diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..5ad7ccd
--- /dev/null
+++ b/go.mod
@@ -0,0 +1 @@
+module golang.org/x/mod
diff --git a/semver/semver.go b/semver/semver.go
new file mode 100644
index 0000000..122e612
--- /dev/null
+++ b/semver/semver.go
@@ -0,0 +1,388 @@
+// 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 semver implements comparison of semantic version strings.
+// In this package, semantic version strings must begin with a leading "v",
+// as in "v1.0.0".
+//
+// The general form of a semantic version string accepted by this package is
+//
+//	vMAJOR[.MINOR[.PATCH[-PRERELEASE][+BUILD]]]
+//
+// where square brackets indicate optional parts of the syntax;
+// MAJOR, MINOR, and PATCH are decimal integers without extra leading zeros;
+// PRERELEASE and BUILD are each a series of non-empty dot-separated identifiers
+// using only alphanumeric characters and hyphens; and
+// all-numeric PRERELEASE identifiers must not have leading zeros.
+//
+// This package follows Semantic Versioning 2.0.0 (see semver.org)
+// with two exceptions. First, it requires the "v" prefix. Second, it recognizes
+// vMAJOR and vMAJOR.MINOR (with no prerelease or build suffixes)
+// as shorthands for vMAJOR.0.0 and vMAJOR.MINOR.0.
+package semver
+
+// parsed returns the parsed form of a semantic version string.
+type parsed struct {
+	major      string
+	minor      string
+	patch      string
+	short      string
+	prerelease string
+	build      string
+	err        string
+}
+
+// IsValid reports whether v is a valid semantic version string.
+func IsValid(v string) bool {
+	_, ok := parse(v)
+	return ok
+}
+
+// Canonical returns the canonical formatting of the semantic version v.
+// It fills in any missing .MINOR or .PATCH and discards build metadata.
+// Two semantic versions compare equal only if their canonical formattings
+// are identical strings.
+// The canonical invalid semantic version is the empty string.
+func Canonical(v string) string {
+	p, ok := parse(v)
+	if !ok {
+		return ""
+	}
+	if p.build != "" {
+		return v[:len(v)-len(p.build)]
+	}
+	if p.short != "" {
+		return v + p.short
+	}
+	return v
+}
+
+// Major returns the major version prefix of the semantic version v.
+// For example, Major("v2.1.0") == "v2".
+// If v is an invalid semantic version string, Major returns the empty string.
+func Major(v string) string {
+	pv, ok := parse(v)
+	if !ok {
+		return ""
+	}
+	return v[:1+len(pv.major)]
+}
+
+// MajorMinor returns the major.minor version prefix of the semantic version v.
+// For example, MajorMinor("v2.1.0") == "v2.1".
+// If v is an invalid semantic version string, MajorMinor returns the empty string.
+func MajorMinor(v string) string {
+	pv, ok := parse(v)
+	if !ok {
+		return ""
+	}
+	i := 1 + len(pv.major)
+	if j := i + 1 + len(pv.minor); j <= len(v) && v[i] == '.' && v[i+1:j] == pv.minor {
+		return v[:j]
+	}
+	return v[:i] + "." + pv.minor
+}
+
+// Prerelease returns the prerelease suffix of the semantic version v.
+// For example, Prerelease("v2.1.0-pre+meta") == "-pre".
+// If v is an invalid semantic version string, Prerelease returns the empty string.
+func Prerelease(v string) string {
+	pv, ok := parse(v)
+	if !ok {
+		return ""
+	}
+	return pv.prerelease
+}
+
+// Build returns the build suffix of the semantic version v.
+// For example, Build("v2.1.0+meta") == "+meta".
+// If v is an invalid semantic version string, Build returns the empty string.
+func Build(v string) string {
+	pv, ok := parse(v)
+	if !ok {
+		return ""
+	}
+	return pv.build
+}
+
+// Compare returns an integer comparing two versions according to
+// according to semantic version precedence.
+// The result will be 0 if v == w, -1 if v < w, or +1 if v > w.
+//
+// An invalid semantic version string is considered less than a valid one.
+// All invalid semantic version strings compare equal to each other.
