Introduce deliberate instability to difference output (#214)

The reporter output is documented as unstable.
The API for custom reporters also specifies that the diffing of
slices is unstable.

Introduce deliberate instability to the diffing algorithm so that
we have the flexibility to improve it in the future.
The current algorithm optimizes for speed, rather than optimality,
so there is much room for improvement.

cmp/internal/diff/diff.go

@@ -12,6 +12,13 @@
 // is more important than obtaining a minimal Levenshtein distance.
 package diff
 
+import (
+	"math/rand"
+	"time"
+
+	"github.com/google/go-cmp/cmp/internal/flags"
+)
+
 // EditType represents a single operation within an edit-script.
 type EditType uint8
 
@@ -112,6 +119,8 @@ func (r Result) Similar() bool {
 	return r.NumSame+1 >= r.NumDiff
 }
 
+var randInt = rand.New(rand.NewSource(time.Now().Unix())).Intn(2)
+
 // Difference reports whether two lists of lengths nx and ny are equal
 // given the definition of equality provided as f.
 //
@@ -159,6 +168,17 @@ 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
@@ -209,7 +229,7 @@ func Difference(nx, ny int, f EqualFunc) (es EditScript) {
 		if fwdFrontier.X >= revFrontier.X || fwdFrontier.Y >= revFrontier.Y || searchBudget == 0 {
 			break
 		}
-		for stop1, stop2, i := false, false, 0; !(stop1 && stop2) && searchBudget > 0; i++ {
+		for stop1, stop2, i := false, false, zigzagInit; !(stop1 && stop2) && searchBudget > 0; i++ {
 			// Search in a diagonal pattern for a match.
 			z := zigzag(i)
 			p := point{fwdFrontier.X + z, fwdFrontier.Y - z}

cmp/internal/diff/diff_test.go

@@ -10,8 +10,14 @@ import (
 	"strings"
 	"testing"
 	"unicode"
+
+	"github.com/google/go-cmp/cmp/internal/flags"
 )
 
+func init() {
+	flags.Deterministic = true
+}
+
 func TestDifference(t *testing.T) {
 	tests := []struct {
 		// Before passing x and y to Difference, we strip all spaces so that