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    use std::io;

    pub struct State(imp::State);

    impl State {
        /// Captures the current state of all CPUs on the system.
        ///
        /// The `State` returned here isn't too meaningful in terms of
        /// interpretation across platforms, but it can be compared to previous
        /// states to get a meaningful cross-platform number.
        pub fn current() -> io::Result<State> {
            imp::current().map(State)
        }

        /// Returns the percentage of time CPUs were idle from the current state
        /// relative to the previous state, as a percentage from 0.0 to 100.0.
        ///
        /// This function will return, as a percentage, the amount of time that the
        /// entire system was idle between the `previous` state and this own state.
        /// This can be useful to compare two snapshots in time of CPU usage to see
        /// how the CPU usage compares between the two.
        pub fn idle_since(&self, previous: &State) -> f64 {
            imp::pct_idle(&previous.0, &self.0)
        }
    }

    #[cfg(target_os = "linux")]
    mod imp {
        use std::{fs, io};

        pub struct State {
            user: u64,
            nice: u64,
            system: u64,
            idle: u64,
            iowait: u64,
            irq: u64,
            softirq: u64,
            steal: u64,
            guest: u64,
            guest_nice: u64,
        }

        pub fn current() -> io::Result<State> {
            let state = fs::read_to_string("/proc/stat")?;

            (|| {
                let mut parts = state.lines().next()?.split_whitespace();
                if parts.next()? != "cpu" {
                    return None;
                }
                Some(State {
                    user: parts.next()?.parse::<u64>().ok()?,
                    nice: parts.next()?.parse::<u64>().ok()?,
                    system: parts.next()?.parse::<u64>().ok()?,
                    idle: parts.next()?.parse::<u64>().ok()?,
                    iowait: parts.next()?.parse::<u64>().ok()?,
                    irq: parts.next()?.parse::<u64>().ok()?,
                    softirq: parts.next()?.parse::<u64>().ok()?,
                    steal: parts.next()?.parse::<u64>().ok()?,
                    guest: parts.next()?.parse::<u64>().ok()?,
                    guest_nice: parts.next()?.parse::<u64>().ok()?,
                })
            })()
            .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "first line of /proc/stat malformed"))
        }

        pub fn pct_idle(prev: &State, next: &State) -> f64 {
            let user = next.user - prev.user;
            let nice = next.nice - prev.nice;
            let system = next.system - prev.system;
            let idle = next.idle - prev.idle;
            let iowait = next.iowait.saturating_sub(prev.iowait);
            let irq = next.irq - prev.irq;
            let softirq = next.softirq - prev.softirq;
            let steal = next.steal - prev.steal;
            let guest = next.guest - prev.guest;
            let guest_nice = next.guest_nice - prev.guest_nice;
            let total =
                user + nice + system + idle + iowait + irq + softirq + steal + guest + guest_nice;

            (idle as f64) / (total as f64) * 100.0
        }
    }

    #[cfg(target_os = "macos")]
    #[allow(bad_style)]
    mod imp {
        use std::io;
        use std::ptr;

        type host_t = u32;
        type mach_port_t = u32;
        type vm_map_t = mach_port_t;
        type vm_offset_t = usize;
        type vm_size_t = usize;
        type vm_address_t = vm_offset_t;
        type processor_flavor_t = i32;
        type natural_t = u32;
        type processor_info_array_t = *mut i32;
        type mach_msg_type_number_t = i32;
        type kern_return_t = i32;

        const PROESSOR_CPU_LOAD_INFO: processor_flavor_t = 2;
        const CPU_STATE_USER: usize = 0;
        const CPU_STATE_SYSTEM: usize = 1;
        const CPU_STATE_IDLE: usize = 2;
        const CPU_STATE_NICE: usize = 3;
        const CPU_STATE_MAX: usize = 4;

        extern "C" {
            static mut mach_task_self_: mach_port_t;

            fn mach_host_self() -> mach_port_t;
            fn host_processor_info(
                host: host_t,
                flavor: processor_flavor_t,
                out_processor_count: *mut natural_t,
                out_processor_info: *mut processor_info_array_t,
                out_processor_infoCnt: *mut mach_msg_type_number_t,
            ) -> kern_return_t;
            fn vm_deallocate(
                target_task: vm_map_t,
                address: vm_address_t,
                size: vm_size_t,
            ) -> kern_return_t;
        }

        pub struct State {
            user: u64,
            system: u64,
            idle: u64,
            nice: u64,
        }