+func Compare(v, w string) int {
+	pv, ok1 := parse(v)
+	pw, ok2 := parse(w)
+	if !ok1 && !ok2 {
+		return 0
+	}
+	if !ok1 {
+		return -1
+	}
+	if !ok2 {
+		return +1
+	}
+	if c := compareInt(pv.major, pw.major); c != 0 {
+		return c
+	}
+	if c := compareInt(pv.minor, pw.minor); c != 0 {
+		return c
+	}
+	if c := compareInt(pv.patch, pw.patch); c != 0 {
+		return c
+	}
+	return comparePrerelease(pv.prerelease, pw.prerelease)
+}
+
+// Max canonicalizes its arguments and then returns the version string
+// that compares greater.
+func Max(v, w string) string {
+	v = Canonical(v)
+	w = Canonical(w)
+	if Compare(v, w) > 0 {
+		return v
+	}
+	return w
+}
+
+func parse(v string) (p parsed, ok bool) {
+	if v == "" || v[0] != 'v' {
+		p.err = "missing v prefix"
+		return
+	}
+	p.major, v, ok = parseInt(v[1:])
+	if !ok {
+		p.err = "bad major version"
+		return
+	}
+	if v == "" {
+		p.minor = "0"
+		p.patch = "0"
+		p.short = ".0.0"
+		return
+	}
+	if v[0] != '.' {
+		p.err = "bad minor prefix"
+		ok = false
+		return
+	}
+	p.minor, v, ok = parseInt(v[1:])
+	if !ok {
+		p.err = "bad minor version"
+		return
+	}
+	if v == "" {
+		p.patch = "0"
+		p.short = ".0"
+		return
+	}
+	if v[0] != '.' {
+		p.err = "bad patch prefix"
+		ok = false
+		return
+	}
+	p.patch, v, ok = parseInt(v[1:])
+	if !ok {
+		p.err = "bad patch version"
+		return
+	}
+	if len(v) > 0 && v[0] == '-' {
+		p.prerelease, v, ok = parsePrerelease(v)
+		if !ok {
+			p.err = "bad prerelease"
+			return
+		}
+	}
+	if len(v) > 0 && v[0] == '+' {
+		p.build, v, ok = parseBuild(v)
+		if !ok {
+			p.err = "bad build"
+			return
+		}
+	}
+	if v != "" {
+		p.err = "junk on end"
+		ok = false
+		return
+	}
+	ok = true
+	return
+}
+
+func parseInt(v string) (t, rest string, ok bool) {
+	if v == "" {
+		return
+	}
+	if v[0] < '0' || '9' < v[0] {
+		return
+	}
+	i := 1
+	for i < len(v) && '0' <= v[i] && v[i] <= '9' {
+		i++
+	}
+	if v[0] == '0' && i != 1 {
+		return
+	}
+	return v[:i], v[i:], true
+}
+
+func parsePrerelease(v string) (t, rest string, ok bool) {
+	// "A pre-release version MAY be denoted by appending a hyphen and
+	// a series of dot separated identifiers immediately following the patch version.
+	// Identifiers MUST comprise only ASCII alphanumerics and hyphen [0-9A-Za-z-].
+	// Identifiers MUST NOT be empty. Numeric identifiers MUST NOT include leading zeroes."
+	if v == "" || v[0] != '-' {
+		return
+	}
+	i := 1
+	start := 1
+	for i < len(v) && v[i] != '+' {
+		if !isIdentChar(v[i]) && v[i] != '.' {
+			return
+		}
+		if v[i] == '.' {
+			if start == i || isBadNum(v[start:i]) {
+				return
+			}
+			start = i + 1
+		}
+		i++
+	}
+	if start == i || isBadNum(v[start:i]) {
+		return
+	}
+	return v[:i], v[i:], true
+}
+
+func parseBuild(v string) (t, rest string, ok bool) {
+	if v == "" || v[0] != '+' {
+		return
+	}
+	i := 1
+	start := 1
+	for i < len(v) {
+		if !isIdentChar(v[i]) && v[i] != '.' {
+			return
+		}
+		if v[i] == '.' {
+			if start == i {
+				return
+			}
+			start = i + 1
+		}
+		i++
+	}
+	if start == i {
+		return
+	}
+	return v[:i], v[i:], true
+}
+
+func isIdentChar(c byte) bool {
+	return 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z' || '0' <= c && c <= '9' || c == '-'
+}
+
+func isBadNum(v string) bool {
+	i := 0
+	for i < len(v) && '0' <= v[i] && v[i] <= '9' {
+		i++
+	}
+	return i == len(v) && i > 1 && v[0] == '0'
+}
+
+func isNum(v string) bool {
+	i := 0
+	for i < len(v) && '0' <= v[i] && v[i] <= '9' {
+		i++
+	}
+	return i == len(v)
+}
+
+func compareInt(x, y string) int {
+	if x == y {
+		return 0
+	}
+	if len(x) < len(y) {
+		return -1
+	}
+	if len(x) > len(y) {
+		return +1
+	}
+	if x < y {
+		return -1
+	} else {
+		return +1
+	}
+}
+
+func comparePrerelease(x, y string) int {
+	// "When major, minor, and patch are equal, a pre-release version has
+	// lower precedence than a normal version.
