// Copyright 2019 The Go Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package span

import (
	"fmt"
	"net/url"
	"os"
	"path"
	"path/filepath"
	"runtime"
	"strings"
	"unicode"
)

const fileScheme = "file"

// URI represents the full URI for a file.
type URI string

// Filename returns the file path for the given URI.
// It is an error to call this on a URI that is not a valid filename.
func (uri URI) Filename() string {
	filename, err := filename(uri)
	if err != nil {
		panic(err)
	}
	return filepath.FromSlash(filename)
}

func filename(uri URI) (string, error) {
	if uri == "" {
		return "", nil
	}
	u, err := url.ParseRequestURI(string(uri))
	if err != nil {
		return "", err
	}
	if u.Scheme != fileScheme {
		return "", fmt.Errorf("only file URIs are supported, got %q from %q", u.Scheme, uri)
	}
	if isWindowsDriveURI(u.Path) {
		u.Path = u.Path[1:]
	}
	return u.Path, nil
}

// NewURI returns a span URI for the string.
// It will attempt to detect if the string is a file path or uri.
func NewURI(s string) URI {
	if u, err := url.PathUnescape(s); err == nil {
		s = u
	}
	if strings.HasPrefix(s, fileScheme+"://") {
		return URI(s)
	}
	return FileURI(s)
}

func CompareURI(a, b URI) int {
	if equalURI(a, b) {
		return 0
	}
	if a < b {
		return -1
	}
	return 1
}

func equalURI(a, b URI) bool {
	if a == b {
		return true
	}
	// If we have the same URI basename, we may still have the same file URIs.
	if !strings.EqualFold(path.Base(string(a)), path.Base(string(b))) {
		return false
	}
	fa, err := filename(a)
	if err != nil {
		return false
	}
	fb, err := filename(b)
	if err != nil {
		return false
	}
	// Stat the files to check if they are equal.
	infoa, err := os.Stat(filepath.FromSlash(fa))
	if err != nil {
		return false
	}
	infob, err := os.Stat(filepath.FromSlash(fb))
	if err != nil {
		return false
	}
	return os.SameFile(infoa, infob)
}

// FileURI returns a span URI for the supplied file path.
// It will always have the file scheme.
func FileURI(path string) URI {
	if path == "" {
		return ""
	}
	// Handle standard library paths that contain the literal "$GOROOT".
	// TODO(rstambler): The go/packages API should allow one to determine a user's $GOROOT.
	const prefix = "$GOROOT"
	if len(path) >= len(prefix) && strings.EqualFold(prefix, path[:len(prefix)]) {
		suffix := path[len(prefix):]
		path = runtime.GOROOT() + suffix
	}
	if !isWindowsDrivePath(path) {
		if abs, err := filepath.Abs(path); err == nil {
			path = abs
		}
	}
	// Check the file path again, in case it became absolute.
	if isWindowsDrivePath(path) {
		path = "/" + path
	}
	path = filepath.ToSlash(path)
	u := url.URL{
		Scheme: fileScheme,
		Path:   path,
	}
	uri := u.String()
	if unescaped, err := url.PathUnescape(uri); err == nil {
		uri = unescaped
	}
	return URI(uri)
}

// isWindowsDrivePath returns true if the file path is of the form used by
// Windows. We check if the path begins with a drive letter, followed by a ":".
func isWindowsDrivePath(path string) bool {
	if len(path) < 4 {
		return false
	}
	return unicode.IsLetter(rune(path[0])) && path[1] == ':'
}

// isWindowsDriveURI returns true if the file URI is of the format used by
// Windows URIs. The url.Parse package does not specially handle Windows paths
// (see https://golang.org/issue/6027). We check if the URI path has
// a drive prefix (e.g. "/C:"). If so, we trim the leading "/".
func isWindowsDriveURI(uri string) bool {
	if len(uri) < 4 {
		return false
	}
	return uri[0] == '/' && unicode.IsLetter(rune(uri[1])) && uri[2] == ':'
}