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    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    /*!
     * An implementation of the Fortuna CSPRNG
     *
     * First create a `FortunaRng` object using either the `new_unseeded`
     * constructor or `SeedableRng::from_seed`. Additional entropy may be
     * added using the method `add_random_event`, or the underlying RNG
     * maybe reseeded directly by `SeedableRng::reseed`. Note that this is
     * not recommended, since the generator automatically reseeds itself
     * using the data provided by `add_random_events` through an
     * accumulator. The accumulator is part of Fortuna's design and using
     * `SeedableRng::reseed` directly bypasses it.
     *
     * Note that the underlying block cipher is `AesSafe256Encryptor` which
     * is designed to be timing-attack resistant. The speed hit from this
     * is in line with a "safety first" API, but be aware of it.
     *
     * Fortuna was originally described in
     *   Practical Cryptography, Niels Ferguson and Bruce Schneier.
     *   John Wiley & Sons, 2003.
     *
     * Comments throughout this file contain references of the form
     * (PC 1.2.3); these refer to sections within this text.
     *
     * # A note on forking
     *
     * Proper behaviour for a CSRNG on a process fork is to reseed itself with
     * the timestamp and new process ID, to ensure that after forking the child
     * process does not share the same RNG state (and therefore the same output)
     * as its parent.
     *
     * However, this appears not to be possible in Rust, due to
     *     https://github.com/rust-lang/rust/issues/16799
     * The reason is that Rust's process management all happens through its
     * stdlib runtime, which explicitly does not support forking, so it provides
     * no mechanism with which to detect forks.
     *
     * What this means is that if you are writing forking code (using `#![no_std]`
     * say) then you need to EXPLICITLY RESEED THE RNG AFTER FORKING.
     */

    use cryptoutil::copy_memory;

    use rand::{Rng, SeedableRng};
    use time::precise_time_s;

    use aessafe::AesSafe256Encryptor;
    use cryptoutil::read_u32_le;
    use digest::Digest;
    use sha2::Sha256;
    use symmetriccipher::BlockEncryptor;

    /// Length in bytes that the first pool must be before a "catastrophic
    /// reseed" is allowed to happen. (A direct reseed through the
    /// `SeedableRng` API is not affected by this limit.)
    pub const MIN_POOL_SIZE: usize = 64;
    /// Maximum number of bytes to generate before rekeying
    const MAX_GEN_SIZE: usize = (1 << 20);
    /// Length in bytes of the AES key
    const KEY_LEN: usize = 32;
    /// Length in bytes of the AES counter
    const CTR_LEN: usize = 16;
    /// Length in bytes of the AES block
    const AES_BLOCK_SIZE: usize = 16;
    /// Number of pools used to accumulate entropy
    const NUM_POOLS: usize = 32;

    /// The underlying PRNG (PC 9.4)
    struct FortunaGenerator {
        key: [u8; KEY_LEN],
        ctr: [u8; CTR_LEN],
    }

    impl FortunaGenerator {
        /// Creates a new generator (PC 9.4.1)
        fn new() -> FortunaGenerator {
            FortunaGenerator {
                key: [0; KEY_LEN],
                ctr: [0; CTR_LEN],
            }
        }

        /// Increments the counter in place
        fn increment_counter(&mut self) {
            for i in 0..self.ctr.len() {
                self.ctr[i] = self.ctr[i].wrapping_add(1);
                // As soon as we don't carry, stop
                if self.ctr[i] != 0 {
                    break;
                }
            }
        }

        /// Reseeds the generator (PC 9.4.2)
        fn reseed(&mut self, s: &[u8]) {
            // Compute key as Sha256d( key || s )
            let mut hasher = Sha256::new();
            hasher.input(&self.key[..]);
            hasher.input(s);
            hasher.result(&mut self.key);
            hasher = Sha256::new();
            hasher.input(&self.key[..]);
            hasher.result(&mut self.key[..]);
            // Increment the counter
            self.increment_counter();
        }

        /// Generates some `k` 16-byte blocks of random output (PC 9.4.3)
        /// This should never be used directly, except by `generate_random_data`.
        fn generate_blocks(&mut self, k: usize, out: &mut [u8]) {
            assert!(self.ctr[..] != [0; CTR_LEN][..]);

