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    // Copyright 2018 Developers of the Rand project.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.
    use core::fmt;
    use core::num::NonZeroU32;

    /// A small and `no_std` compatible error type.
    ///
    /// The [`Error::raw_os_error()`] will indicate if the error is from the OS, and
    /// if so, which error code the OS gave the application. If such an error is
    /// encountered, please consult with your system documentation.
    ///
    /// Internally this type is a NonZeroU32, with certain values reserved for
    /// certain purposes, see [`Error::INTERNAL_START`] and [`Error::CUSTOM_START`].
    #[derive(Copy, Clone, Eq, PartialEq)]
    pub struct Error(NonZeroU32);

    impl Error {
        #[deprecated(since = "0.1.7")]
        pub const UNKNOWN: Error = UNSUPPORTED;
        #[deprecated(since = "0.1.7")]
        pub const UNAVAILABLE: Error = UNSUPPORTED;

        /// Codes below this point represent OS Errors (i.e. positive i32 values).
        /// Codes at or above this point, but below [`Error::CUSTOM_START`] are
        /// reserved for use by the `rand` and `getrandom` crates.
        pub const INTERNAL_START: u32 = 1 << 31;

        /// Codes at or above this point can be used by users to define their own
        /// custom errors.
        pub const CUSTOM_START: u32 = (1 << 31) + (1 << 30);

        /// Extract the raw OS error code (if this error came from the OS)
        ///
        /// This method is identical to `std::io::Error::raw_os_error()`, except
        /// that it works in `no_std` contexts. If this method returns `None`, the
        /// error value can still be formatted via the `Diplay` implementation.
        #[inline]
        pub fn raw_os_error(&self) -> Option<i32> {
            if self.0.get() < Self::INTERNAL_START {
                Some(self.0.get() as i32)
            } else {
                None
            }
        }

        /// Extract the bare error code.
        ///
        /// This code can either come from the underlying OS, or be a custom error.
        /// Use [`Error::raw_os_error()`] to disambiguate.
        #[inline]
        pub fn code(&self) -> NonZeroU32 {
            self.0
        }
    }

    #[cfg(any(unix, target_os = "redox"))]
    fn os_err_desc(errno: i32, buf: &mut [u8]) -> Option<&str> {
        let buf_ptr = buf.as_mut_ptr() as *mut libc::c_char;
        if unsafe { libc::strerror_r(errno, buf_ptr, buf.len()) } != 0 {
            return None;
        }

        // Take up to trailing null byte
        let idx = buf.iter().position(|&b| b == 0).unwrap_or(buf.len());
        core::str::from_utf8(&buf[..idx]).ok()
    }

    #[cfg(not(any(unix, target_os = "redox")))]
    fn os_err_desc(_errno: i32, _buf: &mut [u8]) -> Option<&str> {
        None
    }

    impl fmt::Debug for Error {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let mut dbg = f.debug_struct("Error");
            if let Some(errno) = self.raw_os_error() {
                dbg.field("os_error", &errno);
                let mut buf = [0u8; 128];
                if let Some(desc) = os_err_desc(errno, &mut buf) {
                    dbg.field("description", &desc);
                }
            } else if let Some(desc) = internal_desc(*self) {
                dbg.field("internal_code", &self.0.get());
                dbg.field("description", &desc);
            } else {
                dbg.field("unknown_code", &self.0.get());
            }
            dbg.finish()
        }
    }

    impl fmt::Display for Error {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            if let Some(errno) = self.raw_os_error() {
                let mut buf = [0u8; 128];
                match os_err_desc(errno, &mut buf) {
                    Some(desc) => f.write_str(desc),
                    None => write!(f, "OS Error: {}", errno),
                }
            } else if let Some(desc) = internal_desc(*self) {
                f.write_str(desc)
            } else {
                write!(f, "Unknown Error: {}", self.0.get())
            }
        }
    }

    impl From<NonZeroU32> for Error {
        fn from(code: NonZeroU32) -> Self {
            Self(code)
        }
    }

    // TODO: Convert to a function when min_version >= 1.33
    macro_rules! internal_error {
        ($n:expr) => {
            Error(unsafe { NonZeroU32::new_unchecked(Error::INTERNAL_START + $n as u16 as u32) })
        };
    }

    /// Internal Error constants
    pub(crate) const UNSUPPORTED: Error = internal_error!(0);
    pub(crate) const ERRNO_NOT_POSITIVE: Error = internal_error!(1);
    pub(crate) const UNKNOWN_IO_ERROR: Error = internal_error!(2);
    pub(crate) const SEC_RANDOM_FAILED: Error = internal_error!(3);
    pub(crate) const RTL_GEN_RANDOM_FAILED: Error = internal_error!(4);
    pub(crate) const FAILED_RDRAND: Error = internal_error!(5);
    pub(crate) const NO_RDRAND: Error = internal_error!(6);
    pub(crate) const BINDGEN_CRYPTO_UNDEF: Error = internal_error!(7);
    pub(crate) const BINDGEN_GRV_UNDEF: Error = internal_error!(8);
    pub(crate) const STDWEB_NO_RNG: Error = internal_error!(9);
    pub(crate) const STDWEB_RNG_FAILED: Error = internal_error!(10);

    fn internal_desc(error: Error) -> Option<&'static str> {
        match error {
            UNSUPPORTED => Some("getrandom: this target is not supported"),
            ERRNO_NOT_POSITIVE => Some("errno: did not return a positive value"),
            UNKNOWN_IO_ERROR => Some("Unknown std::io::Error"),
            SEC_RANDOM_FAILED => Some("SecRandomCopyBytes: call failed"),
            RTL_GEN_RANDOM_FAILED => Some("RtlGenRandom: call failed"),
            FAILED_RDRAND => Some("RDRAND: failed multiple times: CPU issue likely"),
            NO_RDRAND => Some("RDRAND: instruction not supported"),
            BINDGEN_CRYPTO_UNDEF => Some("wasm-bindgen: self.crypto is undefined"),
            BINDGEN_GRV_UNDEF => Some("wasm-bindgen: crypto.getRandomValues is undefined"),
            STDWEB_NO_RNG => Some("stdweb: no randomness source available"),
            STDWEB_RNG_FAILED => Some("stdweb: failed to get randomness"),
            _ => None,
        }
    }

    #[cfg(test)]
    mod tests {
        use super::Error;
        use core::mem::size_of;

        #[test]
        fn test_size() {
            assert_eq!(size_of::<Error>(), 4);
            assert_eq!(size_of::<Result<(), Error>>(), 4);
        }
    }

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