add PDF export
This commit is contained in:
Michael Stapelberg
334
internal/pdf/pdf.go
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334
internal/pdf/pdf.go
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// Copyright 2020 Google LLC
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// Package pdf implements a minimal PDF 1.7 writer, just functional enough to
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// create a PDF file containing a QR code encoded as rectangles.
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//
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// It follows the standard “PDF 32000-1:2008 PDF 1.7”:
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// https://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PDF32000_2008.pdf
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package pdf
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import (
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"fmt"
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"io"
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"strings"
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"time"
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)
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func dateString(t time.Time) string {
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return "D:" + t.Format("20060102150405-07'00'")
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}
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// ObjectID is a PDF object id.
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type ObjectID int
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// String implements fmt.Stringer.
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func (o ObjectID) String() string {
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return fmt.Sprintf("%d 0 R", int(o))
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}
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// Object is implemented by all PDF objects.
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type Object interface {
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// Objects returns all Objects which should be encoded into the
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// PDF file.
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Objects() []Object
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// Encode encodes the object into the PDF file w.
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Encode(w io.Writer, ids map[string]ObjectID) error
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// SetID updates the object id.
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SetID(id ObjectID)
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// Name returns the human-readable object name.
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Name() string
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fmt.Stringer
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}
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// Common represents a PDF object.
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type Common struct {
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ObjectName string
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ID ObjectID
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Stream []byte
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}
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// String implements fmt.Stringer.
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func (c *Common) String() string {
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return c.ID.String()
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}
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// SetID implements Object.
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func (c *Common) SetID(id ObjectID) {
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c.ID = id
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}
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// Name implements Object.
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func (c *Common) Name() string {
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return c.ObjectName
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}
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// Objects implements Object.
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func (c *Common) Objects() []Object {
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return []Object{c}
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}
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// Encode implements Object.
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func (c *Common) Encode(w io.Writer, ids map[string]ObjectID) error {
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_, err := fmt.Fprintf(w, `
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%d 0 obj
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<<
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/Length %d
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>>
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stream
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%s
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endstream
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endobj`, c.ID, len(c.Stream), c.Stream)
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return err
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}
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// DocumentInfo represents a PDF document information object.
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type DocumentInfo struct {
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Common
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CreationDate time.Time
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Producer string
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Title string
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}
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// Objects implements Object.
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func (d *DocumentInfo) Objects() []Object {
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return []Object{d}
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}
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// Encode implements Object.
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func (d *DocumentInfo) Encode(w io.Writer, ids map[string]ObjectID) error {
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_, err := fmt.Fprintf(w, `
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%d 0 obj
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<<
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/Title (%s)
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/CreationDate (%s)
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/ModDate (%s)
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/Producer (%s)
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>>
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endobj`,
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int(d.ID),
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d.Title,
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dateString(d.CreationDate),
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dateString(d.CreationDate),
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d.Producer)
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return err
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}
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// Catalog represents a PDF catalog object.
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type Catalog struct {
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Common
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Pages Object // Pages
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}
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// Objects implements Object.
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func (r *Catalog) Objects() []Object {
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return append([]Object{r}, r.Pages.Objects()...)
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}
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// Encode implements Object.
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func (r *Catalog) Encode(w io.Writer, ids map[string]ObjectID) error {
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_, err := fmt.Fprintf(w, `
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%d 0 obj
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<<
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/Type /Catalog
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/Pages %v
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>>
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endobj`, int(r.ID), r.Pages)
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return err
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}
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// Pages represents a PDF pages object
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type Pages struct {
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Common
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Kids []Object // Page
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}
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// Objects implements Object.
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func (p *Pages) Objects() []Object {
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result := []Object{p}
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for _, o := range p.Kids {
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result = append(result, o.Objects()...)
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}
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return result
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}
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// Encode implements Object.
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func (p *Pages) Encode(w io.Writer, ids map[string]ObjectID) error {
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_, err := fmt.Fprintf(w, `
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%d 0 obj
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<<
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/Kids %v
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/Type /Pages
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/Count %d
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>>
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endobj`, int(p.ID), p.Kids, len(p.Kids))
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return err
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}
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// Page represents a PDF page object with size DIN A4
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type Page struct {
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Common
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Resources []Object // Image
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Contents []Object // Common (streams)
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// Parent contains the human-readable name of the parent object,
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// which will be translated into an object ID when encoding.
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Parent string
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}
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// Objects implements Object.
