img2pdf/internal/convert/convert.go

package convert

import (
	"bytes"
	"fmt"
	"image"
	"image/color"
	_ "image/gif"
	"image/jpeg"
	_ "image/png"
	"sort"
	"strconv"
	"strings"
)

// SourceFile represents an uploaded image that should become a PDF page.
type SourceFile struct {
	Name string
	Data []byte
}

// ToPDF converts the given image files into a single PDF document using only
// standard library encoders.
func ToPDF(files []SourceFile) ([]byte, error) {
	if len(files) == 0 {
		return nil, fmt.Errorf("no images provided")
	}

	sort.Slice(files, func(i, j int) bool {
		return NaturalLess(files[i].Name, files[j].Name)
	})

	var pages []pdfImage
	for _, f := range files {
		img, _, err := image.Decode(bytes.NewReader(f.Data))
		if err != nil {
			return nil, fmt.Errorf("decode %s: %w", f.Name, err)
		}
		jpegData, w, h, err := toJPEG(img)
		if err != nil {
			return nil, fmt.Errorf("encode %s: %w", f.Name, err)
		}
		pages = append(pages, pdfImage{
			Width:  w,
			Height: h,
			Data:   jpegData,
		})
	}
	return buildPDF(pages)
}

// toJPEG flattens any transparency onto white and returns JPEG bytes.
func toJPEG(img image.Image) ([]byte, int, int, error) {
	b := img.Bounds()
	rgba := image.NewRGBA(b)
	// Default to white background.
	for y := b.Min.Y; y < b.Max.Y; y++ {
		for x := b.Min.X; x < b.Max.X; x++ {
			c := color.RGBAModel.Convert(img.At(x, y)).(color.RGBA)
			// Premultiply to drop alpha onto white.
			if c.A < 255 {
				alpha := float64(c.A) / 255.0
				c.R = uint8(float64(c.R)*alpha + 255*(1-alpha))
				c.G = uint8(float64(c.G)*alpha + 255*(1-alpha))
				c.B = uint8(float64(c.B)*alpha + 255*(1-alpha))
				c.A = 255
			}
			rgba.SetRGBA(x, y, c)
		}
	}
	var buf bytes.Buffer
	if err := jpeg.Encode(&buf, rgba, &jpeg.Options{Quality: 90}); err != nil {
		return nil, 0, 0, err
	}
	return buf.Bytes(), b.Dx(), b.Dy(), nil
}

// NaturalLess performs a natural, case-insensitive comparison of two strings.
func NaturalLess(a, b string) bool {
	as := splitNumeric(strings.ToLower(a))
	bs := splitNumeric(strings.ToLower(b))

	for i := 0; i < len(as) && i < len(bs); i++ {
		if as[i] == bs[i] {
			continue
		}
		if ia, oka := toInt(as[i]); oka {
			if ib, okb := toInt(bs[i]); okb {
				return ia < ib
			}
		}
		return as[i] < bs[i]
	}
	return len(as) < len(bs)
}

func splitNumeric(s string) []string {
	var parts []string
	curr := strings.Builder{}
	isDigit := func(r rune) bool { return r >= '0' && r <= '9' }

	var digitMode *bool
	for _, r := range s {
		digit := isDigit(r)
		if digitMode == nil {
			digitMode = &digit
		}
		if digit != *digitMode {
			parts = append(parts, curr.String())
			curr.Reset()
			digitMode = &digit
		}
		curr.WriteRune(r)
	}
	if curr.Len() > 0 {
		parts = append(parts, curr.String())
	}
	return parts
}

func toInt(s string) (int, bool) {
	n := 0
	for _, r := range s {
		if r < '0' || r > '9' {
			return 0, false
		}
		n = n*10 + int(r-'0')
	}
	return n, true
}

// pdfImage holds minimal data for embedding into a PDF.
type pdfImage struct {
	Width  int
	Height int
	Data   []byte
}

// buildPDF writes a very small PDF with each image on its own page sized to the image.
func buildPDF(images []pdfImage) ([]byte, error) {
	if len(images) == 0 {
		return nil, fmt.Errorf("no images")
	}
	var buf bytes.Buffer
	write := func(s string) {
		buf.WriteString(s)
		buf.WriteByte('\n')
	}

	type obj struct {
		offset int
	}
	var offsets []obj
	addObject := func(body string) {
		offsets = append(offsets, obj{offset: buf.Len()})
		write(fmt.Sprintf("%d 0 obj", len(offsets)))
		write(body)
		write("endobj")
	}

	write("%PDF-1.4")

	// Placeholder for catalog and pages; will reference counts after we know them.
	addObject("<< /Type /Catalog /Pages 2 0 R >>")
	addObject("") // pages placeholder

	pageObjects := []int{}
	for i, img := range images {
		pageNum := len(offsets) + 1
		contentNum := pageNum + 1
		imageNum := pageNum + 2
		pageObjects = append(pageObjects, pageNum)

		mediaBox := fmt.Sprintf("[0 0 %d %d]", img.Width, img.Height)
		resources := fmt.Sprintf("<< /XObject << /Im%d %d 0 R >> >>", i, imageNum)
		addObject(fmt.Sprintf("<< /Type /Page /Parent 2 0 R /MediaBox %s /Resources %s /Contents %d 0 R >>", mediaBox, resources, contentNum))

		contentStream := fmt.Sprintf("q %d 0 0 %d 0 0 cm /Im%d Do Q", img.Width, img.Height, i)
		addObject(streamObject(contentStream))

		imgDict := fmt.Sprintf("<< /Type /XObject /Subtype /Image /Width %d /Height %d /ColorSpace /DeviceRGB /BitsPerComponent 8 /Filter /DCTDecode /Length %d >>", img.Width, img.Height, len(img.Data))
		offsets = append(offsets, obj{offset: buf.Len()})
		write(fmt.Sprintf("%d 0 obj", len(offsets)))
		write(imgDict)
		write("stream")
		buf.Write(img.Data)
		write("\nendstream")
		write("endobj")
	}

	// Rewrite pages object now that we know kids.
	var kids strings.Builder
	for _, p := range pageObjects {
		kids.WriteString(fmt.Sprintf(" %d 0 R", p))
	}
	offsets[1].offset = buf.Len()
	write("2 0 obj")
	write(fmt.Sprintf("<< /Type /Pages /Count %d /Kids [%s ] >>", len(pageObjects), kids.String()))
	write("endobj")

	// Write xref.
	xrefPos := buf.Len()
	write("xref")
	write(fmt.Sprintf("0 %d", len(offsets)+1))
	write("0000000000 65535 f ")
	for _, o := range offsets {
		write(fmt.Sprintf("%010d 00000 n ", o.offset))
	}
	write("trailer")
	write(fmt.Sprintf("<< /Size %d /Root 1 0 R >>", len(offsets)+1))
	write("startxref")
	write(strconv.Itoa(xrefPos))
	write("%%EOF")
	return buf.Bytes(), nil
}

func streamObject(content string) string {
	return fmt.Sprintf("<< /Length %d >>\nstream\n%s\nendstream", len(content), content)
}