            // Setup AES encryptor
            let block_encryptor = AesSafe256Encryptor::new(&self.key[..]);
            // Concatenate all the blocks
            for j in 0..k {
                block_encryptor.encrypt_block(&self.ctr[..],
                                              &mut out[AES_BLOCK_SIZE * j..AES_BLOCK_SIZE * (j + 1)]);
                self.increment_counter();
            }
        }

        /// Generates `n` bytes of random data (9.4.4)
        fn generate_random_data(&mut self, out: &mut [u8]) {
            let (n, rem) = (out.len() / AES_BLOCK_SIZE, out.len() % AES_BLOCK_SIZE);
            assert!(n <= MAX_GEN_SIZE);

            // Generate output
            self.generate_blocks(n, &mut out[..(n * AES_BLOCK_SIZE)]);
            if rem > 0 {
                let mut buf = [0; AES_BLOCK_SIZE];
                self.generate_blocks(1, &mut buf);
                copy_memory(&buf[..rem], &mut out[(n * AES_BLOCK_SIZE)..]);
            }

            // Rekey
            let mut new_key = [0; KEY_LEN];
            self.generate_blocks(KEY_LEN / AES_BLOCK_SIZE, &mut new_key);
            self.key = new_key;
        }
    }

    /// A single entropy pool (not public)
    #[derive(Clone, Copy)]
    struct Pool {
        state: Sha256,
        count: usize
    }

    impl Pool {
        fn new() -> Pool {
            Pool { state: Sha256::new(), count: 0 }
        }

        fn input(&mut self, data: &[u8]) {
            self.state.input(data);
            self.count += data.len();
        }

        fn result(&mut self, output: &mut [u8]) {
            self.state.result(output);
            // Double-SHA256 it
            self.state = Sha256::new();
            self.state.input(output);
            self.state.result(output);
            // Clear the pool state
            self.state = Sha256::new();
            self.count = 0;
        }
    }

    /// The `Fortuna` CSPRNG (PC 9.5)
    pub struct Fortuna {
        pool: [Pool; NUM_POOLS],
        generator: FortunaGenerator,
        reseed_count: u32,
        last_reseed_time: f64
    }

    impl Fortuna {
        /// Creates a new unseeded `Fortuna` (PC 9.5.4)
        pub fn new_unseeded() -> Fortuna {
            Fortuna {
                pool: [Pool::new(); NUM_POOLS],
                generator: FortunaGenerator::new(),
                reseed_count: 0,
                last_reseed_time: 0.0
            }
        }

        /// Adds a random event `e` from source `s` to entropy pool `i` (PC 9.5.6)
        pub fn add_random_event(&mut self, s: u8, i: usize, e: &[u8]) {
            assert!(i <= NUM_POOLS);
            // These restrictions (and `s` in [0, 255]) are part of the Fortuna spec.
            assert!(e.len() > 0);
            assert!(e.len() <= 32);
            (&mut self.pool[i]).input(&[s]);
            (&mut self.pool[i]).input(&[e.len() as u8]);
            (&mut self.pool[i]).input(e);
        }
    }

    impl Rng for Fortuna {
        /// Generate a bunch of random data into `dest` (PC 9.5.5)
        ///
        /// # Failure modes
        ///
        /// If the RNG has not been seeded, and there is less than
        /// `MIN_POOL_SIZE` bytes of data in the first accumulator
        /// pool, this function will fail the task.
        fn fill_bytes(&mut self, dest: &mut [u8]) {
            // Reseed if necessary
            let now = precise_time_s();
            if self.pool[0].count >= MIN_POOL_SIZE &&
               now - self.last_reseed_time > 0.1 {
                self.reseed_count += 1;
                self.last_reseed_time = now;
                // Compute key as Sha256d( key || s )
                let mut hash = [0; (32 * NUM_POOLS)];
                let mut n_pools = 0;
                while self.reseed_count % (1 << n_pools) == 0 {
                    (&mut self.pool[n_pools]).result(&mut hash[n_pools * 32..(n_pools + 1) * 32]);
                    n_pools += 1;
                    assert!(n_pools < NUM_POOLS);
                    assert!(n_pools < 32); // width of counter
                }
                self.generator.reseed(&hash[..n_pools * 32]);
            }
            // Fail on unseeded RNG
            if self.reseed_count == 0 {
                panic!("rust-crypto: an unseeded Fortuna was asked for random bytes!");
            }
            // Generate return data
            for dest in dest.chunks_mut(MAX_GEN_SIZE) {
                self.generator.generate_random_data(dest);
            }
        }

