      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254
    255
    256
    257
    258
    259
    260
    261
    262
    263
    264
    265
    266
    267
    268
    269
    270
    271
    272
    273
    274
    275
    276
    277
    278
    279
    280
    281
    282
    283
    284
    285
    286
    287
    288
    289
    290
    291
    292
    293
    294
    295
    296
    297
    298
    299
    300
    301
    302
    303
    304
    305
    306
    307
    308
    309
    310
    311
    312
    313
    314
    315
    316
    317
    318
    319
    320
    321
    322
    323
    324
    325
    326
    327
    328
    329
    330
    331
    332
    333
    334
    335
    336
    337
    338
    339
    340
    341
    342
    343
    344
    345
    346
    347
    348
    349
    350
    351
    352
    353
    354
    355
    356
    357
    358
    359
    360
    361
    362
    363
    364
    365
    366
    367
    368
    369
    370
    371
    372
    373
    374
    375
    376
    377
    378
    379
    380
    381
    382
    383
    384
    385
    386
    387
    388
    389
    390
    391
    392
    393
    394
    395
    396
    397
    398
    399
    400
    401
    402
    403
    404
    405
    406
    407
    408
    409
    410
    411
    412
    413
    414
    415
    416
    417
    418
    419
    420
    421
    422
    423
    424
    425
    426
    427
    428
    429
    430
    431
    432
    433
    434
    435
    436
    437
    438
    439
    440
    441
    442
    443
    444
    445
    446
    447
    448
    449
    450
    451
    452
    453
    454
    455
    456
    457
    458
    459
    460
    461
    462
    463
    464
    465
    466
    467
    468
    469
    470
    471
    472
    473
    474
    475
    476
    477
    478
    479
    480
    481
    482
    483
    484
    485
    486
    487
    488
    489
    490
    491
    492
    493
    494
    495
    496
    497
    498
    499
    500
    501
    502
    503
    504
    505
    506
    507
    508
    509
    510
    511
    512
    513
    514
    515
    516
    517
    518
    519
    520
    521
    522
    523
    524
    525
    526
    527
    528
    529
    530
    531
    532
    533
    534
    535
    536
    537
    538
    539
    540
    541
    542
    543
    544
    545
    546
    547
    548
    549
    550
    551
    552
    553
    554
    555
    556
    557
    558
    559
    560
    561
    562
    563
    564
    565
    566
    567
    568
    569
    570
    571
    572
    573
    574
    575
    576
    577
    578
    579
    580
    581
    582
    583
    584
    585
    586
    587
    588
    589
    590
    591
    592
    593
    594
    595
    596
    597
    598
    599
    600
    601
    602
    603
    604
    605
    606
    607
    608
    609
    610
    611
    612
    613
    614
    615
    616
    617
    618
    619
    620
    621
    622
    623
    624
    625
    626
    627
    628
    629
    630
    631
    632
    633
    634
    635
    636
    637
    638
    639
    640
    641
    642
    643
    644
    645
    646
    647
    648
    649
    650
    651
    652
    653
    654
    655
    656
    657
    658
    659
    660
    661
    662
    663
    664
    665
    666
    667
    668
    669
    670
    671
    672
    673
    674
    675
    676
    677
    678
    679
    680
    681
    682
    683
    684
    685
    686
    687
    688
    689
    690
    691
    692
    693
    694
    695
    696
    697
    698
    699
    700
    701
    702
    703
    704
    705
    706
    707
    708
    709
    710
    711
    712
    713
    714
    715
    716
    717
    718
    719
    720
    721
    722
    723
    724
    725
    726
    727
    728
    729
    730
    731
    732
    733
    734
    735
    736
    737
    738
    739
    740
    741
    742
    743
    744
    745
    746
    747
    748
    749
    750
    751
    752
    753
    754
    755
    756
    757
    758
    759
    760
    761
    762
    763
    764
    765
    766
    767
    768
    769
    770
    771
    772
    773
    774
    775
    776
    777
    778
    779
    780
    781
    782
    783
    784
    785
    786
    787
    788
    789
    790
    791
    792
    793
    794
    795
    796
    797
    798
    799
    800
    801
    802
    803
    804
    805
    806
    807
    808
    809
    810
    811
    812
    813
    814
    815
    816
    817
    818
    819
    820
    821
    822
    823
    824
    825
    826
    827
    828
    829
    830
    831
    832
    833
    834
    835
    836
    837
    838
    839
    840
    841
    842
    843
    844
    845
    846
    847
    848
    849
    850
    851
    852
    853
    854
    855
    856
    857
    858
    859
    860
    861
    862
    863
    864
    865
    866
    867
    868
    869
    870
    871
    872
    873

    use crate::{tables, Config, PAD_BYTE};

