1 files changed, 74 insertions, 0 deletions
diff --git a/internal/render/render.go b/internal/render/render.go
new file mode 100644
index 0000000..877e2bd
--- /dev/null
+++ b/internal/render/render.go
@@ -0,0 +1,74 @@
+// Package render renders to the terminal using ANSI escape sequences.
+package render
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"os"
+)
+
+// Renderer updates a terminal UI. Typical usage looks like
+//
+//	r := render.New()
+//	// Game loop
+//	for {
+//	    // Update state
+//
+//	    r.Clear()
+//	    fmt.Fprintf(r, "Render UI by writing to r using io.Writer")
+//	    r.Flush()
+//	}
+type Renderer struct {
+	w         bufio.Writer
+	prevLines int
+}
+
+// New creates a new Renderer
+func New() *Renderer {
+	r := &Renderer{}
+	r.w.Reset(os.Stderr)
+	return r
+}
+
+// Clear clears the screen before rendering a new frame.
+func (r *Renderer) Clear() {
+	if r.prevLines > 0 {
+		fmt.Fprintf(&r.w, "\033[%dA", r.prevLines)
+	}
+	r.prevLines = 0
+	r.w.WriteString("\r")
+}
+
+// Write implements io.Writer.
+func (r *Renderer) Write(buf []byte) (int, error) {
+	totalBytes := 0
+	for len(buf) > 0 {
+		i := bytes.IndexByte(buf, '\n')
+		if i < 0 {
+			n, err := r.w.Write(buf)
+			totalBytes += n
+			return totalBytes, err
+		}
+		line := buf[:i]
+		buf = buf[i+1:]
+		n, err := r.w.Write(line)
+		totalBytes += n
+		if err != nil {
+			return totalBytes, err
+		}
+		if _, err := r.w.WriteString("\033[K\n"); err != nil {
+			return totalBytes, err
+		}
+		totalBytes++
+		r.prevLines++
+	}
+	return totalBytes, nil
+}
+
+// Flush flushes the internal buffer to stdout. Flush should be called at the
+// end of every frame.
+func (r *Renderer) Flush() {
+	r.w.WriteString("\033[J")
+	r.w.Flush()
+}