package xtfs

import (
	"io/fs"
	"os"
	"path"
)

// XTFS is an eXTended File System
type XTFS struct {
	fs      fs.FS
	root    string
	doCache bool
	cache   map[string]bool
}

func (x *XTFS) apn(name string) string {
	return path.Join(x.root, name)
}

func (x *XTFS) exists(name string) bool {
	if has, ok := x.cache[name]; ok && x.doCache {
		return has
	}
	_, err := os.Stat(x.apn(name))
	if err != nil {
		x.cache[name] = false
		return false
	}
	x.cache[name] = true
	return true
}

// Open opens an fs.File, preferring disk
func (x *XTFS) Open(name string) (fs.File, error) {
	if x.exists(name) {
		return os.Open(x.apn(name))
	}
	return x.fs.Open(name)
}

// Option is a functional option for an XTFS
type Option func(*XTFS) error

// New returns a new XTFS
func New(root string, fs fs.FS, opts ...Option) (*XTFS, error) {
	x := &XTFS{
		fs:      fs,
		root:    root,
		doCache: true,
		cache:   make(map[string]bool),
	}

	for _, opt := range opts {
		if err := opt(x); err != nil {
			return x, err
		}
	}

	return x, nil
}

// WithSub sets a fs.Sub for an XTFS
func WithSub(sub string) Option {
	return func(x *XTFS) (err error) {
		x.fs, err = fs.Sub(x.fs, sub)
		return
	}
}

// WithCaching sets a caching mode for an XTFS
// Caching avoids subsequent os.Stat to determine if a file exists on disk
// See bench.txt for differences in usage
func WithCaching(doCache bool) Option {
	return func(x *XTFS) error {
		x.doCache = doCache
		return nil
	}
}