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    // Copyright 2015-2016 Brian Smith.
    // Portions Copyright (c) 2014, 2015, Google Inc.
    //
    // Permission to use, copy, modify, and/or distribute this software for any
    // purpose with or without fee is hereby granted, provided that the above
    // copyright notice and this permission notice appear in all copies.
    //
    // THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
    // WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
    // MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
    // SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    // WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
    // OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
    // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

    // TODO: enforce maximum input length.

    // Work around compiler bug?
    #![allow(non_shorthand_field_patterns)]

    use {c, chacha, constant_time, error, polyfill};
    use core;

    // The assembly functions we call expect the state to be 8-byte aligned. We do
    // this manually, by copying the data into an aligned slice, rather than by
    // asking the compiler to align the `Opaque` struct.
    fn with_aligned<F>(opaque: &mut Opaque, f: F)
        where F: FnOnce(&mut Opaque)
    {
        let mut buf = [0u8; OPAQUE_LEN + 7];
        let aligned_start = (buf.as_ptr() as usize + 7) & !7;
        let offset = aligned_start - (buf.as_ptr() as usize);
        let aligned_buf =
            slice_as_array_ref_mut!(
                &mut buf[offset..(offset + OPAQUE_LEN)], OPAQUE_LEN).unwrap();
        aligned_buf.copy_from_slice(&opaque[..]);
        f(aligned_buf);
        opaque.copy_from_slice(aligned_buf);
    }

    impl SigningContext {
        #[inline]
        pub fn from_key(key: Key) -> SigningContext {
            #[inline]
            fn read_u32(buf: &[u8]) -> u32 {
                polyfill::slice::u32_from_le_u8(slice_as_array_ref!(buf, 4).unwrap())
            }

            let (key, nonce) = key.bytes.split_at(16);
            let key = slice_as_array_ref!(key, 16).unwrap();

            let mut ctx = SigningContext {
                opaque: [0u8; OPAQUE_LEN],
                // TODO: When we can get explicit alignment, make `nonce` an
                // aligned `u8[16]` and get rid of this `u8[16]` -> `u32[4]`
                // conversion.
                nonce: [
                    read_u32(&nonce[0..4]),
                    read_u32(&nonce[4..8]),
                    read_u32(&nonce[8..12]),
                    read_u32(&nonce[12..16]),
                ],
                buf: [0; BLOCK_LEN],
                buf_used: 0,
                func: Funcs {
                    blocks_fn: GFp_poly1305_blocks,
                    emit_fn: GFp_poly1305_emit
                }
            };

            { // borrow ctx
                let &mut SigningContext {
                    opaque: ref mut opaque,
                    func: ref mut func,
                    ..
                } = &mut ctx;
                with_aligned(opaque, |opaque| {
                // On some platforms `init()` doesn't initialize `funcs`. The
                // return value of `init()` indicates whether it did or not. Since
                // we already gave `func` a default value above, we can ignore the
                // return value assuming `init()` doesn't change `func` if it chose
                // not to initialize it. Note that this is different than what
                // BoringSSL does.
                    let _ = init(opaque, key, func);
                });
            }

            ctx
        }

        pub fn update(&mut self, mut input: &[u8]) {
            let &mut SigningContext {
                opaque: ref mut opaque,
                buf: ref mut buf,
                buf_used: ref mut buf_used,
                func: ref func,
                ..
            } = self;
            with_aligned(opaque, |opaque: &mut Opaque| {
                if *buf_used != 0 {
                    let todo = core::cmp::min(input.len(), BLOCK_LEN - *buf_used);

                    buf[*buf_used..(*buf_used + todo)].copy_from_slice(
                        &input[..todo]);
                    *buf_used += todo;
                    input = &input[todo..];

                    if *buf_used == BLOCK_LEN {
                        func.blocks(opaque, buf, Pad::Pad);
                        *buf_used = 0;
                    }
                }

                if input.len() >= BLOCK_LEN {
                    let todo = input.len() & !(BLOCK_LEN - 1);
                    let (complete_blocks, remainder) = input.split_at(todo);
                    func.blocks(opaque, complete_blocks, Pad::Pad);
                    input = remainder;
                }

                if input.len() != 0 {
                    buf[..input.len()].copy_from_slice(input);
                    *buf_used = input.len();
                }
            });
        }

