package engine

import (
	"reflect"
	"slices"
	"strings"

	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
)

const closedCallbackContextMessage = "configkit: callback context is no longer active"

type resolutionState struct {
	active      bool
	schema      *schemaDescriptor
	presence    map[string]inputPresence
	document    *document.Node
	input       resolutionInput
	records     map[string]*resolutionRecord
	diagnostics []Diagnostic
}

type resolutionRecord struct {
	path           Path
	kind           ChangeKind
	reasons        []string
	dependencies   []Path
	helperExpected *document.Node
	reasonSet      map[string]struct{}
	dependencySet  map[string]struct{}
}

func (record *resolutionRecord) addReason(reason string) {
	if record.reasonSet == nil {
		record.reasonSet = make(map[string]struct{}, len(record.reasons)+1)
		for _, existing := range record.reasons {
			record.reasonSet[existing] = struct{}{}
		}
	}
	if _, found := record.reasonSet[reason]; found {
		return
	}
	record.reasonSet[reason] = struct{}{}
	record.reasons = append(record.reasons, reason)
}

func (record *resolutionRecord) addDependency(path Path) {
	pointer := path.String()
	if record.dependencySet == nil {
		record.dependencySet = make(map[string]struct{}, len(record.dependencies)+1)
		for _, existing := range record.dependencies {
			record.dependencySet[existing.String()] = struct{}{}
		}
	}
	if _, found := record.dependencySet[pointer]; found {
		return
	}
	record.dependencySet[pointer] = struct{}{}
	record.dependencies = append(record.dependencies, path.clone())
}

type resolutionSnapshot struct {
	change Change
	before *document.Node
	after  *document.Node
}

type resolutionResult struct {
	final       *document.Node
	deltas      []documentDelta
	snapshots   []resolutionSnapshot
	diagnostics []Diagnostic
}

// IsExplicit reports whether surviving direct or inherited input defined path.
func (r *Resolution) IsExplicit(path Path) bool {
	state := r.activeState()
	if !state.validPath(path) {
		return false
	}
	_, explicit := state.presence[path.String()]
	return explicit
}

// InputInfo returns detached entry-state metadata for path. The entry state is
// the decoded configuration, including retained static defaults, immediately
// before resolution starts. It is fixed while resolution records mutate the
// working configuration; unknown or shape-mismatched paths return false.
//
// InputInfo is valid only during the resolution callback. See the configuration
// contract for source and provenance semantics.
func (r *Resolution) InputInfo(path Path) (FieldInfo, bool) {
	return r.activeState().input.lookup(path)
}

// Source returns a detached record for a source acquired by this load. Empty or
// unknown IDs return false and do not perform input, filesystem, or credential
// access. A source path is present only when that acquired source has one.
//
// Source is valid only during the resolution callback. See the configuration
// contract for source and provenance semantics.
func (r *Resolution) Source(id string) (SourceInfo, bool) {
	return r.activeState().input.source(id)
}

// SourceContext returns a passive context during the active resolution
// callback. Retaining the callback context after return is unsupported.
func (r *Resolution) SourceContext(id string) (ResourceContext, bool) {
	return r.activeState().input.sourceContext(id)
}

// Record declares the intent for a resolver mutation. A container path covers
// mutations to its descendants.
func (r *Resolution) Record(path Path, kind ChangeKind, reason string, dependencies ...Path) {
	state := r.activeState()
	state.addRecord(path, kind, reason, dependencies, nil)
}

// Error reports a safe, value-free resolution error at path.
func (r *Resolution) Error(path Path, message string, related ...Path) {
	r.activeState().report(SeverityError, path, message, related)
}

// Warning reports a safe, value-free resolution warning at path.
func (r *Resolution) Warning(path Path, message string, related ...Path) {
	r.activeState().report(SeverityWarning, path, message, related)
}

