2 files changed, 214 insertions, 0 deletions
diff --git a/internal/client/ui/ui.go b/internal/client/ui/ui.go
index b03df59..355fbfb 100644
--- a/internal/client/ui/ui.go
+++ b/internal/client/ui/ui.go
@@ -1,11 +1,18 @@
 package ui
 
 import (
+	"bytes"
 	"fmt"
 	"strings"
 
 	tea "github.com/charmbracelet/bubbletea"
 	"github.com/charmbracelet/lipgloss"
+	charmansi "github.com/charmbracelet/x/ansi"
+	"github.com/mattn/go-runewidth"
+	"github.com/muesli/ansi"
+	"github.com/muesli/reflow/truncate"
+
+	"github.com/kyren223/eko/pkg/assert"
 )
 
 type ModelTransition struct {
@@ -22,6 +29,8 @@ func AddBorderHeader(header string, headerOffset int, style lipgloss.Style, rend
 
 	leftWidth := headerOffset
 	rightWidth := topWidth - leftWidth - headerWidth
+	assert.Assert(leftWidth >= 0, "left width cannot be negative", "leftWidth", leftWidth)
+	assert.Assert(rightWidth >= 0, "right width cannot be negative", "rightWidth", rightWidth)
 
 	topStyle := lipgloss.NewStyle().
 		Background(style.GetBorderTopBackground()).
@@ -39,3 +48,119 @@ func AddBorderHeader(header string, headerOffset int, style lipgloss.Style, rend
 
