package engine

import (
	"slices"

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

const inheritanceKey = "extends"

// These private records keep structural evidence separate from acquired source
// records. Later inspection can project a transfer onto individual descendants.
type inheritanceTransfer struct {
	from      Path
	to        Path
	reference Path
	locations []Location
}

type inheritanceControl struct {
	path      Path
	locations []Location
	cancelled bool
}

type inheritanceScope struct {
	root      *document.Node
	transfers []inheritanceTransfer
	controls  []inheritanceControl
}

type siblingFrame struct {
	name      string
	parent    string
	hasParent bool
	entered   bool
}

// expandSiblingDefinitions resolves only the mappings directly in scope.
// Nested instructions remain in the returned templates for destination-local
// resolution. This private entry point does not participate in public loading.
func expandSiblingDefinitions(scope *document.Node, scopePath Path) (*inheritanceScope, []Diagnostic) {
	if scope == nil || scope.Kind != document.Mapping {
		return nil, []Diagnostic{{
			Code: CodeInheritanceInvalid, Severity: SeverityError, Path: scopePath,
			Message: "inheritance scope must be a mapping",
		}}
	}

	byName := make(map[string]document.Entry, len(scope.Mapping))
	names := make([]string, 0, len(scope.Mapping))
	for _, entry := range scope.Mapping {
		byName[entry.Key] = entry
		names = append(names, entry.Key)
	}
	slices.Sort(names)

	resolved := make(map[string]*document.Node, len(byName))
	state := make(map[string]uint8, len(byName))
	result := &inheritanceScope{}
	for _, name := range names {
		if byName[name].Value.Kind != document.Mapping || state[name] == 2 {
			continue
		}
		stack := []siblingFrame{{name: name}}
		for len(stack) != 0 {
			frame := &stack[len(stack)-1]
			entry := byName[frame.name]
			path := scopePath.Key(frame.name)
			if !frame.entered {
				state[frame.name] = 1
				frame.entered = true
				reference, found := inheritanceDirective(entry.Value)
				if found {
					referencePath := path.Key(inheritanceKey)
					location := publicLocation(reference.Value.Location)
					result.controls = append(result.controls, inheritanceControl{
						path: referencePath, locations: []Location{location},
						cancelled: reference.Value.Kind == document.Null,
					})
					switch reference.Value.Kind {
					case document.Null:
					case document.String:
						frame.parent, frame.hasParent = reference.Value.Text, true
						parent, exists := byName[frame.parent]
						if !exists {
							return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceMissing, referencePath, location, nil,
								"referenced sibling does not exist")}
						}
						if parent.Value.Kind != document.Mapping {
							related := []Location{publicLocation(parent.Value.Location)}
							return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceInvalid, referencePath, location, related,
								"referenced sibling is not a mapping")}
						}
						if state[frame.parent] == 1 {
							related := []Location{publicLocation(parent.Value.Location)}
							return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceCycle, referencePath, location, related,
								"sibling inheritance contains a cycle")}
						}
						if state[frame.parent] == 0 {
							stack = append(stack, siblingFrame{name: frame.parent})
							continue
						}
					default:
						return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceInvalid, referencePath, location, nil,
							"inheritance reference must be a sibling name or null")}
					}
				}
			}

			payload := withoutInheritanceDirective(entry.Value)
			if frame.hasParent {
				parent := resolved[frame.parent]
				merged := document.Clone(parent)
				mergeRecursiveMappings(merged, payload, path, new(mappingIndex), recursiveMergeHooks{})
				merged.Location = entry.Value.Location
				resolved[frame.name] = merged
				reference := path.Key(inheritanceKey)
				control := inheritanceDirectiveLocation(entry.Value)
				result.transfers = append(result.transfers, inheritanceTransfer{
					from: scopePath.Key(frame.parent), to: path,
					reference: reference, locations: []Location{control},
				})
			} else {
				resolved[frame.name] = document.Clone(payload)
			}
			state[frame.name] = 2
			stack = stack[:len(stack)-1]
		}
	}

	root := *scope
	root.Mapping = make([]document.Entry, len(scope.Mapping))
	for index, entry := range scope.Mapping {
		root.Mapping[index] = entry
		if resolved[entry.Key] != nil {
			root.Mapping[index].Value = resolved[entry.Key]
		} else {
			root.Mapping[index].Value = document.Clone(entry.Value)
		}
	}
	result.root = &root
	return result, nil
}

func inheritanceDirective(node *document.Node) (document.Entry, bool) {
	for _, entry := range node.Mapping {
		if entry.Key == inheritanceKey {
			return entry, true
		}
	}
	return document.Entry{}, false
}

func inheritanceDirectiveLocation(node *document.Node) Location {
	entry, _ := inheritanceDirective(node)
	return publicLocation(entry.Value.Location)
}

func withoutInheritanceDirective(node *document.Node) *document.Node {
	payload := *node
	payload.Mapping = make([]document.Entry, 0, len(node.Mapping))
	for _, entry := range node.Mapping {
		if entry.Key != inheritanceKey {
			payload.Mapping = append(payload.Mapping, entry)
		}
	}
	return &payload
}

func inheritanceDiagnostic(code Code, path Path, primary Location, related []Location, message string) Diagnostic {
	return Diagnostic{
		Code: code, Severity: SeverityError, Path: path, Message: message,
		Primary: &primary, Related: related,
	}
}