        #[repr(C)]
        struct processor_cpu_load_info_data_t {
            cpu_ticks: [u32; CPU_STATE_MAX],
        }

        pub fn current() -> io::Result<State> {
            // There's scant little documentation on `host_processor_info`
            // throughout the internet, so this is just modeled after what everyone
            // else is doing. For now this is modeled largely after libuv.

            unsafe {
                let mut num_cpus_u = 0;
                let mut cpu_info = ptr::null_mut();
                let mut msg_type = 0;
                let err = host_processor_info(
                    mach_host_self(),
                    PROESSOR_CPU_LOAD_INFO,
                    &mut num_cpus_u,
                    &mut cpu_info,
                    &mut msg_type,
                );
                if err != 0 {
                    return Err(io::Error::last_os_error());
                }
                let mut ret = State {
                    user: 0,
                    system: 0,
                    idle: 0,
                    nice: 0,
                };
                let mut current = cpu_info as *const processor_cpu_load_info_data_t;
                for _ in 0..num_cpus_u {
                    ret.user += (*current).cpu_ticks[CPU_STATE_USER] as u64;
                    ret.system += (*current).cpu_ticks[CPU_STATE_SYSTEM] as u64;
                    ret.idle += (*current).cpu_ticks[CPU_STATE_IDLE] as u64;
                    ret.nice += (*current).cpu_ticks[CPU_STATE_NICE] as u64;
                    current = current.offset(1);
                }
                vm_deallocate(mach_task_self_, cpu_info as vm_address_t, msg_type as usize);
                Ok(ret)
            }
        }

        pub fn pct_idle(prev: &State, next: &State) -> f64 {
            let user = next.user - prev.user;
            let system = next.system - prev.system;
            let idle = next.idle - prev.idle;
            let nice = next.nice - prev.nice;
            let total = user + system + idle + nice;
            (idle as f64) / (total as f64) * 100.0
        }
    }

    #[cfg(windows)]
    mod imp {
        use std::io;
        use std::mem;

        use windows_sys::Win32::Foundation::FILETIME;
        use windows_sys::Win32::System::Threading::GetSystemTimes;

        pub struct State {
            idle: FILETIME,
            kernel: FILETIME,
            user: FILETIME,
        }

        pub fn current() -> io::Result<State> {
            unsafe {
                let mut ret = mem::zeroed::<State>();
                let r = GetSystemTimes(&mut ret.idle, &mut ret.kernel, &mut ret.user);
                if r != 0 {
                    Ok(ret)
                } else {
                    Err(io::Error::last_os_error())
                }
            }
        }

        pub fn pct_idle(prev: &State, next: &State) -> f64 {
            fn to_u64(a: &FILETIME) -> u64 {
                ((a.dwHighDateTime as u64) << 32) | (a.dwLowDateTime as u64)
            }

            let idle = to_u64(&next.idle) - to_u64(&prev.idle);
            let kernel = to_u64(&next.kernel) - to_u64(&prev.kernel);
            let user = to_u64(&next.user) - to_u64(&prev.user);
            let total = user + kernel;
            (idle as f64) / (total as f64) * 100.0
        }
    }

    #[cfg(not(any(target_os = "linux", target_os = "macos", windows)))]
    mod imp {
        use std::io;

        pub struct State;

        pub fn current() -> io::Result<State> {
            Err(io::Error::new(
                io::ErrorKind::Other,
                "unsupported platform to learn CPU state",
            ))
        }

        pub fn pct_idle(_prev: &State, _next: &State) -> f64 {
            unimplemented!()
        }
    }