+	// Example: 1.0.0-alpha < 1.0.0.
+	// Precedence for two pre-release versions with the same major, minor,
+	// and patch version MUST be determined by comparing each dot separated
+	// identifier from left to right until a difference is found as follows:
+	// identifiers consisting of only digits are compared numerically and
+	// identifiers with letters or hyphens are compared lexically in ASCII
+	// sort order. Numeric identifiers always have lower precedence than
+	// non-numeric identifiers. A larger set of pre-release fields has a
+	// higher precedence than a smaller set, if all of the preceding
+	// identifiers are equal.
+	// Example: 1.0.0-alpha < 1.0.0-alpha.1 < 1.0.0-alpha.beta <
+	// 1.0.0-beta < 1.0.0-beta.2 < 1.0.0-beta.11 < 1.0.0-rc.1 < 1.0.0."
+	if x == y {
+		return 0
+	}
+	if x == "" {
+		return +1
+	}
+	if y == "" {
+		return -1
+	}
+	for x != "" && y != "" {
+		x = x[1:] // skip - or .
+		y = y[1:] // skip - or .
+		var dx, dy string
+		dx, x = nextIdent(x)
+		dy, y = nextIdent(y)
+		if dx != dy {
+			ix := isNum(dx)
+			iy := isNum(dy)
+			if ix != iy {
+				if ix {
+					return -1
+				} else {
+					return +1
+				}
+			}
+			if ix {
+				if len(dx) < len(dy) {
+					return -1
+				}
+				if len(dx) > len(dy) {
+					return +1
+				}
+			}
+			if dx < dy {
+				return -1
+			} else {
+				return +1
+			}
+		}
+	}
+	if x == "" {
+		return -1
+	} else {
+		return +1
+	}
+}
+
+func nextIdent(x string) (dx, rest string) {
+	i := 0
+	for i < len(x) && x[i] != '.' {
+		i++
+	}
+	return x[:i], x[i:]
+}
diff --git a/semver/semver_test.go b/semver/semver_test.go
new file mode 100644
index 0000000..77025a4
--- /dev/null
+++ b/semver/semver_test.go
@@ -0,0 +1,183 @@
+// 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 semver
+
+import (
+	"strings"
+	"testing"
+)
+
+var tests = []struct {
+	in  string
+	out string
+}{
+	{"bad", ""},
+	{"v1-alpha.beta.gamma", ""},
+	{"v1-pre", ""},
+	{"v1+meta", ""},
+	{"v1-pre+meta", ""},
+	{"v1.2-pre", ""},
+	{"v1.2+meta", ""},
+	{"v1.2-pre+meta", ""},
+	{"v1.0.0-alpha", "v1.0.0-alpha"},
+	{"v1.0.0-alpha.1", "v1.0.0-alpha.1"},
+	{"v1.0.0-alpha.beta", "v1.0.0-alpha.beta"},
+	{"v1.0.0-beta", "v1.0.0-beta"},
+	{"v1.0.0-beta.2", "v1.0.0-beta.2"},
+	{"v1.0.0-beta.11", "v1.0.0-beta.11"},
+	{"v1.0.0-rc.1", "v1.0.0-rc.1"},
+	{"v1", "v1.0.0"},
+	{"v1.0", "v1.0.0"},
+	{"v1.0.0", "v1.0.0"},
+	{"v1.2", "v1.2.0"},
+	{"v1.2.0", "v1.2.0"},
+	{"v1.2.3-456", "v1.2.3-456"},
+	{"v1.2.3-456.789", "v1.2.3-456.789"},
+	{"v1.2.3-456-789", "v1.2.3-456-789"},
+	{"v1.2.3-456a", "v1.2.3-456a"},
+	{"v1.2.3-pre", "v1.2.3-pre"},
+	{"v1.2.3-pre+meta", "v1.2.3-pre"},
+	{"v1.2.3-pre.1", "v1.2.3-pre.1"},
+	{"v1.2.3-zzz", "v1.2.3-zzz"},
+	{"v1.2.3", "v1.2.3"},
+	{"v1.2.3+meta", "v1.2.3"},