    #[cfg(any(feature = "alloc", feature = "std", test))]
    use crate::STANDARD;
    #[cfg(any(feature = "alloc", feature = "std", test))]
    use alloc::vec::Vec;
    use core::fmt;
    #[cfg(any(feature = "std", test))]
    use std::error;

    // decode logic operates on chunks of 8 input bytes without padding
    const INPUT_CHUNK_LEN: usize = 8;
    const DECODED_CHUNK_LEN: usize = 6;
    // we read a u64 and write a u64, but a u64 of input only yields 6 bytes of output, so the last
    // 2 bytes of any output u64 should not be counted as written to (but must be available in a
    // slice).
    const DECODED_CHUNK_SUFFIX: usize = 2;

    // how many u64's of input to handle at a time
    const CHUNKS_PER_FAST_LOOP_BLOCK: usize = 4;
    const INPUT_BLOCK_LEN: usize = CHUNKS_PER_FAST_LOOP_BLOCK * INPUT_CHUNK_LEN;
    // includes the trailing 2 bytes for the final u64 write
    const DECODED_BLOCK_LEN: usize =
        CHUNKS_PER_FAST_LOOP_BLOCK * DECODED_CHUNK_LEN + DECODED_CHUNK_SUFFIX;

    /// Errors that can occur while decoding.
    #[derive(Clone, Debug, PartialEq, Eq)]
    pub enum DecodeError {
        /// An invalid byte was found in the input. The offset and offending byte are provided.
        InvalidByte(usize, u8),
        /// The length of the input is invalid.
        /// A typical cause of this is stray trailing whitespace or other separator bytes.
        /// In the case where excess trailing bytes have produced an invalid length *and* the last byte
        /// is also an invalid base64 symbol (as would be the case for whitespace, etc), `InvalidByte`
        /// will be emitted instead of `InvalidLength` to make the issue easier to debug.
        InvalidLength,
        /// The last non-padding input symbol's encoded 6 bits have nonzero bits that will be discarded.
        /// This is indicative of corrupted or truncated Base64.
        /// Unlike InvalidByte, which reports symbols that aren't in the alphabet, this error is for
        /// symbols that are in the alphabet but represent nonsensical encodings.
        InvalidLastSymbol(usize, u8),
    }

    impl fmt::Display for DecodeError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match *self {
                DecodeError::InvalidByte(index, byte) => {
                    write!(f, "Invalid byte {}, offset {}.", byte, index)
                }
                DecodeError::InvalidLength => write!(f, "Encoded text cannot have a 6-bit remainder."),
                DecodeError::InvalidLastSymbol(index, byte) => {
                    write!(f, "Invalid last symbol {}, offset {}.", byte, index)
                }
            }
        }
    }

    #[cfg(any(feature = "std", test))]
    impl error::Error for DecodeError {
        fn description(&self) -> &str {
            match *self {
                DecodeError::InvalidByte(_, _) => "invalid byte",
                DecodeError::InvalidLength => "invalid length",
                DecodeError::InvalidLastSymbol(_, _) => "invalid last symbol",
            }
        }

        fn cause(&self) -> Option<&dyn error::Error> {
            None
        }
    }

    ///Decode from string reference as octets.
    ///Returns a Result containing a Vec<u8>.
    ///Convenience `decode_config(input, base64::STANDARD);`.
    ///
    ///# Example
    ///
    ///```rust
    ///extern crate base64;
    ///
    ///fn main() {
    ///    let bytes = base64::decode("aGVsbG8gd29ybGQ=").unwrap();
    ///    println!("{:?}", bytes);
    ///}
    ///```
    #[cfg(any(feature = "alloc", feature = "std", test))]
    pub fn decode<T: AsRef<[u8]>>(input: T) -> Result<Vec<u8>, DecodeError> {
        decode_config(input, STANDARD)
    }

