Source Code for Module midi.encoding

 1  """Utilities needed for encoding/decoding MIDI data. 
 2   
 3  """ 
 4   
 5 -def read_varlen(data): 
 6      """ 
 7      Reads a variable-length value from a stream as used in MIDI data. 
 8      Pops all the bytes that form a part of the value and returns the  
 9      decoded numeric value. 
10       
11      """ 
12      NEXTBYTE = 1 
13      value = 0 
14      while NEXTBYTE: 
15          chr = ord(data.next()) 
16          # is the hi-bit set? 
17          if not (chr & 0x80): 
18              # no next BYTE 
19              NEXTBYTE = 0 
20          # mask out the 8th bit 
21          chr = chr & 0x7f 
22          # shift last value up 7 bits 
23          value = value << 7 
24          # add new value 
25          value += chr 
26      return value 
27   
28 -def write_varlen(value): 
29      """ 
30      Encodes the given numeric value as a MIDI variable-length value  
31      and returns the data as a string that should be written to a  
32      data stream to represent that value. 
33       
34      """ 
35      if value>=pow(2,28): 
36          raise ValueError("A varlen must be less than 2^28") 
37      chr1 = chr(value & 0x7F) 
38      value >>= 7 
39      if value: 
40          chr2 = chr((value & 0x7F) | 0x80) 
41          value >>= 7 
42          if value: 
43              chr3 = chr((value & 0x7F) | 0x80) 
44              value >>= 7 
45              if value: 
46                  chr4 = chr((value & 0x7F) | 0x80) 
47                  res = chr4 + chr3 + chr2 + chr1 
48              else: 
49                  res = chr3 + chr2 + chr1 
50          else: 
51              res = chr2 + chr1 
52      else: 
53          res = chr1 
54      return res 
55   
56   
57 -def r_varlen(hexstring): 
58      "reads a hex value as a string, returns an integer" 
59      value = '' 
60       
61      length = len(hexstring) / 2 
62       
63      for i in range(length): 
64          byte = '0x'+hexstring[2*i]+hexstring[2*i+1] 
65          byte = int(byte,16) 
66           
67   
68          binstring = bin(int(byte)) 
69          binstring = binstring[2:] 
70          binstring = '0'*(8-len(binstring))+binstring 
71          binstring = binstring[1:] 
72           
73          value+=binstring 
74           
75      value='0b'+value 
76      value=int(value,2) 
77   
78      return value 
79   
80   
81 -def __test_varlen(): 
82      for value in xrange(0x0FFFFFFF): 
83          if not (value % 0xFFFF): 
84              print hex(value) 
85          datum = write_varlen(value) 
86          newvalue = read_varlen(iter(datum)) 
87          if value != newvalue:  
88              hexstr = str.join('', map(hex, map(ord, datum))) 
89              print "%s != %s (hex: %s)" % (value, newvalue, hexstr) 
90