danger-js: Add DiffForFile method with comprehensive diff parsing

Add DiffForFile method to Git struct that executes git diff for a specific file.
Add FileDiff and DiffLine types to represent parsed diff content.
Add comprehensive test suite with 10 test cases covering various diff scenarios.

AI::Created

Author: echarrod

danger-js/types_danger.go

@@ -1,5 +1,12 @@
 package dangerJs
 
+import (
+	"bytes"
+	"os/exec"
+	"regexp"
+	"strings"
+)
+
 type DSL struct {
 	Git    Git    `json:"git"`
 	GitHub GitHub `json:"github,omitempty"`
@@ -18,6 +25,46 @@ type Git struct {
 	Commits       []GitCommit `json:"commits"`
 }
 
+// FileDiff represents the changes in a file.
+type FileDiff struct {
+	AddedLines   []DiffLine
+	RemovedLines []DiffLine
+}
+
+// DiffLine represents a single line in a file diff.
+type DiffLine struct {
+	Content string
+	Line    int
+}
+
+// DiffForFile executes a git diff command for a specific file and parses its output.
+func (g Git) DiffForFile(filePath string) (FileDiff, error) {
+	cmd := exec.Command("git", "diff", "--unified=0", "HEAD^", "HEAD", filePath)
+	var out bytes.Buffer
+	cmd.Stdout = &out
+	err := cmd.Run()
+	if err != nil {
+		return FileDiff{}, err
+	}
+
+	diffContent := out.String()
+	var fileDiff FileDiff
+	// Only match lines that start with + or - but not +++ or --- (file headers)
+	addedRe := regexp.MustCompile(`^\+([^+].*|$)`)
+	removedRe := regexp.MustCompile(`^-([^-].*|$)`)
+
+	lines := strings.Split(diffContent, "\n")
+	for _, line := range lines {
+		if matches := addedRe.FindStringSubmatch(line); len(matches) > 1 {
+			fileDiff.AddedLines = append(fileDiff.AddedLines, DiffLine{Content: matches[1]})
+		} else if matches := removedRe.FindStringSubmatch(line); len(matches) > 1 {
+			fileDiff.RemovedLines = append(fileDiff.RemovedLines, DiffLine{Content: matches[1]})
+		}
+	}
+
+	return fileDiff, nil
+}
+
 type Settings struct {
 	GitHub struct {
 		AccessToken       string `json:"accessToken"`

