new feature: ascii85 decoder

decoding pdf files encoded with ascii85, including adobe ending

ascii85.go

@@ -0,0 +1,53 @@
+// file with help function for ascii85 decoder
+// later if new decoders is going to add it reasonable to rename file and add them here
+// also create interfaces to switch between them (like in unidoc)
+
+package pdf
+
+import (
+	"io"
+)
+
+type alphaReader struct {
+	reader io.Reader
+}
+
+func newAlphaReader(reader io.Reader) *alphaReader {
+	return &alphaReader{reader: reader}
+}
+
+func checkAscii85(r byte) byte {
+	if r >= '!' && r <= 'u' { // 33 <= ascii85 <=117
+		return r
+	}
+	if r == '~' {
+		return 1 // for marking possible end of data
+	}
+	return 0 // if non-ascii85
+}
+
+func (a *alphaReader) Read(p []byte) (int, error) {
+	n, err := a.reader.Read(p)
+	if err == io.EOF {
+	}
+	if err != nil {
+		return n, err
+	}
+	buf := make([]byte, n)
+	tilda := false
+	for i := 0; i < n; i++ {
+		char := checkAscii85(p[i])
+		if char == '>' && tilda { // end of data
+			break
+		}
+		if char > 1 {
+			buf[i] = char
+		}
+		if char == 1 {
+			tilda = true // possible end of data
+		}
+	}
+
+	copy(p, buf)
+	return n, nil
+}

read.go

@@ -67,6 +67,7 @@ import (
 	"crypto/cipher"
 	"crypto/md5"
 	"crypto/rc4"
+	"encoding/ascii85"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -840,6 +841,16 @@ func applyFilter(rd io.Reader, name string, param Value) io.Reader {
 		case 12:
 			return &pngUpReader{r: zr, hist: make([]byte, 1+columns), tmp: make([]byte, 1+columns)}
 		}
+	case "ASCII85Decode":
+		cleanAscii85 := newAlphaReader(rd)
+		decoder := ascii85.NewDecoder(cleanAscii85)
+
+		if param.Keys() == nil {
+			return decoder
+		} else {
+			fmt.Println("param=", param)
+			panic("not expected DecodeParms for ascii85")
+		}
 	}
 }