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// Copyright lowRISC contributors (OpenTitan project).
// Licensed under the Apache License, Version 2.0, see LICENSE for details.
// SPDX-License-Identifier: Apache-2.0

use std::fmt;
use std::marker::PhantomData;
use std::str::FromStr;

use serde::de::{self, MapAccess, Visitor};
use serde::{Deserialize, Deserializer};

/// Deserialize a type T from either a string or struct by forwarding
/// string forms to `FromStr`.
///
/// The use-case for this is to allow specifying key material in ownership
/// configuration files either directly or by filename:
/// ```
/// key: {
///   Ecdsa: "some/path/to/key.pub.der"
/// }
///
/// key: {
///   Ecdsa: {
///     x: "...",
///     y: "..."
///   }
/// }
/// ```

// This function was taken nearly verbatim from the serde documentation.
// The example in the serde documentation constrains the `FromStr` error
// type to `Void`; we constrain to any type implementing std::error::Error.
pub fn string_or_struct<'de, T, D>(deserializer: D) -> Result<T, D::Error>
where
    T: Deserialize<'de> + FromStr,
    <T as FromStr>::Err: std::error::Error,
    D: Deserializer<'de>,
{
    // This is a Visitor that forwards string types to T's `FromStr` impl and
    // forwards map types to T's `Deserialize` impl. The `PhantomData` is to
    // keep the compiler from complaining about T being an unused generic type
    // parameter. We need T in order to know the Value type for the Visitor
    // impl.
    struct StringOrStruct<T>(PhantomData<fn() -> T>);

    impl<'de, T> Visitor<'de> for StringOrStruct<T>
    where
        T: Deserialize<'de> + FromStr,
        <T as FromStr>::Err: std::error::Error,
    {
        type Value = T;

        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            formatter.write_str("string or map")
        }

        fn visit_str<E>(self, value: &str) -> Result<T, E>
        where
            E: de::Error,
        {
            FromStr::from_str(value).map_err(|e| E::custom(format!("{e:?}")))
        }

        fn visit_map<M>(self, map: M) -> Result<T, M::Error>
        where
            M: MapAccess<'de>,
        {
            Deserialize::deserialize(de::value::MapAccessDeserializer::new(map))
        }
    }
    deserializer.deserialize_any(StringOrStruct(PhantomData))
}