package bookfile

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

// OpenReaderAt opens a book file and returns a ReaderAt + size.
// Consolidates the 3 places that open a book file + stat + get ReaderAt.
func OpenReaderAt(root, rel string) (*os.File, int64, error) {
	abs := filepath.Join(root, filepath.FromSlash(rel))
	f, err := os.Open(abs)
	if err != nil {
		return nil, 0, err
	}
	st, err := f.Stat()
	if err != nil {
		f.Close()
		return nil, 0, err
	}
	return f, st.Size(), nil
}

// Contains reports whether child is inside parent using EvalSymlinks semantics.
// Falls back to Clean if EvalSymlinks fails (e.g., path doesn't exist yet).
func Contains(parent, child string) bool {
	p, err := filepath.EvalSymlinks(parent)
	if err != nil {
		p = filepath.Clean(parent)
	}
	c, err := filepath.EvalSymlinks(child)
	if err != nil {
		c = filepath.Clean(child)
	}
	return c == p || strings.HasPrefix(c, p+string(os.PathSeparator))
}