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    // Copyright 2016 Brian Smith.
    // Portions Copyright (c) 2016, 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 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.

    use c;
    use core;
    use polyfill::slice::u32_from_le_u8;

    pub type Key = [u32; KEY_LEN_IN_BYTES / 4];

    pub fn key_from_bytes(key_bytes: &[u8; KEY_LEN_IN_BYTES]) -> Key {
        let mut key = [0u32; KEY_LEN_IN_BYTES / 4];
        for (key_u32, key_u8_4) in key.iter_mut().zip(key_bytes.chunks(4)) {
            *key_u32 = u32_from_le_u8(slice_as_array_ref!(key_u8_4, 4).unwrap());
        }
        key
    }

    #[inline]
    pub fn chacha20_xor_in_place(key: &Key, counter: &Counter, in_out: &mut [u8]) {
        chacha20_xor_inner(key, counter, in_out.as_ptr(), in_out.len(),
                           in_out.as_mut_ptr());
    }

    pub fn chacha20_xor_overlapping(key: &Key, counter: &Counter,
                                    in_out: &mut [u8], in_prefix_len: usize) {
        // XXX: The x86 and at least one branch of the ARM assembly language
        // code doesn't allow overlapping input and output unless they are
        // exactly overlapping. TODO: Figure out which branch of the ARM code
        // has this limitation and come up with a better solution.
        //
        // https://rt.openssl.org/Ticket/Display.html?id=4362
        let len = in_out.len() - in_prefix_len;
        if cfg!(any(target_arch = "arm", target_arch = "x86")) &&
                in_prefix_len != 0 {
            unsafe {
                core::ptr::copy(in_out[in_prefix_len..].as_ptr(),
                                in_out.as_mut_ptr(), len);
            }
            chacha20_xor_in_place(key, &counter, &mut in_out[..len]);
        } else {
            chacha20_xor_inner(key, counter, in_out[in_prefix_len..].as_ptr(),
                               len, in_out.as_mut_ptr());
        }
    }

    #[inline]
    pub fn chacha20_xor_inner(key: &Key, counter: &Counter, input: *const u8,
                              in_out_len: usize, output: *mut u8) {
        debug_assert!(core::mem::align_of_val(key) >= 4);
        debug_assert!(core::mem::align_of_val(counter) >= 4);
        unsafe {
            GFp_ChaCha20_ctr32(output, input, in_out_len, key, counter);
        }
    }

    pub type Counter = [u32; 4];

    #[inline]
    pub fn make_counter(nonce: &[u8; NONCE_LEN], counter: u32) -> Counter {
        [counter.to_le(),
         u32_from_le_u8(slice_as_array_ref!(&nonce[0..4], 4).unwrap()),
         u32_from_le_u8(slice_as_array_ref!(&nonce[4..8], 4).unwrap()),
         u32_from_le_u8(slice_as_array_ref!(&nonce[8..12], 4).unwrap())]
    }

    extern {
        fn GFp_ChaCha20_ctr32(out: *mut u8, in_: *const u8, in_len: c::size_t,
                              key: &Key, counter: &Counter);
    }

    pub const KEY_LEN_IN_BYTES: usize = 256 / 8;

    pub const NONCE_LEN: usize = 12; /* 96 bits */

    #[cfg(test)]
    mod tests {
        use test;
        use super::*;
        use super::GFp_ChaCha20_ctr32;

        // This verifies the encryption functionality provided by ChaCha20_ctr32
        // is successful when either computed on disjoint input/output buffers,
        // or on overlapping input/output buffers. On some branches of the 32-bit
        // x86 and ARM code the in-place operation fails in some situations where
        // the input/output buffers are not exactly overlapping. Such failures are
        // dependent not only on the degree of overlapping but also the length of
        // the data. `open()` works around that by moving the input data to the
        // output location so that the buffers exactly overlap, for those targets.
        // This test exists largely as a canary for detecting if/when that type of
        // problem spreads to other platforms.
        #[test]
        pub fn chacha20_tests() {
            test::from_file("src/chacha_tests.txt", |section, test_case| {
                assert_eq!(section, "");

                let key = test_case.consume_bytes("Key");
                let key = slice_as_array_ref!(&key, KEY_LEN_IN_BYTES)?;
                let key = key_from_bytes(key);

                let ctr = test_case.consume_usize("Ctr");
                let nonce_bytes = test_case.consume_bytes("Nonce");
                let nonce = slice_as_array_ref!(&nonce_bytes, NONCE_LEN).unwrap();
                let ctr = make_counter(&nonce, ctr as u32);
                let input = test_case.consume_bytes("Input");
                let output = test_case.consume_bytes("Output");

                // Pre-allocate buffer for use in test_cases.
                let mut in_out_buf = vec![0u8; input.len() + 276];

                // Run the test case over all prefixes of the input because the
                // behavior of ChaCha20 implementation changes dependent on the
                // length of the input.
                for len in 0..(input.len() + 1) {
                    chacha20_test_case_inner(&key, &ctr, &input[..len],
                                             &output[..len], len, &mut in_out_buf);
                }

                Ok(())
            });
        }

        fn chacha20_test_case_inner(key: &Key, ctr: &Counter, input: &[u8],
                                    expected: &[u8], len: usize,
                                    in_out_buf: &mut [u8]) {
            // Straightforward encryption into disjoint buffers is computed
            // correctly.
            unsafe {
              GFp_ChaCha20_ctr32(in_out_buf.as_mut_ptr(), input[..len].as_ptr(),
                                 len, key, &ctr);
            }
            assert_eq!(&in_out_buf[..len], expected);

            // Do not test offset buffers for x86 and ARM architectures (see above
            // for rationale).
            let max_offset =
                if cfg!(any(target_arch = "x86", target_arch = "arm")) {
                    0
                } else {
                    259
                };

            // Check that in-place encryption works successfully when the pointers
            // to the input/output buffers are (partially) overlapping.
            for alignment in 0..16 {
                for offset in 0..(max_offset + 1) {
                  in_out_buf[alignment+offset..][..len].copy_from_slice(input);
                  unsafe {
                      GFp_ChaCha20_ctr32(in_out_buf[alignment..].as_mut_ptr(),
                                         in_out_buf[alignment + offset..].as_ptr(),
                                         len, key, ctr);
                      assert_eq!(&in_out_buf[alignment..][..len], expected);
                  }
                }
            }
        }
    }

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