    ///Decode from string reference as octets.
    ///Returns a Result containing a Vec<u8>.
    ///
    ///# Example
    ///
    ///```rust
    ///extern crate base64;
    ///
    ///fn main() {
    ///    let bytes = base64::decode_config("aGVsbG8gd29ybGR+Cg==", base64::STANDARD).unwrap();
    ///    println!("{:?}", bytes);
    ///
    ///    let bytes_url = base64::decode_config("aGVsbG8gaW50ZXJuZXR-Cg==", base64::URL_SAFE).unwrap();
    ///    println!("{:?}", bytes_url);
    ///}
    ///```
    #[cfg(any(feature = "alloc", feature = "std", test))]
    pub fn decode_config<T: AsRef<[u8]>>(input: T, config: Config) -> Result<Vec<u8>, DecodeError> {
        let mut buffer = Vec::<u8>::with_capacity(input.as_ref().len() * 4 / 3);

        decode_config_buf(input, config, &mut buffer).map(|_| buffer)
    }

    ///Decode from string reference as octets.
    ///Writes into the supplied buffer to avoid allocation.
    ///Returns a Result containing an empty tuple, aka ().
    ///
    ///# Example
    ///
    ///```rust
    ///extern crate base64;
    ///
    ///fn main() {
    ///    let mut buffer = Vec::<u8>::new();
    ///    base64::decode_config_buf("aGVsbG8gd29ybGR+Cg==", base64::STANDARD, &mut buffer).unwrap();
    ///    println!("{:?}", buffer);
    ///
    ///    buffer.clear();
    ///
    ///    base64::decode_config_buf("aGVsbG8gaW50ZXJuZXR-Cg==", base64::URL_SAFE, &mut buffer)
    ///        .unwrap();
    ///    println!("{:?}", buffer);
    ///}
    ///```
    #[cfg(any(feature = "alloc", feature = "std", test))]
    pub fn decode_config_buf<T: AsRef<[u8]>>(
        input: T,
        config: Config,
        buffer: &mut Vec<u8>,
    ) -> Result<(), DecodeError> {
        let input_bytes = input.as_ref();

        let starting_output_len = buffer.len();

        let num_chunks = num_chunks(input_bytes);
        let decoded_len_estimate = num_chunks
            .checked_mul(DECODED_CHUNK_LEN)
            .and_then(|p| p.checked_add(starting_output_len))
            .expect("Overflow when calculating output buffer length");
        buffer.resize(decoded_len_estimate, 0);

        let bytes_written;
        {
            let buffer_slice = &mut buffer.as_mut_slice()[starting_output_len..];
            bytes_written = decode_helper(input_bytes, num_chunks, config, buffer_slice)?;
        }

        buffer.truncate(starting_output_len + bytes_written);

        Ok(())
    }

    /// Decode the input into the provided output slice.
    ///
    /// This will not write any bytes past exactly what is decoded (no stray garbage bytes at the end).
    ///
    /// If you don't know ahead of time what the decoded length should be, size your buffer with a
    /// conservative estimate for the decoded length of an input: 3 bytes of output for every 4 bytes of
    /// input, rounded up, or in other words `(input_len + 3) / 4 * 3`.
    ///
    /// If the slice is not large enough, this will panic.
    pub fn decode_config_slice<T: AsRef<[u8]>>(
        input: T,
        config: Config,
        output: &mut [u8],
    ) -> Result<usize, DecodeError> {
        let input_bytes = input.as_ref();

        decode_helper(input_bytes, num_chunks(input_bytes), config, output)
    }

    /// Return the number of input chunks (including a possibly partial final chunk) in the input
    fn num_chunks(input: &[u8]) -> usize {
        input
            .len()
            .checked_add(INPUT_CHUNK_LEN - 1)
            .expect("Overflow when calculating number of chunks in input")
            / INPUT_CHUNK_LEN
    }

    /// Helper to avoid duplicating num_chunks calculation, which is costly on short inputs.
    /// Returns the number of bytes written, or an error.
    // We're on the fragile edge of compiler heuristics here. If this is not inlined, slow. If this is
    // inlined(always), a different slow. plain ol' inline makes the benchmarks happiest at the moment,
    // but this is fragile and the best setting changes with only minor code modifications.
    #[inline]
    fn decode_helper(
        input: &[u8],
        num_chunks: usize,
        config: Config,
        output: &mut [u8],
    ) -> Result<usize, DecodeError> {
        let char_set = config.char_set;
        let decode_table = char_set.decode_table();

        let remainder_len = input.len() % INPUT_CHUNK_LEN;

