package engine

type sequenceSelectorSegmentKind uint8

const (
	sequenceSelectorLiteral sequenceSelectorSegmentKind = iota + 1
	sequenceSelectorAnyMappingKey
)

type sequenceSelectorSegment struct {
	kind sequenceSelectorSegmentKind
	key  string
}

type sequenceIdentitySelector []sequenceSelectorSegment

type sequenceIdentityRegistration struct {
	selector       sequenceIdentitySelector
	identityFields []string
	invalidPath    *Path
}

// SequenceIdentitySelectorSegment carries one root-owned selector segment into
// an engine definition. This transport type remains behind the internal package
// boundary.
type SequenceIdentitySelectorSegment struct {
	Key           string
	AnyMappingKey bool
}

// SequenceIdentityRegistrationInput carries detached root-owned registration
// data into an engine definition without exposing its compiled policy. A
// non-nil ExactPath selects exact-path registration; otherwise Selector is used.
type SequenceIdentityRegistrationInput struct {
	ExactPath      *Path
	Selector       []SequenceIdentitySelectorSegment
	IdentityFields []string
}

type sequenceSelectorTrie struct {
	literals map[string]*sequenceSelectorTrie
	wildcard *sequenceSelectorTrie
	fields   []string
	terminal bool
}

type sequencePolicy struct {
	selectorTrie *sequenceSelectorTrie
}

// DefinitionWithSequenceIdentityRegistrations attaches facade-owned
// registrations to a detached engine definition without exposing compiled
// engine policy.
func DefinitionWithSequenceIdentityRegistrations[T any](definition Definition[T], registrations []SequenceIdentityRegistrationInput) Definition[T] {
	if registrations == nil {
		definition.sequenceIdentityRegistrations = nil
		return definition
	}
	definition.sequenceIdentityRegistrations = make([]sequenceIdentityRegistration, len(registrations))
	for index, registration := range registrations {
		compiled := sequenceIdentityRegistration{identityFields: append([]string(nil), registration.IdentityFields...)}
		if registration.ExactPath != nil {
			path, err := normalizeDestination(registration.ExactPath.clone())
			if err != nil {
				invalid := registration.ExactPath.clone()
				compiled.invalidPath = &invalid
			} else {
				compiled.selector = make(sequenceIdentitySelector, len(path.segments))
				for segmentIndex, segment := range path.segments {
					compiled.selector[segmentIndex] = sequenceSelectorSegment{kind: sequenceSelectorLiteral, key: segment.key}
				}
			}
		} else {
			compiled.selector = make(sequenceIdentitySelector, len(registration.Selector))
			for segmentIndex, segment := range registration.Selector {
				kind := sequenceSelectorLiteral
				if segment.AnyMappingKey {
					kind = sequenceSelectorAnyMappingKey
				}
				compiled.selector[segmentIndex] = sequenceSelectorSegment{kind: kind, key: segment.Key}
			}
		}
		definition.sequenceIdentityRegistrations[index] = compiled
	}
	return definition
}

func compileSequenceIdentityPolicy(registrations []sequenceIdentityRegistration) (sequencePolicy, []Diagnostic) {
	policy := sequencePolicy{}
	compiled := make([]sequenceIdentityRegistration, 0, len(registrations))
	var diagnostics []Diagnostic
	for _, registration := range registrations {
		selector := cloneSequenceIdentitySelector(registration.selector)
		if registration.invalidPath != nil {
			diagnostics = append(diagnostics, Diagnostic{
				Code: CodeOptionsInvalid, Severity: SeverityError, Path: registration.invalidPath.clone(),
				Message: "sequence registration path must contain only non-reserved mapping keys",
			})
			continue
		}
		fields, err := normalizeSequenceIdentityFields(registration.identityFields)
		if err != nil {
			diagnostics = append(diagnostics, Diagnostic{
				Code: CodeOptionsInvalid, Severity: SeverityError, Path: selectorDiagnosticPath(selector),
				Message: "sequence registration requires at least one identity field",
			})
			continue
		}
		if err := validateSequenceIdentitySelector(selector); err != nil {
			diagnostics = append(diagnostics, Diagnostic{
				Code: CodeOptionsInvalid, Severity: SeverityError, Path: selectorDiagnosticPath(selector),
				Message: "sequence registration selector must contain only valid mapping segments",
			})
			continue
		}
		compiled = append(compiled, sequenceIdentityRegistration{selector: selector, identityFields: fields})
	}

