// 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 module defines the module.Version type
// along with support code.
package module

// IMPORTANT NOTE
//
// This file essentially defines the set of valid import paths for the go command.
// There are many subtle considerations, including Unicode ambiguity,
// security, network, and file system representations.
//
// Changes to the semantics in this file require approval from rsc.

import (
	"fmt"
	"sort"
	"strings"
	"unicode/utf8"

	"cmd/go/internal/semver"
)

// A Version is defined by a module path and version pair.
type Version struct {
	Path string

	// Version is usually a semantic version in canonical form.
	// There are two exceptions to this general rule.
	// First, the top-level target of a build has no specific version
	// and uses Version = "".
	// Second, during MVS calculations the version "none" is used
	// to represent the decision to take no version of a given module.
	Version string `json:",omitempty"`
}

// Check checks that a given module path, version pair is valid.
// In addition to the path being a valid module path
// and the version being a valid semantic version,
// the two must correspond.
// For example, the path "yaml/v2" only corresponds to
// semantic versions beginning with "v2.".
func Check(path, version string) error {
	if err := CheckPath(path); err != nil {
		return err
	}
	if !semver.IsValid(version) {
		return fmt.Errorf("malformed semantic version %v", version)
	}
	vm := semver.Major(version)
	_, pathVersion, _ := SplitPathVersion(path)

	if strings.HasPrefix(pathVersion, ".") {
		// Special-case gopkg.in path requirements.
		pathVersion = pathVersion[1:] // cut .
		if vm == pathVersion {
			return nil
		}
	} else {
		// Standard path requirements.
		if pathVersion != "" {
			pathVersion = pathVersion[1:] // cut /
		}
		if vm == "v0" || vm == "v1" {
			vm = ""
		}
		if vm == pathVersion {
			return nil
		}
		if pathVersion == "" {
			pathVersion = "v0 or v1"
		}
	}
	return fmt.Errorf("mismatched module path %v and version %v (want %v)", path, version, pathVersion)
}

// firstPathOK reports whether r can appear in the first element of a module path.
// The first element of the path must be an LDH domain name, at least for now.
// To avoid case ambiguity, the domain name must be entirely lower case.
func firstPathOK(r rune) bool {
	return r == '-' || r == '.' ||
		'0' <= r && r <= '9' ||
		'a' <= r && r <= 'z'
}

// pathOK reports whether r can appear in a module path.
// Paths can be ASCII letters, ASCII digits, and limited ASCII punctuation: + - . / _ and ~.
// This matches what "go get" has historically recognized in import paths.
// TODO(rsc): We would like to allow Unicode letters, but that requires additional
// care in the safe encoding (see note below).
func pathOK(r rune) bool {
	if r < utf8.RuneSelf {
		return r == '+' || r == '-' || r == '.' || r == '/' || r == '_' || r == '~' ||
			'0' <= r && r <= '9' ||
			'A' <= r && r <= 'Z' ||
			'a' <= r && r <= 'z'
	}
	return false
}

// CheckPath checks that a module path is valid.
func CheckPath(path string) error {
	if err := checkImportPath(path); err != nil {
		return fmt.Errorf("malformed module path %q: %v", path, err)
	}
	i := strings.Index(path, "/")
	if i < 0 {
		i = len(path)
	}
	if i == 0 {
		return fmt.Errorf("malformed module path %q: leading slash", path)
	}
	if !strings.Contains(path[:i], ".") {
		return fmt.Errorf("malformed module path %q: missing dot in first path element", path)
	}
	if path[0] == '-' {
		return fmt.Errorf("malformed module path %q: leading dash in first path element", path)
	}
	for _, r := range path[:i] {
		if !firstPathOK(r) {
			return fmt.Errorf("malformed module path %q: invalid char %q in first path element", path, r)
		}
	}
	if _, _, ok := SplitPathVersion(path); !ok {
		return fmt.Errorf("malformed module path %q: invalid version %s", path, path[strings.LastIndex(path, "/")+1:])
	}
	return nil
}

// CheckImportPath checks that an import path is valid.
func CheckImportPath(path string) error {
	if err := checkImportPath(path); err != nil {
		return fmt.Errorf("malformed import path %q: %v", path, err)
	}
	return nil
}

// checkImportPath checks that an import path is valid.
// It returns an error describing why but not mentioning path.
// Because these checks apply to both module paths and import paths,
// the caller is expected to add the "malformed ___ path %q: " prefix.
func checkImportPath(path string) error {
	if !utf8.ValidString(path) {
		return fmt.Errorf("invalid UTF-8")
	}
	if path == "" {
		return fmt.Errorf("empty string")
	}
	if strings.Contains(path, "..") {
		return fmt.Errorf("double dot")
	}
	if strings.Contains(path, "//") {
		return fmt.Errorf("double slash")
	}
	if path[len(path)-1] == '/' {
		return fmt.Errorf("trailing slash")
	}
	elemStart := 0
	for i, r := range path {
		if !pathOK(r) {
			return fmt.Errorf("invalid char %q", r)
		}
		if r == '/' {
			if err := checkElem(path[elemStart:i]); err != nil {
				return err
			}
			elemStart = i + 1
		}
	}
	if err := checkElem(path[elemStart:]); err != nil {
		return err
	}
	return nil
}

