shiny/unit: new package for units of length.

Change-Id: I5b3e90ac5a1eced0eb3ab88683cee5526932d118
Reviewed-on: https://go-review.googlesource.com/21691
Reviewed-by: Andrew Gerrand <[email protected]>

shiny/unit/unit.go

@@ -0,0 +1,137 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package unit defines units of length such as inches or pixels.
+//
+// Functions like Inches and Pixels return a Value in the corresponding unit.
+// For example:
+//
+//	v := unit.Inches(4.5)
+//
+// represents four and a half inches.
+//
+// Converting between pixels (px), physical units (dp, pt, in, mm) and
+// font-face-relative measures (em, ex, ch) depends on the context, such as the
+// screen's DPI resolution and the active font face. That context is
+// represented by the Converter type.
+//
+// Conversions may be lossy. Converting 4.5 inches to pixels and back may
+// result in something slightly different than 4.5. Similarly, converting 4
+// inches and 0.5 inches to pixels and then adding the results won't
+// necessarily equal the conversion of 4.5 inches to pixels.
+//
+// Note that what CSS (Cascading Style Sheets) calls "px" differs from what
+// this package calls "px". For legacy reasons, the CSS semantics are that 1
+// inch should roughly equal 96csspx regardless of the actual DPI resolution,
+// as per https://developer.mozilla.org/en/docs/Web/CSS/length. This package's
+// semantics are that 1px means exactly one physical pixel, always. This
+// package represents 1csspx as 1.666666667dp, since there are 160 density
+// independent pixels per inch, the same definition as Android.
+package unit
+
+import (
+	"fmt"
+
+	"golang.org/x/image/math/fixed"
+)
+
+const (
+	DensityIndependentPixelsPerInch = 160
+	MillimetresPerInch              = 25.4
+	PointsPerInch                   = 72
+)
+
+// Converter converts values from one unit to another. Conversions may be
+// lossy.
+type Converter interface {
+	// Convert converts v to the given unit.
+	Convert(v Value, to Unit) Value
+
+	// Pixels converts v to a 26.6 fixed-point number of physical pixels.
+	Pixels(v Value) fixed.Int26_6
+}
+
+// Value is a number and a unit.
+type Value struct {
+	F float64
+	U Unit
+}
+
+// String implements the fmt.Stringer interface.
+func (v Value) String() string {
+	return fmt.Sprintf("%f%s", v.F, names[v.U])
+}
+
+var names = [...]string{
+	Px: "px",
+	Dp: "dp",
+	Pt: "pt",
+	In: "in",
+	Mm: "mm",
+	Em: "em",
+	Ex: "ex",
+	Ch: "ch",
+}
+
+// Unit is a unit of length, such as inches or pixels.
+type Unit uint8
+
+const (
+	// Px is a physical pixel, regardless of the DPI resolution.
+	Px Unit = iota
+
+	// Dp is 1 density independent pixel: 1/160th of an inch.
+	Dp
+	// Pt is 1 point: 1/72th of an inch.
+	Pt
+	// Mm is 1 millimetre: 1/25.4th of an inch.
+	Mm
+	// In is 1 inch.
+	//
+	// If the context does not specify a DPI resolution, the recommended
+	// fallback value for conversion is 72 pixels per inch.
+	In
+
+	// Em is the height of the active font face, disregarding extra leading
+	// such as from double-spaced lines of text.
+	//
+	// If the context does not specify an active font face, the recommended
+	// fallback value for conversion is 12pt.
+	Em
+	// Ex is the x-height of the active font face.
+	//
+	// If the context does not specify an x-height, the recommended fallback
+	// value for conversion is 0.5em.
+	Ex
+	// Ch is the character width of the numeral zero glyph '0' of the active
+	// font face.
+	//
+	// If the context does not specify a '0' glyph, the recommended fallback
+	// value for conversion is 0.5em.
+	Ch
+)
+
+// Pixels returns the given number of Px as a Value.
+func Pixels(f float64) Value { return Value{f, Px} }
+
+// DIPs returns the given number of Dp as a Value.
+func DIPs(f float64) Value { return Value{f, Dp} }
+
+// Points returns the given number of Pt as a Value.
+func Points(f float64) Value { return Value{f, Pt} }
+
+// Millimetres returns the given number of Mm as a Value.
+func Millimetres(f float64) Value { return Value{f, Mm} }
+
+// Inches returns the given number of In as a Value.
+func Inches(f float64) Value { return Value{f, In} }
+
+// Ems returns the given number of Em as a Value.
+func Ems(f float64) Value { return Value{f, Em} }
+
+// Exs returns the given number of Ex as a Value.
+func Exs(f float64) Value { return Value{f, Ex} }
+
+// Chs returns the given number of Ch as a Value.
+func Chs(f float64) Value { return Value{f, Ch} }