package engine

import (
	"cmp"
	"slices"

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

type inheritanceWalker struct {
	root      **document.Node
	limits    projectionLimits
	transfers []inheritanceTransfer
	controls  []inheritanceControl
}

// expandInheritanceTree resolves each destination scope after its enclosing
// relationships. It remains a private, test-only operation until the loading
// lifecycle also carries its complete provenance and inspection evidence.
func expandInheritanceTree(input *document.Node, limits projectionLimits) (*inheritanceScope, []Diagnostic) {
	if input == nil || input.Kind != document.Mapping {
		return nil, []Diagnostic{{Code: CodeInheritanceInvalid, Severity: SeverityError,
			Message: "inheritance requires a mapping document root"}}
	}
	root := input
	walker := inheritanceWalker{root: &root, limits: limits}
	if problem := walker.visit(&root, Path{}, true); problem != nil {
		return nil, []Diagnostic{*problem}
	}
	if problem := inheritanceFinalBudget(root, limits); problem != nil {
		return nil, []Diagnostic{*problem}
	}
	return &inheritanceScope{root: root, transfers: walker.transfers, controls: walker.controls}, nil
}

func (w *inheritanceWalker) visit(node **document.Node, path Path, unscoped bool) *Diagnostic {
	switch (*node).Kind {
	case document.Mapping:
		if unscoped {
			reference, found := inheritanceDirective(*node)
			if found {
				referencePath := path.Key(inheritanceKey)
				location := publicLocation(reference.Value.Location)
				if reference.Value.Kind != document.Null {
					problem := inheritanceDiagnostic(CodeInheritanceInvalid, referencePath, location, nil,
						"inheritance reference has no enclosing sibling scope")
					return &problem
				}
				w.controls = append(w.controls, inheritanceControl{
					path: referencePath, locations: []Location{location}, cancelled: true,
				})
				*node = withoutInheritanceDirective(*node)
			}
		}
		resolved, diagnostics := expandSiblingDefinitionsWithLimits(*node, path, w.limits)
		if len(diagnostics) != 0 {
			return &diagnostics[0]
		}
		*node = resolved.root
		w.transfers = append(w.transfers, resolved.transfers...)
		w.controls = append(w.controls, resolved.controls...)
		if !inheritanceLowerBoundWithin(*w.root, w.limits.nodes) {
			problem := inheritanceBudgetDiagnostic(path, (*node).Location)
			return &problem
		}
		indices := make([]int, len((*node).Mapping))
		for index := range indices {
			indices[index] = index
		}
		slices.SortFunc(indices, func(left, right int) int {
			return cmp.Compare((*node).Mapping[left].Key, (*node).Mapping[right].Key)
		})
		for _, index := range indices {
			entry := &(*node).Mapping[index]
			if problem := w.visit(&entry.Value, path.Key(entry.Key), false); problem != nil {
				return problem
			}
		}
	case document.Sequence:
		for index := range (*node).Sequence {
			child := &(*node).Sequence[index]
			if problem := w.visit(child, path.Index(index), (*child).Kind == document.Mapping); problem != nil {
				return problem
			}
		}
	}
	return nil
}

// A directive may disappear during later destination resolution, so only
// non-directive nodes form a safe early lower bound on final output size.
func inheritanceLowerBoundWithin(root *document.Node, limit int) bool {
	_, within := inheritanceLowerBoundCount(root, limit)
	return within
}

func inheritanceLowerBoundCount(root *document.Node, limit int) (int, bool) {
	if limit < 0 {
		limit = int(^uint(0) >> 1)
	}
	stack := []*document.Node{root}
	count := 0
	for len(stack) != 0 {
		node := stack[len(stack)-1]
		stack = stack[:len(stack)-1]
		if node == nil {
			continue
		}
		count++
		if count > limit {
			return count, false
		}
		switch node.Kind {
		case document.Mapping:
			for _, entry := range node.Mapping {
				if entry.Key == inheritanceKey {
					continue
				}
				count++ // mapping key
				stack = append(stack, entry.Value)
			}
		case document.Sequence:
			stack = append(stack, node.Sequence...)
		}
		if count > limit {
			return count, false
		}
	}
	return count, true
}

// Count the materialized mapping shape before cloning a memoized parent.
// Relationship fields are excluded because later destination resolution
// consumes them; their omission makes this a safe lower bound.
func inheritanceOverlayLowerBoundWithin(lower, upper *document.Node, limit int) bool {
	if limit < 0 {
		return false
	}
	index := new(mappingIndex)
	count := 0
	var visit func(*document.Node, *document.Node) bool
	visit = func(base, incoming *document.Node) bool {
		if base.Kind != document.Mapping || incoming.Kind != document.Mapping {
			added, within := inheritanceLowerBoundCount(incoming, limit-count)
			count += added
			return within
		}
		count++ // mapping node
		if count > limit {
			return false
		}
		for _, entry := range base.Mapping {
			if entry.Key == inheritanceKey {
				continue
			}
			count++ // mapping key
			if count > limit {
				return false
			}
			position := index.find(incoming, entry.Key)
			if position >= 0 {
				if !visit(entry.Value, incoming.Mapping[position].Value) {
					return false
				}
			} else {
				added, within := inheritanceLowerBoundCount(entry.Value, limit-count)
				count += added
				if !within {
					return false
				}
			}
		}
		for _, entry := range incoming.Mapping {
			if entry.Key == inheritanceKey || index.find(base, entry.Key) >= 0 {
				continue
			}
			count++ // mapping key
			if count > limit {
				return false
			}
			added, within := inheritanceLowerBoundCount(entry.Value, limit-count)
			count += added
			if !within || count > limit {
				return false
			}
		}
		return true
	}
	return visit(lower, upper)
}

func inheritanceFinalBudget(root *document.Node, limits projectionLimits) *Diagnostic {
	type pending struct {
		node *document.Node
		path Path
	}
	stack := []pending{{node: root}}
	count := 0
	for len(stack) != 0 {
		item := stack[len(stack)-1]
		stack = stack[:len(stack)-1]
		if len(item.path.segments) > limits.depth {
			problem := inheritanceBudgetDiagnostic(item.path, item.node.Location)
			return &problem
		}
		count++
		if limits.nodes >= 0 && count > limits.nodes {
			problem := inheritanceBudgetDiagnostic(item.path, item.node.Location)
			return &problem
		}
		switch item.node.Kind {
		case document.Mapping:
			count += len(item.node.Mapping) // mapping keys
			for index := len(item.node.Mapping) - 1; index >= 0; index-- {
				entry := item.node.Mapping[index]
				stack = append(stack, pending{node: entry.Value, path: item.path.Key(entry.Key)})
			}
		case document.Sequence:
			for index := len(item.node.Sequence) - 1; index >= 0; index-- {
				stack = append(stack, pending{node: item.node.Sequence[index], path: item.path.Index(index)})
			}
		}
		if limits.nodes >= 0 && count > limits.nodes {
			problem := inheritanceBudgetDiagnostic(item.path, item.node.Location)
			return &problem
		}
	}
	return nil
}

func inheritanceBudgetDiagnostic(path Path, location document.Location) Diagnostic {
	primary := publicLocation(location)
	return Diagnostic{
		Code: CodeSchemaProjectionLimit, Severity: SeverityError, Path: path,
		Message: "expanded configuration exceeds the projection limit", Primary: &primary,
	}
}