PDFtrim/main.go

package main

import (
	"errors"
	"fmt"
	"math"
	"os"
	"path/filepath"
	"strconv"
	"strings"

	"github.com/pdfcpu/pdfcpu/pkg/api"
	"github.com/pdfcpu/pdfcpu/pkg/pdfcpu/model"
	"github.com/pdfcpu/pdfcpu/pkg/pdfcpu/types"
	"rsc.io/pdf"
)

const (
	paddingPoints = 6.0
	minSidePoints = 1.0
)

type rect struct {
	llx float64
	lly float64
	urx float64
	ury float64
}

type pageCrop struct {
	crop  rect
	apply bool
}

type matrix [3][3]float64

var identity = matrix{{1, 0, 0}, {0, 1, 0}, {0, 0, 1}}

func main() {
	if len(os.Args) < 2 {
		fmt.Fprintf(os.Stderr, "Pouzitie: %s subor.pdf\n", filepath.Base(os.Args[0]))
		os.Exit(1)
	}

	input := os.Args[1]
	if strings.ToLower(filepath.Ext(input)) != ".pdf" {
		fmt.Fprintln(os.Stderr, "Chyba: prvy argument musi byt PDF subor.")
		os.Exit(1)
	}

	if _, err := os.Stat(input); err != nil {
		fmt.Fprintf(os.Stderr, "Chyba: subor %q neexistuje alebo sa neda citat (%v).\n", input, err)
		os.Exit(1)
	}

	output := trimmedPath(input)
	if err := trimPDF(input, output); err != nil {
		fmt.Fprintf(os.Stderr, "Chyba: %v\n", err)
		os.Exit(1)
	}

	fmt.Printf("Hotovo: %s\n", output)
}

func trimPDF(input, output string) error {
	reader, err := pdf.Open(input)
	if err != nil {
		return fmt.Errorf("nepodarilo sa otvorit PDF: %w", err)
	}

	pageCount := reader.NumPage()
	if pageCount == 0 {
		return errors.New("PDF nema ziadne strany")
	}

	plan := make([]pageCrop, pageCount)
	for i := 1; i <= pageCount; i++ {
		p := reader.Page(i)
		if p.V.IsNull() {
			continue
		}

		pageBox, err := effectivePageBox(p.V)
		if err != nil {
			return fmt.Errorf("strana %d: %w", i, err)
		}

		contentBox, found := detectContentBox(p)
		if !found {
			continue
		}

		contentBox = contentBox.expand(paddingPoints).clamp(pageBox)
		if contentBox.width() < minSidePoints || contentBox.height() < minSidePoints {
			continue
		}

		if nearlyEqualRect(contentBox, pageBox, 0.01) {
			continue
		}

		plan[i-1] = pageCrop{crop: contentBox, apply: true}
	}

	if err := copyFile(input, output); err != nil {
		return fmt.Errorf("nepodarilo sa pripravit vystupny subor: %w", err)
	}

	for i, item := range plan {
		if !item.apply {
			continue
		}

		boxDef := item.crop.boxDef()
		cropBox, err := model.ParseBox(boxDef, types.POINTS)
		if err != nil {
			return fmt.Errorf("strana %d: neplatny crop box %s: %w", i+1, boxDef, err)
		}

		pageSel := []string{strconv.Itoa(i + 1)}
		if err := api.CropFile(output, output, pageSel, cropBox, nil); err != nil {
			return fmt.Errorf("strana %d: crop zlyhal: %w", i+1, err)
		}

		pageBounds, err := api.PageBoundaries("media:"+boxDef, types.POINTS)
		if err != nil {
			return fmt.Errorf("strana %d: nepodarilo sa pripravit media box: %w", i+1, err)
		}
		if err := api.AddBoxesFile(output, output, pageSel, pageBounds, nil); err != nil {
			return fmt.Errorf("strana %d: nastavenie media box zlyhalo: %w", i+1, err)
		}
	}

	return nil
}

func detectContentBox(p pdf.Page) (rect, bool) {
	var box rect
	found := false

	add := func(r rect) {
		r = r.normalize()
		if !r.valid() {
			return
		}
		if !found {
			box = r
			found = true
			return
		}
		box = box.union(r)
	}

	if tb, ok := detectTextBox(p); ok {
		add(tb)
	}

	if gb, ok := detectGraphicsBox(p); ok {
		add(gb)
	}

	return box, found
}

type textState struct {
	Tc    float64
	Tw    float64
	Th    float64
	Tl    float64
	Tf    pdf.Font
	Tfs   float64
	Trise float64
	Tm    matrix
	Tlm   matrix
	CTM   matrix
}

