style: unit conversion

Implemented the golang.org/x/exp/shiny/unit.Converter interface on
style.Style.

7 files changed, 465 insertions, 8 deletions

diff --git a/go.mod b/go.mod
index a062971..921713c 100644
--- a/go.mod
+++ b/go.mod
@@ -6,10 +6,9 @@ require (
 	github.com/faiface/mainthread v0.0.0-20171120011319-8b78f0a41ae3
 	github.com/go-gl/gl v0.0.0-20231021071112-07e5d0ea2e71
 	github.com/go-gl/glfw v0.0.0-20250301202403-da16c1255728
+	github.com/lithdew/casso v0.0.0-20200531104607-fe75aa82181f
+	golang.org/x/exp/shiny v0.0.0-20260218203240-3dfff04db8fa
+	golang.org/x/image v0.36.0
 )
 
-require (
-	github.com/lithdew/casso v0.0.0-20200531104607-fe75aa82181f // indirect
-	golang.org/x/exp/shiny v0.0.0-20260218203240-3dfff04db8fa // indirect
-	golang.org/x/image v0.36.0 // indirect
-)
+require golang.org/x/text v0.34.0 // indirect
diff --git a/go.sum b/go.sum
index 025737b..d8426d4 100644
--- a/go.sum
+++ b/go.sum
@@ -1,3 +1,4 @@
+github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/faiface/mainthread v0.0.0-20171120011319-8b78f0a41ae3 h1:baVdMKlASEHrj19iqjARrPbaRisD7EuZEVJj6ZMLl1Q=
 github.com/faiface/mainthread v0.0.0-20171120011319-8b78f0a41ae3/go.mod h1:VEPNJUlxl5KdWjDvz6Q1l+rJlxF2i6xqDeGuGAxa87M=
@@ -7,12 +8,17 @@ github.com/go-gl/glfw v0.0.0-20250301202403-da16c1255728 h1:Ak0LUgy7whfnJGPcjhR4
 github.com/go-gl/glfw v0.0.0-20250301202403-da16c1255728/go.mod h1:fOxQgJvH6dIDHn5YOoXiNC8tUMMNuCgbMK2yZTlZVQA=
 github.com/lithdew/casso v0.0.0-20200531104607-fe75aa82181f h1:BJPLqH5ylnMCrDyViN4w9kHYup2w1BNKbEihSAQpDOk=
 github.com/lithdew/casso v0.0.0-20200531104607-fe75aa82181f/go.mod h1:iaGbC3JZCbafAF9UkQTn2To3bMVOkTXNFTfS2yRvclM=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/testify v1.5.1 h1:nOGnQDM7FYENwehXlg/kFVnos3rEvtKTjRvOWSzb6H4=
 github.com/stretchr/testify v1.5.1/go.mod h1:5W2xD1RspED5o8YsWQXVCued0rvSQ+mT+I5cxcmMvtA=
 golang.org/x/exp/shiny v0.0.0-20260218203240-3dfff04db8fa h1:+7e7RPzOw2fG8DBbddatlOmHGNCg+VlA2Ar0yVMw7sM=
 golang.org/x/exp/shiny v0.0.0-20260218203240-3dfff04db8fa/go.mod h1:zxsA7NyDTOUjcveVwAMFI/YIErWwayTW/4RGqB/RzKk=
 golang.org/x/image v0.36.0 h1:Iknbfm1afbgtwPTmHnS2gTM/6PPZfH+z2EFuOkSbqwc=
 golang.org/x/image v0.36.0/go.mod h1:YsWD2TyyGKiIX1kZlu9QfKIsQ4nAAK9bdgdrIsE7xy4=
+golang.org/x/text v0.34.0 h1:oL/Qq0Kdaqxa1KbNeMKwQq0reLCCaFtqu2eNuSeNHbk=
+golang.org/x/text v0.34.0/go.mod h1:homfLqTYRFyVYemLBFl5GgL/DWEiH5wcsQ5gSh1yziA=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
diff --git a/style/fixed.go b/style/fixed.go
new file mode 100644
index 0000000..fe3f91e
--- /dev/null
+++ b/style/fixed.go
@@ -0,0 +1,11 @@
+package style
+
+import "golang.org/x/image/math/fixed"
+
+func fixed26ToFloat(n fixed.Int26_6) float64 {
+	return float64(n) / (1 << 6)
+}
+
+func floatToFixed26(f float64) fixed.Int26_6 {
+	return fixed.Int26_6(f * (1 << 6))
