cfg_select! {
    // Tier 1
    any(target_os = "linux", target_os = "android") => {
        mod linux;
        pub use linux::{fill_bytes, hashmap_random_keys};
    }
    target_os = "windows" => {
        mod windows;
        pub use windows::fill_bytes;
    }
    target_vendor = "apple" => {
        mod apple;
        pub use apple::fill_bytes;
    // Others, in alphabetical ordering.
    }
    any(
        target_os = "dragonfly",
        target_os = "freebsd",
        target_os = "haiku",
        target_os = "illumos",
        target_os = "netbsd",
        target_os = "openbsd",
        target_os = "rtems",
        target_os = "solaris",
        target_os = "vita",
        target_os = "nuttx",
    ) => {
        mod arc4random;
        pub use arc4random::fill_bytes;
    }
    target_os = "emscripten" => {
        mod getentropy;
        pub use getentropy::fill_bytes;
    }
    target_os = "espidf" => {
        mod espidf;
        pub use espidf::fill_bytes;
    }
    target_os = "fuchsia" => {
        mod fuchsia;
        pub use fuchsia::fill_bytes;
    }
    target_os = "hermit" => {
        mod hermit;
        pub use hermit::fill_bytes;
    }
    any(target_os = "horizon", target_os = "cygwin") => {
        // FIXME(horizon): add arc4random_buf to shim-3ds
        mod getrandom;
        pub use getrandom::fill_bytes;
    }
    any(
        target_os = "aix",
        target_os = "hurd",
        target_os = "nto",
        target_os = "qnx",
    ) => {
        mod unix_legacy;
        pub use unix_legacy::fill_bytes;
    }
    target_os = "redox" => {
        mod redox;
        pub use redox::fill_bytes;
    }
    target_os = "motor" => {
        mod motor;
        pub use motor::fill_bytes;
    }
    all(target_vendor = "fortanix", target_env = "sgx") => {
        mod sgx;
        pub use sgx::fill_bytes;
    }
    target_os = "solid_asp3" => {
        mod solid;
        pub use solid::fill_bytes;
    }
    target_os = "teeos" => {
        mod teeos;
        pub use teeos::fill_bytes;
    }
    target_os = "trusty" => {
        mod trusty;
        pub use trusty::fill_bytes;
    }
    target_os = "uefi" => {
        mod uefi;
        pub use uefi::fill_bytes;
    }
    target_os = "vxworks" => {
        mod vxworks;
        pub use vxworks::fill_bytes;
    }
    all(target_os = "wasi", target_env = "p1") => {
        mod wasip1;
        pub use wasip1::fill_bytes;
    }
    all(target_os = "wasi", any(target_env = "p2", target_env = "p3")) => {
        mod wasi;
        pub use wasi::{fill_bytes, hashmap_random_keys};
    }
    target_os = "zkvm" => {
        mod zkvm;
        pub use zkvm::fill_bytes;
    }
    any(
        all(target_family = "wasm", target_os = "unknown"),
        target_os = "xous",
        target_os = "vexos",
        target_os = "l4re",
    ) => {
        // FIXME: finally remove std support for wasm32-unknown-unknown
        // FIXME: add random data generation to xous
        mod unsupported;
        pub use unsupported::{fill_bytes, hashmap_random_keys};
    }
    _ => {}
}

#[cfg(not(any(
    target_os = "linux",
    target_os = "android",
    all(target_family = "wasm", target_os = "unknown"),
    all(target_os = "wasi", not(target_env = "p1")),
    target_os = "xous",
    target_os = "vexos",
    target_os = "l4re",
)))]
pub fn hashmap_random_keys() -> (u64, u64) {
    let mut buf = [0; 16];
    fill_bytes(&mut buf);
    let k1 = u64::from_ne_bytes(buf[..8].try_into().unwrap());
    let k2 = u64::from_ne_bytes(buf[8..].try_into().unwrap());
    (k1, k2)
}