Initial commit

text_codecs/ascii7.py

@@ -0,0 +1,33 @@
+from custom_codecs.register import register_codec
+from easybits import Bits
+
+def encode(text):
+    """An encoder which only handles ASCII: non-ASCII characters
+    are replaced with '?'. Once all the characters are ASCII, this encoder
+    writes the first seven bits of each byte (the first bit of every 
+    ASCII character is 0, so we can easily reconstruct the full byte 
+    when we decode). Therefore, this encoder compresses ASCII text into
+    7/8 of the usual size.
+    """
+    result = Bits()
+    for char in text:
+        try:
+            b = Bits(char, encoding='ascii')
+        except UnicodeEncodeError:
+            b = Bits('?')
+        result = result.concat(b[1:])
+    return result.bytes
+
+def decode(data):
+    """The matching decoder. Reads seven bits at a time, putting a 0 on 
+    the front to create a full byte. Then converts this byte into its 
+    ASCII representation.
+    """
+    bits = Bits(bytes(data))
+    text = ""
+    for i in range(0, len(bits), 7):
+        byte = Bits('0').concat(bits[i:i+7])
+        text += Bits(byte).ascii
+    return text
+
+register_codec(encode, decode, "ascii7")