// Copyright 2023 Google LLC
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use thiserror::Error;

/// A decodable type can be created from a byte buffer.
/// The type returned is separate (copied) from the buffer once decoded.
pub trait Decodable: ::core::marker::Sized {
    type Error;

    /// Decodes into a new object with the number of bytes that were decoded, or
    /// returns an error.
    fn decode(buf: &[u8]) -> ::core::result::Result<(Self, usize), Self::Error>;
}

/// An encodable type can write itself into a byte buffer.
pub trait Encodable {
    type Error;

    /// Returns the number of bytes necessary to encode |self|.
    fn encoded_len(&self) -> ::core::primitive::usize;

    /// Writes the encoded version of |self| at the start of |buf|.
    /// |buf| must be at least |self.encoded_len()| length.
    fn encode(&self, buf: &mut [u8]) -> ::core::result::Result<(), Self::Error>;
}

/// Generates an enum value where each variant can be converted into a constant in the given
/// raw_type.
///
/// For example:
/// decodable_enum! {
///     pub(crate) enum Color<u8, MyError, Variant> {
///        Red = 1,
///        Blue = 2,
///        Green = 3,
///     }
/// }
///
/// Color::try_from(2) -> Color::Red
/// u8::from(&Color::Red) -> 1.
#[macro_export]
macro_rules! decodable_enum {
    ($(#[$meta:meta])* $visibility:vis enum $name:ident<
        $raw_type:ty,
        $error_type:ty,
        $error_path:ident
    > {
        $($(#[$variant_meta:meta])* $variant:ident = $val:expr),*,
    }) => {
        $(#[$meta])*
        #[derive(
            ::core::clone::Clone,
            ::core::marker::Copy,
            ::core::fmt::Debug,
            ::core::cmp::Eq,
            ::core::hash::Hash,
            ::core::cmp::PartialEq)]
        $visibility enum $name {
            $($(#[$variant_meta])* $variant = $val),*
        }

        impl $name {
            pub const VALUES : &'static [$raw_type] = &[$($val),*,];
            pub const VARIANTS : &'static [$name] = &[$($name::$variant),*,];
            pub fn name(&self) -> &'static ::core::primitive::str {
                match self {
                    $($name::$variant => ::core::stringify!($variant)),*
                }
            }
        }

        impl ::core::convert::From<&$name> for $raw_type {
            fn from(v: &$name) -> $raw_type {
                match v {
                    $($name::$variant => $val),*,
                }
            }
        }

        impl ::core::convert::TryFrom<$raw_type> for $name {
            type Error = $error_type;

            fn try_from(value: $raw_type) -> ::core::result::Result<Self, $error_type> {
                match value {
                    $($val => ::core::result::Result::Ok($name::$variant)),*,
                    _ => ::core::result::Result::Err(<$error_type>::$error_path),
                }
            }
        }
    }
}

#[macro_export]
macro_rules! codable_as_bitmask {
    ($type:ty, $raw_type:ty) => {
        impl $type {
            pub fn from_bits(v: $raw_type) -> impl Iterator<Item = $type> {
                (0..<$raw_type>::BITS)
                    .map(|bit| 1 << bit)
                    .filter(move |val| (v & val) != 0)
                    .filter_map(|val| val.try_into().ok())
            }

            pub fn to_bits<'a>(it: impl Iterator<Item = &'a $type>) -> $raw_type {
                it.fold(0, |acc, item| acc | Into::<$raw_type>::into(item))
            }
        }
    };
}

#[derive(Error, Debug, PartialEq)]
pub enum Error {
    #[error("Parameter is not valid: {0}")]
    InvalidParameter(String),

    #[error("Out-of-range enum value")]
    OutOfRange,

    #[error("Encoding buffer is too small")]
    BufferTooSmall,

    #[error("Buffer being decoded is invalid length")]
    UnexpectedDataLength,

    #[error("Unrecognized type for {0}: {1}")]
    UnrecognizedType(String, u8),
}