pdfread.go

// Copyright (c) 2009 Helmar Wodtke. All rights reserved.
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file.
// The MIT License is an OSI approved license and can
// be found at
//   http://www.opensource.org/licenses/mit-license.php

// Access to PDF files.
package pdfread

import (
  "compress/zlib"
  "encoding/ascii85"
  "fancy"
  "hex"
  "lzw"
  "ps"
  "regexp"
)

// limits

const (
  MAX_PDF_UPDATES   = 1024
  MAX_PDF_ARRAYSIZE = 1024
)

// types

type DictionaryT map[string][]byte

type PdfReaderT struct {
  File      string            // name of the file
  rdr       fancy.Reader      // reader for the contents
  Startxref int               // starting of xref table
  Xref      map[int]int       // "pointers" of the xref table
  Trailer   DictionaryT       // trailer dictionary of the file
  rcache    map[string][]byte // resolver cache
  rncache   map[string]int    // resolver cache (positions in file)
  dicache   map[string]DictionaryT
  pages     [][]byte // pages cache
}

var _Bytes = []byte{}

func max(a, b int) int {
  if a < b {
    return b
  }
  return a
}
func min(a, b int) int {
  if a < b {
    return a
  }
  return b
}
func end(a []byte, n int) int { return max(0, len(a)-n) }

func num(n []byte) (r int) {
  for i := 0; i < len(n); i++ {
    if n[i] >= '0' && n[i] <= '9' {
      r = r*10 + int(n[i]-'0')
    } else {
      break
    }
  }
  return
}

func refToken(f fancy.Reader) ([]byte, int64) {
  tok, p := ps.Token(f)
  if len(tok) > 0 && tok[0] >= '0' && tok[0] <= '9' {
    ps.Token(f)
    r, q := ps.Token(f)
    if string(r) == "R" {
      f.Seek(p, 0)
      tok = f.Slice(int(1 + q - p))
    } else {
      f.Seek(p+int64(len(tok)), 0)
    }
  }
  return tok, p
}

func tupel(f fancy.Reader, count int) [][]byte {
  r := make([][]byte, count)
  for i := 0; i < count; i++ {
    r[i], _ = ps.Token(f)
  }
  return r
}

var xref = regexp.MustCompile(
  "startxref[\t ]*(\r?\n|\r)[\t ]*([0-9]+)[\t ]*(\r?\n|\r)[\t ]*%%EOF")

// xrefStart() queries the start of the xref-table in a PDF file.
func xrefStart(f fancy.Reader) int {
  s := int(f.Size())
  pdf := make([]byte, min(s, 1024))
  f.ReadAt(pdf, int64(max(0, s-1024)))
  ps := xref.AllMatches(pdf, 0)
  if ps == nil {
    return -1
  }
  return num(xref.MatchSlices(ps[len(ps)-1])[2])
}

// xrefSkip() queries the start of the trailer for a (partial) xref-table.
func xrefSkip(f fancy.Reader, xref int) int {
  f.Seek(int64(xref), 0)
  t, p := ps.Token(f)
  if string(t) != "xref" {
    return -1
  }
  for {
    t, p = ps.Token(f)
    if t[0] < '0' || t[0] > '9' {
      f.Seek(p, 0)
      break
    }
    t, _ = ps.Token(f)
    ps.SkipLE(f)
    f.Seek(int64(num(t)*20), 1)
  }
  r, _ := f.Seek(0, 1)
  return int(r)
}

// Dictionary() makes a map/hash from PDF dictionary data.
func Dictionary(s []byte) DictionaryT {
  if len(s) < 4 {
    return nil
  }
  e := len(s) - 1
  if s[0] != s[1] || s[0] != '<' || s[e] != s[e-1] || s[e] != '>' {
    return nil
  }
  r := make(DictionaryT)
  rdr := fancy.SliceReader(s[2 : e-1])
  for {
    t, _ := ps.Token(rdr)
    if len(t) == 0 {
      break
    }
    if t[0] != '/' {
      return nil
    }
    k := string(t)
    t, _ = refToken(rdr)
    r[k] = t
  }
  return r
}

// Array() extracts an array from PDF data.
func Array(s []byte) [][]byte {
  if len(s) < 2 || s[0] != '[' || s[len(s)-1] != ']' {
    return nil
  }
  rdr := fancy.SliceReader(s[1 : len(s)-1])
  r := make([][]byte, MAX_PDF_ARRAYSIZE)
  b := 0
  for {
    r[b], _ = refToken(rdr)
    if len(r[b]) == 0 {
      break
    }
    b++
  }
  if b == 0 {
    return nil
  }
  return r[0:b]
}

