eng-pad-server/internal/render/svg.go

package render

import (
	"encoding/binary"
	"fmt"
	"math"
	"strings"
)

const canonicalToPDF = 72.0 / 300.0 // 0.24

// maxPointDataSize is the maximum allowed size of point data (4 MB).
const maxPointDataSize = 4 * 1024 * 1024

// PageSizePt returns the page dimensions in PDF points (72 DPI).
func PageSizePt(pageSize string) (float64, float64) {
	switch pageSize {
	case "LARGE":
		return 3300 * canonicalToPDF, 5100 * canonicalToPDF // 792 x 1224
	default: // REGULAR
		return 2550 * canonicalToPDF, 3300 * canonicalToPDF // 612 x 792
	}
}

type Stroke struct {
	PenSize     float32
	Color       int32
	Style       string
	PointData   []byte
	StrokeOrder int
}

// RenderSVG renders a page's strokes as an SVG document.
func RenderSVG(pageSize string, strokes []Stroke) (string, error) {
	w, h := PageSizePt(pageSize)
	var b strings.Builder

	fmt.Fprintf(&b, `<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 %.0f %.0f" width="%.0f" height="%.0f">`, w, h, w, h)
	b.WriteString("\n")
	// White background
	fmt.Fprintf(&b, `<rect width="%.0f" height="%.0f" fill="white"/>`, w, h)
	b.WriteString("\n")

	for _, s := range strokes {
		points, err := decodePoints(s.PointData)
		if err != nil {
			continue
		}
		if len(points) < 4 {
			continue
		}

		penW := float64(s.PenSize) * canonicalToPDF
		color := colorToCSS(s.Color)

		// Build path data
		var path strings.Builder
		fmt.Fprintf(&path, "M %.2f %.2f", points[0]*canonicalToPDF, points[1]*canonicalToPDF)
		for i := 2; i < len(points)-1; i += 2 {
			fmt.Fprintf(&path, " L %.2f %.2f", points[i]*canonicalToPDF, points[i+1]*canonicalToPDF)
		}

		attrs := fmt.Sprintf(`stroke="%s" stroke-width="%.2f" stroke-linecap="round" stroke-linejoin="round" fill="none"`, color, penW)
		if s.Style == "dashed" {
			attrs += fmt.Sprintf(` stroke-dasharray="%.1f %.1f"`, 7.2, 4.8)
		}

		fmt.Fprintf(&b, `<path d="%s" %s/>`, path.String(), attrs)
		b.WriteString("\n")

		// Arrow heads
		if (s.Style == "arrow" || s.Style == "double_arrow") && len(points) >= 4 {
			x1, y1 := points[0]*canonicalToPDF, points[1]*canonicalToPDF
			x2, y2 := points[2]*canonicalToPDF, points[3]*canonicalToPDF
			b.WriteString(renderArrowHeads(x1, y1, x2, y2, s.Style, penW, color))
		}
	}

	b.WriteString("</svg>")
	return b.String(), nil
}

func renderArrowHeads(x1, y1, x2, y2 float64, style string, penW float64, color string) string {
	arrowLen := 40.0 * canonicalToPDF
	arrowAngle := 25.0 * math.Pi / 180.0
	dx := x2 - x1
	dy := y2 - y1
	angle := math.Atan2(dy, dx)

	var b strings.Builder
	attrs := fmt.Sprintf(`stroke="%s" stroke-width="%.2f" stroke-linecap="round"`, color, penW)

	// End arrow
	ax1 := x2 - arrowLen*math.Cos(angle-arrowAngle)
	ay1 := y2 - arrowLen*math.Sin(angle-arrowAngle)
	ax2 := x2 - arrowLen*math.Cos(angle+arrowAngle)
	ay2 := y2 - arrowLen*math.Sin(angle+arrowAngle)
	fmt.Fprintf(&b, `<line x1="%.2f" y1="%.2f" x2="%.2f" y2="%.2f" %s/>`, x2, y2, ax1, ay1, attrs)
	b.WriteString("\n")
	fmt.Fprintf(&b, `<line x1="%.2f" y1="%.2f" x2="%.2f" y2="%.2f" %s/>`, x2, y2, ax2, ay2, attrs)
	b.WriteString("\n")

	if style == "double_arrow" {
		revAngle := angle + math.Pi
		bx1 := x1 - arrowLen*math.Cos(revAngle-arrowAngle)
		by1 := y1 - arrowLen*math.Sin(revAngle-arrowAngle)
		bx2 := x1 - arrowLen*math.Cos(revAngle+arrowAngle)
		by2 := y1 - arrowLen*math.Sin(revAngle+arrowAngle)
		fmt.Fprintf(&b, `<line x1="%.2f" y1="%.2f" x2="%.2f" y2="%.2f" %s/>`, x1, y1, bx1, by1, attrs)
		b.WriteString("\n")
		fmt.Fprintf(&b, `<line x1="%.2f" y1="%.2f" x2="%.2f" y2="%.2f" %s/>`, x1, y1, bx2, by2, attrs)
		b.WriteString("\n")
	}

	return b.String()
}

// decodePoints decodes a byte slice of little-endian float32 values into float64.
// Returns an error if the data length is not a multiple of 4, exceeds maxPointDataSize,
// or contains NaN/Inf values.
func decodePoints(data []byte) ([]float64, error) {
	if len(data)%4 != 0 {
		return nil, fmt.Errorf("point data length %d is not a multiple of 4", len(data))
	}
	if len(data) > maxPointDataSize {
		return nil, fmt.Errorf("point data size %d exceeds maximum %d", len(data), maxPointDataSize)
	}

	count := len(data) / 4
	points := make([]float64, 0, count)
	for i := 0; i < count; i++ {
		bits := binary.LittleEndian.Uint32(data[i*4 : (i+1)*4])
		v := float64(math.Float32frombits(bits))
		if math.IsNaN(v) || math.IsInf(v, 0) {
			return nil, fmt.Errorf("point data contains NaN or Inf at index %d", i)
		}
		points = append(points, v)
	}
	return points, nil
}

func colorToCSS(argb int32) string {
	r := (argb >> 16) & 0xFF
	g := (argb >> 8) & 0xFF
	b := argb & 0xFF
	if r == 0 && g == 0 && b == 0 {
		return "black"
	}
	return fmt.Sprintf("rgb(%d,%d,%d)", r, g, b)
}