func detectTextBox(p pdf.Page) (rect, bool) {
	var box rect
	found := false

	add := func(r rect) {
		r = r.normalize()
		if !r.valid() {
			return
		}
		if !found {
			box = r
			found = true
			return
		}
		box = box.union(r)
	}

	g := textState{
		Th:  1,
		CTM: identity,
		Tm:  identity,
		Tlm: identity,
	}

	var enc pdf.TextEncoding
	var gstack []textState

	showText := func(raw string) {
		decoded := raw
		if enc != nil {
			decoded = enc.Decode(raw)
		}

		rawBytes := []byte(raw)
		i := 0

		for _, ch := range decoded {
			trm := matrix{{g.Tfs * g.Th, 0, 0}, {0, g.Tfs, 0}, {0, g.Trise, 1}}.mul(g.Tm).mul(g.CTM)

			w0 := 500.0
			if i < len(rawBytes) && !g.Tf.V.IsNull() {
				w0 = g.Tf.Width(int(rawBytes[i]))
			}
			if i < len(rawBytes) {
				i++
			}

			charWidth := math.Abs(w0 / 1000 * trm[0][0])
			h := math.Abs(g.Tfs)
			if h <= 0 {
				h = 8
			}

			if !unicodeIsSpace(ch) {
				add(rect{
					llx: trm[2][0],
					lly: trm[2][1] - 0.30*h,
					urx: trm[2][0] + math.Max(charWidth, 0.2*h),
					ury: trm[2][1] + 0.90*h,
				})
			}

			tx := w0/1000*g.Tfs + g.Tc
			if ch == ' ' {
				tx += g.Tw
			}
			tx *= g.Th

			g.Tm = matrix{{1, 0, 0}, {0, 1, 0}, {tx, 0, 1}}.mul(g.Tm)
		}
	}

	if !interpretPageContent(p, func(stk *pdf.Stack, op string) {
		args := popArgs(stk)

		switch op {
		case "q":
			gstack = append(gstack, g)
		case "Q":
			if len(gstack) == 0 {
				return
			}
			g = gstack[len(gstack)-1]
			gstack = gstack[:len(gstack)-1]
		case "cm":
			m, ok := matrixFromArgs(args)
			if ok {
				g.CTM = m.mul(g.CTM)
			}
		case "BT":
			g.Tm = identity
			g.Tlm = identity
		case "T*":
			x := matrix{{1, 0, 0}, {0, 1, 0}, {0, -g.Tl, 1}}
			g.Tlm = x.mul(g.Tlm)
			g.Tm = g.Tlm
		case "Tc":
			if len(args) == 1 {
				g.Tc = args[0].Float64()
			}
		case "TD":
			if len(args) != 2 {
				return
			}
			g.Tl = -args[1].Float64()
			fallthrough
		case "Td":
			if len(args) != 2 {
				return
			}
			tx := args[0].Float64()
			ty := args[1].Float64()
			x := matrix{{1, 0, 0}, {0, 1, 0}, {tx, ty, 1}}
			g.Tlm = x.mul(g.Tlm)
			g.Tm = g.Tlm
		case "Tf":
			if len(args) != 2 {
				return
			}
			g.Tf = p.Font(args[0].Name())
			g.Tfs = args[1].Float64()
			enc = nil
			if !g.Tf.V.IsNull() {
				enc = g.Tf.Encoder()
			}
		case "\"":
			if len(args) != 3 {
				return
			}
			g.Tw = args[0].Float64()
			g.Tc = args[1].Float64()
			showText(args[2].RawString())
		case "'":
			if len(args) != 1 {
				return
			}
			x := matrix{{1, 0, 0}, {0, 1, 0}, {0, -g.Tl, 1}}
			g.Tlm = x.mul(g.Tlm)
			g.Tm = g.Tlm
			showText(args[0].RawString())
		case "Tj":
			if len(args) == 1 {
				showText(args[0].RawString())
			}
		case "TJ":
			if len(args) != 1 {
				return
			}
			v := args[0]
			for i := 0; i < v.Len(); i++ {
				x := v.Index(i)
				if x.Kind() == pdf.String {
					showText(x.RawString())
					continue
				}
				tx := -x.Float64() / 1000 * g.Tfs * g.Th
				g.Tm = matrix{{1, 0, 0}, {0, 1, 0}, {tx, 0, 1}}.mul(g.Tm)
			}
		case "TL":
			if len(args) == 1 {
				g.Tl = args[0].Float64()
			}
		case "Tm":
			m, ok := matrixFromArgs(args)
			if ok {
				g.Tm = m
				g.Tlm = m
			}
		case "Ts":
			if len(args) == 1 {
				g.Trise = args[0].Float64()
			}
		case "Tw":
			if len(args) == 1 {
				g.Tw = args[0].Float64()
			}
		case "Tz":
			if len(args) == 1 {
				g.Th = args[0].Float64() / 100
			}
		}
	}) {
		return rect{}, false
	}

