package engine

import (
	"errors"

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

var (
	errInvalidDestination = errors.New("configkit: destination must contain only non-reserved mapping keys")
	errDestinationLimit   = errors.New("configkit: destination exceeds document limits")
)

type placementMode uint8

const (
	placementModeMapping placementMode = iota
	placementModeSequenceItem
)

func normalizeDestination(path Path) (Path, error) {
	if !path.valid() {
		return Path{}, errInvalidDestination
	}

	segments := make([]pathSegment, 0, len(path.segments))
	for _, segment := range path.segments {
		if segment.kind != pathSegmentKey && segment.kind != pathSegmentParsed {
			return Path{}, errInvalidDestination
		}
		if segment.key == "configkit" || segment.key == "extends" {
			return Path{}, errInvalidDestination
		}
		segments = append(segments, pathSegment{kind: pathSegmentKey, key: segment.key})
	}
	return Path{segments: segments}, nil
}

// appendDestination combines validated mapping-key paths with owned segments.
func appendDestination(parent, relative Path) Path {
	segments := clonePathSegments(parent.segments, len(relative.segments))
	segments = append(segments, relative.segments...)
	return Path{segments: segments}
}

func invalidDestinationDiagnostic(destination Path) Diagnostic {
	return Diagnostic{
		Code: CodeOptionsInvalid, Severity: SeverityError,
		Path:    destination.clone(),
		Message: "input destination must contain only non-reserved mapping keys",
	}
}

// placeDocument checks all per-document bounds before allocating destination
// wrappers. Aggregate accounting remains the caller's responsibility.
func placeDocument(body *document.Node, destination Path, sourceRoot document.Location, maxDepth, maxNodes int) (Path, *document.Node, error) {
	return placeDocumentWithMode(body, destination, placementModeMapping, sourceRoot, maxDepth, maxNodes)
}

// placeDocumentWithMode places a source mapping beneath its literal destination.
// Sequence-item placement adds one sequence containing the complete mapping.
func placeDocumentWithMode(body *document.Node, destination Path, mode placementMode, sourceRoot document.Location, maxDepth, maxNodes int) (Path, *document.Node, error) {
	normalized, err := normalizeDestination(destination)
	if err != nil {
		return Path{}, nil, err
	}
	if mode != placementModeMapping && mode != placementModeSequenceItem {
		return Path{}, nil, errInvalidDestination
	}
	if mode == placementModeSequenceItem && len(normalized.segments) == 0 {
		return Path{}, nil, errInvalidDestination
	}
	if body == nil {
		return normalized, nil, nil
	}
	if mode == placementModeSequenceItem && body.Kind != document.Mapping {
		return normalized, nil, errInvalidDestination
	}
	if !destinationWithinLimitsWithMode(body, normalized, mode, maxDepth, maxNodes) {
		return normalized, nil, errDestinationLimit
	}

	placed := body
	if mode == placementModeSequenceItem {
		placed = &document.Node{
			Kind: document.Sequence, Location: sourceRoot,
			Sequence: []*document.Node{placed},
		}
	}
	for index := len(normalized.segments) - 1; index >= 0; index-- {
		key := normalized.segments[index].key
		placed = &document.Node{
			Kind:     document.Mapping,
			Location: sourceRoot,
			Mapping: []document.Entry{{
				Key:           key,
				KeyLocation:   sourceRoot,
				ValueLocation: sourceRoot,
				Value:         placed,
			}},
		}
	}
	return normalized, placed, nil
}

// destinationWithinLimits expects a normalized mapping-key destination.
func destinationWithinLimits(body *document.Node, destination Path, maxDepth, maxNodes int) bool {
	return destinationWithinLimitsWithMode(body, destination, placementModeMapping, maxDepth, maxNodes)
}

func destinationWithinLimitsWithMode(body *document.Node, destination Path, mode placementMode, maxDepth, maxNodes int) bool {
	if body == nil {
		return true
	}
	if maxDepth < 0 || maxNodes < 0 || !destination.valid() || (mode != placementModeMapping && mode != placementModeSequenceItem) {
		return false
	}
	segments := len(destination.segments)
	sequenceNodes := 0
	sequenceDepth := 0
	if mode == placementModeSequenceItem {
		if segments == 0 {
			return false
		}
		sequenceNodes = 1
		sequenceDepth = 1
	}
	if segments > maxDepth-sequenceDepth || segments > (maxNodes-sequenceNodes)/2 {
		return false
	}
	wrapperNodes := destinationWrapperNodeCount(destination) + sequenceNodes
	for _, segment := range destination.segments {
		if segment.kind != pathSegmentKey {
			return false
		}
	}
	if documentDepth(body) > maxDepth-segments-sequenceDepth {
		return false
	}
	return countInputNodes(body) <= maxNodes-wrapperNodes
}

func destinationWrapperNodeCountForMode(destination Path, mode placementMode) int {
	count := destinationWrapperNodeCount(destination)
	if mode == placementModeSequenceItem {
		count++
	}
	return count
}

func destinationWrapperNodeCount(destination Path) int {
	return 2 * len(destination.segments)
}

func documentDepth(node *document.Node) int {
	if node == nil {
		return -1
	}
	depth := 0
	var visit func(*document.Node, int)
	visit = func(current *document.Node, currentDepth int) {
		if current == nil {
			return
		}
		if currentDepth > depth {
			depth = currentDepth
		}
		for _, entry := range current.Mapping {
			visit(entry.Value, currentDepth+1)
		}
		for _, child := range current.Sequence {
			visit(child, currentDepth+1)
		}
	}
	visit(node, 0)
	return depth
}