// checkElem checks whether an individual path element is valid.
func checkElem(elem string) error {
	if elem == "" {
		return fmt.Errorf("empty path element")
	}
	if elem[0] == '.' {
		return fmt.Errorf("leading dot in path element")
	}
	if elem[len(elem)-1] == '.' {
		return fmt.Errorf("trailing dot in path element")
	}

	// Windows disallows a bunch of path elements, sadly.
	// See https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file
	short := elem
	if i := strings.Index(short, "."); i >= 0 {
		short = short[:i]
	}
	for _, bad := range badWindowsNames {
		if strings.EqualFold(bad, short) {
			return fmt.Errorf("disallowed path element %q", elem)
		}
	}
	return nil
}

// badWindowsNames are the reserved file path elements on Windows.
// See https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file
var badWindowsNames = []string{
	"CON",
	"PRN",
	"AUX",
	"NUL",
	"COM1",
	"COM2",
	"COM3",
	"COM4",
	"COM5",
	"COM6",
	"COM7",
	"COM8",
	"COM9",
	"LPT1",
	"LPT2",
	"LPT3",
	"LPT4",
	"LPT5",
	"LPT6",
	"LPT7",
	"LPT8",
	"LPT9",
}

// SplitPathVersion returns prefix and major version such that prefix+pathMajor == path
// and version is either empty or "/vN" for N >= 2.
// As a special case, gopkg.in paths are recognized directly;
// they require ".vN" instead of "/vN", and for all N, not just N >= 2.
func SplitPathVersion(path string) (prefix, pathMajor string, ok bool) {
	if strings.HasPrefix(path, "gopkg.in/") {
		return splitGopkgIn(path)
	}

	i := len(path)
	dot := false
	for i > 0 && ('0' <= path[i-1] && path[i-1] <= '9' || path[i-1] == '.') {
		if path[i-1] == '.' {
			dot = true
		}
		i--
	}
	if i <= 1 || path[i-1] != 'v' || path[i-2] != '/' {
		return path, "", true
	}
	prefix, pathMajor = path[:i-2], path[i-2:]
	if dot || len(pathMajor) <= 2 || pathMajor[2] == '0' || pathMajor == "/v1" {
		return path, "", false
	}
	return prefix, pathMajor, true
}

// splitGopkgIn is like SplitPathVersion but only for gopkg.in paths.
func splitGopkgIn(path string) (prefix, pathMajor string, ok bool) {
	if !strings.HasPrefix(path, "gopkg.in/") {
		return path, "", false
	}
	i := len(path)
	for i > 0 && ('0' <= path[i-1] && path[i-1] <= '9') {
		i--
	}
	if i <= 1 || path[i-1] != 'v' || path[i-2] != '.' {
		// All gopkg.in paths must end in vN for some N.
		return path, "", false
	}
	prefix, pathMajor = path[:i-2], path[i-2:]
	if len(pathMajor) <= 2 || pathMajor[2] == '0' && pathMajor != ".v0" {
		return path, "", false
	}
	return prefix, pathMajor, true
}

// MatchPathMajor reports whether the semantic version v
// matches the path major version pathMajor.
func MatchPathMajor(v, pathMajor string) bool {
	m := semver.Major(v)
	if pathMajor == "" {
		return m == "v0" || m == "v1"
	}
	return (pathMajor[0] == '/' || pathMajor[0] == '.') && m == pathMajor[1:]
}

// Sort sorts the list by Path, breaking ties by comparing Versions.
func Sort(list []Version) {
	sort.Slice(list, func(i, j int) bool {
		mi := list[i]
		mj := list[j]
		if mi.Path != mj.Path {
			return mi.Path < mj.Path
		}
		// To help go.sum formatting, allow version/file.
		// Compare semver prefix by semver rules,
		// file by string order.
		vi := mi.Version
		vj := mj.Version
		var fi, fj string
		if k := strings.Index(vi, "/"); k >= 0 {
			vi, fi = vi[:k], vi[k:]
		}
		if k := strings.Index(vj, "/"); k >= 0 {
			vj, fj = vj[:k], vj[k:]
		}
		if vi != vj {
			return semver.Compare(vi, vj) < 0
		}
		return fi < fj
	})
}

