unit: add Metric.DpToSp and Metric.SpToDp

It's sometimes necessary to specify padding or spacing based on
the text size.

Signed-off-by: Egon Elbre <egonelbre@gmail.com>

unit/unit.go

@@ -44,18 +44,27 @@ type (
 
 // Dp converts v to pixels, rounded to the nearest integer value.
 func (c Metric) Dp(v Dp) int {
-	s := c.PxPerDp
-	if s == 0. {
-		s = 1.
-	}
-	return int(math.Round(float64(s) * float64(v)))
+	return int(math.Round(float64(nonZero(c.PxPerDp)) * float64(v)))
 }
 
 // Sp converts v to pixels, rounded to the nearest integer value.
 func (c Metric) Sp(v Sp) int {
-	s := c.PxPerSp
-	if s == 0. {
-		s = 1.
-	}
-	return int(math.Round(float64(s) * float64(v)))
+	return int(math.Round(float64(nonZero(c.PxPerSp)) * float64(v)))
+}
+
+// DpToSp converts v dp to sp.
+func (c Metric) DpToSp(v Dp) Sp {
+	return Sp(float32(v) * nonZero(c.PxPerDp) / nonZero(c.PxPerSp))
+}
+
+// SpToDp converts v sp to dp.
+func (c Metric) SpToDp(v Sp) Dp {
+	return Dp(float32(v) * nonZero(c.PxPerSp) / nonZero(c.PxPerDp))
+}
+
+func nonZero(v float32) float32 {
+	if v == 0. {
+		return 1
+	}
+	return v
 }

unit/unit_test.go

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package unit_test
+
+import (
+	"testing"
+
+	"gioui.org/unit"
+)
+
+func TestMetric_DpToSp(t *testing.T) {
+	m := unit.Metric{
+		PxPerDp: 2,
+		PxPerSp: 3,
+	}
+
+	{
+		exp := m.Dp(5)
+		got := m.Sp(m.DpToSp(5))
+		if got != exp {
+			t.Errorf("DpToSp conversion mismatch %v != %v", exp, got)
+		}
+	}
+
+	{
+		exp := m.Sp(5)
+		got := m.Dp(m.SpToDp(5))
+		if got != exp {
+			t.Errorf("SpToDp conversion mismatch %v != %v", exp, got)
+		}
+	}
+}