package engine

import (
	"errors"
	"io/fs"
	"path"
	"path/filepath"
	"sort"
	"strings"
)

type importSelectionOutcome uint8

const (
	importSelectionNone importSelectionOutcome = iota
	importSelectionHasCandidates
)

type selectedImportCandidate struct {
	path       string
	accessPath string
	label      string
	site       importFieldSite
}

// importSelection contains lexical candidates only; admission assigns source identity later.
type importSelection struct {
	candidates []selectedImportCandidate
	outcome    importSelectionOutcome
}

func selectNamedImportCandidates(access *filesystemAccess, basePath string, spec importDeclarationSpec, policy resolvedImportPolicy) (importSelection, []Diagnostic) {
	normalized := make([]selectedImportCandidate, 0, len(spec.paths))
	seen := make(map[string]struct{}, len(spec.paths))
	for index, relative := range spec.paths {
		accessPath := access.join(basePath, relative)
		candidatePath := accessPath
		if access.native == nil {
			candidatePath = filepath.ToSlash(filepath.Clean(accessPath))
		}
		if _, exists := seen[candidatePath]; exists {
			continue
		}
		seen[candidatePath] = struct{}{}
		var site importFieldSite
		if index < len(spec.selectorSites) {
			site = spec.selectorSites[index]
		} else {
			site = spec.fieldSites["path"]
		}
		normalized = append(normalized, selectedImportCandidate{path: candidatePath, accessPath: accessPath, label: relativeImportLabel(access, basePath, accessPath), site: site})
	}
	sort.Slice(normalized, func(left, right int) bool {
		return normalized[left].path < normalized[right].path
	})
	candidates := make([]selectedImportCandidate, 0, len(normalized))
	for _, candidate := range normalized {
		missing, _, diagnostic := validateNamedImportTarget(access, candidate, policy.required, false)
		if diagnostic != nil {
			return importSelection{}, []Diagnostic{*diagnostic}
		}
		if missing {
			continue
		}
		if excludedBasename(path.Base(candidate.path), policy.exclude) {
			continue
		}
		candidates = append(candidates, candidate)
	}
	selection := importSelection{candidates: candidates}
	if len(candidates) != 0 {
		selection.outcome = importSelectionHasCandidates
	}
	return selection, nil
}

func selectDirectoryImportCandidates(access *filesystemAccess, basePath string, spec importDeclarationSpec, policy resolvedImportPolicy, limits importLimits, accounting *importAccounting) (importSelection, []Diagnostic) {
	roots := make([]selectedImportCandidate, 0, len(spec.paths))
	seenRoots := make(map[string]struct{}, len(spec.paths))
	for index, relative := range spec.paths {
		accessPath := access.join(basePath, relative)
		candidatePath := normalizedImportCandidatePath(access, accessPath)
		if _, exists := seenRoots[candidatePath]; exists {
			continue
		}
		seenRoots[candidatePath] = struct{}{}
		site := spec.fieldSites["dir"]
		if index < len(spec.selectorSites) {
			site = spec.selectorSites[index]
		}
		roots = append(roots, selectedImportCandidate{path: candidatePath, accessPath: accessPath, site: site})
	}
	sort.Slice(roots, func(left, right int) bool { return roots[left].path < roots[right].path })

