From: Joseph Spiros <joseph.spiros@ithinksw.com>
Date: Sat, 9 Apr 2011 02:21:36 +0000 (-0400)
Subject: Initial commit. Implemented reading of variable size integers, element sizes, and... 
X-Git-Url: http://git.ithinksw.org/~jspiros/python-ebml.git/commitdiff_plain/86596c9b7ed163682a87c604f203dba9e99d49bb?ds=sidebyside;pf=~jspiros

Initial commit. Implemented reading of variable size integers, element sizes, and element IDs.
---

86596c9b7ed163682a87c604f203dba9e99d49bb
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..0d20b64
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+*.pyc
diff --git a/ebml/__init__.py b/ebml/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/ebml/core.py b/ebml/core.py
new file mode 100644
index 0000000..2e0702c
--- /dev/null
+++ b/ebml/core.py
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+import warnings
+from .exceptions import *
+
+
+EBMLMaxSizeLength = 8
+EBMLMaxIDLength = 4
+
+
+def _read_vint_to_bytearray(stream, max_width=EBMLMaxSizeLength):
+	"""
+	
+	Reads a vint from stream and returns a bytearray containing all of the bytes without doing any decoding.
+	
+	:arg stream: the source of the bytes
+	:type stream: a file-like object
+	:arg max_width: the maximum length, in bytes, of the vint (defaults to :data:`EBMLMaxSizeLength`)
+	:type max_width: int
+	:returns: bytearray
+	
+	"""
+	
+	marker_found = False
+	vint_bytes = bytearray()
+	vint_len = -7
+	while not marker_found:
+		vint_len += 8
+		if vint_len > max_width:
+			raise ParseError('vint exceeds max_width (%(max_width)i)' % {
+				'max_width': max_width
+			})
+		byte = ord(stream.read(1))
+		vint_bytes.append(byte)
+		for pos in range(0, 8):
+			mask = 0b10000000 >> pos
+			if byte & mask:
+				vint_len += pos
+				marker_found = True
+				break
+	
+	remaining_bytes_len = vint_len - len(vint_bytes)
+	if remaining_bytes_len > 0:
+		vint_bytes.extend(ord(remaining_byte) for remaining_byte in stream.read(remaining_bytes_len))
+	
+	if len(vint_bytes) != vint_len:
+		raise ParseError('Unable to read truncated vint of width %(vint_len)s from stream (%(vint_bytes)s bytes available)' % {
+			'vint_len': vint_len,
+			'vint_bytes': len(vint_bytes)
+		})
+	
+	return vint_bytes
+
+
+def read_element_size(stream, max_width=EBMLMaxSizeLength):
+	"""
+	
+	Reads an EBML element size vint from stream and returns a tuple containing:
+	
+		* the size as an integer, or None if the size is undefined
+		* the length in bytes of the size descriptor (the vint) itself
+	
+	:arg stream: the source of the bytes
+	:type stream: a file-like object
+	:arg max_width: the maximum length, in bytes, of the vint storing the element size (defaults to :data:`EBMLMaxSizeLength`)
+	:type max_width: int
+	:returns: tuple
+	
+	"""
+	
+	vint_bytes = _read_vint_to_bytearray(stream, max_width)
+	vint_len = len(vint_bytes)
+	
+	int_bytes = vint_bytes[((vint_len - 1) // 8):]
+	first_byte_mask = 0b10000000 >> ((vint_len - 1) % 8)
+	max_bytes = 0
+	
+	value = int_bytes[0] & (first_byte_mask - 1)
+	
+	if value == (first_byte_mask - 1):
+		max_bytes += 1
+	
+	for int_byte in int_bytes[1:]:
+		if int_byte == 0b11111111:
+			max_bytes += 1
+		value = (value << 8) | int_byte
+	
+	if max_bytes == len(int_bytes):
+		value = None
+	
+	return value, vint_len
+
+
+def read_element_id(stream, max_width=EBMLMaxIDLength):
+	"""
+	
+	Reads an EBML element ID vint from stream and returns a tuple containing:
+	
+		* the ID as an integer
+		* the length in bytes of the ID descriptor (the vint) itself
+	
+	:arg stream: the source of the bytes
+	:type stream: a file-like object
+	:arg max_width: the maximum length, in bytes, of the vint storing the element ID (defaults to :data:`EBMLMaxIDLength`)
+	:type max_width: int
+	:returns: tuple
+	
+	"""
+	
+	vint_bytes = _read_vint_to_bytearray(stream, max_width)
+	vint_len = len(vint_bytes)
+	
+	value = 0
+	max_bytes = 0
+	min_bytes = 0
+	
+	for vint_byte in vint_bytes:
+		if vint_byte == 0b11111111:
+			max_bytes += 1
+		elif vint_byte == 0:
+			min_bytes += 1
+		value = (value << 8) | vint_byte
+	
+	if max_bytes == vint_len:
+		raise ReservedElementIDError('All value bits set to 1')
+	elif min_bytes == vint_len:
+		raise ReservedElementIDError('All value bits set to 0')
+	
+	return value, vint_len
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diff --git a/ebml/exceptions.py b/ebml/exceptions.py
new file mode 100644
index 0000000..e501df0
--- /dev/null
+++ b/ebml/exceptions.py
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+class EBMLException(Exception):
+	pass
+
+class ParseError(EBMLException):
+	pass
+
+class ReservedElementIDError(EBMLException):
+	pass
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