 	return lipgloss.JoinVertical(lipgloss.Left, borderTop, body)
 }
+
+// PlaceOverlay places fg on top of bg.
+func PlaceOverlay(x, y int, fg, bg string, opts ...WhitespaceOption) string {
+	fgLines, fgWidth := getLines(fg)
+	bgLines, bgWidth := getLines(bg)
+	bgHeight := len(bgLines)
+	fgHeight := len(fgLines)
+
+	if fgWidth >= bgWidth && fgHeight >= bgHeight {
+		// FIXME: return fg or bg?
+		return fg
+	}
+	// TODO: allow placement outside of the bg box?
+	x = clamp(x, 0, bgWidth-fgWidth)
+	y = clamp(y, 0, bgHeight-fgHeight)
+
+	ws := &whitespace{}
+	for _, opt := range opts {
+		opt(ws)
+	}
+
+	var b strings.Builder
+	for i, bgLine := range bgLines {
+		if i > 0 {
+			b.WriteByte('\n')
+		}
+		if i < y || i >= y+fgHeight {
+			b.WriteString(bgLine)
+			continue
+		}
+
+		pos := 0
+		if x > 0 {
+			left := truncate.String(bgLine, uint(x))
+			pos = ansi.PrintableRuneWidth(left)
+			b.WriteString(left)
+			if pos < x {
+				b.WriteString(ws.render(x - pos))
+				pos = x
+			}
+		}
+
+		fgLine := fgLines[i-y]
+		b.WriteString(fgLine)
+		pos += ansi.PrintableRuneWidth(fgLine)
+
+		right := cutLeft(bgLine, pos)
+		bgWidth := ansi.PrintableRuneWidth(bgLine)
+		rightWidth := ansi.PrintableRuneWidth(right)
+		if rightWidth <= bgWidth-pos {
+			b.WriteString(ws.render(bgWidth - rightWidth - pos))
+		}
+
+		b.WriteString(right)
+	}
+
+	return b.String()
+}
+
+// cutLeft cuts printable characters from the left.
+// This function is heavily based on muesli's ansi and truncate packages.
+func cutLeft(s string, cutWidth int) string {
+	var (
+		pos    int
+		isAnsi bool
+		ab     bytes.Buffer
+		b      bytes.Buffer
+	)
+	for _, c := range s {
+		var w int
+		if c == ansi.Marker || isAnsi {
+			isAnsi = true
+			ab.WriteRune(c)
+			if ansi.IsTerminator(c) {
+				isAnsi = false
+				if bytes.HasSuffix(ab.Bytes(), []byte("[0m")) {
+					ab.Reset()
+				}
+			}
+		} else {
+			w = runewidth.RuneWidth(c)
+		}
+
+		if pos >= cutWidth {
+			if b.Len() == 0 {
+				if ab.Len() > 0 {
+					b.Write(ab.Bytes())
+				}
+				if pos-cutWidth > 1 {
+					b.WriteByte(' ')
+					continue
+				}
+			}
+			b.WriteRune(c)
+		}
+		pos += w
+	}
+	return b.String()
+}
+
+func getLines(s string) (lines []string, widest int) {
+	lines = strings.Split(s, "\n")
+
+	for _, l := range lines {
+		w := charmansi.StringWidth(l)
+		if widest < w {
+			widest = w
+		}
+	}
+
+	return lines, widest
+}
+
+func clamp(v, lower, upper int) int {
+	return min(max(v, lower), upper)
+}
diff --git a/internal/client/ui/whitespace.go b/internal/client/ui/whitespace.go
new file mode 100644
index 0000000..c7c7d88
--- /dev/null
+++ b/internal/client/ui/whitespace.go
@@ -0,0 +1,89 @@
+package ui
+
+import (
+	"strings"
+
+	"github.com/charmbracelet/lipgloss"
+	"github.com/charmbracelet/x/ansi"
+	"github.com/muesli/termenv"
+)
+
+// whitespace is a whitespace renderer.
+type whitespace struct {
+	re    *lipgloss.Renderer
+	style termenv.Style
+	chars string
+}
+
+// newWhitespace creates a new whitespace renderer. The order of the options
+// matters, if you're using WithWhitespaceRenderer, make sure it comes first as
+// other options might depend on it.
+func newWhitespace(r *lipgloss.Renderer, opts ...WhitespaceOption) *whitespace {
+	w := &whitespace{
+		re:    r,
+		style: r.ColorProfile().String(),
+	}
+	for _, opt := range opts {
+		opt(w)
+	}
+	return w
+}
+
+// Render whitespaces.
+func (w whitespace) render(width int) string {
+	if w.chars == "" {
+		w.chars = " "
+	}
+
+	r := []rune(w.chars)
+	j := 0
+	b := strings.Builder{}
+
+	// Cycle through runes and print them into the whitespace.
+	for i := 0; i < width; {
+		b.WriteRune(r[j])
+		j++
+		if j >= len(r) {
+			j = 0
+		}
+		i += ansi.StringWidth(string(r[j]))
+	}
+
+	// Fill any extra gaps white spaces. This might be necessary if any runes
+	// are more than one cell wide, which could leave a one-rune gap.
+	short := width - ansi.StringWidth(b.String())
+	if short > 0 {
+		b.WriteString(strings.Repeat(" ", short))
+	}
+
+	return w.style.Styled(b.String())
+}
+
+// WhitespaceOption sets a styling rule for rendering whitespace.
+type WhitespaceOption func(*whitespace)
+
+// WithWhitespaceForeground sets the color of the characters in the whitespace.
+func WithWhitespaceForeground(c lipgloss.TerminalColor) WhitespaceOption {
+	return func(w *whitespace) {
+		w.style = w.style.Foreground(ToTermenvColor(w.re, c))
+	}
+}
+
+// WithWhitespaceBackground sets the background color of the whitespace.
+func WithWhitespaceBackground(c lipgloss.TerminalColor) WhitespaceOption {
+	return func(w *whitespace) {
+		w.style = w.style.Background(ToTermenvColor(w.re, c))
+	}
+}
+
+// WithWhitespaceChars sets the characters to be rendered in the whitespace.
+func WithWhitespaceChars(s string) WhitespaceOption {
+	return func(w *whitespace) {
+		w.chars = s
+	}
+}
+
+func ToTermenvColor(re *lipgloss.Renderer, c lipgloss.TerminalColor) termenv.Color {
+	color := c.(lipgloss.Color)
+	return re.ColorProfile().Color(string(color))
+}