Burrows–Wheeler transform 算法

Burrows–Wheeler transform 算法

#!/usr/bin/python

# Burrows–Wheeler transform Algorithm

def transform(s, start_symbol, end_symbol):
  s = start_symbol + s + end_symbol
  rotations = []
  for i in range(0, len(s)):
    rotations.append(s[len(s) - i:] + s[0:len(s) - i])
  rotations.sort();
  output = ""
  for item in rotations:
    output += item[-1]
  return output

def inverse_transform(s, start_symbol, end_symbol):
  lines = []
  for c in s:
    lines.append("")
  for times in range(len(s)):
    for i in range(len(s)):
      lines[i] = s[i] + lines[i]
    lines.sort()
  for item in lines:
    if item.startswith(start_symbol) and item.endswith(end_symbol):
      return item[len(start_symbol):-len(end_symbol)]

if __name__ == "__main__":
  start_symbol = "^"
  end_symbol = "@"
  src = "banana"
  print "original str: ", src
  str1 = transform(src, start_symbol, end_symbol)
  print "transformed str: ", str1
  str2 = inverse_transform(str1, start_symbol, end_symbol)
  print "inverse_transformed: ", str2