danger-js/types_danger_test.go

@@ -0,0 +1,234 @@
+package dangerJs
+
+import (
+	"regexp"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// parseDiffContent extracts the diff parsing logic for testing
+func parseDiffContent(diffContent string) FileDiff {
+	var fileDiff FileDiff
+	// Only match lines that start with + or - but not +++ or --- (file headers)
+	addedRe := regexp.MustCompile(`^\+([^+].*|$)`)
+	removedRe := regexp.MustCompile(`^-([^-].*|$)`)
+
+	lines := strings.Split(diffContent, "\n")
+	for _, line := range lines {
+		if matches := addedRe.FindStringSubmatch(line); len(matches) > 1 {
+			fileDiff.AddedLines = append(fileDiff.AddedLines, DiffLine{Content: matches[1]})
+		} else if matches := removedRe.FindStringSubmatch(line); len(matches) > 1 {
+			fileDiff.RemovedLines = append(fileDiff.RemovedLines, DiffLine{Content: matches[1]})
+		}
+	}
+
+	return fileDiff
+}
+
+func TestParseDiffContent(t *testing.T) {
+	tests := []struct {
+		name          string
+		gitDiffOutput string
+		wantFileDiff  FileDiff
+	}{
+		{
+			name: "basic added and removed lines",
+			gitDiffOutput: `diff --git a/test.go b/test.go
+index 123..456 100644
+--- a/test.go
++++ b/test.go
+@@ -1 +1 @@
+-func oldFunction() {
++func newFunction() {
+@@ -5 +5,2 @@
++	return "added line"
+-	fmt.Println("removed line")`,
+			wantFileDiff: FileDiff{
+				AddedLines: []DiffLine{
+					{Content: "func newFunction() {", Line: 0},
+					{Content: "\treturn \"added line\"", Line: 0},
+				},
+				RemovedLines: []DiffLine{
+					{Content: "func oldFunction() {", Line: 0},
+					{Content: "\tfmt.Println(\"removed line\")", Line: 0},
+				},
+			},
+		},
+		{
+			name: "only added lines",
+			gitDiffOutput: `diff --git a/new.go b/new.go
+index 123..456 100644
+--- a/new.go
++++ b/new.go
+@@ -1,0 +1,3 @@
++package main
++
++func main() {}`,
+			wantFileDiff: FileDiff{
+				AddedLines: []DiffLine{
+					{Content: "package main", Line: 0},
+					{Content: "", Line: 0},
+					{Content: "func main() {}", Line: 0},
+				},
+				RemovedLines: nil,
+			},
+		},
+		{
+			name: "only removed lines",
+			gitDiffOutput: `diff --git a/old.go b/old.go
+index 123..456 100644
+--- a/old.go
++++ b/old.go
+@@ -1,3 +0,0 @@
+-package main
+-
+-func old() {}`,
+			wantFileDiff: FileDiff{
+				AddedLines: nil,
+				RemovedLines: []DiffLine{
+					{Content: "package main", Line: 0},
+					{Content: "", Line: 0},
+					{Content: "func old() {}", Line: 0},
+				},
+			},
+		},
+		{
+			name:          "no changes",
+			gitDiffOutput: ``,
+			wantFileDiff: FileDiff{
+				AddedLines:   nil,
+				RemovedLines: nil,
+			},
+		},
+		{
+			name: "complex diff with context lines",
+			gitDiffOutput: `diff --git a/complex.go b/complex.go
+index 123..456 100644
+--- a/complex.go
++++ b/complex.go
+@@ -10,5 +10,6 @@
+ 	unchanged line 1
+ 	unchanged line 2
+-	old implementation
++	new implementation
++	additional line
+ 	unchanged line 3`,
+			wantFileDiff: FileDiff{
+				AddedLines: []DiffLine{
+					{Content: "\tnew implementation", Line: 0},
+					{Content: "\tadditional line", Line: 0},
+				},
+				RemovedLines: []DiffLine{
+					{Content: "\told implementation", Line: 0},
+				},
+			},
+		},
+		{
+			name: "lines with special characters and whitespace",
+			gitDiffOutput: `diff --git a/special.go b/special.go
+index 123..456 100644
+--- a/special.go
++++ b/special.go
+@@ -1,2 +1,2 @@
+-	fmt.Printf("Hello %s\n", name)
++	fmt.Printf("Hi %s!\n", name)`,
+			wantFileDiff: FileDiff{
+				AddedLines: []DiffLine{
+					{Content: "\tfmt.Printf(\"Hi %s!\\n\", name)", Line: 0},
+				},
+				RemovedLines: []DiffLine{
+					{Content: "\tfmt.Printf(\"Hello %s\\n\", name)", Line: 0},
+				},
+			},
+		},
+		{
+			name: "lines with only symbols",
+			gitDiffOutput: `diff --git a/symbols.go b/symbols.go
+index 123..456 100644
+--- a/symbols.go
++++ b/symbols.go
+@@ -1,2 +1,2 @@
+-}
++},`,
+			wantFileDiff: FileDiff{
+				AddedLines: []DiffLine{
+					{Content: "},", Line: 0},
+				},
+				RemovedLines: []DiffLine{
+					{Content: "}", Line: 0},
+				},
+			},
+		},
+		{
+			name: "empty added and removed lines",
+			gitDiffOutput: `diff --git a/empty.go b/empty.go
+index 123..456 100644
+--- a/empty.go
++++ b/empty.go
+@@ -1,2 +1,2 @@
+-
++
+- 
++ `,
+			wantFileDiff: FileDiff{
+				AddedLines: []DiffLine{
+					{Content: "", Line: 0},
+					{Content: " ", Line: 0},
+				},
+				RemovedLines: []DiffLine{
+					{Content: "", Line: 0},
+					{Content: " ", Line: 0},
+				},
+			},
+		},
+		{
+			name: "diff with file headers only",
+			gitDiffOutput: `diff --git a/test.go b/test.go
+index 123..456 100644
+--- a/test.go
++++ b/test.go`,
+			wantFileDiff: FileDiff{
+				AddedLines:   nil,
+				RemovedLines: nil,
+			},
+		},
+		{
+			name: "multiple hunks with mixed changes",
+			gitDiffOutput: `diff --git a/multi.go b/multi.go
+index 123..456 100644
+--- a/multi.go
++++ b/multi.go
+@@ -1,3 +1,4 @@
+ package main
+ 
++import "fmt"
+ func main() {
+@@ -10,6 +11,7 @@
+ 	if true {
+-		fmt.Println("old")
++		fmt.Println("new")
++		fmt.Println("extra")
+ 	}
+ }`,
+			wantFileDiff: FileDiff{
+				AddedLines: []DiffLine{
+					{Content: "import \"fmt\"", Line: 0},
+					{Content: "\t\tfmt.Println(\"new\")", Line: 0},
+					{Content: "\t\tfmt.Println(\"extra\")", Line: 0},
+				},
+				RemovedLines: []DiffLine{
+					{Content: "\t\tfmt.Println(\"old\")", Line: 0},
+				},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotFileDiff := parseDiffContent(tt.gitDiffOutput)
+			require.Equal(t, tt.wantFileDiff, gotFileDiff)
+		})
+	}
+}