widget: test firstPos

This commit adds a test to lock in the correct behavior of the
firstPos helper method.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/label.go

@@ -76,6 +76,14 @@ func subLayout(line text.Line, start, end combinedPos) text.Layout {
 	return line.Layout.Slice(startCluster, endCluster)
 }
 
+// firstPos returns a combinedPos with *only* the x and y
+// fields populated. They will be set to the location of the
+// dot at the beginning of the line, with text alignment taken
+// into account. For RTL text, this will
+// be on the right edge of the available space.
+//
+// The results can be counterinuitive due to the fact that meaning
+// of alignment changes depending on the text direction.
 func firstPos(line text.Line, alignment text.Alignment, width int) combinedPos {
 	p := combinedPos{
 		x: align(alignment, line.Layout.Direction, line.Width, width),
@@ -88,10 +96,6 @@ func firstPos(line text.Line, alignment text.Alignment, width int) combinedPos {
 	return p
 }
 
-func (p1 screenPos) Less(p2 screenPos) bool {
-	return p1.Y < p2.Y || (p1.Y == p2.Y && p1.X < p2.X)
-}
-
 func (l Label) Layout(gtx layout.Context, s text.Shaper, font text.Font, size unit.Sp, txt string) layout.Dimensions {
 	cs := gtx.Constraints
 	textSize := fixed.I(gtx.Sp(size))

widget/text_test.go

@@ -0,0 +1,144 @@
+package widget
+
+import (
+	"strconv"
+	"testing"
+
+	nsareg "eliasnaur.com/font/noto/sans/arabic/regular"
+	"gioui.org/font/opentype"
+	"gioui.org/text"
+	"golang.org/x/image/font/gofont/goregular"
+	"golang.org/x/image/math/fixed"
+)
+
+func TestFirstPos(t *testing.T) {
+	ltrFace, _ := opentype.Parse(goregular.TTF)
+	rtlFace, _ := opentype.Parse(nsareg.TTF)
+
+	shaper := text.NewCache([]text.FontFace{
+		{
+			Font: text.Font{Typeface: "LTR"},
+			Face: ltrFace,
+		},
+		{
+			Font: text.Font{Typeface: "RTL"},
+			Face: rtlFace,
+		},
+	})
+	fontSize := 16
+	lineWidth := int(fontSize) * 10
+	ltrText := shaper.LayoutString(text.Font{Typeface: "LTR"}, fixed.I(fontSize), lineWidth, english, "The quick brown fox\njumps over the lazy dog.")
+	rtlText := shaper.LayoutString(text.Font{Typeface: "RTL"}, fixed.I(fontSize), lineWidth, arabic, "الحب سماء لا\nتمط غير الأحلام")
+
+	type testcase struct {
+		name      string
+		line      text.Line
+		xAlignMap map[text.Alignment]map[int]fixed.Int26_6
+		expected  combinedPos
+	}
+	for _, tc := range []testcase{
+		{
+			name: "ltr line 0",
+			line: ltrText[0],
+			xAlignMap: map[text.Alignment]map[int]fixed.Int26_6{
+				text.Start: {
+					lineWidth:     0,
+					lineWidth * 2: 0,
+				},
+				text.Middle: {
+					lineWidth:     fixed.I(8),
+					lineWidth * 2: fixed.I(88),
+				},
+				text.End: {
+					lineWidth:     fixed.I(16),
+					lineWidth * 2: fixed.I(176),
+				},
+			},
+			expected: combinedPos{
+				x: 0,
+				y: ltrText[0].Ascent.Ceil(),
+			},
+		},
+		{
+			name: "ltr line 1",
+			line: ltrText[1],
+			xAlignMap: map[text.Alignment]map[int]fixed.Int26_6{
+				text.Start: {
+					lineWidth:     0,
+					lineWidth * 2: 0,
+				},
+				text.Middle: {
+					lineWidth:     fixed.I(8),
+					lineWidth * 2: fixed.I(88),
+				},
+				text.End: {
+					lineWidth:     fixed.I(16),
+					lineWidth * 2: fixed.I(176),
+				},
+			},
+			expected: combinedPos{
+				x: 0,
+				y: ltrText[1].Ascent.Ceil(),
+			},
+		},
+		{
+			name: "rtl line 0",
+			line: rtlText[0],
+			xAlignMap: map[text.Alignment]map[int]fixed.Int26_6{
+				text.End: {
+					lineWidth:     rtlText[0].Width,
+					lineWidth * 2: rtlText[0].Width,
+				},
+				text.Middle: {
+					lineWidth:     fixed.Int26_6(7827),
+					lineWidth * 2: fixed.Int26_6(12947),
+				},
+				text.Start: {
+					lineWidth:     fixed.Int26_6(10195),
+					lineWidth * 2: fixed.Int26_6(20435),
+				},
+			},
+			expected: combinedPos{
+				x: 0,
+				y: rtlText[0].Ascent.Ceil(),
+			},
+		},
+		{
+			name: "rtl line 1",
+			line: rtlText[1],
+			xAlignMap: map[text.Alignment]map[int]fixed.Int26_6{
+				text.End: {
+					lineWidth:     rtlText[1].Width,
+					lineWidth * 2: rtlText[1].Width,
+				},
+				text.Middle: {
+					lineWidth:     fixed.Int26_6(8184),
+					lineWidth * 2: fixed.Int26_6(13304),
+				},
+				text.Start: {
+					lineWidth:     fixed.Int26_6(10232),
+					lineWidth * 2: fixed.Int26_6(20472),
+				},
+			},
+			expected: combinedPos{
+				x: 0,
+				y: rtlText[1].Ascent.Ceil(),
+			},
+		},
+	} {
+		for align, cases := range tc.xAlignMap {
+			for width, expectedX := range cases {
+				t.Run(tc.name+" "+align.String()+" "+strconv.Itoa(width), func(t *testing.T) {
+					actual := firstPos(tc.line, align, width)
+					tc.expected.x = expectedX
+					if tc.expected.x != actual.x {
+						t.Errorf("expected x=%s(%d), got %s(%d)", tc.expected.x, tc.expected.x, actual.x, actual.x)
+					}
+					if tc.expected.y != actual.y {
+						t.Errorf("expected y=%d(%d), got %d(%d)", tc.expected.y, tc.expected.y, actual.y, actual.y)
+					}
+				})
+			}
+		}
+	}
+}