        fn next_u32(&mut self) -> u32 {
            let mut ret = [0; 4];
            self.fill_bytes(&mut ret);
            read_u32_le(&ret[..])
        }
    }

    impl<'a> SeedableRng<&'a [u8]> for Fortuna {
        fn from_seed(seed: &'a [u8]) -> Fortuna {
            let mut ret = Fortuna::new_unseeded();
            ret.reseed(seed);
            ret
        }

        fn reseed(&mut self, seed: &'a [u8]) {
            self.reseed_count += 1;
            self.last_reseed_time = precise_time_s();
            self.generator.reseed(seed);
        }
    }

    #[cfg(test)]
    fn test_force_reseed(f: &mut Fortuna) {
        f.last_reseed_time -= 0.2;
    }

    #[cfg(test)]
    mod tests {
        use rand::{SeedableRng, Rng};

        use super::{Fortuna, Pool, NUM_POOLS, test_force_reseed};

        #[test]
        fn test_create_unseeded() {
            let _: Fortuna = Fortuna::new_unseeded();
        }

        #[test]
        #[should_panic]
        fn test_use_unseeded() {
            let mut f: Fortuna = Fortuna::new_unseeded();
            let _ = f.next_u32();
        }

        #[test]
        #[should_panic]
        fn test_badly_seeded() {
            let mut f: Fortuna = Fortuna::new_unseeded();
            f.add_random_event(0, 0, &[10; 32]);
            let _ = f.next_u32();
        }

        #[test]
        #[should_panic]
        fn test_too_big_event() {
            let mut f: Fortuna = Fortuna::new_unseeded();
            f.add_random_event(0, 0, &[10; 33]);
        }

        #[test]
        fn test_seeded() {
            // NB for this test I'm just trusting the output of the RNG to be correct.
            // I do check for some high-level features: changing most anything should
            // change the output, there should be no tests, etc.
            let mut f1: Fortuna = SeedableRng::from_seed(&[0, 1, 2, 3, 4, 5][..]);
            assert_eq!(f1.next_u32(), 3369034117);

            let mut f2: Fortuna = Fortuna::new_unseeded();
            f2.reseed(&[0, 1, 2, 3, 4, 5]);
            assert_eq!(f2.next_u32(), 3369034117);

            // Ensure reseeding doesn't totally reset the seed. That is, this output should
            // be different from the above
            let mut f3: Fortuna = Fortuna::new_unseeded();
            f3.reseed(&[0, 1, 2, 3, 4, 5]);
            f3.reseed(&[0, 1, 2, 3, 4, 5]);
            assert_eq!(f3.next_u32(), 2689122182);

            // These three should all be different
            let mut f4: Fortuna = Fortuna::new_unseeded();
            f4.add_random_event(0, 0, &[10; 32]);
            f4.add_random_event(0, 0, &[10; 32]);
            let x = f4.next_u32();

            let mut f5: Fortuna = Fortuna::new_unseeded();
            f5.add_random_event(0, 0, &[10; 32]);
            f5.add_random_event(0, 0, &[20; 32]);
            let y = f5.next_u32();

            let mut f6: Fortuna = Fortuna::new_unseeded();
            f6.add_random_event(0, 0, &[20; 32]);
            f6.add_random_event(0, 0, &[10; 32]);
            let z = f6.next_u32();

            assert!(x != y);
            assert!(y != z);
            assert!(x != z);
        }

