From 3765727e659402b7901f8e739f769db567404421 Mon Sep 17 00:00:00 2001
From: Christian Rocha <christian@rocha.is>
Date: Mon, 12 Dec 2022 09:52:00 -0500
Subject: [PATCH] docs(examples): add cellbuffer + harmonica example (#409)

---
 examples/cellbuffer/main.go | 199 ++++++++++++++++++++++++++++++++++++
 examples/go.mod             |   1 +
 2 files changed, 200 insertions(+)
 create mode 100644 examples/cellbuffer/main.go

diff --git a/examples/cellbuffer/main.go b/examples/cellbuffer/main.go
new file mode 100644
index 0000000..00de39c
--- /dev/null
+++ b/examples/cellbuffer/main.go
@@ -0,0 +1,199 @@
+package main
+
+// A simple example demonstrating how to draw and animate on a cellular grid.
+// Note that the cellbuffer implementation in this example does not support
+// double-width runes.
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"time"
+
+	tea "github.com/charmbracelet/bubbletea"
+	"github.com/charmbracelet/harmonica"
+)
+
+const (
+	fps       = 60
+	frequency = 7.5
+	damping   = 0.15
+)
+
+func drawEllipse(cb *cellbuffer, xc, yc, rx, ry float64) {
+	const c = "*"
+	var (
+		dx, dy, d1, d2 float64
+		x              float64 = 0
+		y                      = ry
+	)
+
+	d1 = ry*ry - rx*rx*ry + 0.25*rx*rx
+	dx = 2 * ry * ry * x
+	dy = 2 * rx * rx * y
+
+	for dx < dy {
+		cb.set(c, int(x+xc), int(y+yc))
+		cb.set(c, int(-x+xc), int(y+yc))
+		cb.set(c, int(x+xc), int(-y+yc))
+		cb.set(c, int(-x+xc), int(-y+yc))
+		if d1 < 0 {
+			x++
+			dx = dx + (2 * ry * ry)
+			d1 = d1 + dx + (ry * ry)
+		} else {
+			x++
+			y--
+			dx = dx + (2 * ry * ry)
+			dy = dy - (2 * rx * rx)
+			d1 = d1 + dx - dy + (ry * ry)
+		}
+	}
+
+	d2 = ((ry * ry) * ((x + 0.5) * (x + 0.5))) + ((rx * rx) * ((y - 1) * (y - 1))) - (rx * rx * ry * ry)
+
+	for y >= 0 {
+		cb.set(c, int(x+xc), int(y+yc))
+		cb.set(c, int(-x+xc), int(y+yc))
+		cb.set(c, int(x+xc), int(-y+yc))
+		cb.set(c, int(-x+xc), int(-y+yc))
+		if d2 > 0 {
+			y--
+			dy = dy - (2 * rx * rx)
+			d2 = d2 + (rx * rx) - dy
+		} else {
+			y--
+			x++
+			dx = dx + (2 * ry * ry)
+			dy = dy - (2 * rx * rx)
+			d2 = d2 + dx - dy + (rx * rx)
+		}
+	}
+}
+
+type cellbuffer struct {
+	cells  []string
+	stride int
+}
+
+func (c *cellbuffer) init(w, h int) {
+	if w == 0 {
+		return
+	}
+	c.stride = w
+	c.cells = make([]string, w*h)
+	c.wipe()
+}
+
+func (c cellbuffer) set(v string, x, y int) {
+	i := y*c.stride + x
+	if i > len(c.cells)-1 || x < 0 || y < 0 || x >= c.width() || y >= c.height() {
+		return
+	}
+	c.cells[i] = v
+}
+
+func (c *cellbuffer) clear(x, y int) {
+	c.set(" ", x, y)
+}
+
+func (c *cellbuffer) wipe() {
+	for i := range c.cells {
+		c.cells[i] = " "
+	}
+}
+
+func (c cellbuffer) width() int {
+	return c.stride
+}
+
+func (c cellbuffer) height() int {
+	h := len(c.cells) / c.stride
+	if len(c.cells)%c.stride != 0 {
+		h++
+	}
+	return h
+}
+
+func (c cellbuffer) ready() bool {
+	return len(c.cells) > 0
+}
+
+func (c cellbuffer) String() string {
+	var b strings.Builder
+	for i := 0; i < len(c.cells); i++ {
+		if i > 0 && i%c.stride == 0 && i < len(c.cells)-1 {
+			b.WriteRune('\n')
+		}
+		b.WriteString(c.cells[i])
+	}
+	return b.String()
+}
+
+type frameMsg struct{}
+
+func animate() tea.Cmd {
+	return tea.Tick(time.Second/fps, func(_ time.Time) tea.Msg {
+		return frameMsg{}
+	})
+}
+
+type model struct {
+	cells                cellbuffer
+	spring               harmonica.Spring
+	targetX, targetY     float64
+	x, y                 float64
+	xVelocity, yVelocity float64
+}
+
+func (m model) Init() tea.Cmd {
+	return animate()
+}
+
+func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
+	switch msg := msg.(type) {
+	case tea.KeyMsg:
+		return m, tea.Quit
+	case tea.WindowSizeMsg:
+		if !m.cells.ready() {
+			m.targetX, m.targetY = float64(msg.Width)/2, float64(msg.Height)/2
+		}
+		m.cells.init(msg.Width, msg.Height)
+		return m, nil
+	case tea.MouseMsg:
+		if !m.cells.ready() {
+			return m, nil
+		}
+		m.targetX, m.targetY = float64(msg.X), float64(msg.Y)
+		return m, nil
+
+	case frameMsg:
+		if !m.cells.ready() {
+			return m, nil
+		}
+
+		m.cells.wipe()
+		m.x, m.xVelocity = m.spring.Update(m.x, m.xVelocity, m.targetX)
+		m.y, m.yVelocity = m.spring.Update(m.y, m.yVelocity, m.targetY)
+		drawEllipse(&m.cells, m.x, m.y, 16, 8)
+		return m, animate()
+	default:
+		return m, nil
+	}
+}
+
+func (m model) View() string {
+	return m.cells.String()
+}
+
+func main() {
+	m := model{
+		spring: harmonica.NewSpring(harmonica.FPS(fps), frequency, damping),
+	}
+
+	p := tea.NewProgram(m, tea.WithAltScreen(), tea.WithMouseCellMotion())
+	if _, err := p.Run(); err != nil {
+		fmt.Println("Uh oh:", err)
+		os.Exit(1)
+	}
+}
diff --git a/examples/go.mod b/examples/go.mod
index 7465247..8804897 100644
--- a/examples/go.mod
+++ b/examples/go.mod
@@ -6,6 +6,7 @@ require (
 	github.com/charmbracelet/bubbles v0.13.1-0.20220815142520-649f78e1fd8b
 	github.com/charmbracelet/bubbletea v0.22.0
 	github.com/charmbracelet/glamour v0.6.0
+	github.com/charmbracelet/harmonica v0.2.0
 	github.com/charmbracelet/lipgloss v0.6.0
 	github.com/fogleman/ease v0.0.0-20170301025033-8da417bf1776
 	github.com/lucasb-eyer/go-colorful v1.2.0