	return box, found
}

func detectGraphicsBox(p pdf.Page) (rect, bool) {
	var box rect
	found := false

	add := func(r rect) {
		r = r.normalize()
		if !r.valid() {
			return
		}
		if !found {
			box = r
			found = true
			return
		}
		box = box.union(r)
	}

	xObjects := p.Resources().Key("XObject")

	ctm := identity
	var ctmStack []matrix

	if !interpretPageContent(p, func(stk *pdf.Stack, op string) {
		args := popArgs(stk)

		switch op {
		case "q":
			ctmStack = append(ctmStack, ctm)

		case "Q":
			if len(ctmStack) == 0 {
				return
			}
			ctm = ctmStack[len(ctmStack)-1]
			ctmStack = ctmStack[:len(ctmStack)-1]

		case "cm":
			m, ok := matrixFromArgs(args)
			if ok {
				ctm = m.mul(ctm)
			}

		case "re":
			if len(args) != 4 {
				return
			}
			x := args[0].Float64()
			y := args[1].Float64()
			w := args[2].Float64()
			h := args[3].Float64()
			add(transformedRect(ctm, x, y, x+w, y+h))

		case "Do":
			if len(args) != 1 {
				return
			}

			name := args[0].Name()
			if name == "" || xObjects.IsNull() {
				return
			}

			xobj := xObjects.Key(name)
			subtype := xobj.Key("Subtype").Name()
			if subtype == "Image" || subtype == "Form" {
				add(transformedRect(ctm, 0, 0, 1, 1))
			}
		}
	}) {
		return rect{}, false
	}

	return box, found
}

func interpretPageContent(p pdf.Page, fn func(stk *pdf.Stack, op string)) bool {
	contents := p.V.Key("Contents")
	if contents.IsNull() {
		return false
	}

	ok := false
	run := func(stream pdf.Value) {
		if safeInterpretStream(stream, fn) {
			ok = true
		}
	}

	switch contents.Kind() {
	case pdf.Stream:
		run(contents)
	case pdf.Array:
		for i := 0; i < contents.Len(); i++ {
			stream := contents.Index(i)
			if stream.IsNull() || stream.Kind() != pdf.Stream {
				continue
			}
			run(stream)
		}
	}

	return ok
}

func safeInterpretStream(stream pdf.Value, fn func(stk *pdf.Stack, op string)) (ok bool) {
	if stream.IsNull() || stream.Kind() != pdf.Stream {
		return false
	}

	defer func() {
		if recover() != nil {
			ok = false
		}
	}()

	pdf.Interpret(stream, fn)
	return true
}

func unicodeIsSpace(r rune) bool {
	if r == ' ' || r == '\t' || r == '\n' || r == '\r' {
		return true
	}
	return false
}

func popArgs(stk *pdf.Stack) []pdf.Value {
	n := stk.Len()
	if n == 0 {
		return nil
	}

	args := make([]pdf.Value, n)
	for i := n - 1; i >= 0; i-- {
		args[i] = stk.Pop()
	}

	return args
}

func matrixFromArgs(args []pdf.Value) (matrix, bool) {
	if len(args) != 6 {
		return matrix{}, false
	}

	var m matrix
	for i := 0; i < 6; i++ {
		m[i/2][i%2] = args[i].Float64()
	}
	m[2][2] = 1

	return m, true
}

func transformedRect(m matrix, x0, y0, x1, y1 float64) rect {
	p1x, p1y := transformPoint(m, x0, y0)
	p2x, p2y := transformPoint(m, x1, y0)
	p3x, p3y := transformPoint(m, x0, y1)
	p4x, p4y := transformPoint(m, x1, y1)

	minX := math.Min(math.Min(p1x, p2x), math.Min(p3x, p4x))
	minY := math.Min(math.Min(p1y, p2y), math.Min(p3y, p4y))
	maxX := math.Max(math.Max(p1x, p2x), math.Max(p3x, p4x))
	maxY := math.Max(math.Max(p1y, p2y), math.Max(p3y, p4y))