        #[test]
        fn test_generator_correctness() {
            let mut output = [0; 100];
            // Expected output as in http://www.seehuhn.de/pages/fortuna
            let expected = [ 82, 254, 233, 139, 254,  85,   6, 222, 222, 149,
                            120,  35, 173,  71,  89, 232,  51, 182, 252, 139,
                            153, 153, 111,  30,  16,   7, 124, 185, 159,  24,
                             50,  68, 236, 107, 133,  18, 217, 219,  46, 134,
                            169, 156, 211,  74, 163,  17, 100, 173,  26,  70,
                            246, 193,  57, 164, 167, 175, 233, 220, 160, 114,
                              2, 200, 215,  80, 207, 218,  85,  58, 235, 117,
                            177, 223,  87, 192,  50, 251,  61,  65, 141, 100,
                             59, 228,  23, 215,  58, 107, 248, 248, 103,  57,
                            127,  31, 241,  91, 230,  33,   0, 164,  77, 46];
            let mut f: Fortuna = SeedableRng::from_seed(&[1, 2, 3, 4][..]);
            f.fill_bytes(&mut output);
            assert_eq!(&expected[..], &output[..]);

            let mut scratch = [0; (1 << 20)];
            f.generator.generate_random_data(&mut scratch);

            let expected = [122, 164,  26,  67, 102,  65,  30, 217, 219, 113,
                             14,  86, 214, 146, 185,  17, 107, 135, 183,   7,
                             18, 162, 126, 206,  46,  38,  54, 172, 248, 194,
                            118,  84, 162, 146,  83, 156, 152,  96, 192,  15,
                             23, 224, 113,  76,  21,   8, 226,  41, 161, 171,
                            197, 180, 138, 236, 126, 137, 101,  25, 219, 225,
                              3, 189,  16, 242,  33,  91,  34,  27,   8, 171,
                            171, 115, 157, 109, 248, 198, 227,  18, 204, 211,
                             42, 184,  92,  42, 171, 222, 198, 117, 162, 134,
                            116, 109,  77, 195, 187, 139,  37,  78, 224,  63];
            f.fill_bytes(&mut output);
            assert_eq!(&expected[..], &output[..]);

            f.reseed(&[5]);

            let expected = [217, 168, 141, 167,  46,   9, 218, 188,  98, 124,
                            109, 128, 242,  22, 189, 120, 180, 124,  15, 192,
                            116, 149, 211, 136, 253, 132,  60,   3,  29, 250,
                             95,  66, 133, 195,  37,  78, 242, 255, 160, 209,
                            185, 106,  68, 105,  83, 145, 165,  72, 179, 167,
                             53, 254, 183, 251, 128,  69,  78, 156, 219,  26,
                            124, 202,  35,   9, 174, 167,  41, 128, 184,  25,
                              2,   1,  63, 142, 205, 162,  69,  68, 207, 251,
                            101,  10,  29,  33, 133,  87, 189,  36, 229,  56,
                             17, 100, 138,  49,  79, 239, 210, 189, 141,  46];

            f.fill_bytes(&mut output);
            assert_eq!(&expected[..], &output[..]);
        }

        #[test]
        fn test_accumulator_correctness() {
            let mut output = [0; 100];
            // Expected output from experiments with pycryto
            // Note that this does not match the results for the Go implementation
            // as described at http://www.seehuhn.de/pages/fortuna ... I believe
            // this is because the author there is reusing some Fortuna state from
            // the previous test. These results agree with pycrypto on a fresh slate
            let mut f = Fortuna::new_unseeded();
            f.pool = [Pool::new(); NUM_POOLS];
            f.add_random_event(0, 0, &[0; 32]);
            f.add_random_event(0, 0, &[0; 32]);
            for i in 0..32 {
                f.add_random_event(1, i, &[1, 2]);
            }

            // from Crypto.Random.Fortuna import FortunaAccumulator
            // x = FortunaAccumulator.FortunaAccumulator()
            // x.add_random_event(0, 0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0")
            // x.add_random_event(0, 0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0")
            // x.add_random_event(1, 0, "\1\2")
            // x.add_random_event(1, 1, "\1\2")
            // print list(bytearray(x.random_data(100)))
            let expected = [ 21,  42, 103, 180, 211,  46, 177, 231, 172, 210,
                            109, 198,  34,  40, 245, 199,  76, 114, 105, 185,
                            186, 112, 183, 213,  19,  72, 186,  26, 182, 211,
                            254,  88,  67, 142, 246, 102,  80,  93, 144, 152,
                            123, 191, 168,  26,  21, 194,  69, 214, 249,  80,
                            182, 165, 203,  69, 134, 140,  11, 208,  50, 175,
                            180, 210, 110, 119,   3,  75,   1,   8,   5, 142,
                            226, 168, 179, 246,  82,  42, 223, 239, 201,  23,
                             28,  30, 195, 195,   9, 154,  31, 172, 209, 232,
                            238, 111,  75, 251, 196,  43, 217, 241,  93, 237];
            f.fill_bytes(&mut output);
            assert_eq!(&expected[..], &output[..]);

