// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package cache

import (
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
	"runtime"
	"sync"
)

// Default returns the default cache to use, or nil if no cache should be used.
func Default() *Cache {
	defaultOnce.Do(initDefaultCache)
	return defaultCache
}

var (
	defaultOnce  sync.Once
	defaultCache *Cache
)

// cacheREADME is a message stored in a README in the cache directory.
// Because the cache lives outside the normal Go trees, we leave the
// README as a courtesy to explain where it came from.
const cacheREADME = `This directory holds cached build artifacts from the Go build system.
Run "go clean -cache" if the directory is getting too large.
See golang.org to learn more about Go.
`

// initDefaultCache does the work of finding the default cache
// the first time Default is called.
func initDefaultCache() {
	dir := DefaultDir()
	if dir == "off" {
		return
	}
	if err := os.MkdirAll(dir, 0777); err != nil {
		fmt.Fprintf(os.Stderr, "go: disabling cache (%s) due to initialization failure: %s\n", dir, err)
		return
	}
	if _, err := os.Stat(filepath.Join(dir, "README")); err != nil {
		// Best effort.
		ioutil.WriteFile(filepath.Join(dir, "README"), []byte(cacheREADME), 0666)
	}

	c, err := Open(dir)
	if err != nil {
		fmt.Fprintf(os.Stderr, "go: disabling cache (%s) due to initialization failure: %s\n", dir, err)
		return
	}
	defaultCache = c
}

// DefaultDir returns the effective GOCACHE setting.
// It returns "off" if the cache is disabled.
func DefaultDir() string {
	dir := os.Getenv("GOCACHE")
	if dir != "" {
		return dir
	}

	// Compute default location.
	// TODO(rsc): This code belongs somewhere else,
	// like maybe ioutil.CacheDir or os.CacheDir.
	switch runtime.GOOS {
	case "windows":
		dir = os.Getenv("LocalAppData")
		if dir == "" {
			// Fall back to %AppData%, the old name of
			// %LocalAppData% on Windows XP.
			dir = os.Getenv("AppData")
		}
		if dir == "" {
			return "off"
		}

	case "darwin":
		dir = os.Getenv("HOME")
		if dir == "" {
			return "off"
		}
		dir += "/Library/Caches"

	case "plan9":
		dir = os.Getenv("home")
		if dir == "" {
			return "off"
		}
		// Plan 9 has no established per-user cache directory,
		// but $home/lib/xyz is the usual equivalent of $HOME/.xyz on Unix.
		dir += "/lib/cache"

	default: // Unix
		// https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
		dir = os.Getenv("XDG_CACHE_HOME")
		if dir == "" {
			dir = os.Getenv("HOME")
			if dir == "" {
				return "off"
			}
			dir += "/.cache"
		}
	}
	return filepath.Join(dir, "go-build")
}