	if accounting == nil {
		accounting = &importAccounting{}
	}
	discovered := make([]selectedImportCandidate, 0)
	seenCandidates := make(map[string]struct{})
	for _, root := range roots {
		missing, isDirectory, diagnostic := validateNamedImportTarget(access, root, false, true)
		if diagnostic != nil {
			return importSelection{}, []Diagnostic{*diagnostic}
		}
		if missing {
			continue
		}
		if !isDirectory {
			return importSelection{}, []Diagnostic{*importSelectionDiagnostic(root.site, CodeImportInvalid, "directory search root is not a directory")}
		}
		rootTarget, targetErr := access.importEntryTarget(root.accessPath, nil, true)
		if targetErr != nil {
			return importSelection{}, []Diagnostic{*importDirectoryTargetDiagnostic(root.site, targetErr)}
		}
		root.accessPath = rootTarget.identity
		entries, missing, diagnostic := importDirectoryEntries(access, root.accessPath, root.site, limits, accounting)
		if diagnostic != nil {
			return importSelection{}, []Diagnostic{*diagnostic}
		}
		if missing {
			continue
		}
		stack := []importDirectoryFrame{{directory: root, depth: 0, entries: entries, identity: rootTarget.identity}}
		activeDirectories := map[string]struct{}{rootTarget.identity: {}}
		for len(stack) != 0 {
			frameIndex := len(stack) - 1
			frame := &stack[frameIndex]
			if frame.next >= len(frame.entries) {
				delete(activeDirectories, frame.identity)
				stack = stack[:frameIndex]
				continue
			}
			entry := frame.entries[frame.next]
			frame.next++
			accessPath := access.join(frame.directory.accessPath, entry.Name())
			lexicalPath := access.join(frame.directory.path, entry.Name())
			candidate := selectedImportCandidate{
				path: normalizedImportCandidatePath(access, lexicalPath), accessPath: accessPath,
				label: relativeImportLabel(access, basePath, lexicalPath), site: frame.directory.site,
			}
			if !policy.hidden && strings.HasPrefix(entry.Name(), ".") {
				continue
			}
			target, entryErr := access.importEntryTarget(candidate.accessPath, entry, policy.follow)
			if entryErr != nil {
				if errors.Is(entryErr, fs.ErrNotExist) {
					continue
				}
				return importSelection{}, []Diagnostic{*importDirectoryTargetDiagnostic(frame.directory.site, entryErr)}
			}
			if target.symlink && !policy.follow {
				continue
			}
			if target.dir {
				if !directoryDepthAllows(policy, frame.depth) {
					continue
				}
				if _, exists := activeDirectories[target.identity]; exists {
					return importSelection{}, []Diagnostic{*importSelectionDiagnostic(frame.directory.site, CodeImportCycle, "directory traversal cycle detected")}
				}
				childMissing, childIsDirectory, childDiagnostic := validateNamedImportTarget(access, candidate, false, true)
				if childDiagnostic != nil {
					return importSelection{}, []Diagnostic{*childDiagnostic}
				}
				if childMissing || !childIsDirectory {
					continue
				}
				childEntries, vanished, childDiagnostic := importDirectoryEntries(access, candidate.accessPath, candidate.site, limits, accounting)
				if childDiagnostic != nil {
					return importSelection{}, []Diagnostic{*childDiagnostic}
				}
				if !vanished {
					candidate.accessPath = target.identity
					activeDirectories[target.identity] = struct{}{}
					stack = append(stack, importDirectoryFrame{directory: candidate, depth: frame.depth + 1, entries: childEntries, identity: target.identity})
				}
				continue
			}
			if target.identity != "" {
				candidate.accessPath = target.identity
			} else {
				canonical, canonicalErr := access.canonical(accessPath)
				if canonicalErr != nil {
					if errors.Is(canonicalErr, fs.ErrNotExist) {
						continue
					}
					return importSelection{}, []Diagnostic{globFilesystemDiagnostic(canonicalErr, "directory search file could not be canonicalized", frame.directory.site.location)}
				}
				candidate.accessPath = canonical
			}
			if !matchesImportFilename(entry.Name(), policy.pattern) {
				continue
			}
			if _, exists := seenCandidates[candidate.path]; exists {
				continue
			}
			seenCandidates[candidate.path] = struct{}{}
			discovered = append(discovered, candidate)
		}
	}
	sort.Slice(discovered, func(left, right int) bool { return discovered[left].path < discovered[right].path })
	selected := make([]selectedImportCandidate, 0, len(discovered))
	for _, candidate := range discovered {
		missing, isDirectory, diagnostic := validateNamedImportTarget(access, candidate, false, true)
		if diagnostic != nil {
			return importSelection{}, []Diagnostic{*diagnostic}
		}
		if missing || isDirectory || excludedBasename(path.Base(candidate.path), policy.exclude) {
			continue
		}
		selected = append(selected, candidate)
	}
	if policy.required && len(selected) == 0 {
		return importSelection{}, []Diagnostic{*importSelectionDiagnostic(spec.fieldSites["dir"], CodeImportMissing, "required directory import selected no files")}
	}
	selection := importSelection{candidates: selected}
	if len(selected) != 0 {
		selection.outcome = importSelectionHasCandidates
	}
	return selection, nil
}

type importDirectoryFrame struct {
	directory selectedImportCandidate
	depth     int
	entries   []fs.DirEntry
	next      int
	identity  string
}

func importDirectoryTargetDiagnostic(site importFieldSite, err error) *Diagnostic {
	if isImportLinkCycle(err) {
		return importSelectionDiagnostic(site, CodeImportCycle, "directory traversal cycle detected")
	}
	if errors.Is(err, errImportPathOutsideRoot) {
		return importSelectionDiagnostic(site, CodeImportOutsideRoot, "import target resolves outside the configuration root")
	}
	return importSelectionDiagnostic(site, CodeImportInvalid, "directory entry could not be inspected")
}

