feat(parser/diff): limit max diff output in the end

go.mod

@@ -12,6 +12,7 @@ require (
 	github.com/koding/multiconfig v0.0.0-20171124222453-69c27309b2d7
 	github.com/mcuadros/go-defaults v1.2.0
 	github.com/mitchellh/mapstructure v1.5.0
+	github.com/stretchr/testify v1.10.0
 	google.golang.org/grpc v1.73.0
 	google.golang.org/protobuf v1.36.6
 )
@@ -24,6 +25,7 @@ require (
 	github.com/cloudflare/circl v1.6.1 // indirect
 	github.com/cyphar/filepath-securejoin v0.4.1 // indirect
 	github.com/danwakefield/fnmatch v0.0.0-20160403171240-cbb64ac3d964 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/emirpasic/gods v1.18.1 // indirect
 	github.com/fatih/camelcase v1.0.0 // indirect
 	github.com/fatih/structs v1.1.0 // indirect
@@ -33,6 +35,7 @@ require (
 	github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99 // indirect
 	github.com/kevinburke/ssh_config v1.2.0 // indirect
 	github.com/pjbgf/sha1cd v0.3.2 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/sergi/go-diff v1.4.0 // indirect
 	github.com/skeema/knownhosts v1.3.1 // indirect
 	github.com/xanzy/ssh-agent v0.3.3 // indirect
@@ -43,4 +46,5 @@ require (
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20250603155806-513f23925822 // indirect
 	gopkg.in/warnings.v0 v0.1.2 // indirect
 	gopkg.in/yaml.v2 v2.4.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 )

internal/parser/diff/parser.go

@@ -124,16 +124,7 @@ func (d *Diff) generateDiffComment(answerStr, resultStr string, output Output) s
 	if output.MaxDiffLines == 0 { // real default value
 		output.MaxDiffLines = 50
 	}
-
 	truncated := false
-	if len(answerStr) > output.MaxDiffLength {
-		answerStr = answerStr[:output.MaxDiffLength]
-		truncated = true
-	}
-	if len(resultStr) > output.MaxDiffLength {
-		resultStr = resultStr[:output.MaxDiffLength]
-		truncated = true
-	}
 
 	answerLines := strings.Split(answerStr, "\n")
 	resultLines := strings.Split(resultStr, "\n")
@@ -150,8 +141,8 @@ func (d *Diff) generateDiffComment(answerStr, resultStr string, output Output) s
 			); n += 1 {
 		}
 		if n > 0 {
-			answerLines = answerLines[n-1:]
+			answerLines = answerLines[n:]
-			resultLines = resultLines[n-1:]
+			resultLines = resultLines[n:]
 			commonPrefixLineCount = n
 		}
 	}
@@ -173,7 +164,10 @@ func (d *Diff) generateDiffComment(answerStr, resultStr string, output Output) s
 		})
 	diffOutput := diffText(diffs)
 	diffOutput = strings.TrimSuffix(diffOutput, "\n  ")
-
+	if len(diffOutput) > output.MaxDiffLength {
+		diffOutput = diffOutput[:output.MaxDiffLength]
+		truncated = true
+	}
 	if truncated {
 		diffOutput += "\n\n(truncated)"
 	}

internal/parser/diff/parser_test.go

@@ -0,0 +1,67 @@
+package diff
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGenerateDiffComment(t *testing.T) {
+	d := &Diff{}
+
+	t.Run("HideCommonPrefix with common prefix", func(t *testing.T) {
+		answerStr := "line1\nline2\nline3\nline4"
+		resultStr := "line1\nline2\nlineA\nlineB"
+		output := Output{
+			HideCommonPrefix: true,
+		}
+		comment := d.generateDiffComment(answerStr, resultStr, output)
+		expected := "```diff\n(2 line(s) of common prefix hidden)\n\n- line3\n- line4\n+ lineA\n+ lineB\n```\n"
+		assert.Equal(t, expected, comment)
+	})
+
+	t.Run("HideCommonPrefix with no common prefix", func(t *testing.T) {
+		answerStr := "line1\nline2"
+		resultStr := "lineA\nlineB"
+		output := Output{
+			HideCommonPrefix: true,
+		}
+		comment := d.generateDiffComment(answerStr, resultStr, output)
+		expected := "```diff\n- line1\n- line2\n+ lineA\n+ lineB\n```\n"
+		assert.Equal(t, expected, comment)
+	})
+
+	t.Run("HideCommonPrefix with identical content", func(t *testing.T) {
+		answerStr := "line1\nline2"
+		resultStr := "line1\nline2"
+		output := Output{
+			HideCommonPrefix: true,
+		}
+		comment := d.generateDiffComment(answerStr, resultStr, output)
+		expected := "```diff\n(2 line(s) of common prefix hidden)\n\n\n```\n"
+		assert.Equal(t, expected, comment)
+	})
+
+	t.Run("HideCommonPrefix with only common prefix", func(t *testing.T) {
+		answerStr := "line1\nline2"
+		resultStr := "line1\nline2\nlineA"
+		output := Output{
+			HideCommonPrefix: true,
+		}
+		comment := d.generateDiffComment(answerStr, resultStr, output)
+		expected := "```diff\n(2 line(s) of common prefix hidden)\n\n+ lineA\n```\n"
+		assert.Equal(t, expected, comment)
+	})
+
+	t.Run("MaxDiffLines truncation", func(t *testing.T) {
+		answerStr := "line1\nline2\nline3\nline4\nline5\nline6\nline7\nline8"
+		resultStr := "line1\nline2\nline3\nlineA\nlineB\nlineC\nlineD\nlineE"
+		output := Output{
+			MaxDiffLines:     3,
+			HideCommonPrefix: true,
+		}
+		comment := d.generateDiffComment(answerStr, resultStr, output)
+		expected := "```diff\n(3 line(s) of common prefix hidden)\n\n- line4\n- line5\n- line6\n+ lineA\n+ lineB\n+ lineC\n\n(truncated)\n```\n"
+		assert.Equal(t, expected, comment)
+	})
+}