// Default assigns and records value only when path was absent from surviving
// input. Static defaults are not considered explicit.
func Default[V any](resolution *Resolution, path Path, target *V, value V, reason string, dependencies ...Path) {
	state := resolution.activeState()
	descriptor, ok := state.helperDescriptor(path, target)
	if !ok {
		return
	}
	if _, explicit := state.presence[path.String()]; explicit {
		return
	}
	*target = value
	expected, ok := state.projectHelperValue(path, descriptor, reflect.ValueOf(target).Elem())
	if !ok {
		return
	}
	state.addRecord(path, ChangeDerivedDefault, reason, dependencies, expected)
}

// Normalize assigns and records value regardless of input presence.
func Normalize[V any](resolution *Resolution, path Path, target *V, value V, reason string, dependencies ...Path) {
	state := resolution.activeState()
	descriptor, ok := state.helperDescriptor(path, target)
	if !ok {
		return
	}
	*target = value
	expected, ok := state.projectHelperValue(path, descriptor, reflect.ValueOf(target).Elem())
	if !ok {
		return
	}
	state.addRecord(path, ChangeNormalization, reason, dependencies, expected)
}

func (r *Resolution) activeState() *resolutionState {
	if r == nil || r.state == nil || !r.state.active {
		panic(closedCallbackContextMessage)
	}
	return r.state
}

func (state *resolutionState) validPath(path Path) bool {
	if _, ok := resolveSchemaPath(state.schema, path); ok {
		return true
	}
	state.addDiagnostic(CodeResolveInvalidPath, path, "resolution path is not valid for the configuration schema")
	return false
}

func (state *resolutionState) helperDescriptor(path Path, target any) (*typeDescriptor, bool) {
	resolved, ok := resolveSchemaPath(state.schema, path)
	if !ok {
		state.addDiagnostic(CodeResolveInvalidPath, path, "resolution helper path is not valid for the configuration schema")
		return nil, false
	}
	value := reflect.ValueOf(target)
	if !value.IsValid() || value.Kind() != reflect.Pointer || value.IsNil() {
		state.addDiagnostic(CodeResolveInvalidTarget, path, "resolution helper target must be a non-nil pointer")
		return nil, false
	}
	if resolved.descriptor.kind != descriptorDynamic && value.Elem().Type() != resolved.descriptor.typeOf {
		state.addDiagnostic(CodeResolveInvalidTarget, path, "resolution helper target type does not match its configuration path")
		return nil, false
	}
	return resolved.descriptor, true
}

func (state *resolutionState) projectHelperValue(path Path, descriptor *typeDescriptor, value reflect.Value) (*document.Node, bool) {
	if descriptor.kind == descriptorDynamic {
		wrapped := reflect.New(descriptor.typeOf).Elem()
		if value.IsValid() {
			wrapped.Set(value)
		}
		value = wrapped
	}
	operation := valueOperation{
		schema: state.schema, limits: defaultProjectionLimits(), active: make(map[valueVisit]bool),
	}
	node, issue := operation.project(value, descriptor, path, 0)
	if issue != nil {
		state.addDiagnostic(CodeResolveInvalidValue, issue.path, "resolution helper value is not a valid configuration value")
		return nil, false
	}
	return node, true
}

func (state *resolutionState) addRecord(path Path, kind ChangeKind, reason string, dependencies []Path, expected *document.Node) {
	if !state.validPath(path) {
		return
	}
	if kind != ChangeDerivedDefault && kind != ChangeNormalization {
		state.addDiagnostic(CodeResolveInvalidKind, path, "resolution record has an invalid change kind")
		return
	}
	if reason == "" {
		state.addDiagnostic(CodeResolveInvalidRecord, path, "resolution record requires a safe reason")
		return
	}
	for _, dependency := range dependencies {
		if !state.validPath(dependency) {
			return
		}
	}
	pointer := path.String()
	record := state.records[pointer]
	if record == nil {
		record = &resolutionRecord{path: path.clone(), kind: kind}
		state.records[pointer] = record
	}
	if kind == ChangeNormalization {
		record.kind = ChangeNormalization
	}
	record.addReason(reason)
	for _, dependency := range dependencies {
		record.addDependency(dependency)
	}
	if expected != nil {
		record.helperExpected = document.Clone(expected)
	}
}

