#!/usr/bin/env python3
"""Report Linux page-cache residency for files without reading their contents."""

from __future__ import annotations

import argparse
import ctypes
import json
import os


def residency(path: str) -> dict[str, int | float | str]:
    page_size = os.sysconf("SC_PAGE_SIZE")
    size = os.stat(path).st_size
    pages = (size + page_size - 1) // page_size
    fd = os.open(path, os.O_RDONLY)
    libc = ctypes.CDLL(None, use_errno=True)
    libc.mmap.restype = ctypes.c_void_p
    libc.mmap.argtypes = [
        ctypes.c_void_p,
        ctypes.c_size_t,
        ctypes.c_int,
        ctypes.c_int,
        ctypes.c_int,
        ctypes.c_long,
    ]
    libc.mincore.argtypes = [ctypes.c_void_p, ctypes.c_size_t, ctypes.c_void_p]
    libc.munmap.argtypes = [ctypes.c_void_p, ctypes.c_size_t]
    address = libc.mmap(None, size, 0, 1, fd, 0)  # PROT_NONE, MAP_SHARED
    os.close(fd)
    if address == ctypes.c_void_p(-1).value:
        error = ctypes.get_errno()
        raise OSError(error, os.strerror(error), path)
    vector = (ctypes.c_ubyte * pages)()
    try:
        if libc.mincore(address, size, vector) != 0:
            error = ctypes.get_errno()
            raise OSError(error, os.strerror(error), path)
        resident = sum(1 for value in vector if value & 1)
    finally:
        libc.munmap(address, size)
    return {
        "path": path,
        "size_bytes": size,
        "page_size": page_size,
        "pages": pages,
        "resident_pages": resident,
        "resident_bytes": resident * page_size,
        "resident_percent": round(100 * resident / pages, 2) if pages else 100.0,
    }


def main() -> None:
    parser = argparse.ArgumentParser()
    parser.add_argument("paths", nargs="+")
    args = parser.parse_args()
    print(json.dumps([residency(path) for path in args.paths], separators=(",", ":")))


if __name__ == "__main__":
    main()