internal/semver: add SemverToGoTag

Add SemverToGoTag, which reverses GoTagToSemver.
The exception is when converting a go tag to the semver would have
resulted in an invalid semver (such as go RC versions). GoTagToSemver
returns an empty string in that case, and as such is not reversible.

Change-Id: I8d68c2372261b1ec7ec8b72354ac1919ad24d6f6
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/617396
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Zvonimir Pavlinovic <zpavlinovic@google.com>

internal/semver/semver.go

@@ -7,6 +7,7 @@
 package semver
 
 import (
+	"fmt"
 	"regexp"
 	"strings"
 
@@ -93,3 +94,47 @@ func GoTagToSemver(tag string) string {
 	}
 	return version
 }
+
+// This is a modified copy of pkgsite/internal/stlib:TagForVersion
+func SemverToGoTag(v string) string {
+	// Special case: v1.0.0 => go1.
+	if v == "v1.0.0" {
+		return "go1"
+	}
+
+	goVersion := semver.Canonical(v)
+	prerelease := semver.Prerelease(goVersion)
+	versionWithoutPrerelease := strings.TrimSuffix(goVersion, prerelease)
+	patch := strings.TrimPrefix(versionWithoutPrerelease, semver.MajorMinor(goVersion)+".")
+	if patch == "0" && (semver.Compare(v, "v1.21.0") < 0 || prerelease != "") {
+		// Starting with go1.21.0, the first patch version includes .0.
+		// Prereleases do not include .0 (we don't do prereleases for other patch releases).
+		versionWithoutPrerelease = strings.TrimSuffix(versionWithoutPrerelease, ".0")
+	}
+	goVersion = fmt.Sprintf("go%s", strings.TrimPrefix(versionWithoutPrerelease, "v"))
+	if prerelease != "" {
+		i := finalDigitsIndex(prerelease)
+		if i >= 1 {
+			// Remove the dot.
+			prerelease = prerelease[:i-1] + prerelease[i:]
+		}
+		goVersion += prerelease
+	}
+	return goVersion
+}
+
+// finalDigitsIndex returns the index of the first digit in the sequence of digits ending s.
+// If s doesn't end in digits, it returns -1.
+func finalDigitsIndex(s string) int {
+	// Assume ASCII (since the semver package does anyway).
+	var i int
+	for i = len(s) - 1; i >= 0; i-- {
+		if s[i] < '0' || s[i] > '9' {
+			break
+		}
+	}
+	if i == len(s)-1 {
+		return -1
+	}
+	return i + 1
+}

internal/semver/semver_test.go

@@ -39,6 +39,27 @@ func TestGoTagToSemver(t *testing.T) {
 	}
 }
 
+func TestSemverToGoTag(t *testing.T) {
+	for _, test := range []struct {
+		v    string
+		want string
+	}{
+		{"v1.19.0", "go1.19"},
+		{"v1.20.0-pre.4", "go1.20-pre4"},
+		{"v1.0.0", "go1"},
+		{"v1.20.0-rc.1", "go1.20-rc1"},
+		{"v1.20.0-beta.1", "go1.20-beta1"},
+		{"v1.20.0-alpha.1", "go1.20-alpha1"},
+		{"v1.20.0-alpha.1+incompatible", "go1.20-alpha1"},
+		{"v1.20.0-alpha.1-0.20220101230456-1234abcd", "go1.20-alpha.1-0.20220101230456-1234abcd"},
+	} {
+		got := SemverToGoTag(test.v)
+		if got != test.want {
+			t.Errorf("want %s; got %s", test.want, got)
+		}
+	}
+}
+
 func TestLess(t *testing.T) {
 	for _, test := range []struct {
 		v1   string