Initial commit of goapidiff

A Go tool that analyzes API changes between two versions and reports breaking changes.

Features:
- Detects breaking API changes in Go code
- Compares git refs or directories
- Ignores internal/ packages by default
- Multiple output formats: text, JSON, markdown
- CI-friendly exit codes

Built on top of golang.org/x/exp/apidiff which provides the core API analysis.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

testdata/break/new/example.go

@@ -0,0 +1,45 @@
+package example
+
+import "fmt"
+
+// Greeter provides greeting functionality
+type Greeter struct {
+	Name     string
+	Language string // Added field - compatible change
+}
+
+// Greet returns a greeting message
+// Breaking change: added parameter
+func (g *Greeter) Greet(formal bool) string {
+	if formal {
+		return fmt.Sprintf("Good day, %s!", g.Name)
+	}
+	return fmt.Sprintf("Hello, %s!", g.Name)
+}
+
+// Add adds two integers
+func Add(a, b int) int {
+	return a + b
+}
+
+// Multiply multiplies two integers (new function - compatible)
+func Multiply(a, b int) int {
+	return a * b
+}
+
+// Config holds configuration
+type Config struct {
+	Host    string
+	Port    int
+	Timeout int // Added field - compatible change
+}
+
+// Status represents service status
+type Status int
+
+const (
+	StatusUnknown Status = iota
+	StatusReady
+	StatusError
+	StatusStarting // Added constant - compatible change
+)

testdata/break/old/example.go

@@ -0,0 +1,33 @@
+package example
+
+import "fmt"
+
+// Greeter provides greeting functionality
+type Greeter struct {
+	Name string
+}
+
+// Greet returns a greeting message
+func (g *Greeter) Greet() string {
+	return fmt.Sprintf("Hello, %s!", g.Name)
+}
+
+// Add adds two integers
+func Add(a, b int) int {
+	return a + b
+}
+
+// Config holds configuration
+type Config struct {
+	Host string
+	Port int
+}
+
+// Status represents service status
+type Status int
+
+const (
+	StatusUnknown Status = iota
+	StatusReady
+	StatusError
+)

testdata/clean/new/math.go

@@ -0,0 +1,41 @@
+package math
+
+// Add adds two integers
+func Add(a, b int) int {
+	// Implementation changed but API is the same
+	result := a + b
+	return result
+}
+
+// Subtract subtracts b from a
+func Subtract(a, b int) int {
+	// Refactored implementation
+	diff := a - b
+	return diff
+}
+
+// Calculator provides basic math operations
+type Calculator struct {
+	precision int
+}
+
+// NewCalculator creates a new calculator
+func NewCalculator() *Calculator {
+	// Changed internal default but API is the same
+	return &Calculator{precision: 4}
+}
+
+// Compute performs a calculation
+func (c *Calculator) Compute(operation string, a, b float64) float64 {
+	// Refactored with better performance
+	var result float64
+	switch operation {
+	case "add":
+		result = a + b
+	case "subtract":
+		result = a - b
+	default:
+		result = 0
+	}
+	return result
+}

testdata/clean/old/math.go

@@ -0,0 +1,33 @@
+package math
+
+// Add adds two integers
+func Add(a, b int) int {
+	return a + b
+}
+
+// Subtract subtracts b from a
+func Subtract(a, b int) int {
+	return a - b
+}
+
+// Calculator provides basic math operations
+type Calculator struct {
+	precision int
+}
+
+// NewCalculator creates a new calculator
+func NewCalculator() *Calculator {
+	return &Calculator{precision: 2}
+}
+
+// Compute performs a calculation
+func (c *Calculator) Compute(operation string, a, b float64) float64 {
+	switch operation {
+	case "add":
+		return a + b
+	case "subtract":
+		return a - b
+	default:
+		return 0
+	}
+}

testdata/internal/new/internal/helper.go

@@ -0,0 +1,19 @@
+package internal
+
+// Helper is an internal helper function
+// BREAKING: Changed return type (but should be ignored)
+func Helper(s string) (string, error) {
+	return s + "!", nil
+}
+
+// InternalType is an internal type
+// BREAKING: Removed field (but should be ignored)
+type InternalType struct {
+	NewValue string
+}
+
+// Process does internal processing
+// BREAKING: Changed signature (but should be ignored)
+func (i *InternalType) Process(multiplier int) string {
+	return i.NewValue
+}

