patterm/internal/app/viewport_renderer.go

package app

import (
	"fmt"
	"strconv"
	"strings"
	"sync"
)

// viewportRenderer rewrites child PTY output so it lands inside the
// main viewport instead of controlling patterm's full host terminal.
type viewportRenderer struct {
	mu      sync.Mutex
	shifter *cursorShifter
	layout  terminalLayout
	row     int
	col     int

	state   viewportState
	buf     []byte
	pending strings.Builder
}

type viewportState int

const (
	vpNormal viewportState = iota
	vpEsc
	vpCSI
	vpOSC
	vpOSCEsc
	vpDCS
	vpDCSEsc
	vpSOSPMAPC
	vpSOSPMAPCEsc
)

func newViewportRenderer(l terminalLayout) *viewportRenderer {
	return &viewportRenderer{
		shifter: newCursorShifter(int(l.mainTop)-1, int(l.childRows())),
		layout:  l,
		row:     1,
		col:     1,
	}
}

func (vr *viewportRenderer) SetLayout(l terminalLayout) {
	vr.mu.Lock()
	defer vr.mu.Unlock()
	vr.layout = l
	vr.shifter.SetGeometry(int(l.mainTop)-1, int(l.childRows()))
}

func (vr *viewportRenderer) Render(in []byte) []byte {
	vr.mu.Lock()
	defer vr.mu.Unlock()
	vr.pending.Reset()
	for _, b := range in {
		vr.feed(b)
	}
	return []byte(vr.pending.String())
}

func (vr *viewportRenderer) ClearViewport() []byte {
	vr.mu.Lock()
	defer vr.mu.Unlock()
	return []byte(vr.clearViewport())
}

func (vr *viewportRenderer) feed(b byte) {
	switch vr.state {
	case vpNormal:
		if b == 0x1b {
			vr.state = vpEsc
			vr.buf = vr.buf[:0]
			vr.buf = append(vr.buf, b)
			return
		}
		vr.pending.WriteByte(b)
		vr.advancePrintable(b)
	case vpEsc:
		vr.buf = append(vr.buf, b)
		switch b {
		case '[':
			vr.state = vpCSI
		case ']':
			vr.state = vpOSC
		case 'P':
			vr.state = vpDCS
		case 'X', '^', '_':
			vr.state = vpSOSPMAPC
		default:
			vr.pending.Write(vr.buf)
			vr.state = vpNormal
			vr.buf = vr.buf[:0]
		}
	case vpCSI:
		vr.buf = append(vr.buf, b)
		if isCSIFinal(b) {
			vr.emitCSI()
			vr.state = vpNormal
			vr.buf = vr.buf[:0]
		}
	case vpOSC:
		vr.buf = append(vr.buf, b)
		switch b {
		case 0x07:
			vr.pending.Write(vr.buf)
			vr.state = vpNormal
			vr.buf = vr.buf[:0]
		case 0x1b:
			vr.state = vpOSCEsc
		}
	case vpOSCEsc:
		vr.buf = append(vr.buf, b)
		vr.pending.Write(vr.buf)
		vr.state = vpNormal
		vr.buf = vr.buf[:0]
	case vpDCS:
		vr.buf = append(vr.buf, b)
		if b == 0x1b {
			vr.state = vpDCSEsc
		}
	case vpDCSEsc:
		vr.buf = append(vr.buf, b)
		vr.pending.Write(vr.buf)
		vr.state = vpNormal
		vr.buf = vr.buf[:0]
	case vpSOSPMAPC:
		vr.buf = append(vr.buf, b)
		if b == 0x1b {
			vr.state = vpSOSPMAPCEsc
		}
	case vpSOSPMAPCEsc:
		vr.buf = append(vr.buf, b)
		vr.pending.Write(vr.buf)
		vr.state = vpNormal
		vr.buf = vr.buf[:0]
	}
}

func (vr *viewportRenderer) emitCSI() {
	if len(vr.buf) < 3 {
		vr.pending.Write(vr.buf)
		return
	}
	final := vr.buf[len(vr.buf)-1]
	params := vr.buf[2 : len(vr.buf)-1]

	if final == 'h' || final == 'l' {
		if isAltScreenMode(params) {
			return
		}
	}

	switch final {
	case 'J':
		n, ok := parseOneParam(params, 0)
		if !ok {
			vr.pending.Write(vr.shifter.Shift(vr.buf))
			return
		}
		switch n {
		case 0:
			vr.pending.WriteString(vr.clearViewportFromCursor())
		case 1:
			vr.pending.WriteString(vr.clearViewportToCursor())
		case 2, 3:
			vr.pending.WriteString(vr.clearViewport())
		default:
			vr.pending.Write(vr.shifter.Shift(vr.buf))
		}
	case 'K':
		n, ok := parseOneParam(params, 0)
		if !ok {
			vr.pending.Write(vr.shifter.Shift(vr.buf))
			return
		}
		vr.pending.WriteString(vr.clearLine(n))
	default:
		vr.pending.Write(vr.shifter.Shift(vr.buf))
	}
	vr.trackCSI(final, params)
}

