInvalid, ErrInvalidEventSequence
	}
	if last != ^uint64(0) && incoming == last+1 {
		return EventSequenceAppend, nil
	}
	if incoming > last {
		return EventSequenceInvalid, fmt.Errorf("%w: have %d, received %d", ErrEventSequenceGap, last, incoming)
	}
	if stored == nil {
		return EventSequenceInvalid, ErrMissingDuplicate
	}
	if *stored != candidate {
		return EventSequenceInvalid, ErrConflictingDuplicate
	}
	return EventSequenceDuplicate, nil
}

// ValidateOutputTruncation checks the two allowed forms: both range endpoints
// absent for pre-sequencing client loss, or a complete nonempty retained range.
func ValidateOutputTruncation(value *rvboxv1.OutputTruncation) error {
	if value == nil {
		return ErrInvalidTruncationRange
	}
	first, last := value.FirstRemovedEventSeq, value.LastRemovedEventSeq
	if first == nil && last == nil {
		return nil
	}
	if first == nil || last == nil || *first == 0 || *last == 0 || *first > *last {
		return ErrInvalidTruncationRange
	}
	return nil
}