From 8ffae86ca4ddec375d07cd0f2b7ea9d96ea1d5ef Mon Sep 17 00:00:00 2001
From: Cameron Clark <cameron.clark@hey.com>
Date: Mon, 11 Aug 2025 18:05:55 +0100
Subject: [PATCH 1/3] perf(tspath): optimize the fast path case for
 `ToFileNameLowerCase`

---
 internal/tspath/path.go | 35 ++++++++++++++++++++++++++++++++---
 1 file changed, 32 insertions(+), 3 deletions(-)

diff --git a/internal/tspath/path.go b/internal/tspath/path.go
index a0d722b233..4813543a5c 100644
--- a/internal/tspath/path.go
+++ b/internal/tspath/path.go
@@ -533,13 +533,42 @@ func GetCanonicalFileName(fileName string, useCaseSensitiveFileNames bool) strin
 // Rest special characters are either already in lower case format or
 // they have corresponding upper case character so they dont need special handling
 
-func ToFileNameLowerCase(fileName string) string {
+func ToFileNameLowerCase(s string) string {
+	const IWithDot = '\u0130'
+
+	ascii := true
+	needsLower := false
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= 0x80 {
+			ascii = false
+			break
+		}
+		if 'A' <= c && c <= 'Z' {
+			needsLower = true
+		}
+	}
+	if ascii {
+		if !needsLower {
+			return s
+		}
+		b := make([]byte, len(s))
+		for i := 0; i < len(s); i++ {
+			c := s[i]
+			if 'A' <= c && c <= 'Z' {
+				c += 'a' - 'A' // +32
+			}
+			b[i] = c
+		}
+		return string(b)
+	}
+
 	return strings.Map(func(r rune) rune {
-		if r == '\u0130' {
+		if r == IWithDot {
 			return r
 		}
 		return unicode.ToLower(r)
-	}, fileName)
+	}, s)
 }
 
 func ToPath(fileName string, basePath string, useCaseSensitiveFileNames bool) Path {

From 2462166aa1750d082e3737c0d647d8caf965c91d Mon Sep 17 00:00:00 2001
From: Cameron Clark <cameron.clark@hey.com>
Date: Mon, 11 Aug 2025 23:52:40 +0100
Subject: [PATCH 2/3] lint

---
 internal/tspath/path.go | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/internal/tspath/path.go b/internal/tspath/path.go
index 4813543a5c..1bb78533a3 100644
--- a/internal/tspath/path.go
+++ b/internal/tspath/path.go
@@ -538,8 +538,7 @@ func ToFileNameLowerCase(s string) string {
 
 	ascii := true
 	needsLower := false
-	for i := 0; i < len(s); i++ {
-		c := s[i]
+	for _, c := range []byte(s) {
 		if c >= 0x80 {
 			ascii = false
 			break
@@ -553,8 +552,7 @@ func ToFileNameLowerCase(s string) string {
 			return s
 		}
 		b := make([]byte, len(s))
-		for i := 0; i < len(s); i++ {
-			c := s[i]
+		for i, c := range []byte(s) {
 			if 'A' <= c && c <= 'Z' {
 				c += 'a' - 'A' // +32
 			}

From dcb78e8afda3c11175a704b9a3194029cd125f7e Mon Sep 17 00:00:00 2001
From: Cameron Clark <cameron.clark@hey.com>
Date: Tue, 12 Aug 2025 18:50:54 +0100
Subject: [PATCH 3/3] rename fn arg from `s` to `fileName`

---
 internal/tspath/path.go | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/internal/tspath/path.go b/internal/tspath/path.go
index 1bb78533a3..d5fe169564 100644
--- a/internal/tspath/path.go
+++ b/internal/tspath/path.go
@@ -533,12 +533,12 @@ func GetCanonicalFileName(fileName string, useCaseSensitiveFileNames bool) strin
 // Rest special characters are either already in lower case format or
 // they have corresponding upper case character so they dont need special handling
 
-func ToFileNameLowerCase(s string) string {
+func ToFileNameLowerCase(fileName string) string {
 	const IWithDot = '\u0130'
 
 	ascii := true
 	needsLower := false
-	for _, c := range []byte(s) {
+	for _, c := range []byte(fileName) {
 		if c >= 0x80 {
 			ascii = false
 			break
@@ -549,10 +549,10 @@ func ToFileNameLowerCase(s string) string {
 	}
 	if ascii {
 		if !needsLower {
-			return s
+			return fileName
 		}
-		b := make([]byte, len(s))
-		for i, c := range []byte(s) {
+		b := make([]byte, len(fileName))
+		for i, c := range []byte(fileName) {
 			if 'A' <= c && c <= 'Z' {
 				c += 'a' - 'A' // +32
 			}
@@ -566,7 +566,7 @@ func ToFileNameLowerCase(s string) string {
 			return r
 		}
 		return unicode.ToLower(r)
-	}, s)
+	}, fileName)
 }
 
 func ToPath(fileName string, basePath string, useCaseSensitiveFileNames bool) Path {