func isAltScreenMode(params []byte) bool {
	s := string(params)
	if !strings.HasPrefix(s, "?") {
		return false
	}
	for _, p := range strings.Split(strings.TrimPrefix(s, "?"), ";") {
		switch p {
		case "47", "1047", "1049":
			return true
		}
	}
	return false
}

func (vr *viewportRenderer) clearViewport() string {
	var b strings.Builder
	b.WriteString("\x1b7")
	for r := uint16(0); r < vr.layout.childRows(); r++ {
		fmt.Fprintf(&b, "\x1b[%d;%dH\x1b[%dX", int(vr.layout.mainTop+r), int(vr.layout.mainLeft), int(vr.layout.childCols()))
	}
	b.WriteString("\x1b8")
	return b.String()
}

// clearViewportFromCursor implements `CSI 0 J` clamped to the viewport.
// Without clamping, the child's "clear to end of screen" would reach the
// rightmost columns and erase the sidebar.
func (vr *viewportRenderer) clearViewportFromCursor() string {
	row, col := vr.row, vr.col
	cols := int(vr.layout.childCols())
	rows := int(vr.layout.childRows())
	if row < 1 {
		row = 1
	}
	if col < 1 {
		col = 1
	}
	var b strings.Builder
	b.WriteString("\x1b7")
	if remaining := cols - col + 1; remaining > 0 {
		fmt.Fprintf(&b, "\x1b[%dX", remaining)
	}
	for r := row + 1; r <= rows; r++ {
		fmt.Fprintf(&b, "\x1b[%d;%dH\x1b[%dX",
			int(vr.layout.mainTop)+r-1, int(vr.layout.mainLeft), cols)
	}
	b.WriteString("\x1b8")
	return b.String()
}

// clearViewportToCursor implements `CSI 1 J` clamped to the viewport.
func (vr *viewportRenderer) clearViewportToCursor() string {
	row, col := vr.row, vr.col
	cols := int(vr.layout.childCols())
	if row < 1 {
		row = 1
	}
	if col < 1 {
		col = 1
	}
	var b strings.Builder
	b.WriteString("\x1b7")
	for r := 1; r < row; r++ {
		fmt.Fprintf(&b, "\x1b[%d;%dH\x1b[%dX",
			int(vr.layout.mainTop)+r-1, int(vr.layout.mainLeft), cols)
	}
	fmt.Fprintf(&b, "\x1b[%d;%dH\x1b[%dX",
		int(vr.layout.mainTop)+row-1, int(vr.layout.mainLeft), col)
	b.WriteString("\x1b8")
	return b.String()
}

func (vr *viewportRenderer) clearLine(n int) string {
	right := int(vr.layout.childCols())
	if vr.col < 1 {
		vr.col = 1
	}
	if vr.col > right {
		vr.col = right
	}
	switch n {
	case 0:
		return "\x1b[" + strconv.Itoa(right-vr.col+1) + "X"
	case 1:
		return "\x1b7\r\x1b[" + strconv.Itoa(vr.col) + "X\x1b8"
	case 2:
		return "\x1b7\r\x1b[" + strconv.Itoa(right) + "X\x1b8"
	default:
		return "\x1b[" + strconv.Itoa(right-vr.col+1) + "X"
	}
}

func (vr *viewportRenderer) advancePrintable(b byte) {
	switch b {
	case '\r':
		vr.col = 1
	case '\n':
		vr.row++
	case '\b':
		if vr.col > 1 {
			vr.col--
		}
	case '\t':
		vr.col += 8 - ((vr.col - 1) % 8)
	default:
		if b >= 0x20 && b != 0x7f {
			vr.col++
		}
	}
	vr.clampCursor()
}

func (vr *viewportRenderer) trackCSI(final byte, params []byte) {
	switch final {
	case 'H', 'f':
		r, c, ok := parseTwoParams(params)
		if ok {
			vr.row, vr.col = r, c
		}
	case 'G', '`':
		c, ok := parseOneParam(params, 1)
		if ok {
			vr.col = c
		}
	case 'd':
		r, ok := parseOneParam(params, 1)
		if ok {
			vr.row = r
		}
	case 'A':
		n, ok := parseOneParam(params, 1)
		if ok {
			vr.row -= n
		}
	case 'B', 'e':
		n, ok := parseOneParam(params, 1)
		if ok {
			vr.row += n
		}
	case 'C', 'a':
		n, ok := parseOneParam(params, 1)
		if ok {
			vr.col += n
		}
	case 'D':
		n, ok := parseOneParam(params, 1)
		if ok {
			vr.col -= n
		}
	}
	vr.clampCursor()
}

func (vr *viewportRenderer) clampCursor() {
	if vr.row < 1 {
		vr.row = 1
	}
	if vr.col < 1 {
		vr.col = 1
	}
	if max := int(vr.layout.childRows()); vr.row > max {
		vr.row = max
	}
	if max := int(vr.layout.childCols()); vr.col > max {
		vr.col = max
	}
}