// Safe encodings
//
// Module paths appear as substrings of file system paths
// (in the download cache) and of web server URLs in the proxy protocol.
// In general we cannot rely on file systems to be case-sensitive,
// nor can we rely on web servers, since they read from file systems.
// That is, we cannot rely on the file system to keep rsc.io/QUOTE
// and rsc.io/quote separate. Windows and macOS don't.
// Instead, we must never require two different casings of a file path.
// Because we want the download cache to match the proxy protocol,
// and because we want the proxy protocol to be possible to serve
// from a tree of static files (which might be stored on a case-insensitive
// file system), the proxy protocol must never require two different casings
// of a URL path either.
//
// One possibility would be to make the safe encoding be the lowercase
// hexadecimal encoding of the actual path bytes. This would avoid ever
// needing different casings of a file path, but it would be fairly illegible
// to most programmers when those paths appeared in the file system
// (including in file paths in compiler errors and stack traces)
// in web server logs, and so on. Instead, we want a safe encoding that
// leaves most paths unaltered.
//
// The safe encoding is this:
// replace every uppercase letter with an exclamation mark
// followed by the letter's lowercase equivalent.
//
// For example,
// github.com/Azure/azure-sdk-for-go ->  github.com/!azure/azure-sdk-for-go.
// github.com/GoogleCloudPlatform/cloudsql-proxy -> github.com/!google!cloud!platform/cloudsql-proxy
// github.com/Sirupsen/logrus -> github.com/!sirupsen/logrus.
//
// Import paths that avoid upper-case letters are left unchanged.
// Note that because import paths are ASCII-only and avoid various
// problematic punctuation (like : < and >), the safe encoding is also ASCII-only
// and avoids the same problematic punctuation.
//
// Import paths have never allowed exclamation marks, so there is no
// need to define how to encode a literal !.
//
// Although paths are disallowed from using Unicode (see pathOK above),
// the eventual plan is to allow Unicode letters as well, to assume that
// file systems and URLs are Unicode-safe (storing UTF-8), and apply
// the !-for-uppercase convention. Note however that not all runes that
// are different but case-fold equivalent are an upper/lower pair.
// For example, U+004B ('K'), U+006B ('k'), and U+212A ('K' for Kelvin)
// are considered to case-fold to each other. When we do add Unicode
// letters, we must not assume that upper/lower are the only case-equivalent pairs.
// Perhaps the Kelvin symbol would be disallowed entirely, for example.
// Or perhaps it would encode as "!!k", or perhaps as "(212A)".
//
// Also, it would be nice to allow Unicode marks as well as letters,
// but marks include combining marks, and then we must deal not
// only with case folding but also normalization: both U+00E9 ('é')
// and U+0065 U+0301 ('e' followed by combining acute accent)
// look the same on the page and are treated by some file systems
// as the same path. If we do allow Unicode marks in paths, there
// must be some kind of normalization to allow only one canonical
// encoding of any character used in an import path.

// EncodePath returns the safe encoding of the given module path.
// It fails if the module path is invalid.
func EncodePath(path string) (encoding string, err error) {
	if err := CheckPath(path); err != nil {
		return "", err
	}

	haveUpper := false
	for _, r := range path {
		if r == '!' || r >= utf8.RuneSelf {
			// This should be disallowed by CheckPath, but diagnose anyway.
			// The correctness of the encoding loop below depends on it.
			return "", fmt.Errorf("internal error: inconsistency in EncodePath")
		}
		if 'A' <= r && r <= 'Z' {
			haveUpper = true
		}
	}

	if !haveUpper {
		return path, nil
	}

	var buf []byte
	for _, r := range path {
		if 'A' <= r && r <= 'Z' {
			buf = append(buf, '!', byte(r+'a'-'A'))
		} else {
			buf = append(buf, byte(r))
		}
	}
	return string(buf), nil
}

// DecodePath returns the module path of the given safe encoding.
// It fails if the encoding is invalid.
func DecodePath(encoding string) (path string, err error) {
	var buf []byte

	bang := false
	for _, r := range encoding {
		if r >= utf8.RuneSelf {
			goto BadEncoding
		}
		if bang {
			bang = false
			if r < 'a' || 'z' < r {
				goto BadEncoding
			}
			buf = append(buf, byte(r+'A'-'a'))
			continue
		}
		if r == '!' {
			bang = true
			continue
		}
		if 'A' <= r && r <= 'Z' {
			goto BadEncoding
		}
		buf = append(buf, byte(r))
	}
	if bang {
		goto BadEncoding
	}
	path = string(buf)
	if err := CheckPath(path); err != nil {
		return "", fmt.Errorf("invalid module path encoding %q: %v", encoding, err)
	}
	return path, nil

BadEncoding:
	return "", fmt.Errorf("invalid module path encoding %q", encoding)
}