title: Efficient way to append bytes to a word and reverse them?
body: <p>I have an array of bytes like this, in Python 2.7 (the actual object is a bytearray, but I write it as a common array for better readability):</p>
  
  <pre><code>array = [0x01, 0x02, 0x03, 0x04]
  # actual thing is b'\x01\x02\x03\x04'
  
  # its bit representation would be:
  bit_array = [0b00000001, 0b00000010, 0b00000011, 0b00000100]
  </code></pre>
  
  <p>I would like to construct another variable, called <em>word</em>, that is the combination of the bytes in the array, like this:</p>
  
  <pre><code>word = 0x01020304  # this would be a 32-bit int
  </code></pre>
  
  <p>However, in my case, I also need to reverse the bytes (as they are received by another system that works in a reversed bit-order than mine), so instead of the original byte array, I should combine this one in order to obtain what I need:</p>
  
  <pre><code>reversed_bit_array = [0b10000000, 0b01000000, 0b11000000, 0b00100000]
  #reversed_word = whatever
  </code></pre>
  
  <p>What is the most <em>efficient</em> way to do this? The code should be called multiple times in a loop, so it should be as fast as possible. I've tried to use a lookup table but I'm a little confused about the results, so there is definitely something wrong.
  Below is my code:</p>
  
  <pre><code>a = bytearray([0x01,0x02,0x03, 0x04])
  b = 0x01020304 # this is the combined array without reversal
  c = 0x10203040 # this is what I want
  word = 0
  
  for j in range(len(a)):
      word = word &lt;&lt; 8
      word += bitReverseTable256[a[j]] 
  
  print(bin(word))
  print(bin(c))
  
  &gt;&gt;0b10000000010000001100000000100000
  &gt;&gt;0b10000001000000011000001000000
  </code></pre>
  
  <p>Where <code>bitReverseTable256</code> is the following lookup table:</p>
  
  <pre><code>bitReverseTable256 = (
      0x00, 0x80, 0x40, 0xC0, 0x20, 0xA0, 0x60, 0xE0, 0x10, 0x90, 0x50, 0xD0, 0x30, 0xB0, 0x70, 0xF0,
      0x08, 0x88, 0x48, 0xC8, 0x28, 0xA8, 0x68, 0xE8, 0x18, 0x98, 0x58, 0xD8, 0x38, 0xB8, 0x78, 0xF8,
      0x04, 0x84, 0x44, 0xC4, 0x24, 0xA4, 0x64, 0xE4, 0x14, 0x94, 0x54, 0xD4, 0x34, 0xB4, 0x74, 0xF4,
      0x0C, 0x8C, 0x4C, 0xCC, 0x2C, 0xAC, 0x6C, 0xEC, 0x1C, 0x9C, 0x5C, 0xDC, 0x3C, 0xBC, 0x7C, 0xFC,
      0x02, 0x82, 0x42, 0xC2, 0x22, 0xA2, 0x62, 0xE2, 0x12, 0x92, 0x52, 0xD2, 0x32, 0xB2, 0x72, 0xF2,
      0x0A, 0x8A, 0x4A, 0xCA, 0x2A, 0xAA, 0x6A, 0xEA, 0x1A, 0x9A, 0x5A, 0xDA, 0x3A, 0xBA, 0x7A, 0xFA,
      0x06, 0x86, 0x46, 0xC6, 0x26, 0xA6, 0x66, 0xE6, 0x16, 0x96, 0x56, 0xD6, 0x36, 0xB6, 0x76, 0xF6,
      0x0E, 0x8E, 0x4E, 0xCE, 0x2E, 0xAE, 0x6E, 0xEE, 0x1E, 0x9E, 0x5E, 0xDE, 0x3E, 0xBE, 0x7E, 0xFE,
      0x01, 0x81, 0x41, 0xC1, 0x21, 0xA1, 0x61, 0xE1, 0x11, 0x91, 0x51, 0xD1, 0x31, 0xB1, 0x71, 0xF1,
      0x09, 0x89, 0x49, 0xC9, 0x29, 0xA9, 0x69, 0xE9, 0x19, 0x99, 0x59, 0xD9, 0x39, 0xB9, 0x79, 0xF9,
      0x05, 0x85, 0x45, 0xC5, 0x25, 0xA5, 0x65, 0xE5, 0x15, 0x95, 0x55, 0xD5, 0x35, 0xB5, 0x75, 0xF5,
      0x0D, 0x8D, 0x4D, 0xCD, 0x2D, 0xAD, 0x6D, 0xED, 0x1D, 0x9D, 0x5D, 0xDD, 0x3D, 0xBD, 0x7D, 0xFD,
      0x03, 0x83, 0x43, 0xC3, 0x23, 0xA3, 0x63, 0xE3, 0x13, 0x93, 0x53, 0xD3, 0x33, 0xB3, 0x73, 0xF3,
      0x0B, 0x8B, 0x4B, 0xCB, 0x2B, 0xAB, 0x6B, 0xEB, 0x1B, 0x9B, 0x5B, 0xDB, 0x3B, 0xBB, 0x7B, 0xFB,
      0x07, 0x87, 0x47, 0xC7, 0x27, 0xA7, 0x67, 0xE7, 0x17, 0x97, 0x57, 0xD7, 0x37, 0xB7, 0x77, 0xF7,
      0x0F, 0x8F, 0x4F, 0xCF, 0x2F, 0xAF, 0x6F, 0xEF, 0x1F, 0x9F, 0x5F, 0xDF, 0x3F, 0xBF, 0x7F, 0xFF
  )
  </code></pre>
  
  <p>As it can be seen, the results are not what I expected. I don't know what is wrong, and I also ask myself how efficient is this (I suppose there is a faster way to do it).</p>