+}
diff --git a/style/fixed_test.go b/style/fixed_test.go
new file mode 100644
index 0000000..5cae4bf
--- /dev/null
+++ b/style/fixed_test.go
@@ -0,0 +1,74 @@
+package style
+
+import (
+	"testing"
+
+	"golang.org/x/image/math/fixed"
+)
+
+func TestFixed26ToFloat(t *testing.T) {
+	t.Parallel()
+
+	var (
+		in        fixed.Int26_6
+		out, want float64
+	)
+
+	in = 0
+	out = fixed26ToFloat(in)
+	want = 0
+	if out != want {
+		t.Errorf("float(%#v) = %v; want %v", in, out, want)
+	}
+
+	in = fixed.I(123)
+	out = fixed26ToFloat(in)
+	want = 123
+	if out != want {
+		t.Errorf("float(%#v) = %v; want %v", in, out, want)
+	}
+
+	in = fixed.I(-123)
+	out = fixed26ToFloat(in)
+	want = -123
+	if out != want {
+		t.Errorf("float(%#v) = %v; want %v", in, out, want)
+	}
+
+	in = fixed.Int26_6(1<<6 + 1<<4)
+	out = fixed26ToFloat(in)
+	want = 1.25
+	if out != want {
+		t.Errorf("float(%#v) = %v; want %v", in, out, want)
+	}
+}
+
+func TestFloatToFixed26(t *testing.T) {
+	t.Parallel()
+
+	var (
+		in        float64
+		out, want fixed.Int26_6
+	)
+
+	in = 0
+	out = floatToFixed26(in)
+	want = 0
+	if out != want {
+		t.Errorf("fixed(%#v) = %v; want %v", in, out, want)
+	}
+
+	in = 1.25
+	out = floatToFixed26(in)
+	want = fixed.Int26_6(1<<6 + 1<<4)
+	if out != want {
+		t.Errorf("fixed(%#v) = %v; want %v", in, out, want)
+	}
+
+	in = -1.25
+	out = floatToFixed26(in)
+	want = -fixed.Int26_6(1<<6 + 1<<4)
+	if out != want {
+		t.Errorf("fixed(%#v) = %v; want %v", in, out, want)
+	}
+}
diff --git a/style/options.go b/style/options.go
new file mode 100644
index 0000000..c1514c0
--- /dev/null
+++ b/style/options.go
@@ -0,0 +1,62 @@
+package style
+
+import (
+	"golang.org/x/image/font"
+	"golang.org/x/image/font/gofont/goregular"
+	"golang.org/x/image/font/opentype"
+)
+
+var defaultOptions = options{
+	font: goregular.TTF,
+	FaceOptions: opentype.FaceOptions{
+		DefaultFontSize,
+		DefaultDpi,
+		DefaultHinting,
+	},
+}
+
+// Option is a functional option to Style constructor, New.
+type Option func(*options)
+
+type options struct {
+	font []byte
+	opentype.FaceOptions
+}
+
+func parseOpts(opts ...Option) options {
+	o := defaultOptions
+	for i := range opts {
+		opts[i](&o)
+	}
+	return o
+}
+
+// Font option sets the font source to use. It takes TTF or OTF data
+// (see golang.org/x/image/font/opentype.Parse).
+func Font(src []byte) Option {
+	return func(o *options) {
+		o.font = src
+	}
+}
+
+// FontSize option sets the font size in points (1/72").
+func FontSize(size float64) Option {
+	return func(o *options) {
+		o.FaceOptions.Size = size
+	}
+}
+
+// Dpi option sets the dots per inch resolution of the display.
+func Dpi(dpi float64) Option {
+	return func(o *options) {
+		o.FaceOptions.DPI = dpi
+	}
+}
+
+// Hinting options selects how to quantize a vector font's glyph
+// nodes.
