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use crate::bits::FixedBytes;

#[cfg(all(feature = "native-keccak", not(feature = "tiny-keccak")))]
#[link(wasm_import_module = "vm_hooks")]
extern "C" {
    /// When targeting VMs with native keccak hooks, the `native-keccak` feature
    /// can be enabled to import and use the host environment's implementation
    /// of [`keccak256`] in place of [`tiny_keccak`]. This is overridden when
    /// the `tiny-keccak` feature is enabled.
    ///
    /// # SAFETY
    ///
    /// The VM accepts the preimage by pointer and length, and writes the
    /// 32-byte hash.
    /// - `bytes` must point to an input buffer at least `len` long.
    /// - `output` must point to a buffer that is at least 32-bytes long.
    ///
    /// [`keccak256`]: https://en.wikipedia.org/wiki/SHA-3
    /// [`tiny_keccak`]: https://docs.rs/tiny-keccak/latest/tiny_keccak/
    fn native_keccak256(bytes: *const u8, len: usize, output: *mut u8);
}

/// Simple interface to the [`keccak256`] hash function.
///
/// [`keccak256`]: https://en.wikipedia.org/wiki/SHA-3
pub fn keccak256<T: AsRef<[u8]>>(bytes: T) -> FixedBytes<32> {
    cfg_if::cfg_if! {
        if #[cfg(all(feature = "native-keccak", not(feature = "tiny-keccak")))] {
            /// Calls an external native keccak hook when `native-keccak` is enabled.
            /// This is overridden when `tiny-keccak` is enabled.
            fn keccak256(bytes: &[u8]) -> FixedBytes<32> {
                let mut output = [0u8; 32];

                // SAFETY: The output is 32-bytes, and the input comes from a slice.
                unsafe { native_keccak256(bytes.as_ptr(), bytes.len(), output.as_mut_ptr()) };
                output.into()
            }
        } else {
            /// Calls [`tiny-keccak`] when the `tiny-keccak` feature is enabled or
            /// when no particular keccak feature flag is specified.
            ///
            /// [`tiny_keccak`]: https://docs.rs/tiny-keccak/latest/tiny_keccak/
            fn keccak256(bytes: &[u8]) -> FixedBytes<32> {
                use tiny_keccak::{Hasher, Keccak};

                let mut output = [0u8; 32];
                let mut hasher = Keccak::v256();
                hasher.update(bytes);
                hasher.finalize(&mut output);
                output.into()
            }
        }
    }

    keccak256(bytes.as_ref())
}