Add testable example of diffing

Signed-off-by: Andrew Z Allen <me@andrewzallen.com>

Author: achew22

example_test.go

@@ -6,17 +6,74 @@ import (
 	"io/ioutil"
 	"log"
 	"os"
+	"path"
 	"path/filepath"
+	"strconv"
+	"testing"
 
-	"gopkg.in/src-d/go-git.v4"
+	"github.com/src-d/go-git/plumbing/object"
+	git "gopkg.in/src-d/go-git.v4"
 	"gopkg.in/src-d/go-git.v4/config"
 	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/transport/http"
 	"gopkg.in/src-d/go-git.v4/storage/memory"
+	"gopkg.in/src-d/go-git.v4/utils/merkletrie"
 
 	"gopkg.in/src-d/go-billy.v4/memfs"
 )
 
+// repository is a boilerplate sample repository (go-git itself).
+var repository *git.Repository
+
+func init() {
+	cwd, err := os.Getwd()
+	if err == nil {
+		for true {
+			files, err := ioutil.ReadDir(cwd)
+			if err != nil {
+				break
+			}
+			found := false
+			for _, f := range files {
+				if f.Name() == "README.md" {
+					found = true
+					break
+				}
+			}
+			if found {
+				break
+			}
+			oldCwd := cwd
+			cwd = path.Dir(cwd)
+			if oldCwd == cwd {
+				break
+			}
+		}
+		Repository, err = git.PlainOpen(cwd)
+		if err == nil {
+			iter, err := Repository.CommitObjects()
+			if err == nil {
+				commits := -1
+				for ; err != io.EOF; _, err = iter.Next() {
+					if err != nil {
+						panic(err)
+					}
+					commits++
+					if commits >= 100 {
+						return
+					}
+				}
+			}
+		}
+	}
+	Repository, err = git.Clone(memory.NewStorage(), nil, &git.CloneOptions{
+		URL: "https://github.com/src-d/go-git",
+	})
+	if err != nil {
+		panic(err)
+	}
+}
+
 func ExampleClone() {
 	// Filesystem abstraction based on memory
 	fs := memfs.New()
@@ -164,3 +221,78 @@ func ExampleRepository_CreateRemote() {
 	// example https://github.com/git-fixtures/basic.git (fetch)
 	// example https://github.com/git-fixtures/basic.git (push)
 }
+
+func ExampleDiffRepo(t *testing.T) {
+	var hash plumbing.Hash
+
+	prevHash := plumbing.NewHash("5fddbeb678bd2c36c5e5c891ab8f2b143ced5baf")
+	hash := plumbing.NewHash("c088fd6a7e1a38e9d5a9815265cb575bb08d08ff")
+
+	prevCommit, err := repo.CommitObject(prevHash)
+	if err != nil {
+		t.Fatalf("repo.CommitObject(%q) error: %v", prevHash, err)
+	}
+
+	commit, err := repo.CommitObject(hash)
+	if err != nil {
+		t.Fatalf("repo.CommitObject(%q) error: %v", hash, err)
+	}
+
+	fmt.Println("Comparing from:" + prevCommit.Hash.String() + " to:" + commit.Hash.String())
+
+	isAncestor, err := commit.IsAncestor(prevCommit)
+	if err != nil {
+		t.Fatalf("commit.IsAncestor(%q) error: %v", prevCommit, err)
+	}
+
+	fmt.Printf("Is the prevCommit an ancestor of commit? : %v %v\n", isAncestor)
+
+	currentTree, err := commit.Tree()
+	if err != nil {
+		t.Errorf("commit.Tree() error: %v", err)
+	}
+
+	prevTree, err := prevCommit.Tree()
+	if err != nil {
+		t.Errorf("prevCommit.Tree() error: %v", err)
+	}
+
+	patch, err := currentTree.Patch(prevTree)
+	if err != nil {
+		t.Errorf("currentTree.Patch(%q) error: %v", prevTree, err)
+	}
+	fmt.Println("Got here" + strconv.Itoa(len(patch.Stats())))
+
+	var changedFiles []string
+	for _, fileStat := range patch.Stats() {
+		fmt.Println(fileStat.Name)
+		changedFiles = append(changedFiles, fileStat.Name)
+	}
+
+	changes, err := currentTree.Diff(prevTree)
+	if err != nil {
+		t.Errorf("currentTree.Diff(%v) error: %v", prevTree, err)
+	}
+
+	fmt.Println("Got here!")
+	for _, change := range changes {
+		// Ignore deleted files
+		action, err := change.Action()
+		if err != nil {
+			t.Errorf("change.Action() error: %v", err)
+		}
+		if action == merkletrie.Delete {
+			fmt.Println("Skipping delete")
+			continue
+		}
+
+		// Get list of involved files
+		name := change.To.Name
+
+		var empty = object.ChangeEntry{}
+		if change.From != empty {
+			name = change.From.Name
+		}
+		fmt.Println(name)
+	}
+}