/
encode.go
149 lines (129 loc) · 3.48 KB
/
encode.go
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// Copyright (C) 2011, Ross Light
package pdf
import (
"fmt"
"io"
)
// encoder writes the PDF file format structure.
type encoder struct {
objects []interface{}
root Reference
}
type trailer struct {
Size int
Root Reference
}
// add appends an object to the file. The object is marshalled only when an
// encoding is requested.
func (enc *encoder) add(v interface{}) Reference {
enc.objects = append(enc.objects, v)
return Reference{uint(len(enc.objects)), 0}
}
const (
header = "%PDF-1.7" + newline + "%\x93\x8c\x8b\x9e" + newline
newline = "\r\n"
)
// Cross reference strings
const (
crossReferenceSectionHeader = "xref" + newline
crossReferenceSubsectionFormat = "%d %d" + newline
crossReferenceFormat = "%010d %05d n" + newline
crossReferenceFreeFormat = "%010d %05d f" + newline
)
const trailerHeader = "trailer" + newline
const startxrefFormat = "startxref" + newline + "%d" + newline
const eofString = "%%EOF" + newline
// encode writes an entire PDF document by marshalling the added objects.
func (enc *encoder) encode(wr io.Writer) error {
w := &offsetWriter{Writer: wr}
if err := enc.writeHeader(w); err != nil {
return err
}
objectOffsets, err := enc.writeBody(w)
if err != nil {
return err
}
tableOffset := w.offset
if err := enc.writeXrefTable(w, objectOffsets); err != nil {
return err
}
if err := enc.writeTrailer(w); err != nil {
return err
}
if err := enc.writeStartxref(w, tableOffset); err != nil {
return err
}
if err := enc.writeEOF(w); err != nil {
return err
}
return nil
}
func (enc *encoder) writeHeader(w *offsetWriter) error {
_, err := io.WriteString(w, header)
return err
}
func (enc *encoder) writeBody(w *offsetWriter) ([]int64, error) {
objectOffsets := make([]int64, len(enc.objects))
for i, obj := range enc.objects {
// TODO: Use same buffer for writing across objects
objectOffsets[i] = w.offset
data, err := marshal(nil, indirectObject{Reference{uint(i + 1), 0}, obj})
if err != nil {
return nil, err
}
if _, err = w.Write(append(data, newline...)); err != nil {
return nil, err
}
}
return objectOffsets, nil
}
func (enc *encoder) writeXrefTable(w *offsetWriter, objectOffsets []int64) error {
if _, err := io.WriteString(w, crossReferenceSectionHeader); err != nil {
return err
}
if _, err := fmt.Fprintf(w, crossReferenceSubsectionFormat, 0, len(enc.objects)+1); err != nil {
return err
}
if _, err := fmt.Fprintf(w, crossReferenceFreeFormat, 0, 65535); err != nil {
return err
}
for _, offset := range objectOffsets {
if _, err := fmt.Fprintf(w, crossReferenceFormat, offset, 0); err != nil {
return err
}
}
return nil
}
func (enc *encoder) writeTrailer(w *offsetWriter) error {
var err error
dict := trailer{
Size: len(enc.objects) + 1,
Root: enc.root,
}
data := make([]byte, 0, len(trailerHeader)+len(newline))
data = append(data, trailerHeader...)
if data, err = marshal(data, dict); err != nil {
return err
}
data = append(data, newline...)
_, err = w.Write(data)
return err
}
func (enc *encoder) writeStartxref(w *offsetWriter, tableOffset int64) error {
_, err := fmt.Fprintf(w, startxrefFormat, tableOffset)
return err
}
func (enc *encoder) writeEOF(w *offsetWriter) error {
_, err := io.WriteString(w, eofString)
return err
}
// offsetWriter tracks how many bytes have been written to it.
type offsetWriter struct {
io.Writer
offset int64
}
func (w *offsetWriter) Write(p []byte) (n int, err error) {
n, err = w.Writer.Write(p)
w.offset += int64(n)
return
}