package builder

import (
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"os/exec"
	"path/filepath"
	"regexp"
	"sort"
	"strings"
)

// getGorootVersion returns the major and minor version for a given GOROOT path.
// If the goroot cannot be determined, (0, 0) is returned.
func getGorootVersion(goroot string) (major, minor int, err error) {
	s, err := GorootVersionString(goroot)
	if err != nil {
		return 0, 0, err
	}

	if s == "" || s[:2] != "go" {
		return 0, 0, errors.New("could not parse Go version: version does not start with 'go' prefix")
	}

	parts := strings.Split(s[2:], ".")
	if len(parts) < 2 {
		return 0, 0, errors.New("could not parse Go version: version has less than two parts")
	}

	// Ignore the errors, we don't really handle errors here anyway.
	var trailing string
	n, err := fmt.Sscanf(s, "go%d.%d%s", &major, &minor, &trailing)
	if n == 2 && err == io.EOF {
		// Means there were no trailing characters (i.e., not an alpha/beta)
		err = nil
	}
	if err != nil {
		return 0, 0, fmt.Errorf("failed to parse version: %s", err)
	}
	return
}

// GorootVersionString returns the version string as reported by the Go
// toolchain for the given GOROOT path. It is usually of the form `go1.x.y` but
// can have some variations (for beta releases, for example).
func GorootVersionString(goroot string) (string, error) {
	if data, err := ioutil.ReadFile(filepath.Join(
		goroot, "src", "runtime", "internal", "sys", "zversion.go")); err == nil {

		r := regexp.MustCompile("const TheVersion = `(.*)`")
		matches := r.FindSubmatch(data)
		if len(matches) != 2 {
			return "", errors.New("Invalid go version output:\n" + string(data))
		}

		return string(matches[1]), nil

	} else if data, err := ioutil.ReadFile(filepath.Join(goroot, "VERSION")); err == nil {
		return string(data), nil

	} else {
		return "", err
	}
}

// getClangHeaderPath returns the path to the built-in Clang headers. It tries
// multiple locations, which should make it find the directory when installed in
// various ways.
func getClangHeaderPath(TINYGOROOT string) string {
	// Check whether we're running from the source directory.
	path := filepath.Join(TINYGOROOT, "llvm", "tools", "clang", "lib", "Headers")
	if _, err := os.Stat(path); !os.IsNotExist(err) {
		return path
	}

	// Check whether we're running from the installation directory.
	path = filepath.Join(TINYGOROOT, "lib", "clang", "include")
	if _, err := os.Stat(path); !os.IsNotExist(err) {
		return path
	}

	// It looks like we are built with a system-installed LLVM. Do a last
	// attempt: try to use Clang headers relative to the clang binary.
	for _, cmdName := range commands["clang"] {
		binpath, err := exec.LookPath(cmdName)
		if err == nil {
			// This should be the command that will also be used by
			// execCommand. To avoid inconsistencies, make sure we use the
			// headers relative to this command.
			binpath, err = filepath.EvalSymlinks(binpath)
			if err != nil {
				// Unexpected.
				return ""
			}
			// Example executable:
			//     /usr/lib/llvm-8/bin/clang
			// Example include path:
			//     /usr/lib/llvm-8/lib/clang/8.0.1/include/
			llvmRoot := filepath.Dir(filepath.Dir(binpath))
			clangVersionRoot := filepath.Join(llvmRoot, "lib", "clang")
			dirs, err := ioutil.ReadDir(clangVersionRoot)
			if err != nil {
				// Unexpected.
				continue
			}
			dirnames := make([]string, len(dirs))
			for i, d := range dirs {
				dirnames[i] = d.Name()
			}
			sort.Strings(dirnames)
			// Check for the highest version first.
			for i := len(dirnames) - 1; i >= 0; i-- {
				path := filepath.Join(clangVersionRoot, dirnames[i], "include")
				_, err := os.Stat(filepath.Join(path, "stdint.h"))
				if err == nil {
					return path
				}
			}
		}
	}

	// Could not find it.
	return ""
}