// Copyright 2018 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 main

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

// Copies of functions from src/cmd/go/internal/cfg/cfg.go for
// finding the GOROOT.
// Keep them in sync until support is moved to a common place, if ever.

func findGOROOT() string {
	if env := os.Getenv("GOROOT"); env != "" {
		return filepath.Clean(env)
	}
	def := filepath.Clean(runtime.GOROOT())
	if runtime.Compiler == "gccgo" {
		// gccgo has no real GOROOT, and it certainly doesn't
		// depend on the executable's location.
		return def
	}
	exe, err := os.Executable()
	if err == nil {
		exe, err = filepath.Abs(exe)
		if err == nil {
			if dir := filepath.Join(exe, "../.."); isGOROOT(dir) {
				// If def (runtime.GOROOT()) and dir are the same
				// directory, prefer the spelling used in def.
				if isSameDir(def, dir) {
					return def
				}
				return dir
			}
			exe, err = filepath.EvalSymlinks(exe)
			if err == nil {
				if dir := filepath.Join(exe, "../.."); isGOROOT(dir) {
					if isSameDir(def, dir) {
						return def
					}
					return dir
				}
			}
		}
	}
	return def
}

// isGOROOT reports whether path looks like a GOROOT.
//
// It does this by looking for the path/pkg/tool directory,
// which is necessary for useful operation of the cmd/go tool,
// and is not typically present in a GOPATH.
func isGOROOT(path string) bool {
	stat, err := os.Stat(filepath.Join(path, "pkg", "tool"))
	if err != nil {
		return false
	}
	return stat.IsDir()
}

// isSameDir reports whether dir1 and dir2 are the same directory.
func isSameDir(dir1, dir2 string) bool {
	if dir1 == dir2 {
		return true
	}
	info1, err1 := os.Stat(dir1)
	info2, err2 := os.Stat(dir2)
	return err1 == nil && err2 == nil && os.SameFile(info1, info2)
}