feat: implement method for reading file line at a time

internal/file/file.go

@@ -3,13 +3,32 @@ package file
 
 import (
 	"bufio"
+	"iter"
 	"os"
 )
 
-// TODO: Implement a StreamLines method
-// in the file package
+func ReadLines(filePath string) iter.Seq[string] {
+	return func(yield func(string) bool) {
+		file, err := os.Open(filePath)
 
-func ReadLines(filePath string) []string {
+		if err != nil {
+			yield("")
+			return
+		}
+
+		defer file.Close()
+
+		scanner := bufio.NewScanner(file)
+
+		for scanner.Scan() {
+			if !yield(scanner.Text()) {
+				return
+			}
+		}
+	}
+}
+
+func ReadAllLines(filePath string) []string {
 	lines := []string{}
 
 	file, openErr := os.Open(filePath)
@@ -20,7 +39,7 @@ func ReadLines(filePath string) []string {
 
 	scanner := bufio.NewScanner(file)
 
-	for hasLine := scanner.Scan(); hasLine; hasLine = scanner.Scan() {
+	for scanner.Scan() {
 		line := scanner.Text()
 		lines = append(lines, line)
 	}

internal/file/file_test.go

@@ -7,13 +7,13 @@ import (
 	"github.com/StevanFreeborn/advent-of-code-2025/internal/file"
 )
 
-func TestReadLines(t *testing.T) {
+func TestAllReadLines(t *testing.T) {
 	expectedLines := []string{
 		"hello",
 		"world",
 	}
 
-	result := file.ReadLines("test.txt")
+	result := file.ReadAllLines("test.txt")
 
 	if slices.Equal(result, expectedLines) == false {
 		t.Errorf("got %s but expected %s", result, expectedLines)
@@ -29,3 +29,55 @@ func TestReadAllText(t *testing.T) {
 		t.Errorf("got %q but expected %q", result, expectedText)
 	}
 }
+
+func TestReadLines(t *testing.T) {
+	t.Run("it should return an empty line if the file does not exist", func(t *testing.T) {
+		expectedLines := []string{""}
+		lines := []string{}
+
+		for line := range file.ReadLines("non-existent-file.txt") {
+			lines = append(lines, line)
+		}
+
+		if slices.Equal(lines, expectedLines) == false {
+			t.Errorf("got %s but expected %s", lines, expectedLines)
+		}
+	})
+
+	t.Run("it should read all lines from the file", func(t *testing.T) {
+		expectedLines := []string{
+			"hello",
+			"world",
+		}
+
+		lines := []string{}
+
+		for line := range file.ReadLines("test.txt") {
+			lines = append(lines, line)
+		}
+
+		if slices.Equal(lines, expectedLines) == false {
+			t.Errorf("got %s but expected %s", lines, expectedLines)
+		}
+	})
+
+	t.Run("it should stop reading lines when iteration is stopped", func(t *testing.T) {
+		expectedLines := []string{
+			"hello",
+		}
+
+		lines := []string{}
+
+		for line := range file.ReadLines("test.txt") {
+			if len(lines) >= 1 {
+				break
+			}
+
+			lines = append(lines, line)
+		}
+
+		if slices.Equal(lines, expectedLines) == false {
+			t.Errorf("got %s but expected %s", lines, expectedLines)
+		}
+	})
+}