func importDirectoryEntries(access *filesystemAccess, directory string, site importFieldSite, limits importLimits, accounting *importAccounting) ([]fs.DirEntry, bool, *Diagnostic) {
	target, targetErr := access.importEntryTarget(directory, nil, true)
	if targetErr != nil {
		if errors.Is(targetErr, fs.ErrNotExist) {
			return nil, true, nil
		}
		return nil, false, importDirectoryTargetDiagnostic(site, targetErr)
	}
	readPath := target.identity
	if readPath == "" {
		readPath = directory
	}
	entries := make([]fs.DirEntry, 0)
	err := visitBoundedImportDirectory(access, readPath, limits.entries, accounting, func(entry fs.DirEntry) error {
		entries = append(entries, entry)
		return nil
	})
	if errors.Is(err, errImportDirectoryEntryLimit) {
		return nil, false, importSelectionDiagnostic(site, CodeImportLimitExceeded, "directory search exceeds the directory-entry visit limit")
	}
	if errors.Is(err, fs.ErrNotExist) {
		return nil, true, nil
	}
	if err != nil {
		return nil, false, importSelectionDiagnostic(site, CodeImportInvalid, "directory search root could not be read")
	}
	sort.Slice(entries, func(left, right int) bool { return entries[left].Name() < entries[right].Name() })
	return entries, false, nil
}

func directoryDepthAllows(policy resolvedImportPolicy, currentDepth int) bool {
	if !policy.recursive {
		return false
	}
	childDepth := currentDepth + 1
	return !policy.depthSet || policy.depth == 0 || childDepth < policy.depth
}

func normalizedImportCandidatePath(access *filesystemAccess, accessPath string) string {
	if access.native != nil {
		return accessPath
	}
	return filepath.ToSlash(filepath.Clean(accessPath))
}

func relativeImportLabel(access *filesystemAccess, basePath, candidatePath string) string {
	if access.native != nil {
		if candidatePath == basePath {
			return "."
		}
		prefix := strings.TrimSuffix(basePath, "/") + "/"
		if basePath == "." {
			return candidatePath
		}
		if strings.HasPrefix(candidatePath, prefix) {
			return strings.TrimPrefix(candidatePath, prefix)
		}
		return candidatePath
	}
	label, err := filepath.Rel(basePath, candidatePath)
	if err == nil {
		return filepath.ToSlash(label)
	}
	return filepath.ToSlash(candidatePath)
}

func matchesImportFilename(name string, patterns []string) bool {
	if patterns == nil {
		return true
	}
	for _, pattern := range patterns {
		matched, err := path.Match(pattern, name)
		if err == nil && matched {
			return true
		}
	}
	return false
}

func validateNamedImportTarget(access *filesystemAccess, candidate selectedImportCandidate, required, allowDirectory bool) (bool, bool, *Diagnostic) {
	if !access.contains(candidate.accessPath) {
		return false, false, importSelectionDiagnostic(candidate.site, CodeImportOutsideRoot, "import target is outside the configuration root")
	}
	canonical, err := access.canonical(candidate.accessPath)
	if err != nil {
		if errors.Is(err, fs.ErrNotExist) {
			if required {
				return false, false, importSelectionDiagnostic(candidate.site, CodeImportMissing, "required import target does not exist")
			}
			return true, false, nil
		}
		if errors.Is(err, fs.ErrPermission) {
			return false, false, importSelectionDiagnostic(candidate.site, CodeImportInvalid, "import target cannot be accessed")
		}
		return false, false, importSelectionDiagnostic(candidate.site, CodeImportInvalid, "import target could not be canonicalized")
	}
	if !access.contains(canonical) {
		return false, false, importSelectionDiagnostic(candidate.site, CodeImportOutsideRoot, "import target resolves outside the configuration root")
	}
	info, err := access.stat(canonical)
	if err != nil {
		if errors.Is(err, fs.ErrNotExist) {
			if required {
				return false, false, importSelectionDiagnostic(candidate.site, CodeImportMissing, "required import target does not exist")
			}
			return true, false, nil
		}
		if errors.Is(err, fs.ErrPermission) {
			return false, false, importSelectionDiagnostic(candidate.site, CodeImportInvalid, "import target cannot be accessed")
		}
		return false, false, importSelectionDiagnostic(candidate.site, CodeImportInvalid, "import target could not be inspected")
	}
	if info.IsDir() && allowDirectory {
		return false, true, nil
	}
	if !info.Mode().IsRegular() {
		return false, false, importSelectionDiagnostic(candidate.site, CodeSourceNotRegular, "source is not a regular file")
	}
	return false, false, nil
}

func excludedBasename(name string, patterns []string) bool {
	for _, pattern := range patterns {
		matched, err := path.Match(pattern, name)
		if err == nil && matched {
			return true
		}
	}
	return false
}

func importSelectionDiagnostic(site importFieldSite, code Code, message string) *Diagnostic {
	location := site.location
	return &Diagnostic{
		Code: code, Severity: SeverityError, Path: site.path.clone(),
		Message: message, Primary: &location,
	}
}