+func Hinting(h font.Hinting) Option {
+	return func(o *options) {
+		o.FaceOptions.Hinting = h
+	}
+}
diff --git a/style/style.go b/style/style.go
index 5b74952..fe10833 100644
--- a/style/style.go
+++ b/style/style.go
@@ -1,22 +1,125 @@
 package style
 
 import (
+	"fmt"
+
 	"golang.org/x/exp/shiny/unit"
+	"golang.org/x/image/font"
+	"golang.org/x/image/font/opentype"
 	"golang.org/x/image/math/fixed"
 )
 
-// TODO
+const (
+	DefaultFontSize = 12.0 // points (1/72")
+	DefaultDpi      = 72.0 // dots per inch
+	DefaultHinting  = font.HintingFull
+)
+
+// Style defines the colors and font faces that widgets are drawn with.
 type Style struct {
+	font     *opentype.Font
+	faceOpts opentype.FaceOptions
+	face     font.Face // for measurements
+
+	// TODO
+}
+
+// New returns a new Style with the given options.
+// The Style should be closed after use.
+func New(opts ...Option) (*Style, error) {
+	o := parseOpts(opts...)
+
+	fnt, err := opentype.Parse(o.font)
+	if err != nil {
+		return nil, err
+	}
+	face, err := opentype.NewFace(fnt, &o.FaceOptions)
+	if err != nil {
+		return nil, err
+	}
+
+	return &Style{
+		fnt,
+		o.FaceOptions,
+		face,
+	}, nil
+}
+
+func (s *Style) Close() error {
+	return s.face.Close()
+}
+
+// NewFace returns a new font.Face using the Style's Font and FaceOptions.
+func (s *Style) NewFace() (font.Face, error) {
+	return opentype.NewFace(s.font, &s.faceOpts)
 }
 
 // Convert implements the golang.org/x/exp/shiny/unit.Converter
 // interface.
 func (s *Style) Convert(v unit.Value, to unit.Unit) unit.Value {
-	// TODO
+	px := fixed26ToFloat(s.Pixels(v))
+	var f float64
+	switch to {
+	case unit.Px:
+		f = px
+	case unit.Dp:
+		f = s.pixelsToInches(px) * unit.DensityIndependentPixelsPerInch
+	case unit.Pt:
+		f = s.pixelsToInches(px) * unit.PointsPerInch
+	case unit.Mm:
+		f = s.pixelsToInches(px) * unit.MillimetresPerInch
+	case unit.In:
+		f = s.pixelsToInches(px)
+	case unit.Em:
+		f = px / fixed26ToFloat(s.face.Metrics().Height)
+	case unit.Ex:
+		f = px / fixed26ToFloat(s.face.Metrics().XHeight)
+	case unit.Ch:
+		f = px / fixed26ToFloat(s.zeroWidth())
+	default:
+		panic(fmt.Sprintf("unreachable: impossible %T: %v", to, to))
+	}
+	return unit.Value{f, to}
 }
 
 // Pixels implements the golang.org/x/exp/shiny/unit.Converter
 // interface.
 func (s *Style) Pixels(v unit.Value) fixed.Int26_6 {
-	// TODO
+	f := floatToFixed26(v.F)
+	switch v.U {
+	case unit.Px:
+		return f
+	case unit.Dp:
+		return s.inchesToPixels(v.F / unit.DensityIndependentPixelsPerInch)
+	case unit.Pt:
+		return s.inchesToPixels(v.F / unit.PointsPerInch)
+	case unit.Mm:
+		return s.inchesToPixels(v.F / unit.MillimetresPerInch)
+	case unit.In:
+		return s.inchesToPixels(v.F)
+	case unit.Em:
+		return f.Mul(s.face.Metrics().Height)
+	case unit.Ex:
+		return f.Mul(s.face.Metrics().XHeight)
+	case unit.Ch:
+		return f.Mul(s.zeroWidth())
+	default:
+		panic(fmt.Sprintf("unreachable: impossible %T: %v", v.U, v.U))
+	}
+}
+
+func (s *Style) pixelsToInches(px float64) (in float64) {
+	return px / s.faceOpts.DPI
+}
+
+func (s *Style) inchesToPixels(in float64) (px fixed.Int26_6) {
+	return floatToFixed26(in * s.faceOpts.DPI)