	for leftIndex := 0; leftIndex < len(compiled); leftIndex++ {
		for rightIndex := leftIndex + 1; rightIndex < len(compiled); rightIndex++ {
			left, right := compiled[leftIndex], compiled[rightIndex]
			if !sequenceIdentitySelectorsOverlap(left.selector, right.selector) || equalSequenceIdentityFields(left.identityFields, right.identityFields) {
				continue
			}
			diagnostics = append(diagnostics, Diagnostic{
				Code: CodeOptionsInvalid, Severity: SeverityError, Path: selectorDiagnosticPath(left.selector),
				Message: "overlapping sequence registrations must use the same identity fields",
			})
		}
	}
	if len(diagnostics) != 0 {
		return policy, canonicalDiagnostics(diagnostics)
	}

	unique := make([]sequenceIdentityRegistration, 0, len(compiled))
	for _, registration := range compiled {
		duplicate := false
		for _, existing := range unique {
			if sequenceIdentitySelectorsEqual(existing.selector, registration.selector) && equalSequenceIdentityFields(existing.identityFields, registration.identityFields) {
				duplicate = true
				break
			}
		}
		if duplicate {
			continue
		}
		unique = append(unique, registration)
	}
	for _, registration := range unique {
		policy.addRegistration(registration)
	}
	return policy, nil
}

func (p *sequencePolicy) addRegistration(registration sequenceIdentityRegistration) {
	if p.selectorTrie == nil {
		p.selectorTrie = &sequenceSelectorTrie{}
	}
	node := p.selectorTrie
	for _, segment := range registration.selector {
		if segment.kind == sequenceSelectorAnyMappingKey {
			if node.wildcard == nil {
				node.wildcard = &sequenceSelectorTrie{}
			}
			node = node.wildcard
			continue
		}
		if node.literals == nil {
			node.literals = make(map[string]*sequenceSelectorTrie)
		}
		if node.literals[segment.key] == nil {
			node.literals[segment.key] = &sequenceSelectorTrie{}
		}
		node = node.literals[segment.key]
	}
	node.fields = append([]string(nil), registration.identityFields...)
	node.terminal = true
}

func (p sequencePolicy) identityFieldsAt(path Path) ([]string, bool) {
	if !path.valid() {
		return nil, false
	}
	if p.selectorTrie == nil {
		return nil, false
	}
	current := []*sequenceSelectorTrie{p.selectorTrie}
	for _, segment := range path.segments {
		if segment.kind != pathSegmentKey && segment.kind != pathSegmentParsed {
			return nil, false
		}
		next := make([]*sequenceSelectorTrie, 0, len(current)*2)
		for _, node := range current {
			if literal := node.literals[segment.key]; literal != nil {
				next = append(next, literal)
			}
			if node.wildcard != nil {
				next = append(next, node.wildcard)
			}
		}
		if len(next) == 0 {
			return nil, false
		}
		current = next
	}
	for _, node := range current {
		if node.terminal {
			return append([]string(nil), node.fields...), true
		}
	}
	return nil, false
}

func (p sequencePolicy) registered(path Path) bool {
	_, ok := p.identityFieldsAt(path)
	return ok
}

func validateSequenceIdentitySelector(selector sequenceIdentitySelector) error {
	for _, segment := range selector {
		switch segment.kind {
		case sequenceSelectorLiteral:
			if _, err := normalizeDestination(Keys(segment.key)); err != nil {
				return err
			}
		case sequenceSelectorAnyMappingKey:
		default:
			return errInvalidDestination
		}
	}
	return nil
}

func sequenceIdentitySelectorsEqual(left, right sequenceIdentitySelector) bool {
	if len(left) != len(right) {
		return false
	}
	for index := range left {
		if left[index] != right[index] {
			return false
		}
	}
	return true
}

func sequenceIdentitySelectorsOverlap(left, right sequenceIdentitySelector) bool {
	if len(left) != len(right) {
		return false
	}
	for index := range left {
		if left[index].kind == sequenceSelectorLiteral && right[index].kind == sequenceSelectorLiteral && left[index].key != right[index].key {
			return false
		}
	}
	return true
}

func equalSequenceIdentityFields(left, right []string) bool {
	if len(left) != len(right) {
		return false
	}
	for index := range left {
		if left[index] != right[index] {
			return false
		}
	}
	return true
}

func cloneSequenceIdentitySelector(selector sequenceIdentitySelector) sequenceIdentitySelector {
	return append(sequenceIdentitySelector(nil), selector...)
}

func selectorDiagnosticPath(selector sequenceIdentitySelector) Path {
	path := Path{}
	for _, segment := range selector {
		if segment.kind != sequenceSelectorLiteral {
			return Path{}
		}
		path = path.Key(segment.key)
	}
	return path
}