package fileops

import (
	"fmt"
	"os"
	"path/filepath"
	"strings"

	"gitea.maximumdirect.net/eric/narratio/internal/pathsafe"
)

// OpenConfinedRegularFile opens a declared file beneath root without following
// symlinked ancestors or leaf entries. The returned descriptor remains valid if
// the pathname is later replaced.
func OpenConfinedRegularFile(rootPath, relativePath string) (*os.File, error) {
	rootPath = filepath.Clean(strings.TrimSpace(rootPath))
	if rootPath == "" || rootPath == "." {
		return nil, fmt.Errorf("source root is required")
	}
	relativePath, err := pathsafe.NormalizeRelativeDestination(relativePath)
	if err != nil {
		return nil, fmt.Errorf("source path: %w", err)
	}

	root, err := openConfinedDirectory(rootPath)
	if err != nil {
		return nil, fmt.Errorf("open source root: %w", err)
	}
	defer root.Close()

	parts := strings.Split(filepath.FromSlash(relativePath), string(filepath.Separator))
	parent := root
	for _, part := range parts[:len(parts)-1] {
		child, err := openConfinedChild(parent, part, false, 0)
		if err != nil {
			if parent != root {
				_ = parent.Close()
			}
			return nil, fmt.Errorf("open source ancestor %q: %w", part, err)
		}
		if parent != root {
			_ = parent.Close()
		}
		parent = child
	}
	if parent != root {
		defer parent.Close()
	}

	name := parts[len(parts)-1]
	declared, err := parent.Lstat(name)
	if err != nil {
		return nil, fmt.Errorf("inspect source %q: %w", relativePath, err)
	}
	if declared.Mode()&os.ModeSymlink != 0 || !declared.Mode().IsRegular() {
		return nil, fmt.Errorf("source %q is not a regular file", relativePath)
	}

	file, err := parent.Open(name)
	if err != nil {
		return nil, fmt.Errorf("open source %q: %w", relativePath, err)
	}
	opened, err := file.Stat()
	if err != nil {
		_ = file.Close()
		return nil, fmt.Errorf("inspect opened source %q: %w", relativePath, err)
	}
	current, err := parent.Lstat(name)
	if err != nil || current.Mode()&os.ModeSymlink != 0 || !current.Mode().IsRegular() ||
		!opened.Mode().IsRegular() || !os.SameFile(opened, declared) || !os.SameFile(opened, current) {
		_ = file.Close()
		if err != nil {
			return nil, fmt.Errorf("reinspect source %q: %w", relativePath, err)
		}
		return nil, fmt.Errorf("source %q changed while being opened", relativePath)
	}
	return file, nil
}