            // Immediately (less than 100ms)
            f.add_random_event(0, 0, &[0; 32]);
            f.add_random_event(0, 0, &[0; 32]);

            // x.add_random_event(0, 0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0")
            // x.add_random_event(0, 0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0")
            // print list(bytearray(x.random_data(100)))
            let expected = [101, 123, 175, 157, 142, 202, 211,  47, 149, 214,
                            135, 249, 148,  19,  50, 116, 169, 188, 240, 218,
                             91,  62,  35,  44, 142, 108,  95,  20,  37, 185,
                             19, 121, 128, 231, 213,  23,  94, 147,  14,  41,
                            199, 253, 246,  14, 230, 152,  11,  17, 118, 254,
                             96, 251, 171, 115,  66,  21, 196, 164,  82,   6,
                            139, 238, 135,  22, 179,   6,   6, 252, 115,  87,
                             19, 167,  56, 192, 140,  93, 132,  78,  22,  16,
                            114,  68, 123, 200,  37, 183, 163, 224, 201, 155,
                            233,  71, 111,  26,   8, 114, 232, 181,  13,  51];
            f.fill_bytes(&mut output);
            assert_eq!(&expected[..], &output[..]);

            // Simulate more than 100 ms passing
            test_force_reseed(&mut f);
            // time.sleep(0.2)
            // print list(bytearray(x.random_data(100)))
            let expected = [ 62, 147, 205, 228,  22,   3, 225, 217, 211, 202,
                             49, 148, 236, 125, 132,  43,  25, 177, 172,  93,
                             98, 177, 112, 160,  76, 101,  60,  98, 225,   9,
                            223, 120, 161,  98, 173, 178,  71,  15,  90, 153,
                             64, 179, 143,  22,  43, 165,  87, 147, 177, 128,
                             21, 105, 214, 197, 224, 187,  22, 139,  16, 153,
                            251,  48, 244,  87,  10, 104, 119, 179,  27, 255,
                             67, 148, 192,  52, 147, 216,  79, 204, 106, 112,
                            238,   0, 239,  99, 159,  96, 184,  90,  54, 122,
                            184, 241, 221, 151, 169,  29, 197,  45,  80,   6];
            f.fill_bytes(&mut output);
            assert_eq!(&expected[..], &output[..]);
        }
    }

    #[cfg(all(test, feature = "with-bench"))]
    mod bench {
        use rand::{SeedableRng, Rng};
        use test::Bencher;

        use super::Fortuna;

        #[bench]
        pub fn fortuna_new_32(bh: &mut Bencher) {
            let mut f: Fortuna = SeedableRng::from_seed(&[100; 64][..]);
            bh.iter( || {
                f.next_u32();
            });
            bh.bytes = 4;
        }

        #[bench]
        pub fn fortuna_new_64(bh: &mut Bencher) {
            let mut f: Fortuna = SeedableRng::from_seed(&[100; 64][..]);
            bh.iter( || {
                f.next_u64();
            });
            bh.bytes = 8;
        }

        #[bench]
        pub fn fortuna_new_1k(bh: &mut Bencher) {
            let mut f: Fortuna = SeedableRng::from_seed(&[100; 64][..]);
            let mut bytes = [0u8; 1024];
            bh.iter( || {
                f.fill_bytes(&mut bytes);
            });
            bh.bytes = bytes.len() as u64;
        }

        #[bench]
        pub fn fortuna_new_64k(bh: &mut Bencher) {
            let mut f: Fortuna = SeedableRng::from_seed(&[100; 64][..]);
            let mut bytes = [0u8; 65536];
            bh.iter( || {
                f.fill_bytes(&mut bytes);
            });
            bh.bytes = bytes.len() as u64;
        }
    }

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