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    use crate::{
        encode::{add_padding, encode_to_slice},
        Config,
    };
    #[cfg(any(feature = "alloc", feature = "std", test))]
    use alloc::string::String;
    use core::cmp;
    #[cfg(any(feature = "alloc", feature = "std", test))]
    use core::str;

    /// The output mechanism for ChunkedEncoder's encoded bytes.
    pub trait Sink {
        type Error;

        /// Handle a chunk of encoded base64 data (as UTF-8 bytes)
        fn write_encoded_bytes(&mut self, encoded: &[u8]) -> Result<(), Self::Error>;
    }

    const BUF_SIZE: usize = 1024;

    /// A base64 encoder that emits encoded bytes in chunks without heap allocation.
    pub struct ChunkedEncoder {
        config: Config,
        max_input_chunk_len: usize,
    }

    impl ChunkedEncoder {
        pub fn new(config: Config) -> ChunkedEncoder {
            ChunkedEncoder {
                config,
                max_input_chunk_len: max_input_length(BUF_SIZE, config),
            }
        }

        pub fn encode<S: Sink>(&self, bytes: &[u8], sink: &mut S) -> Result<(), S::Error> {
            let mut encode_buf: [u8; BUF_SIZE] = [0; BUF_SIZE];
            let encode_table = self.config.char_set.encode_table();

            let mut input_index = 0;

            while input_index < bytes.len() {
                // either the full input chunk size, or it's the last iteration
                let input_chunk_len = cmp::min(self.max_input_chunk_len, bytes.len() - input_index);

                let chunk = &bytes[input_index..(input_index + input_chunk_len)];

                let mut b64_bytes_written = encode_to_slice(chunk, &mut encode_buf, encode_table);

                input_index += input_chunk_len;
                let more_input_left = input_index < bytes.len();

                if self.config.pad && !more_input_left {
                    // no more input, add padding if needed. Buffer will have room because
                    // max_input_length leaves room for it.
                    b64_bytes_written += add_padding(bytes.len(), &mut encode_buf[b64_bytes_written..]);
                }

                sink.write_encoded_bytes(&encode_buf[0..b64_bytes_written])?;
            }

            Ok(())
        }
    }

    /// Calculate the longest input that can be encoded for the given output buffer size.
    ///
    /// If the config requires padding, two bytes of buffer space will be set aside so that the last
    /// chunk of input can be encoded safely.
    ///
    /// The input length will always be a multiple of 3 so that no encoding state has to be carried over
    /// between chunks.
    fn max_input_length(encoded_buf_len: usize, config: Config) -> usize {
        let effective_buf_len = if config.pad {
            // make room for padding
            encoded_buf_len
                .checked_sub(2)
                .expect("Don't use a tiny buffer")
        } else {
            encoded_buf_len
        };

        // No padding, so just normal base64 expansion.
        (effective_buf_len / 4) * 3
    }

    // A really simple sink that just appends to a string
    #[cfg(any(feature = "alloc", feature = "std", test))]
    pub(crate) struct StringSink<'a> {
        string: &'a mut String,
    }

    #[cfg(any(feature = "alloc", feature = "std", test))]
    impl<'a> StringSink<'a> {
        pub(crate) fn new(s: &mut String) -> StringSink {
            StringSink { string: s }
        }
    }

    #[cfg(any(feature = "alloc", feature = "std", test))]
    impl<'a> Sink for StringSink<'a> {
        type Error = ();

        fn write_encoded_bytes(&mut self, s: &[u8]) -> Result<(), Self::Error> {
            self.string.push_str(str::from_utf8(s).unwrap());

            Ok(())
        }
    }

    #[cfg(test)]
    pub mod tests {
        use super::*;
        use crate::{encode_config_buf, tests::random_config, CharacterSet, STANDARD};

        use rand::{
            distributions::{Distribution, Uniform},
            FromEntropy, Rng,
        };

        #[test]
        fn chunked_encode_empty() {
            assert_eq!("", chunked_encode_str(&[], STANDARD));
        }

