Add code components 366-385; ScrollArea Follow, editor sync

A terminal with history, a Go-colored code block, a log viewer that
follows, a hex viewer, a memory map, a JSON tree, a line diff, a GAME
OVER crash screen, an FPS meter, profiler bars, breakpoints,
variables, command history, a shortcut list, a debug grid, a pixel
editor that exports string art, a palette editor, a sprite sheet, a
glyph map and a 6502 register view.

layout.ScrollArea takes Follow, and ChatLog now uses it. data.TreeNode
takes a Value drawn in its Slot. pixel gains Has, and Sprite.Art
writes MustSprite art. pixel.Editor takes the caller's value at each
input event, so a value changed between frames is never typed onto.

code/diffview.go

@@ -0,0 +1,94 @@
+package code
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/layout"
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type DiffViewOptions struct{}
+
+// change is a diff line's sign, text and line numbers.
+type change struct {
+	sign     byte
+	text     string
+	old, new int
+}
+
+// DiffView marks lines removed with -, added with +.
+func DiffView(c *ui.Context, text, before, after string, o DiffViewOptions) *ui.Element {
+	k := pixel.K(c)
+	changes := diff(strings.Split(tidy(before), "\n"), strings.Split(tidy(after), "\n"))
+	// Rows share the longest width, so tints line up.
+	w := 0
+	for _, ch := range changes {
+		w = max(w, 32+pixel.Measure(ch.text))
+	}
+	return layout.Panel(c, layout.PanelOptions{Style: pixel.Sunken, Fill: pixel.Well, Padding: 2}, func() {
+		for _, ch := range changes {
+			ui.Box(c).Size(k.D(w), k.D(pixel.LineHeight)).Role(ui.RoleText).Label(string(ch.sign) + " " + ch.text).Draw(func(p *ui.Painter, r ui.Rect) {
+				g := k.Canvas(p, r)
+				// Changed lines take their color and a left bar.
+				ink, text := pixel.Faint, pixel.Ink
+				switch ch.sign {
+				case '+':
+					ink, text = pixel.Good, pixel.Good
+				case '-':
+					ink, text = pixel.Bad, pixel.Bad
+				}
+				if ch.sign != ' ' {
+					g.Rect(0, 0, 2, g.H, ink)
+				}
+				num := func(x, n int) {
+					if n > 0 {
+						s := fmt.Sprint(n)
+						g.Micro(x-pixel.MicroWidth(s), 4, s, pixel.Faint, 1)
+					}
+				}
+				num(11, ch.old)
+				num(23, ch.new)
+				g.Text(26, 0, string(ch.sign), ink, 1)
+				g.Text(32, 0, ch.text, text, 1)
+			})
+		}
+	}).Role(ui.RoleGroup).Label(text)
+}
+
+// diff walks the longest common run of lines.
+func diff(a, b []string) []change {
+	// lcs[i][j] is the common length of a[i:] and b[j:].
+	lcs := make([][]int, len(a)+1)
+	for i := range lcs {
+		lcs[i] = make([]int, len(b)+1)
+	}
+	for i := len(a) - 1; i >= 0; i-- {
+		for j := len(b) - 1; j >= 0; j-- {
+			if a[i] == b[j] {
+				lcs[i][j] = lcs[i+1][j+1] + 1
+			} else {
+				lcs[i][j] = max(lcs[i+1][j], lcs[i][j+1])
+			}
+		}
+	}
+	var out []change
+	i, j := 0, 0
+	for i < len(a) || j < len(b) {
+		switch {
+		case i < len(a) && j < len(b) && a[i] == b[j]:
+			out = append(out, change{' ', a[i], i + 1, j + 1})
+			i, j = i+1, j+1
+		// Taken out before put in, as diffs read.
+		case i < len(a) && (j == len(b) || lcs[i+1][j] >= lcs[i][j+1]):
+			out = append(out, change{'-', a[i], i + 1, 0})
+			i++
+		default:
+			out = append(out, change{'+', b[j], 0, j + 1})
+			j++
+		}
+	}
+	return out
+}