        // Because the fast decode loop writes in groups of 8 bytes (unrolled to
        // CHUNKS_PER_FAST_LOOP_BLOCK times 8 bytes, where possible) and outputs 8 bytes at a time (of
        // which only 6 are valid data), we need to be sure that we stop using the fast decode loop
        // soon enough that there will always be 2 more bytes of valid data written after that loop.
        let trailing_bytes_to_skip = match remainder_len {
            // if input is a multiple of the chunk size, ignore the last chunk as it may have padding,
            // and the fast decode logic cannot handle padding
            0 => INPUT_CHUNK_LEN,
            // 1 and 5 trailing bytes are illegal: can't decode 6 bits of input into a byte
            1 | 5 => {
                // trailing whitespace is so common that it's worth it to check the last byte to
                // possibly return a better error message
                if let Some(b) = input.last() {
                    if *b != PAD_BYTE && decode_table[*b as usize] == tables::INVALID_VALUE {
                        return Err(DecodeError::InvalidByte(input.len() - 1, *b));
                    }
                }

                return Err(DecodeError::InvalidLength);
            }
            // This will decode to one output byte, which isn't enough to overwrite the 2 extra bytes
            // written by the fast decode loop. So, we have to ignore both these 2 bytes and the
            // previous chunk.
            2 => INPUT_CHUNK_LEN + 2,
            // If this is 3 unpadded chars, then it would actually decode to 2 bytes. However, if this
            // is an erroneous 2 chars + 1 pad char that would decode to 1 byte, then it should fail
            // with an error, not panic from going past the bounds of the output slice, so we let it
            // use stage 3 + 4.
            3 => INPUT_CHUNK_LEN + 3,
            // This can also decode to one output byte because it may be 2 input chars + 2 padding
            // chars, which would decode to 1 byte.
            4 => INPUT_CHUNK_LEN + 4,
            // Everything else is a legal decode len (given that we don't require padding), and will
            // decode to at least 2 bytes of output.
            _ => remainder_len,
        };

        // rounded up to include partial chunks
        let mut remaining_chunks = num_chunks;

        let mut input_index = 0;
        let mut output_index = 0;

        {
            let length_of_fast_decode_chunks = input.len().saturating_sub(trailing_bytes_to_skip);

            // Fast loop, stage 1
            // manual unroll to CHUNKS_PER_FAST_LOOP_BLOCK of u64s to amortize slice bounds checks
            if let Some(max_start_index) = length_of_fast_decode_chunks.checked_sub(INPUT_BLOCK_LEN) {
                while input_index <= max_start_index {
                    let input_slice = &input[input_index..(input_index + INPUT_BLOCK_LEN)];
                    let output_slice = &mut output[output_index..(output_index + DECODED_BLOCK_LEN)];

                    decode_chunk(
                        &input_slice[0..],
                        input_index,
                        decode_table,
                        &mut output_slice[0..],
                    )?;
                    decode_chunk(
                        &input_slice[8..],
                        input_index + 8,
                        decode_table,
                        &mut output_slice[6..],
                    )?;
                    decode_chunk(
                        &input_slice[16..],
                        input_index + 16,
                        decode_table,
                        &mut output_slice[12..],
                    )?;
                    decode_chunk(
                        &input_slice[24..],
                        input_index + 24,
                        decode_table,
                        &mut output_slice[18..],
                    )?;

                    input_index += INPUT_BLOCK_LEN;
                    output_index += DECODED_BLOCK_LEN - DECODED_CHUNK_SUFFIX;
                    remaining_chunks -= CHUNKS_PER_FAST_LOOP_BLOCK;
                }
            }

            // Fast loop, stage 2 (aka still pretty fast loop)
            // 8 bytes at a time for whatever we didn't do in stage 1.
            if let Some(max_start_index) = length_of_fast_decode_chunks.checked_sub(INPUT_CHUNK_LEN) {
                while input_index < max_start_index {
                    decode_chunk(
                        &input[input_index..(input_index + INPUT_CHUNK_LEN)],
                        input_index,
                        decode_table,
                        &mut output
                            [output_index..(output_index + DECODED_CHUNK_LEN + DECODED_CHUNK_SUFFIX)],
                    )?;

                    output_index += DECODED_CHUNK_LEN;
                    input_index += INPUT_CHUNK_LEN;
                    remaining_chunks -= 1;
                }
            }
        }

