package engine

import (
	"bytes"
	"errors"
	"io/fs"
	"os"
	"path/filepath"
	"strconv"
	"strings"

	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
)

func activeOverlay(name string) bool {
	return name != "" && name != "default"
}

func validOverlayName(name string) bool {
	if !activeOverlay(name) {
		return true
	}
	for _, character := range name {
		if (character >= 'a' && character <= 'z') ||
			(character >= 'A' && character <= 'Z') ||
			(character >= '0' && character <= '9') || character == '_' || character == '-' {
			continue
		}
		return false
	}
	return true
}

func loadSelectedOverlayDocument(source Source, context *FilesystemContext, main parsedSource, base *composedDocument, sources []SourceInfo, name string, rule *VersionRule, limits importLimits) (*composedDocument, []SourceInfo, []Diagnostic) {
	rootPath, _, _, diagnostic := importFilesystem(source, context, main)
	if diagnostic != nil {
		return nil, cloneSourceInfos(sources), []Diagnostic{*diagnostic}
	}
	overlayPath := filepath.Join(rootPath, "overlays", name+".yml")
	canonical, err := canonicalExistingPath(overlayPath)
	if err != nil {
		code := CodeOverlayInvalid
		message := "selected overlay could not be resolved"
		if errors.Is(err, fs.ErrNotExist) {
			code, message = CodeOverlayMissing, "selected overlay does not exist"
		} else if errors.Is(err, fs.ErrPermission) {
			code, message = CodeImportPermission, "selected overlay cannot be accessed"
		}
		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(code, message, nil)}
	}
	if !containedPath(rootPath, canonical) {
		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayOutsideRoot, "selected overlay resolves outside the configuration root", nil)}
	}
	for _, existing := range sources {
		if existing.Path != "" && existing.Path == canonical {
			return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayRepeated, "selected overlay is already present in the base source graph", nil)}
		}
	}
	if len(sources)+1 > limits.sources {
		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayLimitExceeded, "selected overlay exceeds the source-count limit", nil)}
	}

	root, err := os.OpenRoot(rootPath)
	if err != nil {
		value := globFilesystemDiagnostic(err, "configuration root could not be opened for the selected overlay", Location{})
		value.Primary = nil
		return nil, cloneSourceInfos(sources), []Diagnostic{value}
	}
	rootClosed := false
	defer func() {
		if !rootClosed {
			_ = root.Close()
		}
	}()
	relative, err := filepath.Rel(rootPath, canonical)
	if err != nil || filepath.IsAbs(relative) || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeOverlayOutsideRoot, "selected overlay could not be addressed inside the configuration root", nil)}
	}

	sourceID := "source-" + strconv.Itoa(len(sources)+1)
	overlayLocation := &Location{SourceID: sourceID}
	overlaySource := SourceInfo{ID: sourceID, Label: filepath.ToSlash(filepath.Join("overlays", name+".yml")), Path: canonical, Layer: LayerOverlay}
	sources = append(cloneSourceInfos(sources), overlaySource)
	data, diagnostic := readOverlayFile(root, relative, sourceID, limits.bytes-base.inputBytes)
	if diagnostic != nil {
		return nil, sources, []Diagnostic{*diagnostic}
	}
	overlayRoot, issues := yamladapter.Parse(bytes.NewReader(data), sourceID, yamladapter.Limits{Depth: maxDocumentDepth, Nodes: maxDocumentNodes})
	if len(issues) != 0 {
		return nil, sources, parserDiagnostics(sourceID, issues)
	}
	nodeCount := countInputNodes(overlayRoot)
	if base.inputNodes+nodeCount > limits.nodes {
		return nil, sources, []Diagnostic{importDiagnostic(CodeOverlayLimitExceeded, "selected overlay exceeds the aggregate node limit", overlayLocation)}
	}
	if diagnostics := overlayDirectiveDiagnostics(overlayRoot, Path{}); len(diagnostics) != 0 {
		return nil, sources, diagnostics
	}
	if versionNode := fragmentVersionClaim(overlayRoot, rule); versionNode != nil {
		location := publicLocation(versionNode.Location)
		return nil, sources, []Diagnostic{{
			Code: CodeOverlayVersion, Severity: SeverityError, Path: rule.Path.clone(),
			Message: "overlays cannot contribute to or replace the schema-version path", Primary: &location,
		}}
	}
	closeErr := root.Close()
	rootClosed = true
	if closeErr != nil {
		return nil, sources, []Diagnostic{importDiagnostic(CodeImportRead, "configuration root could not be closed after reading the selected overlay", nil)}
	}

	overlayd, diagnostics := overlayDocuments(base, overlayRoot)
	if overlayd != nil {
		overlayd.inputBytes = base.inputBytes + int64(len(data))
		overlayd.inputNodes = base.inputNodes + nodeCount
	}
	return overlayd, sources, diagnostics
}

func readOverlayFile(root *os.Root, relative, sourceID string, remaining int64) ([]byte, *Diagnostic) {
	limit := min(int64(maxSourceBytes), remaining)
	if limit < 0 {
		limit = 0
	}
	data, failure := readRegularFile(relative, root.Stat, root.Open, limit)
	if failure != nil {
		value := sourceReadDiagnostic(failure, sourceID, CodeOverlayLimitExceeded, "selected overlay exceeds a source or aggregate byte limit")
		return nil, &value
	}
	return data, nil
}