pub fn hashmap_random_keys() -> (u64, u64) {
    const KEY_LEN: usize = core::mem::size_of::<u64>();

    let mut v = [0u8; KEY_LEN * 2];
    imp::fill_bytes(&mut v);

    let key1 = v[0..KEY_LEN].try_into().unwrap();
    let key2 = v[KEY_LEN..].try_into().unwrap();

    (u64::from_ne_bytes(key1), u64::from_ne_bytes(key2))
}

#[cfg(all(
    unix,
    not(target_os = "macos"),
    not(target_os = "ios"),
    not(target_os = "watchos"),
    not(target_os = "openbsd"),
    not(target_os = "freebsd"),
    not(target_os = "netbsd"),
    not(target_os = "fuchsia"),
    not(target_os = "redox"),
    not(target_os = "vxworks")
))]
mod imp {
    use crate::fs::File;
    use crate::io::Read;

    #[cfg(any(target_os = "linux", target_os = "android"))]
    use crate::sys::weak::syscall;

    #[cfg(any(target_os = "linux", target_os = "android"))]
    fn getrandom(buf: &mut [u8]) -> libc::ssize_t {
        use crate::sync::atomic::{AtomicBool, Ordering};
        use crate::sys::os::errno;

        // A weak symbol allows interposition, e.g. for perf measurements that want to
        // disable randomness for consistency. Otherwise, we'll try a raw syscall.
        // (`getrandom` was added in glibc 2.25, musl 1.1.20, android API level 28)
        syscall! {
            fn getrandom(
                buffer: *mut libc::c_void,
                length: libc::size_t,
                flags: libc::c_uint
            ) -> libc::ssize_t
        }

        // This provides the best quality random numbers available at the given moment
        // without ever blocking, and is preferable to falling back to /dev/urandom.
        static GRND_INSECURE_AVAILABLE: AtomicBool = AtomicBool::new(true);
        if GRND_INSECURE_AVAILABLE.load(Ordering::Relaxed) {
            let ret = unsafe { getrandom(buf.as_mut_ptr().cast(), buf.len(), libc::GRND_INSECURE) };
            if ret == -1 && errno() as libc::c_int == libc::EINVAL {
                GRND_INSECURE_AVAILABLE.store(false, Ordering::Relaxed);
            } else {
                return ret;
            }
        }

        unsafe { getrandom(buf.as_mut_ptr().cast(), buf.len(), libc::GRND_NONBLOCK) }
    }

    #[cfg(any(target_os = "espidf", target_os = "horizon"))]
    fn getrandom(buf: &mut [u8]) -> libc::ssize_t {
        unsafe { libc::getrandom(buf.as_mut_ptr().cast(), buf.len(), 0) }
    }

    #[cfg(not(any(
        target_os = "linux",
        target_os = "android",
        target_os = "espidf",
        target_os = "horizon"
    )))]
    fn getrandom_fill_bytes(_buf: &mut [u8]) -> bool {
        false
    }

    #[cfg(any(
        target_os = "linux",
        target_os = "android",
        target_os = "espidf",
        target_os = "horizon"
    ))]
    fn getrandom_fill_bytes(v: &mut [u8]) -> bool {
        use crate::sync::atomic::{AtomicBool, Ordering};
        use crate::sys::os::errno;

        static GETRANDOM_UNAVAILABLE: AtomicBool = AtomicBool::new(false);
        if GETRANDOM_UNAVAILABLE.load(Ordering::Relaxed) {
            return false;
        }

        let mut read = 0;
        while read < v.len() {
            let result = getrandom(&mut v[read..]);
            if result == -1 {
                let err = errno() as libc::c_int;
                if err == libc::EINTR {
                    continue;
                } else if err == libc::ENOSYS || err == libc::EPERM {
                    // Fall back to reading /dev/urandom if `getrandom` is not
                    // supported on the current kernel.
                    //
                    // Also fall back in case it is disabled by something like
                    // seccomp or inside of virtual machines.
                    GETRANDOM_UNAVAILABLE.store(true, Ordering::Relaxed);
                    return false;
                } else if err == libc::EAGAIN {
                    return false;
                } else {
                    panic!("unexpected getrandom error: {err}");
                }
            } else {
                read += result as usize;
            }
        }
        true
    }

    pub fn fill_bytes(v: &mut [u8]) {
        // getrandom_fill_bytes here can fail if getrandom() returns EAGAIN,
        // meaning it would have blocked because the non-blocking pool (urandom)
        // has not initialized in the kernel yet due to a lack of entropy. The
        // fallback we do here is to avoid blocking applications which could
        // depend on this call without ever knowing they do and don't have a
        // work around. The PRNG of /dev/urandom will still be used but over a
        // possibly predictable entropy pool.
        if getrandom_fill_bytes(v) {
            return;
        }

        // getrandom failed because it is permanently or temporarily (because
        // of missing entropy) unavailable. Open /dev/urandom, read from it,
        // and close it again.
        let mut file = File::open("/dev/urandom").expect("failed to open /dev/urandom");
        file.read_exact(v).expect("failed to read /dev/urandom")
    }
}

#[cfg(any(target_os = "macos", target_os = "ios", target_os = "watchos"))]
mod imp {
    use crate::io;
    use crate::sys::weak::weak;
    use libc::{c_int, c_void, size_t};

    fn getentropy_fill_bytes(v: &mut [u8]) -> bool {
        weak!(fn getentropy(*mut c_void, size_t) -> c_int);

        getentropy
            .get()
            .map(|f| {
                // getentropy(2) permits a maximum buffer size of 256 bytes
                for s in v.chunks_mut(256) {
                    let ret = unsafe { f(s.as_mut_ptr() as *mut c_void, s.len()) };
                    if ret == -1 {
                        panic!("unexpected getentropy error: {}", io::Error::last_os_error());
                    }
                }
                true
            })
            .unwrap_or(false)
    }

