import struct
# ported directly from the PalmDoc Perl library
# http://kobesearch.cpan.org/htdocs/EBook-Tools/EBook/Tools/PalmDoc.pm.html
# ... and reported to py 3

def uncompress_lz77(data):
  length = len(data);
  offset = 0;   # Current offset into data
  # char;      # Character being examined
  # ord;      # Ordinal of $char
  # lz77;      # 16-bit Lempel-Ziv 77 length-offset pair
  # lz77offset;   # LZ77 offset
  # lz77length;   # LZ77 length
  # lz77pos;    # Position inside $lz77length
  text = b'';   # Output (uncompressed) text
  # textlength;   # Length of uncompressed text during LZ77 pass
  # textpos;    # Position inside $text during LZ77 pass

  while offset < length:
    # char = substr($data, $offset++, 1);
    ord_ = data[offset];
    byte = ord_.to_bytes(1, byteorder='big')
    offset += 1;

    # The long if-elsif chain is the best logic for $ord handling
    ## no critic (Cascading if-elsif chain)
    if (ord_ == 0):
      # Nulls are literal
      text += byte;
    elif (ord_ <= 8):
      # Next $ord bytes are literal
      text += data[offset:offset + ord_] # text .=substr($data, $offset, ord);
      offset += ord_;
    elif (ord_ <= 0x7f):
      # Values from 0x09 through 0x7f are literal
      text += byte;
    elif (ord_ <= 0xbf):
      # Data is LZ77-compressed

      # From Wikipedia:
      # "A length-distance pair is always encoded by a two-byte
      # sequence. Of the 16 bits that make up these two bytes,
      # 11 bits go to encoding the distance, 3 go to encoding
      # the length, and the remaining two are used to make sure
      # the decoder can identify the first byte as the beginning
      # of such a two-byte sequence."

      offset += 1;
      if (offset > len(data)):
        print("WARNING: offset to LZ77 bits is outside of the data: %d" % offset);
        return text;

      lz77, = struct.unpack('>H', data[offset-2:offset])

      # Leftmost two bits are ID bits and need to be dropped
      lz77 &= 0x3fff;

      # Length is rightmost 3 bits + 3
      lz77length = (lz77 & 0x0007) + 3;

      # Remaining 11 bits are offset
      lz77offset = lz77 >> 3;
      if (lz77offset < 1):
        print("WARNING: LZ77 decompression offset is invalid!");
        return text;

      # Getting text from the offset is a little tricky, because
      # in theory you can be referring to characters you haven't
      # actually decompressed yet. You therefore have to check
      # the reference one character at a time.
      textlength = len(text);
      for lz77pos in range(lz77length): # for($lz77pos = 0; $lz77pos < $lz77length; $lz77pos++)
        textpos = textlength - lz77offset;
        if (textpos < 0):
          print("WARNING: LZ77 decompression reference is before"+
                " beginning of text! %x" % lz77);
          return;

        text += text[textpos:textpos + 1]; #text .= substr($text, $textpos, 1);
        textlength += 1;
    else:
      # 0xc0 - 0xff are single characters (XOR 0x80) preceded by
      # a space
      text += b' ' + (ord_ ^ 0x80).to_bytes(1, byteorder='big');
  return text;