unicode/bidi: fix Direction zero value to Neutral

The Direction constants had LeftToRight as iota (value 0), making it
indistinguishable from the zero value of an unset Direction field.
This caused DefaultDirection(LeftToRight) to be silently ignored in
Paragraph.Order(), as the code could not distinguish between "user
explicitly set LTR" and "no direction was set."

Reorder the Direction constants so that Neutral is the zero value,
and add an explicit case for LeftToRight in Order() to set the
paragraph embedding level to 0.

Fixes golang/go#71809

Author: pgundlach

unicode/bidi/bidi.go

@@ -29,10 +29,14 @@ import (
 type Direction int
 
 const (
+	// Neutral means that text contains no left-to-right and right-to-left
+	// characters and that no default direction has been set.
+	Neutral Direction = iota
+
 	// LeftToRight indicates the text contains no right-to-left characters and
 	// that either there are some left-to-right characters or the option
 	// DefaultDirection(LeftToRight) was passed.
-	LeftToRight Direction = iota
+	LeftToRight
 
 	// RightToLeft indicates the text contains no left-to-right characters and
 	// that either there are some right-to-left characters or the option
@@ -42,10 +46,6 @@ const (
 	// Mixed indicates text contains both left-to-right and right-to-left
 	// characters.
 	Mixed
-
-	// Neutral means that text contains no left-to-right and right-to-left
-	// characters and that no default direction has been set.
-	Neutral
 )
 
 type options struct {
@@ -219,8 +219,11 @@ func (p *Paragraph) Order() (Ordering, error) {
 		fn(&p.options)
 	}
 	lvl := level(-1)
-	if p.options.defaultDirection == RightToLeft {
+	switch p.options.defaultDirection {
+	case RightToLeft:
 		lvl = 1
+	case LeftToRight:
+		lvl = 0
 	}
 	para, err := newParagraph(p.types, p.pairTypes, p.pairValues, lvl)
 	if err != nil {

unicode/bidi/bidi_test.go

@@ -254,6 +254,38 @@ func TestMixedSimple(t *testing.T) {
 	}
 }
 
+func TestDirectionLTR(t *testing.T) {
+	str := "ع a"
+	p := Paragraph{}
+	p.SetString(str, DefaultDirection(LeftToRight))
+	order, err := p.Order()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	expectedRuns := []runInformation{
+		{"ع", RightToLeft, 0, 0},
+		{" a", LeftToRight, 1, 2},
+	}
+
+	if nr, want := order.NumRuns(), len(expectedRuns); nr != want {
+		t.Errorf("order.NumRuns() = %d; want %d", nr, want)
+	}
+
+	for i, want := range expectedRuns {
+		r := order.Run(i)
+		if got := r.String(); got != want.str {
+			t.Errorf("Run(%d) = %q; want %q", i, got, want.str)
+		}
+		if s, e := r.Pos(); s != want.start || e != want.end {
+			t.Errorf("Run(%d).start = %d, .end = %d; want start = %d, end = %d", i, s, e, want.start, want.end)
+		}
+		if d := r.Direction(); d != want.dir {
+			t.Errorf("Run(%d).Direction = %d; want %d", i, d, want.dir)
+		}
+	}
+}
+
 func TestDefaultDirection(t *testing.T) {
 	str := "+"
 	p := Paragraph{}