testdata/internal/new/public.go

@@ -0,0 +1,16 @@
+package example
+
+// PublicFunc is a public function
+// BREAKING: Added required parameter
+func PublicFunc(s string, uppercase bool) string {
+	if uppercase {
+		return s
+	}
+	return s
+}
+
+// PublicType is a public type
+type PublicType struct {
+	Name  string
+	Email string // Added field - compatible
+}

testdata/internal/old/internal/helper.go

@@ -0,0 +1,16 @@
+package internal
+
+// Helper is an internal helper function
+func Helper(s string) string {
+	return s + "!"
+}
+
+// InternalType is an internal type
+type InternalType struct {
+	Value int
+}
+
+// Process does internal processing
+func (i *InternalType) Process() int {
+	return i.Value * 2
+}

testdata/internal/old/public.go

@@ -0,0 +1,11 @@
+package example
+
+// PublicFunc is a public function
+func PublicFunc(s string) string {
+	return s
+}
+
+// PublicType is a public type
+type PublicType struct {
+	Name string
+}

testdata/safe/new/api.go

@@ -0,0 +1,78 @@
+package api
+
+import "time"
+
+// User represents a user in the system
+type User struct {
+	ID        string
+	Name      string
+	Email     string    // Added field - compatible change
+	CreatedAt time.Time // Added field - compatible change
+}
+
+// GetUser retrieves a user by ID
+func GetUser(id string) (*User, error) {
+	// Mock implementation
+	return &User{ID: id, Name: "John Doe"}, nil
+}
+
+// GetUsers retrieves all users (new function - compatible)
+func GetUsers() ([]*User, error) {
+	return []*User{}, nil
+}
+
+// UpdateUser updates a user (new function - compatible)
+func UpdateUser(user *User) error {
+	return nil
+}
+
+// Status represents the service status
+type Status string
+
+const (
+	StatusOK      Status = "ok"
+	StatusError   Status = "error"
+	StatusPending Status = "pending" // Added constant - compatible change
+	StatusUnknown Status = "unknown" // Added constant - compatible change
+)
+
+// Service provides API functionality
+type Service struct {
+	endpoint string
+	timeout  time.Duration // Added field - compatible change
+}
+
+// NewService creates a new service
+func NewService(endpoint string) *Service {
+	return &Service{endpoint: endpoint}
+}
+
+// NewServiceWithTimeout creates a new service with timeout (new function - compatible)
+func NewServiceWithTimeout(endpoint string, timeout time.Duration) *Service {
+	return &Service{endpoint: endpoint, timeout: timeout}
+}
+
+// Call makes an API call
+func (s *Service) Call(method string) error {
+	// Mock implementation
+	return nil
+}
+
+// CallWithContext makes an API call with context (new method - compatible)
+func (s *Service) CallWithContext(method string, timeout time.Duration) error {
+	return nil
+}
+
+// GetStatus returns the service status (new method - compatible)
+func (s *Service) GetStatus() Status {
+	return StatusOK
+}
+
+// ErrorCode represents error codes (new type - compatible)
+type ErrorCode int
+
+const (
+	ErrNone    ErrorCode = 0 // New constant - compatible
+	ErrInvalid ErrorCode = 1 // New constant - compatible
+	ErrTimeout ErrorCode = 2 // New constant - compatible
+)

testdata/safe/old/api.go

@@ -0,0 +1,37 @@
+package api
+
+// User represents a user in the system
+type User struct {
+	ID   string
+	Name string
+}
+
+// GetUser retrieves a user by ID
+func GetUser(id string) (*User, error) {
+	// Mock implementation
+	return &User{ID: id, Name: "John Doe"}, nil
+}
+
+// Status represents the service status
+type Status string
+
+const (
+	StatusOK    Status = "ok"
+	StatusError Status = "error"
+)
+
+// Service provides API functionality
+type Service struct {
+	endpoint string
+}
+
+// NewService creates a new service
+func NewService(endpoint string) *Service {
+	return &Service{endpoint: endpoint}
+}
+
+// Call makes an API call
+func (s *Service) Call(method string) error {
+	// Mock implementation
+	return nil
+}