all: rename the gioui.org/ui module to gioui.org

The "ui" is redundant and stutters.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

internal/fling/animation.go

@@ -0,0 +1,98 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package fling
+
+import (
+	"math"
+	"runtime"
+	"time"
+
+	"gioui.org/ui"
+)
+
+type Animation struct {
+	// Current offset in pixels.
+	x float32
+	// Initial time.
+	t0 time.Time
+	// Initial velocity in pixels pr second.
+	v0 float32
+}
+
+var (
+	// Pixels/second.
+	minFlingVelocity = ui.Dp(50)
+	maxFlingVelocity = ui.Dp(8000)
+)
+
+const (
+	thresholdVelocity = 1
+)
+
+// Start a fling given a starting velocity. Returns whether a
+// fling was started.
+func (f *Animation) Start(c ui.Config, velocity float32) bool {
+	min := float32(c.Px(minFlingVelocity))
+	v := velocity
+	if -min <= v && v <= min {
+		return false
+	}
+	max := float32(c.Px(maxFlingVelocity))
+	if v > max {
+		v = max
+	} else if v < -max {
+		v = -max
+	}
+	f.init(c.Now(), v)
+	return true
+}
+
+func (f *Animation) init(now time.Time, v0 float32) {
+	f.t0 = now
+	f.v0 = v0
+	f.x = 0
+}
+
+func (f *Animation) Active() bool {
+	return f.v0 != 0
+}
+
+// Tick computes and returns a fling distance since
+// the last time Tick was called.
+func (f *Animation) Tick(now time.Time) int {
+	if !f.Active() {
+		return 0
+	}
+	var k float32
+	if runtime.GOOS == "darwin" {
+		k = -2 // iOS
+	} else {
+		k = -4.2 // Android and default
+	}
+	t := now.Sub(f.t0)
+	// The acceleration x''(t) of a point mass with a drag
+	// force, f, proportional with velocity, x'(t), is
+	// governed by the equation
+	//
+	// x''(t) = kx'(t)
+	//
+	// Given the starting position x(0) = 0, the starting
+	// velocity x'(0) = v0, the position is then
+	// given by
+	//
+	// x(t) = v0*e^(k*t)/k - v0/k
+	//
+	ekt := float32(math.Exp(float64(k) * t.Seconds()))
+	x := f.v0*ekt/k - f.v0/k
+	dist := x - f.x
+	idist := int(math.Round(float64(dist)))
+	f.x += float32(idist)
+	// Solving for the velocity x'(t) gives us
+	//
+	// x'(t) = v0*e^(k*t)
+	v := f.v0 * ekt
+	if -thresholdVelocity < v && v < thresholdVelocity {
+		f.v0 = 0
+	}
+	return idist
+}