+}
+
+func (s *Style) zeroWidth() (px fixed.Int26_6) {
+	if advance, ok := s.face.GlyphAdvance('0'); ok {
+		return advance
+	} else {
+		return floatToFixed26(0.5).Mul(s.face.Metrics().Height) // 0.5em
+	}
 }
diff --git a/style/style_test.go b/style/style_test.go
new file mode 100644
index 0000000..14f00e2
--- /dev/null
+++ b/style/style_test.go
@@ -0,0 +1,202 @@
+package style
+
+import (
+	"testing"
+
+	"golang.org/x/exp/shiny/unit"
+	"golang.org/x/image/font/gofont/gomono"
+	"golang.org/x/image/math/fixed"
+)
+
+const (
+	dpi     = 150.0
+	epsilon = 0.5 // TODO: optimize precision
+)
+
+var (
+	fontTtf = gomono.TTF
+	units   = []unit.Unit{unit.Px, unit.Dp, unit.Pt, unit.Mm, unit.In, unit.Em, unit.Ex, unit.Ch}
+)
+
+func TestConvert(t *testing.T) {
+	t.Parallel()
+
+	s, err := New(Font(fontTtf), Dpi(dpi))
+	if err != nil {
+		t.Error(err)
+	}
+	defer func() {
+		if err := s.Close(); err != nil {
+			t.Fatal(err)
+		}
+	}()
+
+	// Trivial
+	for _, from := range units {
+		for _, to := range units {
+			testConvert(t, s, unit.Value{0, from}, to, 0)
+		}
+	}
+
+	pxPerEm := fixed26ToFloat(s.face.Metrics().Height)
+	pxPerEx := fixed26ToFloat(s.face.Metrics().XHeight)
+	advance, ok := s.face.GlyphAdvance('0')
+	if !ok {
+		t.Fail()
+	}
+	pxPerCh := fixed26ToFloat(advance)
+
+	// px → all
+	px := 123.4
+	in := px / dpi
+	from := unit.Value{px, unit.Px}
+	testConvert(t, s, from, unit.Px, px)
+	testConvert(t, s, from, unit.Dp, in*160)
+	testConvert(t, s, from, unit.Pt, in*72)
+	testConvert(t, s, from, unit.Mm, in*25.4)
+	testConvert(t, s, from, unit.In, in)
+	testConvert(t, s, from, unit.Em, px/pxPerEm)
+	testConvert(t, s, from, unit.Ex, px/pxPerEx)
+	testConvert(t, s, from, unit.Ch, px/pxPerCh)
+
+	// dp → all
+	dp := 123.4
+	in = dp / 160
+	px = in * dpi
+	from = unit.Value{dp, unit.Dp}
+	testConvert(t, s, from, unit.Px, px)
+	testConvert(t, s, from, unit.Dp, dp)
+	testConvert(t, s, from, unit.Pt, in*72)
+	testConvert(t, s, from, unit.Mm, in*25.4)
+	testConvert(t, s, from, unit.In, in)
+	testConvert(t, s, from, unit.Em, px/pxPerEm)
+	testConvert(t, s, from, unit.Ex, px/pxPerEx)
+	testConvert(t, s, from, unit.Ch, px/pxPerCh)
+
+	// Pt → all
+	pt := 123.4
+	in = pt / 72
+	px = in * dpi
+	from = unit.Value{pt, unit.Pt}
+	testConvert(t, s, from, unit.Px, px)
+	testConvert(t, s, from, unit.Dp, in*160)
+	testConvert(t, s, from, unit.Pt, pt)
+	testConvert(t, s, from, unit.Mm, in*25.4)
+	testConvert(t, s, from, unit.In, in)
+	testConvert(t, s, from, unit.Em, px/pxPerEm)
+	testConvert(t, s, from, unit.Ex, px/pxPerEx)
+	testConvert(t, s, from, unit.Ch, px/pxPerCh)
+
+	// Mm → all
+	mm := 123.4
+	in = mm / 25.4
+	px = in * dpi
+	from = unit.Value{mm, unit.Mm}
+	testConvert(t, s, from, unit.Px, px)
+	testConvert(t, s, from, unit.Dp, in*160)
+	testConvert(t, s, from, unit.Pt, in*72)
+	testConvert(t, s, from, unit.Mm, mm)
+	testConvert(t, s, from, unit.In, in)
+	testConvert(t, s, from, unit.Em, px/pxPerEm)
+	testConvert(t, s, from, unit.Ex, px/pxPerEx)
+	testConvert(t, s, from, unit.Ch, px/pxPerCh)
+
+	// In → all
+	in = 123.4
+	px = in * dpi
+	from = unit.Value{in, unit.In}