    #[cfg(target_os = "macos")]
    fn fallback_fill_bytes(v: &mut [u8]) {
        use crate::fs::File;
        use crate::io::Read;

        let mut file = File::open("/dev/urandom").expect("failed to open /dev/urandom");
        file.read_exact(v).expect("failed to read /dev/urandom")
    }

    // On iOS and MacOS `SecRandomCopyBytes` calls `CCRandomCopyBytes` with
    // `kCCRandomDefault`. `CCRandomCopyBytes` manages a CSPRNG which is seeded
    // from `/dev/random` and which runs on its own thread accessed via GCD.
    //
    // This is very heavyweight compared to the alternatives, but they may not be usable:
    // - `getentropy` was added in iOS 10, but we support a minimum of iOS 7
    // - `/dev/urandom` is not accessible inside the iOS app sandbox.
    //
    // Therefore `SecRandomCopyBytes` is only used on older iOS versions where no
    // better options are present.
    #[cfg(target_os = "ios")]
    fn fallback_fill_bytes(v: &mut [u8]) {
        use crate::ptr;

        enum SecRandom {}

        #[allow(non_upper_case_globals)]
        const kSecRandomDefault: *const SecRandom = ptr::null();

        extern "C" {
            fn SecRandomCopyBytes(rnd: *const SecRandom, count: size_t, bytes: *mut u8) -> c_int;
        }

        let ret = unsafe { SecRandomCopyBytes(kSecRandomDefault, v.len(), v.as_mut_ptr()) };
        if ret == -1 {
            panic!("couldn't generate random bytes: {}", io::Error::last_os_error());
        }
    }

    // All supported versions of watchOS (>= 5) have support for `getentropy`.
    #[cfg(target_os = "watchos")]
    #[cold]
    fn fallback_fill_bytes(_: &mut [u8]) {
        unreachable!()
    }

    pub fn fill_bytes(v: &mut [u8]) {
        if getentropy_fill_bytes(v) {
            return;
        }

        // Older macOS versions (< 10.12) don't support `getentropy`. Fallback to
        // reading from `/dev/urandom` on these systems.
        //
        // Older iOS versions (< 10) don't support it either. Fallback to
        // `SecRandomCopyBytes` on these systems. On watchOS, this is unreachable
        // because the minimum supported version is 5 while `getentropy` became accessible
        // in 3.
        fallback_fill_bytes(v)
    }
}

#[cfg(target_os = "openbsd")]
mod imp {
    use crate::sys::os::errno;

    pub fn fill_bytes(v: &mut [u8]) {
        // getentropy(2) permits a maximum buffer size of 256 bytes
        for s in v.chunks_mut(256) {
            let ret = unsafe { libc::getentropy(s.as_mut_ptr() as *mut libc::c_void, s.len()) };
            if ret == -1 {
                panic!("unexpected getentropy error: {}", errno());
            }
        }
    }
}

#[cfg(any(target_os = "freebsd", target_os = "netbsd"))]
mod imp {
    use crate::ptr;

    pub fn fill_bytes(v: &mut [u8]) {
        let mib = [libc::CTL_KERN, libc::KERN_ARND];
        // kern.arandom permits a maximum buffer size of 256 bytes
        for s in v.chunks_mut(256) {
            let mut s_len = s.len();
            let ret = unsafe {
                libc::sysctl(
                    mib.as_ptr(),
                    mib.len() as libc::c_uint,
                    s.as_mut_ptr() as *mut _,
                    &mut s_len,
                    ptr::null(),
                    0,
                )
            };
            if ret == -1 || s_len != s.len() {
                panic!(
                    "kern.arandom sysctl failed! (returned {}, s.len() {}, oldlenp {})",
                    ret,
                    s.len(),
                    s_len
                );
            }
        }
    }
}

#[cfg(target_os = "fuchsia")]
mod imp {
    #[link(name = "zircon")]
    extern "C" {
        fn zx_cprng_draw(buffer: *mut u8, len: usize);
    }

    pub fn fill_bytes(v: &mut [u8]) {
        unsafe { zx_cprng_draw(v.as_mut_ptr(), v.len()) }
    }
}

#[cfg(target_os = "redox")]
mod imp {
    use crate::fs::File;
    use crate::io::Read;

    pub fn fill_bytes(v: &mut [u8]) {
        // Open rand:, read from it, and close it again.
        let mut file = File::open("rand:").expect("failed to open rand:");
        file.read_exact(v).expect("failed to read rand:")
    }
}

#[cfg(target_os = "vxworks")]
mod imp {
    use crate::io;
    use core::sync::atomic::{AtomicBool, Ordering::Relaxed};

    pub fn fill_bytes(v: &mut [u8]) {
        static RNG_INIT: AtomicBool = AtomicBool::new(false);
        while !RNG_INIT.load(Relaxed) {
            let ret = unsafe { libc::randSecure() };
            if ret < 0 {
                panic!("couldn't generate random bytes: {}", io::Error::last_os_error());
            } else if ret > 0 {
                RNG_INIT.store(true, Relaxed);
                break;
            }
            unsafe { libc::usleep(10) };
        }
        let ret = unsafe {
            libc::randABytes(v.as_mut_ptr() as *mut libc::c_uchar, v.len() as libc::c_int)
        };
        if ret < 0 {
            panic!("couldn't generate random bytes: {}", io::Error::last_os_error());
        }
    }
}