From 0eec33f14b3d7e28c0eaa8586c2e8bb22dee034d Mon Sep 17 00:00:00 2001
From: Kevin Wan <wanjunfeng@gmail.com>
Date: Sat, 20 Jul 2024 22:44:13 +0800
Subject: [PATCH] chore: optimize file reading (#4264)

---
 core/filex/file.go      | 27 ++++++++++-----------
 core/filex/file_test.go | 54 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 67 insertions(+), 14 deletions(-)

diff --git a/core/filex/file.go b/core/filex/file.go
index 904accf9..442f6274 100644
--- a/core/filex/file.go
+++ b/core/filex/file.go
@@ -61,25 +61,26 @@ func lastLine(filename string, file *os.File) (string, error) {
 		return "", err
 	}
 
-	bf := int64(bufSize)
 	var last []byte
+	bufLen := int64(bufSize)
 	offset := info.Size()
-	for {
-		if offset < bufSize {
-			bf = offset
+
+	for offset > 0 {
+		if offset < bufLen {
+			bufLen = offset
 			offset = 0
 		} else {
-			offset -= bf
+			offset -= bufLen
 		}
 
-		buf := make([]byte, bf)
+		buf := make([]byte, bufLen)
 		n, err := file.ReadAt(buf, offset)
 		if err != nil && err != io.EOF {
 			return "", err
 		}
 
 		if n == 0 {
-			return "", nil
+			break
 		}
 
 		if buf[n-1] == '\n' {
@@ -89,16 +90,14 @@ func lastLine(filename string, file *os.File) (string, error) {
 			buf = buf[:n]
 		}
 
-		for n--; n >= 0; n-- {
-			if buf[n] == '\n' {
-				return string(append(buf[n+1:], last...)), nil
+		for i := n - 1; i >= 0; i-- {
+			if buf[i] == '\n' {
+				return string(append(buf[i+1:], last...)), nil
 			}
 		}
 
 		last = append(buf, last...)
-
-		if offset == 0 {
-			return string(last), nil
-		}
 	}
+
+	return string(last), nil
 }
diff --git a/core/filex/file_test.go b/core/filex/file_test.go
index f0660d7e..76cc78d0 100644
--- a/core/filex/file_test.go
+++ b/core/filex/file_test.go
@@ -164,3 +164,57 @@ func TestLastLineEmptyFile(t *testing.T) {
 	assert.Nil(t, err)
 	assert.Equal(t, "", val)
 }
+
+func TestFirstLineExactlyBufSize(t *testing.T) {
+	content := make([]byte, bufSize)
+	for i := range content {
+		content[i] = 'a'
+	}
+	content[bufSize-1] = '\n' // Ensure there is a newline at the edge
+
+	filename, err := fs.TempFilenameWithText(string(content))
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := FirstLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, string(content[:bufSize-1]), val)
+}
+
+func TestLastLineExactlyBufSize(t *testing.T) {
+	content := make([]byte, bufSize)
+	for i := range content {
+		content[i] = 'a'
+	}
+	content[bufSize-1] = '\n' // Ensure there is a newline at the edge
+
+	filename, err := fs.TempFilenameWithText(string(content))
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := LastLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, string(content[:bufSize-1]), val)
+}
+
+func TestFirstLineLargeFile(t *testing.T) {
+	content := text + text + text + "\n" + "extra"
+	filename, err := fs.TempFilenameWithText(content)
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := FirstLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, "first line", val)
+}
+
+func TestLastLineLargeFile(t *testing.T) {
+	content := text + text + text + "\n" + "extra"
+	filename, err := fs.TempFilenameWithText(content)
+	assert.Nil(t, err)
+	defer os.Remove(filename)
+
+	val, err := LastLine(filename)
+	assert.Nil(t, err)
+	assert.Equal(t, "extra", val)
+}