Add signed support to BitStream and BitWriter

libqpdf/BitStream.cc

@@ -30,6 +30,23 @@ BitStream::getBits(int nbits)
 		     this->bits_available, nbits);
 }
 
+long long
+BitStream::getBitsSigned(int nbits)
+{
+    unsigned long long bits = read_bits(this->p, this->bit_offset,
+                                        this->bits_available, nbits);
+    long long result = 0;
+    if (static_cast<long long>(bits) > 1 << (nbits - 1))
+    {
+        result = static_cast<long long>(bits - (1 << nbits));
+    }
+    else
+    {
+        result = static_cast<long long>(bits);
+    }
+    return result;
+}
+
 void
 BitStream::skipToNextByte()
 {

libqpdf/BitWriter.cc

@@ -17,6 +17,21 @@ BitWriter::writeBits(unsigned long long val, unsigned int bits)
     write_bits(this->ch, this->bit_offset, val, bits, this->pl);
 }
 
+void
+BitWriter::writeBitsSigned(long long val, unsigned int bits)
+{
+    unsigned long long uval = 0;
+    if (val < 0)
+    {
+        uval = static_cast<unsigned long long>((1 << bits) + val);
+    }
+    else
+    {
+        uval = static_cast<unsigned long long>(val);
+    }
+    writeBits(uval, bits);
+}
+
 void
 BitWriter::flush()
 {

libqpdf/qpdf/BitStream.hh

@@ -15,6 +15,8 @@ class BitStream
     QPDF_DLL
     unsigned long long getBits(int nbits);
     QPDF_DLL
+    long long getBitsSigned(int nbits);
+    QPDF_DLL
     void skipToNextByte();
 
   private:

libqpdf/qpdf/BitWriter.hh

@@ -16,6 +16,8 @@ class BitWriter
     BitWriter(Pipeline* pl);
     QPDF_DLL
     void writeBits(unsigned long long val, unsigned int bits);
+    QPDF_DLL
+    void writeBitsSigned(long long val, unsigned int bits);
     // Force any partial byte to be written to the pipeline.
     QPDF_DLL
     void flush();

libtests/bits.cc

@@ -124,6 +124,13 @@ test()
     b.skipToNextByte();
     std::cout << b.getBits(8) << std::endl;
     std::cout << std::endl;
+    b.reset();
+    std::cout << b.getBitsSigned(3) << std::endl;
+    std::cout << b.getBitsSigned(6) << std::endl;
+    std::cout << b.getBitsSigned(5) << std::endl;
+    std::cout << b.getBitsSigned(1) << std::endl;
+    std::cout << b.getBitsSigned(17) << std::endl;
+    std::cout << std::endl;
 
     // Write tests
 
@@ -159,6 +166,13 @@ test()
     bw.writeBits(0xABUL, 8);
     bw.flush();
     print_buffer(bp);
+    bw.writeBitsSigned(-1, 3);  // 111
+    bw.writeBitsSigned(-12, 6); // 110100
+    bw.writeBitsSigned(4, 3);   // 100
+    bw.writeBitsSigned(-4, 3);  // 100
+    bw.writeBitsSigned(-1, 1);  // 1
+    bw.flush();
+    print_buffer(bp);
 
     delete bp;
 }

libtests/qtest/bits/bits.out

@@ -36,6 +36,12 @@ byte offset = 8, bit offset = 7, bits available = 0
 21
 101
 
+-1
+-22
+5
+0
+-39559
+
 ch = f0, bit_offset = 2
 ch = 00, bit_offset = 6
 ch = 14, bit_offset = 0
@@ -56,4 +62,6 @@ f5 15 65 79 12 89 75 4b
 
 f0 ab
 
+fa 49
+
 done