Add editor packages

internal/editor/buffer/render.go

@@ -0,0 +1,51 @@
+package buffer
+
+// Output holds the visible lines for a terminal viewport and the cursor
+// position adjusted for scrolling.
+// Unlike Cursor, Glyph and Line are 1-indexed terminal coordinates for ANSI
+// cursor positioning.
+type Output struct {
+	Glyph int
+	Line  int
+	Lines []string
+}
+
+// TODO: wrap lines.
+// TODO: add highlighting.
+func (b *Buffer) Render(height, width int) *Output {
+	// Vertical scroll.
+	if b.cursor.line < b.offset.line {
+		b.offset.line = b.cursor.line
+	}
+	if b.cursor.line >= height+b.offset.line {
+		b.offset.line = b.cursor.line - height + 1
+	}
+
+	// Horizontal scroll.
+	if b.cursor.glyph < b.offset.glyph {
+		b.offset.glyph = b.cursor.glyph
+	}
+	if b.cursor.glyph >= width+b.offset.glyph {
+		b.offset.glyph = b.cursor.glyph - width + 1
+	}
+
+	// Generate lines.
+	var lines = []string{}
+	for i := b.offset.line; i < height+b.offset.line; i++ {
+		// Return empty lines for indexes past the buffer's lines.
+		if i >= len(b.lines) {
+			lines = append(lines, "")
+			continue
+		}
+
+		line := b.lines[i].Render(b.offset.glyph, width)
+		lines = append(lines, line)
+	}
+
+	return &Output{
+		// Terminals are 1-indexed.
+		Glyph: b.cursor.glyph - b.offset.glyph + 1,
+		Line:  b.cursor.line - b.offset.line + 1,
+		Lines: lines,
+	}
+}