//! Define helpers for working with types in constant time.

use derive_deftly::{Deftly, define_derive_deftly};

use subtle::{Choice, ConditionallySelectable, ConstantTimeEq};

use zeroize::Zeroize;

#[cfg(feature = "memquota-memcost")]

use tor_memquota_cost::derive_deftly_template_HasMemoryCost;

define_derive_deftly! {

    /// Derives [`subtle::ConstantTimeEq`] on structs for which all fields

    /// already implement it. Note that this does NOT work on fields which are

    /// arrays of type `T`, even if `T` implements [`subtle::ConstantTimeEq`].

    /// Arrays do not directly implement [`subtle::ConstantTimeEq`] and instead

    /// dereference to a slice, `[T]`, which does. See subtle!114 for a possible

    /// future resolution.

    export ConstantTimeEq for struct:

    impl<$tgens> ConstantTimeEq for $ttype

    where $twheres

          $( $ftype : ConstantTimeEq , )

    {

            match (self, other) {

                $(

                    (${vpat fprefix=self_}, ${vpat fprefix=other_}) => {

                        $(

                            $<self_ $fname>.ct_eq($<other_ $fname>) &

                        )

                        subtle::Choice::from(1)

                    },

                )

            }

        }

    }

}

define_derive_deftly! {

    /// Derives [`core::cmp::PartialEq`] on types which implement

    /// [`subtle::ConstantTimeEq`] by calling [`subtle::ConstantTimeEq::ct_eq`].

    export PartialEqFromCtEq:

    impl<$tgens> PartialEq for $ttype

    where $twheres

          $ttype : ConstantTimeEq

    {

            self.ct_eq(other).into()

        }

    }

}

pub(crate) use {derive_deftly_template_ConstantTimeEq, derive_deftly_template_PartialEqFromCtEq};

/// A byte array of length N for which comparisons are performed in constant

/// time.

///

/// # Limitations

///

/// It is possible to avoid constant time comparisons here, just by using the

/// `as_ref()` and `as_mut()` methods.  They should therefore be approached with

/// some caution.

///

/// (The decision to avoid implementing `Deref`/`DerefMut` is deliberate.)

#[allow(clippy::derived_hash_with_manual_eq)]

#[derive(Clone, Copy, Debug, Hash, Zeroize, Deftly)]

#[cfg_attr(feature = "memquota-memcost", derive_deftly(HasMemoryCost))]

pub struct CtByteArray<const N: usize>([u8; N]);

impl<const N: usize> ConstantTimeEq for CtByteArray<N> {

}

impl<const N: usize> PartialEq for CtByteArray<N> {

}

impl<const N: usize> Eq for CtByteArray<N> {}

impl<const N: usize> From<[u8; N]> for CtByteArray<N> {

}

impl<const N: usize> From<CtByteArray<N>> for [u8; N] {

}

impl<const N: usize> Ord for CtByteArray<N> {

        // At every point, this value will be set to:

        //       0 if a[i]==b[i] for all i considered so far.

        //       a[i] - b[i] for the lowest i that has a nonzero a[i] - b[i].

        // This comparison with zero is not itself constant-time, but that's

        // okay: we only want our Ord function not to leak the array values.

}

impl<const N: usize> PartialOrd for CtByteArray<N> {

}

impl<const N: usize> AsRef<[u8; N]> for CtByteArray<N> {

}

impl<const N: usize> AsMut<[u8; N]> for CtByteArray<N> {

}

/// Try to find an item in a slice without leaking where and whether the

/// item was found.

///

/// If there is any item `x` in the `array` for which `matches(x)`

/// is true, this function will return a reference to one such

/// item.  (We don't specify which.)

///

/// Otherwise, this function returns none.

///

/// We evaluate `matches` on every item of the array, and try not to

/// leak by timing which element (if any) matched.  Note that if

/// `matches` itself has side channels, this function can't hide them.

///

/// Note that this doesn't necessarily do a constant-time comparison,

/// and that it is not constant-time for the found/not-found case.

{

    // ConditionallySelectable isn't implemented for usize, so we need

    // to use u64.

    } else {

    }

#[cfg(test)]

mod test {

    // @@ begin test lint list maintained by maint/add_warning @@

    #![allow(clippy::bool_assert_comparison)]

    #![allow(clippy::clone_on_copy)]

    #![allow(clippy::dbg_macro)]

    #![allow(clippy::mixed_attributes_style)]

    #![allow(clippy::print_stderr)]

    #![allow(clippy::print_stdout)]

    #![allow(clippy::single_char_pattern)]

    #![allow(clippy::unwrap_used)]

    #![allow(clippy::unchecked_time_subtraction)]

    #![allow(clippy::useless_vec)]

    #![allow(clippy::needless_pass_by_value)]

    //! <!-- @@ end test lint list maintained by maint/add_warning @@ -->

    use super::*;

    use rand::Rng;

    use tor_basic_utils::test_rng;

    #[allow(clippy::nonminimal_bool)]

    #[test]

    fn test_comparisons() {

        let num = 200;

        let mut rng = test_rng::testing_rng();

        let mut array: Vec<CtByteArray<32>> =

            (0..num).map(|_| rng.random::<[u8; 32]>().into()).collect();

        array.sort();

        for i in 0..num {

            assert_eq!(array[i], array[i]);

            assert!(!(array[i] < array[i]));

            assert!(!(array[i] > array[i]));

            for j in (i + 1)..num {

                // Note that this test will behave incorrectly if the rng

                // generates the same 256 value twice, but that's ridiculously

                // implausible.

                assert!(array[i] < array[j]);

                assert_ne!(array[i], array[j]);

                assert!(array[j] > array[i]);

                assert_eq!(

                    array[i].cmp(&array[j]),

                    array[j].as_ref().cmp(array[i].as_ref()).reverse()

                );

            }

        }

    }

    #[test]

    fn test_lookup() {

        use super::ct_lookup as lookup;

        use subtle::ConstantTimeEq;

        let items = vec![

            "One".to_string(),

            "word".to_string(),

            "of".to_string(),

            "every".to_string(),

            "length".to_string(),

        ];

        let of_word = lookup(&items[..], |i| i.len().ct_eq(&2));

        let every_word = lookup(&items[..], |i| i.len().ct_eq(&5));

        let no_word = lookup(&items[..], |i| i.len().ct_eq(&99));

        assert_eq!(of_word.unwrap(), "of");

        assert_eq!(every_word.unwrap(), "every");

        assert_eq!(no_word, None);

    }

}