        pub fn sign(mut self, tag_out: &mut Tag) {
            let &mut SigningContext {
                opaque: ref mut opaque,
                nonce: ref nonce,
                buf: ref mut buf,
                buf_used: buf_used,
                func: ref func,
            } = &mut self;
            with_aligned(opaque, |opaque| {
                if buf_used != 0 {
                    buf[buf_used] = 1;
                    for byte in &mut buf[(buf_used + 1)..] {
                        *byte = 0;
                    }
                    func.blocks(opaque, &buf[..], Pad::AlreadyPadded);
                }

                func.emit(opaque, tag_out, nonce);
            });
        }
    }

    pub fn verify(key: Key, msg: &[u8], tag: &Tag)
                  -> Result<(), error::Unspecified> {
        let mut calculated_tag = [0u8; TAG_LEN];
        sign(key, msg, &mut calculated_tag);
        constant_time::verify_slices_are_equal(&calculated_tag[..], tag)
    }

    pub fn sign(key: Key, msg: &[u8], tag: &mut Tag) {
        let mut ctx = SigningContext::from_key(key);
        ctx.update(msg);
        ctx.sign(tag)
    }

    #[cfg(test)]
    pub fn check_state_layout() {
        let required_state_size =
            if cfg!(target_arch = "x86") {
                // See comment above `_poly1305_init_sse2` in poly1305-x86.pl.
                Some(4 * (5 + 1 + 4 + 2 + 4 * 9))
            } else if cfg!(target_arch = "x86_64") {
                // See comment above `__poly1305_block` in poly1305-x86_64.pl.
                Some(4 * (5 + 1 + 2 * 2 + 2 + 4 * 9))
            } else {
                // TODO(davidben): Figure out the layout of the struct. For now,
                // `OPAQUE_LEN` is taken from OpenSSL.
                None
            };

        if let Some(required_state_size) = required_state_size {
            assert!(core::mem::size_of::<Opaque>() >= required_state_size);
        }
    }

    /// A Poly1305 key.
    pub struct Key {
        bytes: KeyAndNonceBytes,
    }

    impl Key {
        pub fn derive_using_chacha(chacha20_key: &chacha::Key,
                                   counter: &chacha::Counter) -> Key {
            let mut bytes = [0u8; KEY_LEN];
            chacha::chacha20_xor_in_place(chacha20_key, counter, &mut bytes);
            Key { bytes }
        }

        #[cfg(test)]
        pub fn from_test_vector(bytes: &[u8; KEY_LEN]) -> Key {
            Key { bytes: *bytes }
        }
    }

    type KeyAndNonceBytes = [u8; 2 * BLOCK_LEN];

    type KeyBytes = [u8; BLOCK_LEN];
    type Nonce = [u32; BLOCK_LEN / 4];

    /// The length of a `key`.
    pub const KEY_LEN: usize = 32;

    /// A Poly1305 tag.
    pub type Tag = [u8; TAG_LEN];

    /// The length of a `Tag`.
    pub const TAG_LEN: usize = BLOCK_LEN;

    const BLOCK_LEN: usize = 16;

    /// The memory manipulated by the assembly.
    type Opaque = [u8; OPAQUE_LEN];

    const OPAQUE_LEN: usize = 192;

    #[repr(C)]
    struct Funcs {
        blocks_fn: unsafe extern fn(&mut Opaque, input: *const u8,
                                    input_len: c::size_t, should_pad: Pad),
        emit_fn: unsafe extern fn(&mut Opaque, &mut Tag, nonce: &Nonce),
    }

    #[inline]
    fn init(state: &mut Opaque, key: &KeyBytes, func: &mut Funcs) -> i32 {
        debug_assert_eq!(state.as_ptr() as usize % 8, 0);
        unsafe {
            GFp_poly1305_init_asm(state, key, func)
        }
    }

    #[repr(u32)]
    enum Pad {
        AlreadyPadded = 0,
        Pad = 1,
    }

    impl Funcs {
        #[inline]
        fn blocks(&self, state: &mut Opaque, data: &[u8], should_pad: Pad) {
            debug_assert_eq!(state.as_ptr() as usize % 8, 0);
            unsafe {
                (self.blocks_fn)(state, data.as_ptr(), data.len(), should_pad);
            }
        }

