1 files changed, 50 insertions, 0 deletions
diff --git a/dedup/filters.py b/dedup/filters.py
new file mode 100644
index 0000000..c5b3251
--- /dev/null
+++ b/dedup/filters.py
@@ -0,0 +1,50 @@
+import struct
+
+class PNGFilter:
+    """Skips non-critical chunks in a PNG file."""
+    magic = b"\x89PNG\r\n\x1a\n"
+    def __init__(self):
+        self.inbuffer = b""
+        self.critchunk = False
+        self.chunkleft = None
+
+    def filter(self, data):
+        self.inbuffer += data
+        if self.chunkleft is None:
+            if len(self.inbuffer) < 8:
+                return b""
+            if not self.inbuffer.startswith(self.magic):
+                raise ValueError("PNG file magic not found")
+            self.inbuffer = self.inbuffer[8:]
+            self.chunkleft = 0
+        ret = b""
+        while True:
+            if self.chunkleft == 0:
+                if len(self.inbuffer) < 8:
+                    break
+                self.chunkleft, chunktype = struct.unpack(">I4s",
+                        self.inbuffer[:8])
+                self.chunkleft += 12 # len, type, crc
+                self.critchunk = chunktype[0].isupper()
+                if self.critchunk:
+                    print("critical chunk %s %r" % (chunktype, self.inbuffer[8:16]))
+            n = min(self.chunkleft, len(self.inbuffer))
+            if self.critchunk:
+                ret += self.inbuffer[:n]
+            self.inbuffer = self.inbuffer[n:]
+            self.chunkleft -= n
+            if self.chunkleft:
+                break
+        return ret
+
+    def flush(self):
+        ret = self.inbuffer
+        self.inbuffer = b""
+        return ret
+
+    def copy(self):
+        new = PNGFilter()
+        new.inbuffer = self.inbuffer
+        new.critchunk = self.critchunk
+        new.chunkleft = self.chunkleft
+        return new