#!/usr/bin/env python3
"""
Extract the first N lines from a huge file without loading it into RAM.

Works because Python file objects are iterators: reading line-by-line
only ever keeps one line (plus a small internal read-ahead buffer) in
memory at a time, regardless of the file's total size.

Usage:
    python extract_lines.py input.txt output.txt --lines 500

    # Start from a specific line (e.g. skip a header, or resume mid-file)
    python extract_lines.py input.txt output.txt --lines 500 --start 1000
"""

import argparse
from itertools import islice


def extract_lines(input_path: str, output_path: str, start: int, count: int) -> int:
    written = 0
    with open(input_path, "r", encoding="utf-8", errors="replace", buffering=1024 * 1024) as fin, \
         open(output_path, "w", encoding="utf-8") as fout:
        for line in islice(fin, start, start + count):
            fout.write(line)
            written += 1
    return written


def main():
    parser = argparse.ArgumentParser(description="Extract a slice of lines from a huge file, streaming (low RAM).")
    parser.add_argument("input", help="Path to the big input file")
    parser.add_argument("output", help="Path to write the extracted lines to")
    parser.add_argument("--lines", type=int, default=500, help="Number of lines to extract (default: 500)")
    parser.add_argument("--start", type=int, default=0, help="Line number to start at, 0-indexed (default: 0)")
    args = parser.parse_args()

    n = extract_lines(args.input, args.output, args.start, args.lines)
    print(f"Wrote {n} lines (lines {args.start}-{args.start + n - 1}) to {args.output}")

if __name__ == "__main__":
    main()