func (state *resolutionState) report(severity Severity, path Path, message string, related []Path) {
	code := CodeResolveError
	if severity == SeverityWarning {
		code = CodeResolveWarning
	}
	if !state.validPath(path) {
		return
	}
	diagnostic := Diagnostic{Code: code, Severity: severity, Path: path.clone(), Message: message}
	if location, ok := nearestDocumentLocation(state.document, path); ok {
		diagnostic.Primary = documentLocationPointer(location)
	}
	for _, relatedPath := range related {
		if !state.validPath(relatedPath) {
			continue
		}
		if location, ok := nearestDocumentLocation(state.document, relatedPath); ok {
			diagnostic.Related = append(diagnostic.Related, Location{
				SourceID: location.SourceID, Line: location.Line, Column: location.Column,
			})
		}
	}
	state.diagnostics = append(state.diagnostics, diagnostic)
}

func (state *resolutionState) addDiagnostic(code Code, path Path, message string) {
	diagnostic := Diagnostic{Code: code, Severity: SeverityError, Path: path.clone(), Message: message}
	if location, ok := nearestDocumentLocation(state.document, path); ok {
		diagnostic.Primary = documentLocationPointer(location)
	}
	state.diagnostics = append(state.diagnostics, diagnostic)
}

func runResolution[T any](configuration *T, resolver func(*T, *Resolution) error, schema *schemaDescriptor, decoded decodedConfiguration, input *document.Node, entry resolutionInput) resolutionResult {
	state := &resolutionState{
		active: true, schema: schema, presence: decoded.presence, document: input,
		input:   entry,
		records: make(map[string]*resolutionRecord),
	}
	context := &Resolution{state: state}
	var err error
	func() {
		defer func() { state.active = false }()
		err = resolver(configuration, context)
	}()
	if err != nil && !hasErrors(state.diagnostics) {
		state.addDiagnostic(CodeResolveHookFailed, Path{}, "application resolution failed")
	}
	if hasErrors(state.diagnostics) {
		return resolutionResult{diagnostics: canonicalDiagnostics(state.diagnostics)}
	}

	after, issue := projectConfiguration(schema, reflect.ValueOf(*configuration), defaultProjectionLimits())
	if issue != nil {
		state.addDiagnostic(CodeResolveInvalidValue, issue.path, "application resolution produced an invalid configuration value")
		return resolutionResult{diagnostics: canonicalDiagnostics(state.diagnostics)}
	}
	deltas := compareConfigurationValues(decoded.final, after)
	ordered := orderedResolutionRecords(state.records)
	coverage := newResolutionCoverageIndexFromRecords(ordered)
	beforeLookup := buildDocumentLookup(decoded.final)
	afterLookup := buildDocumentLookup(after)
	for _, delta := range deltas {
		if _, ok := coverage.deepestCoveringID(delta.path); !ok {
			state.addDiagnostic(CodeResolveUnrecordedChange, delta.path, "application resolution changed a value without a covering record")
		}
	}
	for _, record := range ordered {
		_, beforePresent := documentAtPathWithLookup(decoded.final, record.path, beforeLookup)
		afterNode, afterPresent := documentAtPathWithLookup(after, record.path, afterLookup)
		if !beforePresent && !afterPresent {
			state.addDiagnostic(CodeResolveInvalidRecord, record.path, "resolution record does not identify an existing or removed value")
			continue
		}
		if record.helperExpected != nil && (!afterPresent || !configurationValuesEqual(record.helperExpected, afterNode)) {
			state.addDiagnostic(CodeResolveInvalidTarget, record.path, "resolution helper assignment does not match its declared configuration path")
		}
	}
	if hasErrors(state.diagnostics) {
		return resolutionResult{diagnostics: canonicalDiagnostics(state.diagnostics)}
	}

