test/reverse: cover Git-derived reversal semantics

diff/reverse_git_test.go

@@ -0,0 +1,215 @@
+package diff
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+// These fixtures are reduced from Git output. They exercise complete parsed
+// FileDiffs rather than individual extended-header rewrites.
+func TestReverseFileDiffGitSemantics(t *testing.T) {
+	tests := []struct {
+		name    string
+		forward string
+		reverse string
+		crlf    bool
+	}{
+		{
+			name: "addition",
+			forward: "diff --git a/added.txt b/added.txt\n" +
+				"new file mode 100644\n" +
+				"index 0000000..76d4bb8\n" +
+				"--- /dev/null\n" +
+				"+++ b/added.txt\n" +
+				"@@ -0,0 +1 @@\n" +
+				"+add\n",
+			reverse: "diff --git b/added.txt a/added.txt\n" +
+				"deleted file mode 100644\n" +
+				"index 76d4bb8..0000000\n" +
+				"--- b/added.txt\n" +
+				"+++ /dev/null\n" +
+				"@@ -1 +0,0 @@\n" +
+				"-add\n",
+		},
+		{
+			name: "deletion",
+			forward: "diff --git a/deleted.txt b/deleted.txt\n" +
+				"deleted file mode 100644\n" +
+				"index c8b1b42..0000000\n" +
+				"--- a/deleted.txt\n" +
+				"+++ /dev/null\n" +
+				"@@ -1 +0,0 @@\n" +
+				"-delete\n",
+			reverse: "diff --git b/deleted.txt a/deleted.txt\n" +
+				"new file mode 100644\n" +
+				"index 0000000..c8b1b42\n" +
+				"--- /dev/null\n" +
+				"+++ a/deleted.txt\n" +
+				"@@ -0,0 +1 @@\n" +
+				"+delete\n",
+		},
+		{
+			name: "rename",
+			forward: "diff --git a/old.txt b/renamed.txt\n" +
+				"similarity index 100%\n" +
+				"rename from old.txt\n" +
+				"rename to renamed.txt\n",
+			reverse: "diff --git b/renamed.txt a/old.txt\n" +
+				"similarity index 100%\n" +
+				"rename from renamed.txt\n" +
+				"rename to old.txt\n",
+		},
+		{
+			name: "mode only",
+			forward: "diff --git a/mode.txt b/mode.txt\n" +
+				"old mode 100644\n" +
+				"new mode 100755\n",
+			reverse: "diff --git b/mode.txt a/mode.txt\n" +
+				"old mode 100755\n" +
+				"new mode 100644\n",
+		},
+		{
+			name: "quoted path",
+			forward: "diff --git \"a/quote\\\\tname.txt\" \"b/quote\\\\tname.txt\"\n" +
+				"index dcae08f..b46c5e4 100644\n" +
+				"--- \"a/quote\\\\tname.txt\"\n" +
+				"+++ \"b/quote\\\\tname.txt\"\n" +
+				"@@ -1 +1 @@\n" +
+				"-quoted old\n" +
+				"+quoted new\n",
+			reverse: "diff --git \"b/quote\\\\tname.txt\" \"a/quote\\\\tname.txt\"\n" +
+				"index b46c5e4..dcae08f 100644\n" +
+				"--- \"b/quote\\\\tname.txt\"\n" +
+				"+++ \"a/quote\\\\tname.txt\"\n" +
+				"@@ -1 +1 @@\n" +
+				"-quoted new\n" +
+				"+quoted old\n",
+		},
+		{
+			name: "CRLF",
+			forward: "diff --git a/crlf.txt b/crlf.txt\n" +
+				"index 9cca7e3..d2eb92c 100644\n" +
+				"--- a/crlf.txt\n" +
+				"+++ b/crlf.txt\n" +
+				"@@ -1 +1 @@\n" +
+				"-old\n" +
+				"+new\n",
+			reverse: "diff --git b/crlf.txt a/crlf.txt\n" +
+				"index d2eb92c..9cca7e3 100644\n" +
+				"--- b/crlf.txt\n" +
+				"+++ a/crlf.txt\n" +
+				"@@ -1 +1 @@\n" +
+				"-new\n" +
+				"+old\n",
+			crlf: true,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			assertGitReverse(t, test.forward, test.reverse, test.crlf)
+		})
+	}
+}
+
+// Git binary patches contain both directions. Reversing the index hashes makes
+// git apply select the inverse payload without reordering the encoded sections.