        #[inline]
        fn emit(&self, state: &mut Opaque, tag_out: &mut Tag, nonce: &Nonce) {
            debug_assert_eq!(state.as_ptr() as usize % 8, 0);
            unsafe {
                 (self.emit_fn)(state, tag_out, nonce);
            }
        }
    }

    pub struct SigningContext {
        opaque: Opaque,
        nonce: [u32; 4],
        buf: [u8; BLOCK_LEN],
        buf_used: usize,
        func: Funcs
    }

    extern {
        fn GFp_poly1305_init_asm(state: &mut Opaque, key: &KeyBytes,
                                 out_func: &mut Funcs) -> c::int;
        fn GFp_poly1305_blocks(state: &mut Opaque, input: *const u8, len: c::size_t,
                               should_pad: Pad);
        fn GFp_poly1305_emit(state: &mut Opaque, mac: &mut Tag, nonce: &Nonce);
    }

    #[cfg(test)]
    mod tests {
        use {error, test};
        use core;
        use super::*;

        #[test]
        pub fn test_state_layout() {
            check_state_layout();
        }

        // Adapted from BoringSSL's crypto/poly1305/poly1305_test.cc.
        #[test]
        pub fn test_poly1305() {
            test::from_file("src/poly1305_test.txt", |section, test_case| {
                assert_eq!(section, "");
                let key = test_case.consume_bytes("Key");
                let key = slice_as_array_ref!(&key, KEY_LEN).unwrap();
                let input = test_case.consume_bytes("Input");
                let expected_mac = test_case.consume_bytes("MAC");
                let expected_mac =
                    slice_as_array_ref!(&expected_mac, TAG_LEN).unwrap();

                // Test single-shot operation.
                {
                    let key = Key::from_test_vector(&key);
                    let mut ctx = SigningContext::from_key(key);
                    ctx.update(&input);
                    let mut actual_mac = [0; TAG_LEN];
                    ctx.sign(&mut actual_mac);
                    assert_eq!(&expected_mac[..], &actual_mac[..]);
                }
                {
                    let key = Key::from_test_vector(&key);
                    let mut actual_mac = [0; TAG_LEN];
                    sign(key, &input, &mut actual_mac);
                    assert_eq!(&expected_mac[..], &actual_mac[..]);
                }
                {
                    let key = Key::from_test_vector(&key);
                    assert_eq!(Ok(()), verify(key, &input, &expected_mac));
                }

                // Test streaming byte-by-byte.
                {
                    let key = Key::from_test_vector(&key);
                    let mut ctx = SigningContext::from_key(key);
                    for chunk in input.chunks(1) {
                        ctx.update(chunk);
                    }
                    let mut actual_mac = [0u8; TAG_LEN];
                    ctx.sign(&mut actual_mac);
                    assert_eq!(&expected_mac[..], &actual_mac[..]);
                }

                test_poly1305_simd(0, key, &input, expected_mac)?;
                test_poly1305_simd(16, key, &input, expected_mac)?;
                test_poly1305_simd(32, key, &input, expected_mac)?;
                test_poly1305_simd(48, key, &input, expected_mac)?;

                Ok(())
            })
        }

        fn test_poly1305_simd(excess: usize, key: &[u8; KEY_LEN], input: &[u8],
                              expected_mac: &[u8; TAG_LEN])
                              -> Result<(), error::Unspecified> {
            let key = Key::from_test_vector(&key);
            let mut ctx = SigningContext::from_key(key);

            // Some implementations begin in non-SIMD mode and upgrade on demand.
            // Stress the upgrade path.
            let init = core::cmp::min(input.len(), 16);
            ctx.update(&input[..init]);

            let long_chunk_len = 128 + excess;
            for chunk in input[init..].chunks(long_chunk_len + excess) {
                if chunk.len() > long_chunk_len {
                    let (long, short) = chunk.split_at(long_chunk_len);

                    // Feed 128 + |excess| bytes to test SIMD mode.
                    ctx.update(long);

                    // Feed |excess| bytes to ensure SIMD mode can handle short
                    // inputs.
                    ctx.update(short);
                } else {
                    // Handle the last chunk.
                    ctx.update(chunk);
                }
            }

            let mut actual_mac = [0u8; TAG_LEN];
            ctx.sign(&mut actual_mac);
            assert_eq!(&expected_mac[..], &actual_mac);

            Ok(())
        }
    }

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