Fix non-determinism in diffing algorithm

A previous attempt to add non-determinism to the diffing algorithm
unfortunately broke the algorithm for half the cases.

This change modifies the algorithm to truly switch between starting
with a forward search versus a reverse search.
The main for-loop of Difference would switch repeatedly between
performing a forward search, then a reverse search, and vice-versa.
Since we can't jump into the middle of a for-loop to start with the
reverse search first, we use a series of labels and goto statements
to accomplish the same effect.

Fixes #238

cmp/internal/diff/diff.go

@@ -119,7 +119,7 @@ func (r Result) Similar() bool {
 	return r.NumSame+1 >= r.NumDiff
 }
 
-var randInt = rand.New(rand.NewSource(time.Now().Unix())).Intn(2)
+var randBool = rand.New(rand.NewSource(time.Now().Unix())).Intn(2) == 0
 
 // Difference reports whether two lists of lengths nx and ny are equal
 // given the definition of equality provided as f.
@@ -168,17 +168,6 @@ func Difference(nx, ny int, f EqualFunc) (es EditScript) {
 	// A vertical edge is equivalent to inserting a symbol from list Y.
 	// A diagonal edge is equivalent to a matching symbol between both X and Y.
 
-	// To ensure flexibility in changing the algorithm in the future,
-	// introduce some degree of deliberate instability.
-	// This is achieved by fiddling the zigzag iterator to start searching
-	// the graph starting from the bottom-right versus than the top-left.
-	// The result may differ depending on the starting search location,
-	// but still produces a valid edit script.
-	zigzagInit := randInt // either 0 or 1
-	if flags.Deterministic {
-		zigzagInit = 0
-	}
-
 	// Invariants:
 	//	• 0 ≤ fwdPath.X ≤ (fwdFrontier.X, revFrontier.X) ≤ revPath.X ≤ nx
 	//	• 0 ≤ fwdPath.Y ≤ (fwdFrontier.Y, revFrontier.Y) ≤ revPath.Y ≤ ny
@@ -197,6 +186,11 @@ func Difference(nx, ny int, f EqualFunc) (es EditScript) {
 	// approximately the square-root of the search budget.
 	searchBudget := 4 * (nx + ny) // O(n)
 
+	// Running the tests with the "cmp_debug" build tag prints a visualization
+	// of the algorithm running in real-time. This is educational for
+	// understanding how the algorithm works. See debug_enable.go.
+	f = debug.Begin(nx, ny, f, &fwdPath.es, &revPath.es)
+
 	// The algorithm below is a greedy, meet-in-the-middle algorithm for
 	// computing sub-optimal edit-scripts between two lists.
 	//
@@ -214,22 +208,28 @@ func Difference(nx, ny int, f EqualFunc) (es EditScript) {
 	//	frontier towards the opposite corner.
 	//	• This algorithm terminates when either the X coordinates or the
 	//	Y coordinates of the forward and reverse frontier points ever intersect.
-	//
+
 	// This algorithm is correct even if searching only in the forward direction
 	// or in the reverse direction. We do both because it is commonly observed
 	// that two lists commonly differ because elements were added to the front
 	// or end of the other list.
 	//
-	// Running the tests with the "cmp_debug" build tag prints a visualization
-	// of the algorithm running in real-time. This is educational for
-	// understanding how the algorithm works. See debug_enable.go.
-	f = debug.Begin(nx, ny, f, &fwdPath.es, &revPath.es)
-	for {
+	// Non-deterministically start with either the forward or reverse direction
+	// to introduce some deliberate instability so that we have the flexibility
+	// to change this algorithm in the future.
+	if flags.Deterministic || randBool {
+		goto forwardSearch
+	} else {
+		goto reverseSearch
+	}
+
+forwardSearch:
+	{
 		// Forward search from the beginning.
 		if fwdFrontier.X >= revFrontier.X || fwdFrontier.Y >= revFrontier.Y || searchBudget == 0 {
-			break
+			goto finishSearch
 		}
-		for stop1, stop2, i := false, false, zigzagInit; !(stop1 && stop2) && searchBudget > 0; i++ {
+		for stop1, stop2, i := false, false, 0; !(stop1 && stop2) && searchBudget > 0; i++ {
 			// Search in a diagonal pattern for a match.
 			z := zigzag(i)
 			p := point{fwdFrontier.X + z, fwdFrontier.Y - z}
@@ -262,10 +262,14 @@ func Difference(nx, ny int, f EqualFunc) (es EditScript) {
 		} else {
 			fwdFrontier.Y++
 		}
+		goto reverseSearch
+	}
 