        // Stage 3
        // If input length was such that a chunk had to be deferred until after the fast loop
        // because decoding it would have produced 2 trailing bytes that wouldn't then be
        // overwritten, we decode that chunk here. This way is slower but doesn't write the 2
        // trailing bytes.
        // However, we still need to avoid the last chunk (partial or complete) because it could
        // have padding, so we always do 1 fewer to avoid the last chunk.
        for _ in 1..remaining_chunks {
            decode_chunk_precise(
                &input[input_index..],
                input_index,
                decode_table,
                &mut output[output_index..(output_index + DECODED_CHUNK_LEN)],
            )?;

            input_index += INPUT_CHUNK_LEN;
            output_index += DECODED_CHUNK_LEN;
        }

        // always have one more (possibly partial) block of 8 input
        debug_assert!(input.len() - input_index > 1 || input.is_empty());
        debug_assert!(input.len() - input_index <= 8);

        // Stage 4
        // Finally, decode any leftovers that aren't a complete input block of 8 bytes.
        // Use a u64 as a stack-resident 8 byte buffer.
        let mut leftover_bits: u64 = 0;
        let mut morsels_in_leftover = 0;
        let mut padding_bytes = 0;
        let mut first_padding_index: usize = 0;
        let mut last_symbol = 0_u8;
        let start_of_leftovers = input_index;
        for (i, b) in input[start_of_leftovers..].iter().enumerate() {
            // '=' padding
            if *b == PAD_BYTE {
                // There can be bad padding in a few ways:
                // 1 - Padding with non-padding characters after it
                // 2 - Padding after zero or one non-padding characters before it
                //     in the current quad.
                // 3 - More than two characters of padding. If 3 or 4 padding chars
                //     are in the same quad, that implies it will be caught by #2.
                //     If it spreads from one quad to another, it will be caught by
                //     #2 in the second quad.

                if i % 4 < 2 {
                    // Check for case #2.
                    let bad_padding_index = start_of_leftovers
                        + if padding_bytes > 0 {
                            // If we've already seen padding, report the first padding index.
                            // This is to be consistent with the faster logic above: it will report an
                            // error on the first padding character (since it doesn't expect to see
                            // anything but actual encoded data).
                            first_padding_index
                        } else {
                            // haven't seen padding before, just use where we are now
                            i
                        };
                    return Err(DecodeError::InvalidByte(bad_padding_index, *b));
                }

                if padding_bytes == 0 {
                    first_padding_index = i;
                }

                padding_bytes += 1;
                continue;
            }

            // Check for case #1.
            // To make '=' handling consistent with the main loop, don't allow
            // non-suffix '=' in trailing chunk either. Report error as first
            // erroneous padding.
            if padding_bytes > 0 {
                return Err(DecodeError::InvalidByte(
                    start_of_leftovers + first_padding_index,
                    PAD_BYTE,
                ));
            }
            last_symbol = *b;

            // can use up to 8 * 6 = 48 bits of the u64, if last chunk has no padding.
            // To minimize shifts, pack the leftovers from left to right.
            let shift = 64 - (morsels_in_leftover + 1) * 6;
            // tables are all 256 elements, lookup with a u8 index always succeeds
            let morsel = decode_table[*b as usize];
            if morsel == tables::INVALID_VALUE {
                return Err(DecodeError::InvalidByte(start_of_leftovers + i, *b));
            }

            leftover_bits |= (morsel as u64) << shift;
            morsels_in_leftover += 1;
        }

        let leftover_bits_ready_to_append = match morsels_in_leftover {
            0 => 0,
            2 => 8,
            3 => 16,
            4 => 24,
            6 => 32,
            7 => 40,
            8 => 48,
            _ => unreachable!(
                "Impossible: must only have 0 to 8 input bytes in last chunk, with no invalid lengths"
            ),
        };

        // if there are bits set outside the bits we care about, last symbol encodes trailing bits that
        // will not be included in the output
        let mask = !0 >> leftover_bits_ready_to_append;
        if !config.decode_allow_trailing_bits && (leftover_bits & mask) != 0 {
            // last morsel is at `morsels_in_leftover` - 1
            return Err(DecodeError::InvalidLastSymbol(
                start_of_leftovers + morsels_in_leftover - 1,
                last_symbol,
            ));
        }

        let mut leftover_bits_appended_to_buf = 0;
        while leftover_bits_appended_to_buf < leftover_bits_ready_to_append {
            // `as` simply truncates the higher bits, which is what we want here
            let selected_bits = (leftover_bits >> (56 - leftover_bits_appended_to_buf)) as u8;
            output[output_index] = selected_bits;
            output_index += 1;

            leftover_bits_appended_to_buf += 8;
        }

        Ok(output_index)
    }

    #[inline]
    fn write_u64(output: &mut [u8], value: u64) {
        output[..8].copy_from_slice(&value.to_be_bytes());
    }