+func TestReverseFileDiffGitBinarySemantics(t *testing.T) {
+	tests := []struct {
+		name    string
+		forward string
+		reverse string
+	}{
+		{
+			name: "literal",
+			forward: "diff --git a/literal.bin b/literal.bin\n" +
+				"index acdc746399905e5ea0d9d4163d2b3c4d04b05123..e0d84f457d2b5c802ecd286e7e576365d94af3b9 100644\n" +
+				"GIT binary patch\n" +
+				"literal 8\n" +
+				"PcmZQzWMXDvWn%{b09*ha\n\n" +
+				"literal 64\n" +
+				"LcmZ>CqznK65PlFr\n\n",
+			reverse: "diff --git b/literal.bin a/literal.bin\n" +
+				"index e0d84f457d2b5c802ecd286e7e576365d94af3b9..acdc746399905e5ea0d9d4163d2b3c4d04b05123 100644\n" +
+				"GIT binary patch\n" +
+				"literal 8\n" +
+				"PcmZQzWMXDvWn%{b09*ha\n\n" +
+				"literal 64\n" +
+				"LcmZ>CqznK65PlFr\n\n",
+		},
+		{
+			name: "delta",
+			forward: "diff --git a/delta.bin b/delta.bin\n" +
+				"index df437f42c808d41dec5d543d60ce94c8cb8a044a..b762194ffce13918858bfd54f1c4ded144c7e3b4 100644\n" +
+				"GIT binary patch\n" +
+				"delta 20\n" +
+				"bcmZorXi(U2ft@2cBQY;MHAQjb4Gj(ePVNVE\n\n" +
+				"delta 10\n" +
+				"PcmZorXi!+h!2v`75gY=g\n\n",
+			reverse: "diff --git b/delta.bin a/delta.bin\n" +
+				"index b762194ffce13918858bfd54f1c4ded144c7e3b4..df437f42c808d41dec5d543d60ce94c8cb8a044a 100644\n" +
+				"GIT binary patch\n" +
+				"delta 20\n" +
+				"bcmZorXi(U2ft@2cBQY;MHAQjb4Gj(ePVNVE\n\n" +
+				"delta 10\n" +
+				"PcmZorXi!+h!2v`75gY=g\n\n",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			assertGitReverse(t, test.forward, test.reverse, false)
+		})
+	}
+}
+
+func TestReverseFileDiffRejectsGitCopy(t *testing.T) {
+	forward := "diff --git a/source.txt b/copied.txt\n" +
+		"similarity index 100%\n" +
+		"copy from source.txt\n" +
+		"copy to copied.txt\n"
+
+	fd, err := ParseFileDiff([]byte(forward))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := ReverseFileDiff(fd); err == nil {
+		t.Fatal("ReverseFileDiff succeeded for a Git copy patch")
+	}
+}
+
+func assertGitReverse(t *testing.T, forward, reverse string, crlf bool) {
+	t.Helper()
+	if diff := gitReverseDiff(t, forward, reverse, crlf); diff != "" {
+		t.Fatalf("reversed FileDiff differs from expected Git semantics (-want +got):\n%s", diff)
+	}
+}
+
+func gitReverseDiff(t *testing.T, forward, reverse string, crlf bool) string {
+	t.Helper()
+	opts := ParseOptions{KeepCR: crlf}
+	if crlf {
+		forward = strings.ReplaceAll(forward, "\n", "\r\n")
+		reverse = strings.ReplaceAll(reverse, "\n", "\r\n")
+	}
+
+	fd, err := ParseFileDiffOptions([]byte(forward), opts)
+	if err != nil {
+		t.Fatalf("parse forward fixture: %s", err)
+	}
+	want, err := ParseFileDiffOptions([]byte(reverse), opts)
+	if err != nil {
+		t.Fatalf("parse reverse fixture: %s", err)
+	}
+	got, err := ReverseFileDiff(fd)
+	if err != nil {
+		t.Fatalf("ReverseFileDiff: %s", err)
+	}
+
+	// diff --git argument rendering is orthogonal to the parsed directional data
+	// covered here, so this remains compatible with either argument order.
+	got.Extended[0], want.Extended[0] = "", ""
+	return cmp.Diff(want, got)
+}