+reverseSearch:
+	{
 		// Reverse search from the end.
 		if fwdFrontier.X >= revFrontier.X || fwdFrontier.Y >= revFrontier.Y || searchBudget == 0 {
-			break
+			goto finishSearch
 		}
 		for stop1, stop2, i := false, false, 0; !(stop1 && stop2) && searchBudget > 0; i++ {
 			// Search in a diagonal pattern for a match.
@@ -300,8 +304,10 @@ func Difference(nx, ny int, f EqualFunc) (es EditScript) {
 		} else {
 			revFrontier.Y--
 		}
+		goto forwardSearch
 	}
 
+finishSearch:
 	// Join the forward and reverse paths and then append the reverse path.
 	fwdPath.connect(revPath.point, f)
 	for i := len(revPath.es) - 1; i >= 0; i-- {

cmp/internal/diff/diff_test.go

@@ -10,12 +10,10 @@ import (
 	"strings"
 	"testing"
 	"unicode"
-
-	"github.com/google/go-cmp/cmp/internal/flags"
 )
 
 func init() {
-	flags.Deterministic = true
+	// flags.Deterministic = true
 }
 
 func TestDifference(t *testing.T) {
@@ -24,7 +22,7 @@ func TestDifference(t *testing.T) {
 		// they can be used by the test author to indicate a missing symbol
 		// in one of the lists.
 		x, y string
-		want string
+		want string // '|' separated list of possible outputs
 	}{{
 		x:    "",
 		y:    "",
@@ -36,7 +34,7 @@ func TestDifference(t *testing.T) {
 	}, {
 		x:    "##",
 		y:    "# ",
-		want: ".X",
+		want: ".X|X.",
 	}, {
 		x:    "a#",
 		y:    "A ",
@@ -48,7 +46,7 @@ func TestDifference(t *testing.T) {
 	}, {
 		x:    "# ",
 		y:    "##",
-		want: ".Y",
+		want: ".Y|Y.",
 	}, {
 		x:    " #",
 		y:    "@#",
@@ -148,7 +146,7 @@ func TestDifference(t *testing.T) {
 	}, {
 		x:    "ABCAB BA ",
 		y:    "  C BABAC",
-		want: "XX.X.Y..Y",
+		want: "XX.X.Y..Y|XX.Y.X..Y",
 	}, {
 		x:    "# ####  ###",
 		y:    "#y####yy###",
@@ -164,7 +162,7 @@ func TestDifference(t *testing.T) {
 	}, {
 		x:    "0 12z3x 456789 x x 0",
 		y:    "0y12Z3 y456789y y y0",
-		want: ".Y..M.XY......YXYXY.",
+		want: ".Y..M.XY......YXYXY.|.Y..M.XY......XYXYY.",
 	}, {
 		x:    "0 2 4 6 8 ..................abXXcdEXF.ghXi",
 		y:    " 1 3 5 7 9..................AB  CDE F.GH I",
@@ -216,7 +214,7 @@ func TestDifference(t *testing.T) {
 	}, {
 		x:    "0123456789     ",
 		y:    "     5678901234",
-		want: "XXXXX.....YYYYY",
+		want: "XXXXX.....YYYYY|YYYYY.....XXXXX",
 	}, {
 		x:    "0123456789    ",
 		y:    "    4567890123",
@@ -252,9 +250,14 @@ func TestDifference(t *testing.T) {
 			x := strings.Replace(tt.x, " ", "", -1)
 			y := strings.Replace(tt.y, " ", "", -1)
 			es := testStrings(t, x, y)
-			if got := es.String(); got != tt.want {
-				t.Errorf("Difference(%s, %s):\ngot  %s\nwant %s", x, y, got, tt.want)
+			var want string
+			got := es.String()
+			for _, want = range strings.Split(tt.want, "|") {
+				if got == want {
+					return
+				}
 			}
+			t.Errorf("Difference(%s, %s):\ngot  %s\nwant %s", x, y, got, want)
 		})
 	}
 }