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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, 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`].
    ///
    /// *If this crate's `"std"` Cargo feature is enabled*, then:
    /// - [`getrandom::Error`][Error] implements
    ///   [`std::error::Error`](https://doc.rust-lang.org/std/error/trait.Error.html)
    /// - [`std::io::Error`](https://doc.rust-lang.org/std/io/struct.Error.html) implements
    ///   [`From<getrandom::Error>`](https://doc.rust-lang.org/std/convert/trait.From.html).
    #[derive(Copy, Clone, Eq, PartialEq)]
    pub struct Error(NonZeroU32);

    const fn internal_error(n: u16) -> Error {
        // SAFETY: code > 0 as INTERNAL_START > 0 and adding n won't overflow a u32.
        let code = Error::INTERNAL_START + (n as u32);
        Error(unsafe { NonZeroU32::new_unchecked(code) })
    }

    impl Error {
        /// This target/platform is not supported by `getrandom`.
        pub const UNSUPPORTED: Error = internal_error(0);
        /// The platform-specific `errno` returned a non-positive value.
        pub const ERRNO_NOT_POSITIVE: Error = internal_error(1);
        /// Call to iOS [`SecRandomCopyBytes`](https://developer.apple.com/documentation/security/1399291-secrandomcopybytes) failed.
        pub const IOS_SEC_RANDOM: Error = internal_error(3);
        /// Call to Windows [`RtlGenRandom`](https://docs.microsoft.com/en-us/windows/win32/api/ntsecapi/nf-ntsecapi-rtlgenrandom) failed.
        pub const WINDOWS_RTL_GEN_RANDOM: Error = internal_error(4);
        /// RDRAND instruction failed due to a hardware issue.
        pub const FAILED_RDRAND: Error = internal_error(5);
        /// RDRAND instruction unsupported on this target.
        pub const NO_RDRAND: Error = internal_error(6);
        /// The environment does not support the Web Crypto API.
        pub const WEB_CRYPTO: Error = internal_error(7);
        /// Calling Web Crypto API `crypto.getRandomValues` failed.
        pub const WEB_GET_RANDOM_VALUES: Error = internal_error(8);
        /// On VxWorks, call to `randSecure` failed (random number generator is not yet initialized).
        pub const VXWORKS_RAND_SECURE: Error = internal_error(11);
        /// Node.js does not have the `crypto` CommonJS module.
        pub const NODE_CRYPTO: Error = internal_error(12);
        /// Calling Node.js function `crypto.randomFillSync` failed.
        pub const NODE_RANDOM_FILL_SYNC: Error = internal_error(13);
        /// Called from an ES module on Node.js. This is unsupported, see:
        /// <https://docs.rs/getrandom#nodejs-es-module-support>.
        pub const NODE_ES_MODULE: Error = internal_error(14);

        /// 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()`][1], except
        /// that it works in `no_std` contexts. If this method returns `None`, the
        /// error value can still be formatted via the `Display` implementation.
        ///
        /// [1]: https://doc.rust-lang.org/std/io/struct.Error.html#method.raw_os_error
        #[inline]
        pub fn raw_os_error(self) -> Option<i32> {
            if self.0.get() < Self::INTERNAL_START {
                match () {
                    #[cfg(target_os = "solid_asp3")]
                    // On SOLID, negate the error code again to obtain the original
                    // error code.
                    () => Some(-(self.0.get() as i32)),
                    #[cfg(not(target_os = "solid_asp3"))]
                    () => 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 const fn code(self) -> NonZeroU32 {
            self.0
        }
    }

    cfg_if! {
        if #[cfg(unix)] {
            fn os_err(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 n = buf.len();
                let idx = buf.iter().position(|&b| b == 0).unwrap_or(n);
                core::str::from_utf8(&buf[..idx]).ok()
            }
        } else {
            fn os_err(_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(err) = os_err(errno, &mut buf) {
                    dbg.field("description", &err);
                }
            } 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(errno, &mut buf) {
                    Some(err) => err.fmt(f),
                    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)
        }
    }

    fn internal_desc(error: Error) -> Option<&'static str> {
        match error {
            Error::UNSUPPORTED => Some("getrandom: this target is not supported"),
            Error::ERRNO_NOT_POSITIVE => Some("errno: did not return a positive value"),
            Error::IOS_SEC_RANDOM => Some("SecRandomCopyBytes: iOS Security framework failure"),
            Error::WINDOWS_RTL_GEN_RANDOM => Some("RtlGenRandom: Windows system function failure"),
            Error::FAILED_RDRAND => Some("RDRAND: failed multiple times: CPU issue likely"),
            Error::NO_RDRAND => Some("RDRAND: instruction not supported"),
            Error::WEB_CRYPTO => Some("Web Crypto API is unavailable"),
            Error::WEB_GET_RANDOM_VALUES => Some("Calling Web API crypto.getRandomValues failed"),
            Error::VXWORKS_RAND_SECURE => Some("randSecure: VxWorks RNG module is not initialized"),
            Error::NODE_CRYPTO => Some("Node.js crypto CommonJS module is unavailable"),
            Error::NODE_RANDOM_FILL_SYNC => Some("Calling Node.js API crypto.randomFillSync failed"),
            Error::NODE_ES_MODULE => Some("Node.js ES modules are not directly supported, see https://docs.rs/getrandom#nodejs-es-module-support"),
            _ => 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);
        }
    }