	snapshots := make([]resolutionSnapshot, 0, len(ordered))
	for _, record := range ordered {
		beforeNode, _ := documentAtPathWithLookup(decoded.final, record.path, beforeLookup)
		afterNode, _ := documentAtPathWithLookup(after, record.path, afterLookup)
		snapshots = append(snapshots, resolutionSnapshot{
			change: Change{
				Path:         record.path.clone(),
				Origin:       Origin{Layer: LayerResolution, Operation: record.kind},
				Reasons:      append([]string(nil), record.reasons...),
				Dependencies: clonePaths(record.dependencies),
			},
			before: document.Clone(beforeNode), after: document.Clone(afterNode),
		})
	}
	return resolutionResult{
		final: after, deltas: deltas, snapshots: snapshots,
		diagnostics: canonicalDiagnostics(state.diagnostics),
	}
}

func orderedResolutionRecords(records map[string]*resolutionRecord) []*resolutionRecord {
	type orderedRecord struct {
		record  *resolutionRecord
		pointer string
	}
	ordered := make([]orderedRecord, 0, len(records))
	for _, record := range records {
		ordered = append(ordered, orderedRecord{record: record, pointer: record.path.String()})
	}
	slices.SortFunc(ordered, func(left, right orderedRecord) int {
		return strings.Compare(left.pointer, right.pointer)
	})
	result := make([]*resolutionRecord, len(ordered))
	for i, item := range ordered {
		result[i] = item.record
	}
	return result
}

type resolutionCoverageIndex struct {
	prefix pathPrefixIndex
	depths []int
}

func newResolutionCoverageIndexFromRecords(records []*resolutionRecord) *resolutionCoverageIndex {
	index := &resolutionCoverageIndex{depths: make([]int, len(records))}
	for id, record := range records {
		index.depths[id] = len(record.path.segments)
		if record.path.valid() {
			index.prefix.insert(record.path.String(), id)
		}
	}
	return index
}

func newResolutionCoverageIndexFromSnapshots(snapshots []resolutionSnapshot) (*resolutionCoverageIndex, []string) {
	index := &resolutionCoverageIndex{depths: make([]int, len(snapshots))}
	pointers := make([]string, len(snapshots))
	for id, snapshot := range snapshots {
		path := snapshot.change.Path
		index.depths[id] = len(path.segments)
		pointers[id] = path.String()
		if path.valid() {
			index.prefix.insert(pointers[id], id)
		}
	}
	return index, pointers
}

func (index *resolutionCoverageIndex) deepestCoveringID(path Path) (int, bool) {
	if !path.valid() {
		return 0, false
	}
	return index.deepestCoveringPointerID(path.String())
}

func (index *resolutionCoverageIndex) deepestCoveringPointerID(pointer string) (int, bool) {
	bestID, bestDepth, found := 0, -1, false
	index.prefix.visitAncestors(pointer, func(id int) {
		depth := index.depths[id]
		if !found || depth > bestDepth || (depth == bestDepth && id < bestID) {
			bestID, bestDepth, found = id, depth, true
		}
	})
	return bestID, found
}

func pathContains(container, descendant Path) bool {
	if !container.valid() || !descendant.valid() || len(container.segments) > len(descendant.segments) {
		return false
	}
	for i, segment := range container.segments {
		if !pathTokenSpellingEqual(segment, descendant.segments[i]) {
			return false
		}
	}
	return true
}

func clonePaths(paths []Path) []Path {
	if paths == nil {
		return nil
	}
	cloned := make([]Path, len(paths))
	for index, path := range paths {
		cloned[index] = path.clone()
	}
	return cloned
}