filebeat: minor cleanup and performance improvement (#37644)

* filebeat/generator/fields: replace for loop with single instruction

Improve performance by replacing a loop with a single instruction.

Signed-off-by: Florian Lehner <florian.lehner@elastic.co>

* filebeat/fileset/flags: remove duplicate import

Simplify code by replacing a duplicate import.

Signed-off-by: Florian Lehner <florian.lehner@elastic.co>

* filebeat/generator/fields: precompile regex

Precompile the fixed regex once instead for each iteration.

Signed-off-by: Florian Lehner <florian.lehner@elastic.co>

---------

Signed-off-by: Florian Lehner <florian.lehner@elastic.co>
Co-authored-by: Denis <denis.rechkunov@elastic.co>

filebeat/fileset/flags.go

@@ -23,7 +23,6 @@ import (
 	"strings"
 
 	"github.com/elastic/elastic-agent-libs/config"
-	conf "github.com/elastic/elastic-agent-libs/config"
 )
 
 // Modules related command line flags.
@@ -32,11 +31,11 @@ var (
 	moduleOverrides = config.SettingFlag(nil, "M", "Module configuration overwrite")
 )
 
-type ModuleOverrides map[string]map[string]*conf.C // module -> fileset -> Config
+type ModuleOverrides map[string]map[string]*config.C // module -> fileset -> Config
 
 // Get returns an array of configuration overrides that should be merged in order.
-func (mo *ModuleOverrides) Get(module, fileset string) []*conf.C {
-	ret := []*conf.C{}
+func (mo *ModuleOverrides) Get(module, fileset string) []*config.C {
+	ret := []*config.C{}
 
 	moduleWildcard := (*mo)["*"]["*"]
 	if moduleWildcard != nil {

filebeat/generator/fields/fields.go

@@ -179,15 +179,12 @@ func addNewField(fs []field, f field) []field {
 	return append(fs, f)
 }
 
-func getSemanticElementsFromPatterns(patterns []string) ([]field, error) {
-	r, err := regexp.Compile("{[\\.\\w\\:]*}")
-	if err != nil {
-		return nil, err
-	}
+var semanticElementsRegex = regexp.MustCompile(`{[\.\w\:]*}`)
 
+func getSemanticElementsFromPatterns(patterns []string) ([]field, error) {
 	var fs []field
 	for _, lp := range patterns {
-		pp := r.FindAllString(lp, -1)
+		pp := semanticElementsRegex.FindAllString(lp, -1)
 		for _, p := range pp {
 			f := newField(p)
 			if f.SemanticElements == nil {
@@ -221,9 +218,7 @@ func accumulateRemoveFields(remove interface{}, out []string) []string {
 			case string:
 				return append(out, vs)
 			case []string:
-				for _, vv := range vs {
-					out = append(out, vv)
-				}
+				out = append(out, vs...)
 			case []interface{}:
 				for _, vv := range vs {
 					vvs := vv.(string)