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// Copyright 2026 Rose Hogenson <r@roseh.moe>
//
// This file is part of ccp.
//
// Ccp is free software: you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the Free Software
// Foundation, either version 3 of the License, or (at your option) any later
// version.
//
// Ccp is distributed in the hope that it will be useful, but WITHOUT ANY
// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
// FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
// details.
//
// You should have received a copy of the GNU General Public License along with
// ccp. If not, see <https://www.gnu.org/licenses/>.
// Package render renders to the terminal using ANSI escape sequences.
package render
import (
"bufio"
"bytes"
"fmt"
"os"
runewidth "github.com/mattn/go-runewidth"
)
// 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
width int
partialLineLen 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(width int) {
r.width = width
if r.prevLines > 0 {
fmt.Fprintf(&r.w, "\033[%dA", r.prevLines)
}
r.w.WriteString("\r")
r.prevLines = 0
r.partialLineLen = 0
}
func truncate(b []byte, width int) ([]byte, int) {
str := string(b)
currentWidth := 0
for i := 0; i < len(b); {
chunkBytes := bytes.Index(b[i:], []byte("\033["))
if chunkBytes < 0 {
chunkBytes = len(b) - i
} else if chunkBytes == 0 {
// An escape sequence usually starts with [, then has one or two numbers
// separated by semicolon, and ends with some terminating character. To
// try and munch the whole sequence, skip over any numbers and
// semicolon here.
for i += 2; i < len(b)-1 && ('0' <= b[i] && b[i] <= '9' || b[i] == ';'); i++ {
}
// Skip the terminating character.
i++
continue
}
chunkWidth := runewidth.StringWidth(str[i : i+chunkBytes])
if currentWidth+chunkWidth <= width {
i += chunkBytes
currentWidth += chunkWidth
continue
}
lastChunk := runewidth.Truncate(str[i:i+chunkBytes], width-currentWidth, "")
return b[:i+len(lastChunk)], width
}
return b, currentWidth
}
// 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 {
line, lineWidth := truncate(buf, r.width-r.partialLineLen)
r.partialLineLen += lineWidth
if n, err := r.w.Write(line); err != nil {
return totalBytes + n, err
}
totalBytes += len(buf)
return totalBytes, nil
}
line, _ := truncate(buf[:i], r.width)
buf = buf[i+1:]
if n, err := r.w.Write(line); err != nil {
return totalBytes + n, err
}
totalBytes += i
if _, err := r.w.WriteString("\033[K\n"); err != nil {
return totalBytes, err
}
totalBytes++
r.prevLines++
r.partialLineLen = 0
}
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()
}
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