+	{"v1.2.3+meta-pre", "v1.2.3"},
+	{"v1.2.3+meta-pre.sha.256a", "v1.2.3"},
+}
+
+func TestIsValid(t *testing.T) {
+	for _, tt := range tests {
+		ok := IsValid(tt.in)
+		if ok != (tt.out != "") {
+			t.Errorf("IsValid(%q) = %v, want %v", tt.in, ok, !ok)
+		}
+	}
+}
+
+func TestCanonical(t *testing.T) {
+	for _, tt := range tests {
+		out := Canonical(tt.in)
+		if out != tt.out {
+			t.Errorf("Canonical(%q) = %q, want %q", tt.in, out, tt.out)
+		}
+	}
+}
+
+func TestMajor(t *testing.T) {
+	for _, tt := range tests {
+		out := Major(tt.in)
+		want := ""
+		if i := strings.Index(tt.out, "."); i >= 0 {
+			want = tt.out[:i]
+		}
+		if out != want {
+			t.Errorf("Major(%q) = %q, want %q", tt.in, out, want)
+		}
+	}
+}
+
+func TestMajorMinor(t *testing.T) {
+	for _, tt := range tests {
+		out := MajorMinor(tt.in)
+		var want string
+		if tt.out != "" {
+			want = tt.in
+			if i := strings.Index(want, "+"); i >= 0 {
+				want = want[:i]
+			}
+			if i := strings.Index(want, "-"); i >= 0 {
+				want = want[:i]
+			}
+			switch strings.Count(want, ".") {
+			case 0:
+				want += ".0"
+			case 1:
+				// ok
+			case 2:
+				want = want[:strings.LastIndex(want, ".")]
+			}
+		}
+		if out != want {
+			t.Errorf("MajorMinor(%q) = %q, want %q", tt.in, out, want)
+		}
+	}
+}
+
+func TestPrerelease(t *testing.T) {
+	for _, tt := range tests {
+		pre := Prerelease(tt.in)
+		var want string
+		if tt.out != "" {
+			if i := strings.Index(tt.out, "-"); i >= 0 {
+				want = tt.out[i:]
+			}
+		}
+		if pre != want {
+			t.Errorf("Prerelease(%q) = %q, want %q", tt.in, pre, want)
+		}
+	}
+}
+
+func TestBuild(t *testing.T) {
+	for _, tt := range tests {
+		build := Build(tt.in)
+		var want string
+		if tt.out != "" {
+			if i := strings.Index(tt.in, "+"); i >= 0 {
+				want = tt.in[i:]
+			}
+		}
+		if build != want {
+			t.Errorf("Build(%q) = %q, want %q", tt.in, build, want)
+		}
+	}
+}
+
+func TestCompare(t *testing.T) {
+	for i, ti := range tests {
+		for j, tj := range tests {
+			cmp := Compare(ti.in, tj.in)
+			var want int
+			if ti.out == tj.out {
+				want = 0
+			} else if i < j {
+				want = -1
+			} else {
+				want = +1
+			}
+			if cmp != want {
+				t.Errorf("Compare(%q, %q) = %d, want %d", ti.in, tj.in, cmp, want)
+			}
+		}
+	}
+}
+
+func TestMax(t *testing.T) {
+	for i, ti := range tests {
+		for j, tj := range tests {
+			max := Max(ti.in, tj.in)
+			want := Canonical(ti.in)
+			if i < j {
+				want = Canonical(tj.in)
+			}
+			if max != want {
+				t.Errorf("Max(%q, %q) = %q, want %q", ti.in, tj.in, max, want)
+			}
+		}
+	}
+}
+
+var (
+	v1 = "v1.0.0+metadata-dash"
+	v2 = "v1.0.0+metadata-dash1"
+)
+
+func BenchmarkCompare(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		if Compare(v1, v2) != 0 {
+			b.Fatalf("bad compare")
+		}
+	}
+}