	return rect{llx: minX, lly: minY, urx: maxX, ury: maxY}
}

func transformPoint(m matrix, x, y float64) (float64, float64) {
	px := x*m[0][0] + y*m[1][0] + m[2][0]
	py := x*m[0][1] + y*m[1][1] + m[2][1]
	return px, py
}

func effectivePageBox(page pdf.Value) (rect, error) {
	if r, ok := inheritedRect(page, "CropBox"); ok {
		return r, nil
	}
	if r, ok := inheritedRect(page, "MediaBox"); ok {
		return r, nil
	}
	return rect{}, errors.New("chybajuci MediaBox/CropBox")
}

func inheritedRect(v pdf.Value, key string) (rect, bool) {
	for cur := v; !cur.IsNull(); cur = cur.Key("Parent") {
		candidate := cur.Key(key)
		r, err := rectFromArray(candidate)
		if err == nil {
			return r, true
		}
	}
	return rect{}, false
}

func rectFromArray(v pdf.Value) (rect, error) {
	if v.IsNull() || v.Len() != 4 {
		return rect{}, errors.New("neplatny rectangle")
	}

	r := rect{
		llx: v.Index(0).Float64(),
		lly: v.Index(1).Float64(),
		urx: v.Index(2).Float64(),
		ury: v.Index(3).Float64(),
	}.normalize()

	if !r.valid() {
		return rect{}, errors.New("neplatny rectangle")
	}

	return r, nil
}

func copyFile(src, dst string) error {
	in, err := os.Open(src)
	if err != nil {
		return err
	}
	defer in.Close()

	out, err := os.Create(dst)
	if err != nil {
		return err
	}

	_, err = out.ReadFrom(in)
	if closeErr := out.Close(); closeErr != nil && err == nil {
		err = closeErr
	}

	if err != nil {
		_ = os.Remove(dst)
	}
	return err
}

func trimmedPath(input string) string {
	ext := filepath.Ext(input)
	base := strings.TrimSuffix(input, ext)
	return base + "-trimmed.pdf"
}

func nearlyEqualRect(a, b rect, eps float64) bool {
	return math.Abs(a.llx-b.llx) <= eps &&
		math.Abs(a.lly-b.lly) <= eps &&
		math.Abs(a.urx-b.urx) <= eps &&
		math.Abs(a.ury-b.ury) <= eps
}

func (m matrix) mul(other matrix) matrix {
	var out matrix
	for i := 0; i < 3; i++ {
		for j := 0; j < 3; j++ {
			for k := 0; k < 3; k++ {
				out[i][j] += m[i][k] * other[k][j]
			}
		}
	}
	return out
}

func (r rect) normalize() rect {
	if r.llx > r.urx {
		r.llx, r.urx = r.urx, r.llx
	}
	if r.lly > r.ury {
		r.lly, r.ury = r.ury, r.lly
	}
	return r
}

func (r rect) valid() bool {
	return !(math.IsNaN(r.llx) || math.IsNaN(r.lly) || math.IsNaN(r.urx) || math.IsNaN(r.ury) ||
		math.IsInf(r.llx, 0) || math.IsInf(r.lly, 0) || math.IsInf(r.urx, 0) || math.IsInf(r.ury, 0))
}

func (r rect) width() float64 {
	return r.urx - r.llx
}

func (r rect) height() float64 {
	return r.ury - r.lly
}

func (r rect) union(other rect) rect {
	return rect{
		llx: math.Min(r.llx, other.llx),
		lly: math.Min(r.lly, other.lly),
		urx: math.Max(r.urx, other.urx),
		ury: math.Max(r.ury, other.ury),
	}
}

func (r rect) expand(padding float64) rect {
	return rect{
		llx: r.llx - padding,
		lly: r.lly - padding,
		urx: r.urx + padding,
		ury: r.ury + padding,
	}
}

func (r rect) clamp(limit rect) rect {
	return rect{
		llx: clamp(r.llx, limit.llx, limit.urx),
		lly: clamp(r.lly, limit.lly, limit.ury),
		urx: clamp(r.urx, limit.llx, limit.urx),
		ury: clamp(r.ury, limit.lly, limit.ury),
	}
}

func (r rect) boxDef() string {
	return fmt.Sprintf("[%.4f %.4f %.4f %.4f]", r.llx, r.lly, r.urx, r.ury)
}

func clamp(v, minV, maxV float64) float64 {
	if v < minV {
		return minV
	}
	if v > maxV {
		return maxV
	}
	return v
}