Optimize focus to avoid allocations

Both regex creation and ConcatenatedString() are expensive. Memoize
them within the spec iterator so we don't recalculate them later.

internal/spec/specs.go

@@ -7,14 +7,21 @@ import (
 )
 
 type Specs struct {
-	specs                []*Spec
+	specs []*Spec
+	names []string
+
 	hasProgrammaticFocus bool
 	RegexScansFilePath   bool
 }
 
 func NewSpecs(specs []*Spec) *Specs {
+	names := make([]string, len(specs))
+	for i, spec := range specs {
+		names[i] = spec.ConcatenatedString()
+	}
 	return &Specs{
 		specs: specs,
+		names: names,
 	}
 }
 
@@ -30,10 +37,13 @@ func (e *Specs) Shuffle(r *rand.Rand) {
 	sort.Sort(e)
 	permutation := r.Perm(len(e.specs))
 	shuffledSpecs := make([]*Spec, len(e.specs))
+	names := make([]string, len(e.specs))
 	for i, j := range permutation {
 		shuffledSpecs[i] = e.specs[j]
+		names[i] = e.names[j]
 	}
 	e.specs = shuffledSpecs
+	e.names = names
 }
 
 func (e *Specs) ApplyFocus(description string, focusString string, skipString string) {
@@ -64,33 +74,43 @@ func (e *Specs) applyProgrammaticFocus() {
 
 // toMatch returns a byte[] to be used by regex matchers.  When adding new behaviours to the matching function,
 // this is the place which we append to.
-func (e *Specs) toMatch(description string, spec *Spec) []byte {
+func (e *Specs) toMatch(description string, i int) []byte {
+	if i > len(e.names) {
+		return nil
+	}
 	if e.RegexScansFilePath {
 		return []byte(
 			description + " " +
-				spec.ConcatenatedString() + " " +
-				spec.subject.CodeLocation().FileName)
+				e.names[i] + " " +
+				e.specs[i].subject.CodeLocation().FileName)
 	} else {
 		return []byte(
 			description + " " +
-				spec.ConcatenatedString())
+				e.names[i])
 	}
 }
 
 func (e *Specs) applyRegExpFocusAndSkip(description string, focusString string, skipString string) {
-	for _, spec := range e.specs {
+	var focusFilter *regexp.Regexp
+	if focusString != "" {
+		focusFilter = regexp.MustCompile(focusString)
+	}
+	var skipFilter *regexp.Regexp
+	if skipString != "" {
+		skipFilter = regexp.MustCompile(skipString)
+	}
+
+	for i, spec := range e.specs {
 		matchesFocus := true
 		matchesSkip := false
 
-		toMatch := e.toMatch(description, spec)
+		toMatch := e.toMatch(description, i)
 
-		if focusString != "" {
-			focusFilter := regexp.MustCompile(focusString)
+		if focusFilter != nil {
 			matchesFocus = focusFilter.Match([]byte(toMatch))
 		}
 
-		if skipString != "" {
-			skipFilter := regexp.MustCompile(skipString)
+		if skipFilter != nil {
 			matchesSkip = skipFilter.Match([]byte(toMatch))
 		}
 
@@ -115,9 +135,10 @@ func (e *Specs) Len() int {
 }
 
 func (e *Specs) Less(i, j int) bool {
-	return e.specs[i].ConcatenatedString() < e.specs[j].ConcatenatedString()
+	return e.names[i] < e.names[j]
 }
 
 func (e *Specs) Swap(i, j int) {
+	e.names[i], e.names[j] = e.names[j], e.names[i]
 	e.specs[i], e.specs[j] = e.specs[j], e.specs[i]
 }