//! macOS: hint the kernel to prefetch a generous chunk. macOS has no
//! direct `POSIX_FADV_SEQUENTIAL` equivalent; the idiomatic hint is
//! `fcntl(F_RDADVISE, &radvisory)` describing the byte range you
//! intend to read soon. We point it at the whole file (clamped to a
//! ceiling so a multi-TB ISO doesn't ask the kernel to prefetch
//! everything at once).

use std::fs::File;
use std::os::unix::io::AsRawFd;

/// Cap on the byte length we pass to `F_RDADVISE`. Asking for a
/// multi-GB readahead window is counterproductive — the OS doesn't
/// have that much cache to throw at one fd. 64 MiB is generous for
/// our use case (sweep, mux) so the kernel's prefetch ≥ our app-level
/// pipeline depth.
const RDADVISE_MAX_BYTES: i64 = 64 * 1024 * 1024;

pub(super) fn hint_sequential(file: &File, len_bytes: u64) {
    let bytes = (len_bytes as i64).min(RDADVISE_MAX_BYTES);
    let mut ra = libc::radvisory {
        ra_offset: 0,
        ra_count: bytes as libc::c_int,
    };
    // Best-effort.
    unsafe {
        libc::fcntl(file.as_raw_fd(), libc::F_RDADVISE, &mut ra);
    }
}

/// macOS has no direct `POSIX_FADV_DONTNEED` equivalent for a byte
/// range. `fcntl(F_NOCACHE)` would disable caching globally on the fd
/// (too coarse — we want the unread region to still benefit). Best
/// approximation: no-op. macOS's unified buffer cache is generally
/// less prone to the pin-everything pathology that triggers the
/// regression on Linux NFS clients.
pub(super) fn drop_window(_file: &File, _start: u64, _len: u64) {}

/// Async-prefetch the byte range `[offset, offset+len)`. macOS uses
/// the same `fcntl(F_RDADVISE, &radvisory)` primitive as the open-
/// time sequential hint, just targeted at a moving window instead of
/// the whole file. The kernel queues I/O for the requested range and
/// returns immediately.
pub(super) fn prefetch(file: &File, offset: u64, len: u64) {
    let bytes = (len as i64).min(RDADVISE_MAX_BYTES);
    let mut ra = libc::radvisory {
        ra_offset: offset as libc::off_t,
        ra_count: bytes as libc::c_int,
    };
    // Best-effort — kernel hint only.
    unsafe {
        libc::fcntl(file.as_raw_fd(), libc::F_RDADVISE, &mut ra);
    }
}