// xrefRead() reads the xref table(s) of a PDF file. This is not recursive
// in favour of not to have to keep track of already used starting points
// for xrefs.
func xrefRead(f fancy.Reader, p int) map[int]int {
  var back [MAX_PDF_UPDATES]int
  b := 0
  s := _Bytes
  for ok := true; ok; {
    back[b] = p
    b++
    p = xrefSkip(f, p)
    f.Seek(int64(p), 0)
    s, _ = ps.Token(f)
    if string(s) != "trailer" {
      return nil
    }
    s, _ = ps.Token(f)
    s, ok = Dictionary(s)["/Prev"]
    p = num(s)
  }
  r := make(map[int]int)
  for b != 0 {
    b--
    f.Seek(int64(back[b]), 0)
    ps.Token(f) // skip "xref"
    for {
      m := tupel(f, 2)
      if string(m[0]) == "trailer" {
        break
      }
      ps.SkipLE(f)
      o := num(m[0])
      dat := f.Slice(num(m[1]) * 20)
      for i := 0; i < len(dat); i += 20 {
        if dat[i+17] != 'n' {
          r[o] = 0, false
        } else {
          r[o] = num(dat[i : i+10])
        }
        o++
      }
    }
  }
  return r
}

// object() extracts the top informations of a PDF "object". For streams
// this would be the dictionary as bytes.  It also returns the position in
// binary data where one has to continue to read for this "object".
func (pd *PdfReaderT) object(o int) (int, []byte) {
  p, ok := pd.Xref[o]
  if !ok {
    return -1, _Bytes
  }
  pd.rdr.Seek(int64(p), 0)
  m := tupel(pd.rdr, 3)
  if num(m[0]) != o {
    return -1, _Bytes
  }
  r, np := refToken(pd.rdr)
  return int(np) + len(r), r
}

// pd.Resolve() resolves a reference in the PDF file. You'll probably need
// this method for reading streams only.
func (pd *PdfReaderT) Resolve(s []byte) (int, []byte) {
  if len(s) < 5 || s[len(s)-1] != 'R' {
    return -1, s
  }
  done := make(map[int]int)
  var resolve func(s []byte) (int, []byte)
  resolve = func(s []byte) (int, []byte) {
    n := -1
    if len(s) >= 5 && s[0] >= '0' && s[0] <= '9' && s[len(s)-1] == 'R' {
      z, ok := pd.rcache[string(s)]
      if ok {
        return pd.rncache[string(s)], z
      }
      orig := s
      o := num(s)
      if _, ok = done[o]; ok {
        return -1, _Bytes
      }
      done[o] = 1
      n, s = pd.object(o)
      if s[0] >= '0' && s[0] <= '9' && s[len(s)-1] == 'R' {
        n, s = resolve(s)
      }
      pd.rcache[string(orig)] = s
      pd.rncache[string(orig)] = n
    }
    return n, s
  }
  return resolve(s)
}

// pd.Obj() is the universal method to access contents of PDF objects or
// data tokens in i.e.  dictionaries.  For reading streams you'll have to
// utilize pd.Resolve().
func (pd *PdfReaderT) Obj(reference []byte) []byte {
  _, r := pd.Resolve(reference)
  return r
}

// pd.Num() queries integer data from a reference.
func (pd *PdfReaderT) Num(reference []byte) int {
  return num(pd.Obj(reference))
}

// pd.Dic() queries dictionary data from a reference.
func (pd *PdfReaderT) Dic(reference []byte) DictionaryT {
  d, ok := pd.dicache[string(reference)]
  if !ok {
    d = Dictionary(pd.Obj(reference))
    pd.dicache[string(reference)] = d
  }
  return d
}

// pd.Arr() queries array data from a reference.
func (pd *PdfReaderT) Arr(reference []byte) [][]byte {
  return Array(pd.Obj(reference))
}

// pd.ForcedArray() queries array data. If reference does not refer to an
// array, reference is taken as element of the returned array.
func (pd *PdfReaderT) ForcedArray(reference []byte) [][]byte {
  nr := pd.Obj(reference)
  if nr[0] != '[' {
    return [][]byte{reference}
  }
  return Array(nr)
}

