gistfile1.nim

# nimrod c -d:release fasta.nim
# time ./fasta > /dev/null
# 3.635s

import parseutils
import strutils

proc process(filename: string) =
  var gcCount = 0
  var totalBaseCount = 0

  var input = open(filename)
  finally: input.close()

  while not input.EndOfFile():
    var line = input.readLine()
    if not line.startsWith(">"):
      for letter in line:
        case letter
        of 'T', 'A':
          totalBaseCount += 1
        of 'G', 'C':
          gcCount += 1
          totalBaseCount += 1
        else:
          nil

  let gcFraction = gcCount / totalBaseCount
  echo formatFloat(gcFraction * 100, ffDecimal, 4)


when isMainModule:
    process("Homo_sapiens.GRCh37.67.dna_rm.chromosome.Y.fa")