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use core::convert::Infallible;

use super::{FieldCopyAccess, PrimitiveField};
use crate::endianness::Endianness;
use crate::fields::primitive::view::FieldView;
use crate::fields::{Field, StorageIntoFieldView, StorageToFieldView};

impl<E: Endianness, const OFFSET_: usize> FieldCopyAccess for PrimitiveField<(), E, OFFSET_> {
    /// See [FieldCopyAccess::ReadError]
    type ReadError = Infallible;
    /// See [FieldCopyAccess::WriteError]
    type WriteError = Infallible;
    /// See [FieldCopyAccess::HighLevelType]
    type HighLevelType = ();

    doc_comment::doc_comment! {
        concat! {"
                'Read' the `", stringify!(()), "`-typed field from a given data region, assuming the defined layout, using the [Field] API.

                # Example:

                ```
                use binary_layout::prelude::*;

                binary_layout!(my_layout, LittleEndian, {
                    //... other fields ...
                    some_zst_field: ", stringify!(()), "
                    //... other fields ...
                });

                fn func(storage_data: &[u8]) {
                    let read: ", stringify!(()), " = my_layout::some_zst_field::try_read(storage_data).unwrap();
                    read
                }
                ```

                In reality, this method doesn't do any work; `",
                stringify!(()), "` is a zero-sized type, so there's no work to
                do. This implementation exists solely to make writing derive
                macros simpler.
                "},
        #[inline(always)]
        #[allow(clippy::unused_unit)] // I don't want to remove this as it's part of the trait.
        fn try_read(_storage: &[u8]) -> Result<(), Infallible> {
            Ok(())
        }
    }

    doc_comment::doc_comment! {
        concat! {"
                'Write' the `", stringify!(()), "`-typed field to a given data region, assuming the defined layout, using the [Field] API.

                # Example:

                ```
                use binary_layout::prelude::*;

                binary_layout!(my_layout, LittleEndian, {
                    //... other fields ...
                    some_zst_field: ", stringify!(()), "
                    //... other fields ...
                });

                fn func(storage_data: &mut [u8]) {
                    my_layout::some_zst_field::try_write(storage_data, ()).unwrap();
                }
                ```

                # WARNING

                In reality, this method doesn't do any work; `",
                stringify!(()), "` is a zero-sized type, so there's no work to
                do. This implementation exists solely to make writing derive
                macros simpler.
                "},
        #[inline(always)]
        #[allow(clippy::unused_unit)] // I don't want to remove this as it's part of the trait.
        fn try_write(_storage: &mut [u8], _value: ()) -> Result<(), Infallible> {
            Ok(())
        }
    }
}

impl_field_traits!(());

#[cfg(test)]
mod tests {
    use crate::prelude::*;
    use crate::PrimitiveField;

    macro_rules! test_unit_copy_access {
        ($endian:ident, $endian_type:ty) => {
            $crate::internal::paste! {
                #[test]
                fn [<test_unit_ $endian endian_metadata>]() {
                    type Field1 = PrimitiveField<(), $endian_type, 5>;
                    type Field2 = PrimitiveField<(), $endian_type, 123>;

                    assert_eq!(Some(0), Field1::SIZE);
                    assert_eq!(5, Field1::OFFSET);
                    assert_eq!(Some(0), Field2::SIZE);
                    assert_eq!(123, Field2::OFFSET);
                }

                #[allow(clippy::unit_cmp)]
                #[test]
                fn [<test_unit_ $endian endian_fieldapi>]() {
                    let mut storage = [0; 1024];

                    type Field1 = PrimitiveField<(), $endian_type, 5>;
                    type Field2 = PrimitiveField<(), $endian_type, 123>;

                    Field1::write(&mut storage, ());
                    Field2::write(&mut storage, ());

                    assert_eq!((), Field1::read(&storage));
                    assert_eq!((), Field2::read(&storage));

                    // Zero-sized types do not mutate the storage, so it should remain
                    // unchanged for all of time.
                    assert_eq!(storage, [0; 1024]);
                }

                #[allow(clippy::unit_cmp)]
                #[test]
                fn [<test_unit_ $endian endian_viewapi>]() {
                    binary_layout!(layout, $endian_type, {
                        field1: (),
                        field2: (),
                    });
                    let mut storage = [0; 1024];
                    let mut view = layout::View::new(&mut storage);

                    view.field1_mut().write(());
                    view.field2_mut().write(());

                    assert_eq!((), view.field1().read());
                    assert_eq!((), view.field2().read());

                    // Zero-sized types do not mutate the storage, so it should remain
                    // unchanged for all of time.
                    assert_eq!(storage, [0; 1024]);
                }
            }
        };
    }

    test_unit_copy_access!(little, LittleEndian);
    test_unit_copy_access!(big, BigEndian);
    test_unit_copy_access!(native, NativeEndian);
}