        #[test]
        fn chunked_encode_intermediate_fast_loop() {
            // > 8 bytes input, will enter the pretty fast loop
            assert_eq!(
                "Zm9vYmFyYmF6cXV4",
                chunked_encode_str(b"foobarbazqux", STANDARD)
            );
        }

        #[test]
        fn chunked_encode_fast_loop() {
            // > 32 bytes input, will enter the uber fast loop
            assert_eq!(
                "Zm9vYmFyYmF6cXV4cXV1eGNvcmdlZ3JhdWx0Z2FycGx5eg==",
                chunked_encode_str(b"foobarbazquxquuxcorgegraultgarplyz", STANDARD)
            );
        }

        #[test]
        fn chunked_encode_slow_loop_only() {
            // < 8 bytes input, slow loop only
            assert_eq!("Zm9vYmFy", chunked_encode_str(b"foobar", STANDARD));
        }

        #[test]
        fn chunked_encode_matches_normal_encode_random_string_sink() {
            let helper = StringSinkTestHelper;
            chunked_encode_matches_normal_encode_random(&helper);
        }

        #[test]
        fn max_input_length_no_pad() {
            let config = config_with_pad(false);
            assert_eq!(768, max_input_length(1024, config));
        }

        #[test]
        fn max_input_length_with_pad_decrements_one_triple() {
            let config = config_with_pad(true);
            assert_eq!(765, max_input_length(1024, config));
        }

        #[test]
        fn max_input_length_with_pad_one_byte_short() {
            let config = config_with_pad(true);
            assert_eq!(765, max_input_length(1025, config));
        }

        #[test]
        fn max_input_length_with_pad_fits_exactly() {
            let config = config_with_pad(true);
            assert_eq!(768, max_input_length(1026, config));
        }

        #[test]
        fn max_input_length_cant_use_extra_single_encoded_byte() {
            let config = Config::new(crate::CharacterSet::Standard, false);
            assert_eq!(300, max_input_length(401, config));
        }

        pub fn chunked_encode_matches_normal_encode_random<S: SinkTestHelper>(sink_test_helper: &S) {
            let mut input_buf: Vec<u8> = Vec::new();
            let mut output_buf = String::new();
            let mut rng = rand::rngs::SmallRng::from_entropy();
            let input_len_range = Uniform::new(1, 10_000);

            for _ in 0..5_000 {
                input_buf.clear();
                output_buf.clear();

                let buf_len = input_len_range.sample(&mut rng);
                for _ in 0..buf_len {
                    input_buf.push(rng.gen());
                }

                let config = random_config(&mut rng);

                let chunk_encoded_string = sink_test_helper.encode_to_string(config, &input_buf);
                encode_config_buf(&input_buf, config, &mut output_buf);

                assert_eq!(
                    output_buf, chunk_encoded_string,
                    "input len={}, config: pad={}",
                    buf_len, config.pad
                );
            }
        }

        fn chunked_encode_str(bytes: &[u8], config: Config) -> String {
            let mut s = String::new();
            {
                let mut sink = StringSink::new(&mut s);
                let encoder = ChunkedEncoder::new(config);
                encoder.encode(bytes, &mut sink).unwrap();
            }

            return s;
        }

        fn config_with_pad(pad: bool) -> Config {
            Config::new(CharacterSet::Standard, pad)
        }

        // An abstraction around sinks so that we can have tests that easily to any sink implementation
        pub trait SinkTestHelper {
            fn encode_to_string(&self, config: Config, bytes: &[u8]) -> String;
        }

        struct StringSinkTestHelper;

        impl SinkTestHelper for StringSinkTestHelper {
            fn encode_to_string(&self, config: Config, bytes: &[u8]) -> String {
                let encoder = ChunkedEncoder::new(config);
                let mut s = String::new();
                {
                    let mut sink = StringSink::new(&mut s);
                    encoder.encode(bytes, &mut sink).unwrap();
                }

                s
            }
        }
    }