package sysfs

import (
	"os"
	"syscall"
	"unsafe"
)

// A File represents basic IO interactions with the underlying file system
type File interface {
	Write(b []byte) (n int, err error)
	WriteString(s string) (ret int, err error)
	Sync() (err error)
	Read(b []byte) (n int, err error)
	ReadAt(b []byte, off int64) (n int, err error)
	Seek(offset int64, whence int) (ret int64, err error)
	Fd() uintptr
	Close() error
}

// filesystem is a unexposed interface to allow the switch between the native file system or a mocked implementation
type filesystem interface {
	openFile(name string, flag int, perm os.FileMode) (file File, err error)
	stat(name string) (os.FileInfo, error)
	find(baseDir string, pattern string) (dirs []string, err error)
	readFile(name string) (content []byte, err error)
}

// systemCaller represents unexposed Syscall interface to allow the switch between native and mocked implementation
// Prevent unsafe call, since go 1.15, see "Pattern 4" in: https://go101.org/article/unsafe.html
type systemCaller interface {
	syscall(trap uintptr, f File, signal uintptr, payload unsafe.Pointer) (r1, r2 uintptr, err syscall.Errno)
}

// Accesser provides access to system calls and filesystem
type Accesser struct {
	sys systemCaller
	fs  filesystem
}

// NewAccesser returns a accesser to native system call and native file system
func NewAccesser() *Accesser {
	s := Accesser{
		sys: &nativeSyscall{},
		fs:  &nativeFilesystem{},
	}
	return &s
}

// UseMockSyscall sets the Syscall implementation of the accesser to the mocked one. Used only for tests.
func (a *Accesser) UseMockSyscall() *mockSyscall {
	msc := &mockSyscall{}
	a.sys = msc
	return msc
}

// UseMockFilesystem sets the filesystem implementation of the accesser to the mocked one. Used only for tests.
func (a *Accesser) UseMockFilesystem(files []string) *MockFilesystem {
	fs := newMockFilesystem(files)
	a.fs = fs
	return fs
}

// OpenFile opens file of given name from native or the mocked file system
func (a *Accesser) OpenFile(name string, flag int, perm os.FileMode) (File, error) {
	return a.fs.openFile(name, flag, perm)
}

// Stat returns a generic FileInfo, if the file with given name exists. It uses the native or the mocked file system.
func (a *Accesser) Stat(name string) (os.FileInfo, error) {
	return a.fs.stat(name)
}

// Find finds file from native or the mocked file system
func (a *Accesser) Find(baseDir string, pattern string) ([]string, error) {
	return a.fs.find(baseDir, pattern)
}

// ReadFile reads the named file and returns the contents. A successful call returns err == nil, not err == EOF.
// Because ReadFile reads the whole file, it does not treat an EOF from Read as an error to be reported.
func (a *Accesser) ReadFile(name string) ([]byte, error) {
	return a.fs.readFile(name)
}