#!/usr/bin/env python3
"""
Generate a >64 KB test file with predictable position-encoded content.

Every 256-byte block starts with a 32-character ASCII line
    "[off=NNNNNNN] block #BBBB filler...\n"
where NNNNNNN is the absolute byte offset of the block start (padded to 7
decimal digits) and BBBB is the 4-digit block index.  The line is padded
to 256 bytes with '.' so that seeking to any 256-aligned offset gives an
immediately-readable label.
"""
import sys
import os

def main():
    path = sys.argv[1] if len(sys.argv) > 1 else "big.txt"
    target_size = int(sys.argv[2]) if len(sys.argv) > 2 else 100 * 1024

    block_size = 256
    nblocks = (target_size + block_size - 1) // block_size

    with open(path, "wb") as f:
        for b in range(nblocks):
            offset = b * block_size
            header = f"[off={offset:07d}] block #{b:04d} "
            body = header + ("." * (block_size - len(header) - 1)) + "\n"
            assert len(body) == block_size, len(body)
            f.write(body.encode("ascii"))

    actual = os.path.getsize(path)
    print(f"wrote {path}: {actual} bytes ({nblocks} blocks of {block_size} each)")


if __name__ == "__main__":
    main()