// pd.Pages() returns an array with references to the pages of the PDF.
func (pd *PdfReaderT) Pages() [][]byte {
  if pd.pages != nil {
    return pd.pages
  }
  pages := pd.Dic(pd.Dic(pd.Trailer["/Root"])["/Pages"])
  pd.pages = make([][]byte, pd.Num(pages["/Count"]))
  cp := 0
  done := make(map[string]int)
  var q func(p [][]byte)
  q = func(p [][]byte) {
    for k := range p {
      if _, wrong := done[string(p[k])]; !wrong {
        done[string(p[k])] = 1
        if kids, ok := pd.Dic(p[k])["/Kids"]; ok {
          q(pd.Arr(kids))
        } else {
          pd.pages[cp] = p[k]
          cp++
        }
      } else {
        panic("Bad Page-Tree!")
      }
    }
  }
  q(pd.Arr(pages["/Kids"]))
  return pd.pages
}

// pd.Attribute() tries to get an attribute definition from a page
// reference.  Note that the attribute definition is not resolved - so it's
// possible to get back a reference here.
func (pd *PdfReaderT) Attribute(a string, src []byte) []byte {
  d := pd.Dic(src)
  done := make(map[string]int)
  r, ok := d[a]
  for !ok {
    r, ok = d["/Parent"]
    if _, wrong := done[string(r)]; wrong || !ok {
      return _Bytes
    }
    done[string(r)] = 1
    d = pd.Dic(r)
    r, ok = d[a]
  }
  return r
}

// pd.Att() tries to get an attribute from a page reference.  The
// attribute will be resolved.
func (pd *PdfReaderT) Att(a string, src []byte) []byte {
  return pd.Obj(pd.Attribute(a, src))
}

// pd.Stream() returns contents of a stream.
func (pd *PdfReaderT) Stream(reference []byte) (DictionaryT, []byte) {
  q, d := pd.Resolve(reference)
  dic := pd.Dic(d)
  l := pd.Num(dic["/Length"])
  pd.rdr.Seek(int64(q), 0)
  t, _ := ps.Token(pd.rdr)
  if string(t) != "stream" {
    return nil, []byte{}
  }
  ps.SkipLE(pd.rdr)
  return dic, pd.rdr.Slice(l)
}

// pd.DecodedStream() returns decoded contents of a stream.
func (pd *PdfReaderT) DecodedStream(reference []byte) (DictionaryT, []byte) {
  dic, data := pd.Stream(reference)
  if f, ok := dic["/Filter"]; ok {
    filter := pd.ForcedArray(f)
    var decos [][]byte
    if d, ok := dic["/DecodeParams"]; ok {
      decos = pd.ForcedArray(d)
    } else {
      decos = make([][]byte, len(filter))
    }
    for ff := range filter {
      deco := pd.Dic(decos[ff])
      switch string(filter[ff]) {
      case "/FlateDecode":
        data = fancy.ReadAndClose(zlib.NewInflater(fancy.SliceReader(data)))
      case "/LZWDecode":
        early := true
        if deco != nil {
          if s, ok := deco["/EarlyChange"]; ok {
            early = pd.Num(s) == 1
          }
        }
        data = lzw.Decode(data, early)
      case "/ASCII85Decode":
        ds := data
        for len(ds) > 1 && ds[len(ds)-1] < 33 {
          ds = ds[0 : len(ds)-1]
        }
        if len(ds) >= 2 && ds[len(ds)-1] == '>' && ds[len(ds)-2] == '~' {
          ds = ds[0 : len(ds)-2]
        }
        data = fancy.ReadAll(ascii85.NewDecoder(fancy.SliceReader(ds)))
      case "/ASCIIHexDecode":
        data = hex.Decode(string(data))
      default:
        data = []byte{}
      }
    }
  }
  return dic, data
}

// pd.PageFonts() returns references to the fonts defined for a page.
func (pd *PdfReaderT) PageFonts(page []byte) DictionaryT {
  fonts, _ := pd.Dic(pd.Attribute("/Resources", page))["/Font"]
  if fonts == nil {
    return nil
  }
  return pd.Dic(fonts)
}

// Load() loads a PDF file of a given name.
func Load(fn string) *PdfReaderT {
  r := new(PdfReaderT)
  r.File = fn
  r.rdr = fancy.FileReader(fn)
  if r.Startxref = xrefStart(r.rdr); r.Startxref == -1 {
    return nil
  }
  if r.Xref = xrefRead(r.rdr, r.Startxref); r.Xref == nil {
    return nil
  }
  r.rdr.Seek(int64(xrefSkip(r.rdr, r.Startxref)), 0)
  s, _ := ps.Token(r.rdr)
  if string(s) != "trailer" {
    return nil
  }
  s, _ = ps.Token(r.rdr)
  if r.Trailer = Dictionary(s); r.Trailer == nil {
    return nil
  }
  r.rcache = make(map[string][]byte)
  r.rncache = make(map[string]int)
  r.dicache = make(map[string]DictionaryT)
  return r
}