import sys

# Make sure the command line is valid.
if (len(sys.argv) != 4):
  print "USAGE: compute-fdr.py <threshold> <target> <decoy>"
  sys.exit(1)

#############################################################################
# Define a function to read values from a file and report how many are above
# a given threshold.
#############################################################################
def readFileWithThreshold (fileName, threshold):
  # Counter of values above threshold
  returnValue = 0

  # Read the file line by line.
  myFile = open(fileName, "r")
  for line in myFile:
    if (len(line.split()) != 1):
      print "Invalid line (%s).\n" % line
      sys.exit(1)
    thisValue = float(line)
    if (thisValue >= threshold):
      returnValue += 1

  myFile.close()
  return(returnValue)
  
#############################################################################
# MAIN PROCEDURE
#############################################################################

# Read the arguments.
threshold = float(sys.argv[1])
targetFilename = sys.argv[2]
decoyFilename = sys.argv[3]

# Parse both files and store the count above threshold.
numTargets = readFileWithThreshold(targetFilename, threshold)
numDecoys = readFileWithThreshold(decoyFilename, threshold)

# Check for divide by zero and print FDR.
if (numTargets == 0):
  print "No targets above threshold. FDR undefined.\n"
else:
  print "%g" % (float(numDecoys) / float(numTargets))