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func (p *Page) Objects() []Object {
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result := []Object{p}
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for _, o := range p.Resources {
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result = append(result, o.Objects()...)
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}
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for _, o := range p.Contents {
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result = append(result, o.Objects()...)
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}
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return result
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}
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// Encode implements Object.
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func (p *Page) Encode(w io.Writer, ids map[string]ObjectID) error {
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xObjects := make([]string, len(p.Resources))
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for idx, o := range p.Resources {
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xObjects[idx] = fmt.Sprintf("/%s %v", o.Name(), ids[o.Name()])
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}
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_, err := fmt.Fprintf(w, `
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%d 0 obj
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<<
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/Resources <<
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/XObject <<
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%s
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>>
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>>
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/Contents %v
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/Parent %v
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/Type /Page
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/MediaBox [ 0 0 152 152 ]
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>>
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endobj`, int(p.ID), strings.Join(xObjects, "\n"), p.Contents, ids[p.Parent])
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return err
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}
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type countingWriter struct {
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cnt int
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w io.Writer
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}
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func (cw *countingWriter) Write(p []byte) (n int, err error) {
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n, err = cw.w.Write(p)
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cw.cnt += n
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return n, err
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}
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// Encoder is a PDF writer.
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type Encoder struct {
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w *countingWriter
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}
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// NewEncoder returns a ready-to-use Encoder writing to w.
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func NewEncoder(w io.Writer) *Encoder {
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return &Encoder{
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w: &countingWriter{w: w},
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}
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}
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// writeXrefTable writes a cross-reference table to e.w. See also “PDF
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// 32000-1:2008 PDF 1.7” section “7.5.4 Cross-Reference Table”
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func (e *Encoder) writeXrefTable(Objects []Object, xrefOffsets []int) error {
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if _, err := fmt.Fprintf(e.w, "\nxref\n0 %d\n", len(Objects)+1); err != nil {
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return err
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}
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// index 0 can never point to a valid Object, so print an invalid entry:
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if _, err := fmt.Fprintf(e.w, "%010d %05d %s \n", 0, 65535, "f"); err != nil {
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return err
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}
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const generation = 0
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for _, offset := range xrefOffsets {
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if _, err := fmt.Fprintf(e.w, "%010d %05d %s \n", offset, generation, "n"); err != nil {
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return err
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}
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}
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return nil
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}
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// Encode writes the PDF file represented by the specified catalog.
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func (e *Encoder) Encode(r *Catalog, info *DocumentInfo) error {
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// As per “PDF Explained: How a PDF File is Written”:
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// https://www.geekbooks.me/book/view/pdf-explained
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// (1.) Output the header.
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// Byte sequence 0xE2E3CFD3 as per the recommendation from
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// “Developing with PDF”. See “Chapter 1. PDF Syntax”:
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// https://www.safaribooksonline.com/library/view/developing-with-pdf/9781449327903/ch01.html#_header
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if _, err := e.w.Write(append([]byte("%PDF-1.4\n%"), 0xe2, 0xe3, 0xcf, 0xd3)); err != nil {
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return err
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}
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// Flatten the Object graph into a slice
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objects := append(r.Objects(), info.Objects()...)
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// (3.) Assign ids from 1 to n and store them in a lookup table
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// (some Objects need to resolve name references when encoding).
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ids := make(map[string]ObjectID, len(objects))
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for idx, obj := range objects {
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id := ObjectID(idx + 1)
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obj.SetID(id)
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ids[obj.Name()] = id
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}
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// (4.) Output the Objects one by one, starting with Object number
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// one, recording the byte offset of each for the cross-reference
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// table.
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xrefOffsets := make([]int, len(objects))
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for idx, obj := range objects {
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xrefOffsets[idx] = e.w.cnt + 1
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if err := obj.Encode(e.w, ids); err != nil {
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return err
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}
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}
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// (5.) Write the cross-reference table.
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xrefOffset := e.w.cnt
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if err := e.writeXrefTable(objects, xrefOffsets); err != nil {
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return err
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}
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// (6.) Write the trailer, trailer dictionary, and end-of-file marker.
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if _, err := fmt.Fprintf(e.w, `trailer
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<<
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/Root %v
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/Size %d
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/Info %v
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>>
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startxref
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%d
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%%%%EOF
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`, ids["catalog"], len(objects)+1, ids["info"], xrefOffset); err != nil {
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return err
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}
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return nil
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}
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