+	testConvert(t, s, from, unit.Px, px)
+	testConvert(t, s, from, unit.Dp, in*160)
+	testConvert(t, s, from, unit.Pt, in*72)
+	testConvert(t, s, from, unit.Mm, in*25.4)
+	testConvert(t, s, from, unit.In, in)
+	testConvert(t, s, from, unit.Em, px/pxPerEm)
+	testConvert(t, s, from, unit.Ex, px/pxPerEx)
+	testConvert(t, s, from, unit.Ch, px/pxPerCh)
+
+	// Em → all
+	em := 123.4
+	px = em * pxPerEm
+	in = px / dpi
+	from = unit.Value{em, unit.Em}
+	testConvert(t, s, from, unit.Px, px)
+	testConvert(t, s, from, unit.Dp, in*160)
+	testConvert(t, s, from, unit.Pt, in*72)
+	testConvert(t, s, from, unit.Mm, in*25.4)
+	testConvert(t, s, from, unit.In, in)
+	testConvert(t, s, from, unit.Em, em)
+	testConvert(t, s, from, unit.Ex, px/pxPerEx)
+	testConvert(t, s, from, unit.Ch, px/pxPerCh)
+
+	// Ex → all
+	ex := 123.4
+	px = ex * pxPerEx
+	in = px / dpi
+	from = unit.Value{ex, unit.Ex}
+	testConvert(t, s, from, unit.Px, px)
+	testConvert(t, s, from, unit.Dp, in*160)
+	testConvert(t, s, from, unit.Pt, in*72)
+	testConvert(t, s, from, unit.Mm, in*25.4)
+	testConvert(t, s, from, unit.In, in)
+	testConvert(t, s, from, unit.Em, px/pxPerEm)
+	testConvert(t, s, from, unit.Ex, ex)
+	testConvert(t, s, from, unit.Ch, px/pxPerCh)
+
+	// Ch → all
+	ch := 123.4
+	px = ch * pxPerCh
+	in = px / dpi
+	from = unit.Value{ch, unit.Ch}
+	testConvert(t, s, from, unit.Px, px)
+	testConvert(t, s, from, unit.Dp, in*160)
+	testConvert(t, s, from, unit.Pt, in*72)
+	testConvert(t, s, from, unit.Mm, in*25.4)
+	testConvert(t, s, from, unit.In, in)
+	testConvert(t, s, from, unit.Em, px/pxPerEm)
+	testConvert(t, s, from, unit.Ex, px/pxPerEx)
+	testConvert(t, s, from, unit.Ch, ch)
+}
+
+func testConvert(t *testing.T, s *Style, from unit.Value, to unit.Unit, want float64) {
+	out := s.Convert(from, to)
+	if out.U != to || out.F < want-epsilon || out.F > want+epsilon {
+		t.Errorf("Convert(%v, %v) = %v; want %v", from, to, out, unit.Value{want, to})
+	}
+}
+
+func TestPixels(t *testing.T) {
+	t.Parallel()
+
+	s, err := New(Font(fontTtf), Dpi(dpi))
+	if err != nil {
+		t.Error(err)
+	}
+	defer func() {
+		if err := s.Close(); err != nil {
+			t.Fatal(err)
+		}
+	}()
+
+	for _, u := range units {
+		testPixels(t, s, unit.Value{0, u}, 0)
+	}
+	testPixels(t, s, unit.Value{123.4, unit.Px}, floatToFixed26(123.4))
+	testPixels(t, s, unit.Value{123.4, unit.Dp}, floatToFixed26(123.4/160*dpi))
+	testPixels(t, s, unit.Value{123.4, unit.Pt}, floatToFixed26(123.4/72*dpi))
+	testPixels(t, s, unit.Value{123.4, unit.Mm}, floatToFixed26(123.4/25.4*dpi))
+	testPixels(t, s, unit.Value{123.4, unit.In}, floatToFixed26(123.4*dpi))
+	testPixels(t, s, unit.Value{123.4, unit.Em}, floatToFixed26(123.4).Mul(s.face.Metrics().Height))
+	testPixels(t, s, unit.Value{123.4, unit.Ex}, floatToFixed26(123.4).Mul(s.face.Metrics().XHeight))
+	if advance, ok := s.face.GlyphAdvance('0'); ok {
+		testPixels(t, s, unit.Value{123.4, unit.Ch}, floatToFixed26(123.4).Mul(advance))
+	} else {
+		t.Fail()
+	}
+}
+
+func testPixels(t *testing.T, s *Style, in unit.Value, want fixed.Int26_6) {
+	out := s.Pixels(in)
+	if out != want {
+		t.Errorf("Pixels(%#v) = %v; want %v", in, out, want)
+	}
+}