drop init()

Initializing a package global map and compiling regexes increases the init
functions and adds up to the start time of executables.

This change implements two approaches:

- compile the regex on its first use
- rewrite the map to a switch case statement

Minimal example:
```
package main

import "github.com/hashicorp/go-version"

func main() {
	v, err := version.NewSemver("1.2.3")
	if err != nil {
		panic(err)
	}
	println(v.String())
}
```

On Go 1.25.4 linux/amd64 the above code compiles to ~2728103 bytes and
GODEBUG=inittrace=1 shows the following:

init github.com/hashicorp/go-version @0.90 ms, 0.37 ms clock, 80992 bytes, 504 allocs

With this change applied on the same system, the above code compiles to ~2703283
bytes (reduction of ~24820 bytes) and GODEBUG=inittrace=1 does not list any init entry for
github.com/hashicorp/go-version anymore.

Signed-off-by: Florian Lehner <[email protected]>

Author: florianl

constraint.go

@@ -8,8 +8,26 @@ import (
 	"regexp"
 	"sort"
 	"strings"
+	"sync"
 )
 
+var (
+	constraintRegexp     *regexp.Regexp
+	constraintRegexpOnce sync.Once
+)
+
+func getConstraintRegexp() *regexp.Regexp {
+	constraintRegexpOnce.Do(func() {
+		// This heavy lifting only happens the first time this function is called
+		constraintRegexp = regexp.MustCompile(fmt.Sprintf(
+			`^\s*(%s)\s*(%s)\s*$`,
+			`<=|>=|!=|~>|<|>|=|`,
+			VersionRegexpRaw,
+		))
+	})
+	return constraintRegexp
+}
+
 // Constraint represents a single constraint for a version, such as
 // ">= 1.0".
 type Constraint struct {
@@ -29,38 +47,11 @@ type Constraints []*Constraint
 
 type constraintFunc func(v, c *Version) bool
 
-var constraintOperators map[string]constraintOperation
-
 type constraintOperation struct {
 	op operator
 	f  constraintFunc
 }
 
-var constraintRegexp *regexp.Regexp
-
-func init() {
-	constraintOperators = map[string]constraintOperation{
-		"":   {op: equal, f: constraintEqual},
-		"=":  {op: equal, f: constraintEqual},
-		"!=": {op: notEqual, f: constraintNotEqual},
-		">":  {op: greaterThan, f: constraintGreaterThan},
-		"<":  {op: lessThan, f: constraintLessThan},
-		">=": {op: greaterThanEqual, f: constraintGreaterThanEqual},
-		"<=": {op: lessThanEqual, f: constraintLessThanEqual},
-		"~>": {op: pessimistic, f: constraintPessimistic},
-	}
-
-	ops := make([]string, 0, len(constraintOperators))
-	for k := range constraintOperators {
-		ops = append(ops, regexp.QuoteMeta(k))
-	}
-
-	constraintRegexp = regexp.MustCompile(fmt.Sprintf(
-		`^\s*(%s)\s*(%s)\s*$`,
-		strings.Join(ops, "|"),
-		VersionRegexpRaw))
-}
-
 // NewConstraint will parse one or more constraints from the given
 // constraint string. The string must be a comma-separated list of
 // constraints.
@@ -176,7 +167,7 @@ func (c *Constraint) String() string {
 }
 
 func parseSingle(v string) (*Constraint, error) {
-	matches := constraintRegexp.FindStringSubmatch(v)
+	matches := getConstraintRegexp().FindStringSubmatch(v)
 	if matches == nil {
 		return nil, fmt.Errorf("malformed constraint: %s", v)
 	}
@@ -186,7 +177,25 @@ func parseSingle(v string) (*Constraint, error) {
 		return nil, err
 	}
 
-	cop := constraintOperators[matches[1]]
+	var cop constraintOperation
+	switch matches[1] {
+	case "=":
+		cop = constraintOperation{op: equal, f: constraintEqual}
+	case "!=":
+		cop = constraintOperation{op: notEqual, f: constraintNotEqual}
+	case ">":
+		cop = constraintOperation{op: greaterThan, f: constraintGreaterThan}
+	case "<":
+		cop = constraintOperation{op: lessThan, f: constraintLessThan}
+	case ">=":
+		cop = constraintOperation{op: greaterThanEqual, f: constraintGreaterThanEqual}
+	case "<=":
+		cop = constraintOperation{op: lessThanEqual, f: constraintLessThanEqual}
+	case "~>":
+		cop = constraintOperation{op: pessimistic, f: constraintPessimistic}
+	default:
+		cop = constraintOperation{op: equal, f: constraintEqual}
+	}
 
 	return &Constraint{
 		f:        cop.f,

version.go

@@ -9,14 +9,31 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"sync"
 )
 
 // The compiled regular expression used to test the validity of a version.
 var (
-	versionRegexp *regexp.Regexp
-	semverRegexp  *regexp.Regexp
+	versionRegexp     *regexp.Regexp
+	versionRegexpOnce sync.Once
+	semverRegexp      *regexp.Regexp
+	semverRegexpOnce  sync.Once
 )
 
+func getVersionRegexp() *regexp.Regexp {
+	versionRegexpOnce.Do(func() {
+		versionRegexp = regexp.MustCompile("^" + VersionRegexpRaw + "$")
+	})
+	return versionRegexp
+}
+
+func getSemverRegexp() *regexp.Regexp {
+	semverRegexpOnce.Do(func() {
+		semverRegexp = regexp.MustCompile("^" + SemverRegexpRaw + "$")
+	})
+	return semverRegexp
+}
+
 // The raw regular expression string used for testing the validity
 // of a version.
 const (
@@ -41,22 +58,17 @@ type Version struct {
 	original string
 }
 
-func init() {
-	versionRegexp = regexp.MustCompile("^" + VersionRegexpRaw + "$")
-	semverRegexp = regexp.MustCompile("^" + SemverRegexpRaw + "$")
-}
-
 // NewVersion parses the given version and returns a new
 // Version.
 func NewVersion(v string) (*Version, error) {
-	return newVersion(v, versionRegexp)
+	return newVersion(v, getVersionRegexp())
 }
 
 // NewSemver parses the given version and returns a new
 // Version that adheres strictly to SemVer specs
 // https://semver.org/
 func NewSemver(v string) (*Version, error) {
-	return newVersion(v, semverRegexp)
+	return newVersion(v, getSemverRegexp())
 }
 
 func newVersion(v string, pattern *regexp.Regexp) (*Version, error) {