#!/usr/bin/env python
# Kanwei Li, 2010
# Selects N random lines from a file and outputs to another file

import random
import sys


def main():
    infile = open(sys.argv[1])
    total_lines = int(sys.argv[2])

    if total_lines < 1:
        sys.stderr.write("Must select at least one line.")
        sys.exit()

    kept = []
    n = 0
    for line in infile:
        line = line.rstrip("\n")
        n += 1
        if n <= total_lines:
            kept.append(line)
        elif random.randint(1, n) <= total_lines:
            kept.pop(random.randint(0, total_lines - 1))
            kept.append(line)

    if n < total_lines:
        sys.stderr.write("Error: asked to select more lines than there were in the file.")
        sys.exit()

    open(sys.argv[3], "w").write("\n".join(kept))


if __name__ == "__main__":
    main()