Initial commit

ackit/ogg.py

@@ -0,0 +1,72 @@
+"""Minimal Ogg page reader/writer."""
+import struct
+
+
+def _crc_table():
+    t = []
+    for i in range(256):
+        r = i << 24
+        for _ in range(8):
+            r = ((r << 1) ^ 0x04C11DB7) if r & 0x80000000 else (r << 1)
+        t.append(r & 0xFFFFFFFF)
+    return t
+
+
+_T = _crc_table()
+
+
+def ogg_crc(b: bytes) -> int:
+    c = 0
+    for x in b:
+        c = ((c << 8) & 0xFFFFFFFF) ^ _T[((c >> 24) & 0xFF) ^ x]
+    return c
+
+
+def read_packets(b: bytes):
+    """Return list of (packet_bytes, granule_of_page_if_packet_ends_page_else_None)."""
+    pos = 0
+    out = []
+    cur = b""
+    while pos + 27 <= len(b):
+        if b[pos:pos + 4] != b"OggS":
+            raise ValueError("lost Ogg sync at %d" % pos)
+        htype, gran, serial, seq, crc, nseg = struct.unpack_from("<BqIIIB", b, pos + 5)
+        segs = b[pos + 27: pos + 27 + nseg]
+        p = pos + 27 + nseg
+        ends = []
+        for s in segs:
+            cur += b[p:p + s]
+            p += s
+            if s < 255:
+                ends.append(cur)
+                cur = b""
+        for i, pk in enumerate(ends):
+            out.append((pk, gran if i == len(ends) - 1 else None))
+        pos = p
+    return out
+
+
+class OggWriter:
+    def __init__(self, serial=0x41434B54):
+        self.serial = serial
+        self.seq = 0
+        self.out = bytearray()
+
+    def page(self, packets, granule, bos=False, eos=False):
+        segs = bytearray()
+        body = bytearray()
+        for pk in packets:
+            n = len(pk)
+            while n >= 255:
+                segs.append(255); n -= 255
+            segs.append(n)
+            body += pk
+        hdr = struct.pack("<4sBBqIIIB", b"OggS", 0, (2 if bos else 0) | (4 if eos else 0),
+                          granule, self.serial, self.seq, 0, len(segs)) + bytes(segs)
+        page = bytearray(hdr + body)
+        struct.pack_into("<I", page, 22, ogg_crc(page))
+        self.out += page
+        self.seq += 1
+
+    def bytes(self):
+        return bytes(self.out)