    /// Decode 8 bytes of input into 6 bytes of output. 8 bytes of output will be written, but only the
    /// first 6 of those contain meaningful data.
    ///
    /// `input` is the bytes to decode, of which the first 8 bytes will be processed.
    /// `index_at_start_of_input` is the offset in the overall input (used for reporting errors
    /// accurately)
    /// `decode_table` is the lookup table for the particular base64 alphabet.
    /// `output` will have its first 8 bytes overwritten, of which only the first 6 are valid decoded
    /// data.
    // yes, really inline (worth 30-50% speedup)
    #[inline(always)]
    fn decode_chunk(
        input: &[u8],
        index_at_start_of_input: usize,
        decode_table: &[u8; 256],
        output: &mut [u8],
    ) -> Result<(), DecodeError> {
        let mut accum: u64;

        let morsel = decode_table[input[0] as usize];
        if morsel == tables::INVALID_VALUE {
            return Err(DecodeError::InvalidByte(index_at_start_of_input, input[0]));
        }
        accum = (morsel as u64) << 58;

        let morsel = decode_table[input[1] as usize];
        if morsel == tables::INVALID_VALUE {
            return Err(DecodeError::InvalidByte(
                index_at_start_of_input + 1,
                input[1],
            ));
        }
        accum |= (morsel as u64) << 52;

        let morsel = decode_table[input[2] as usize];
        if morsel == tables::INVALID_VALUE {
            return Err(DecodeError::InvalidByte(
                index_at_start_of_input + 2,
                input[2],
            ));
        }
        accum |= (morsel as u64) << 46;

        let morsel = decode_table[input[3] as usize];
        if morsel == tables::INVALID_VALUE {
            return Err(DecodeError::InvalidByte(
                index_at_start_of_input + 3,
                input[3],
            ));
        }
        accum |= (morsel as u64) << 40;

        let morsel = decode_table[input[4] as usize];
        if morsel == tables::INVALID_VALUE {
            return Err(DecodeError::InvalidByte(
                index_at_start_of_input + 4,
                input[4],
            ));
        }
        accum |= (morsel as u64) << 34;

        let morsel = decode_table[input[5] as usize];
        if morsel == tables::INVALID_VALUE {
            return Err(DecodeError::InvalidByte(
                index_at_start_of_input + 5,
                input[5],
            ));
        }
        accum |= (morsel as u64) << 28;

        let morsel = decode_table[input[6] as usize];
        if morsel == tables::INVALID_VALUE {
            return Err(DecodeError::InvalidByte(
                index_at_start_of_input + 6,
                input[6],
            ));
        }
        accum |= (morsel as u64) << 22;

        let morsel = decode_table[input[7] as usize];
        if morsel == tables::INVALID_VALUE {
            return Err(DecodeError::InvalidByte(
                index_at_start_of_input + 7,
                input[7],
            ));
        }
        accum |= (morsel as u64) << 16;

        write_u64(output, accum);

        Ok(())
    }

    /// Decode an 8-byte chunk, but only write the 6 bytes actually decoded instead of including 2
    /// trailing garbage bytes.
    #[inline]
    fn decode_chunk_precise(
        input: &[u8],
        index_at_start_of_input: usize,
        decode_table: &[u8; 256],
        output: &mut [u8],
    ) -> Result<(), DecodeError> {
        let mut tmp_buf = [0_u8; 8];

        decode_chunk(
            input,
            index_at_start_of_input,
            decode_table,
            &mut tmp_buf[..],
        )?;

        output[0..6].copy_from_slice(&tmp_buf[0..6]);

        Ok(())
    }

    #[cfg(test)]
    mod tests {
        use super::*;
        use crate::{
            encode::encode_config_buf,
            encode::encode_config_slice,
            tests::{assert_encode_sanity, random_config},
        };

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

        #[test]
        fn decode_chunk_precise_writes_only_6_bytes() {
            let input = b"Zm9vYmFy"; // "foobar"
            let mut output = [0_u8, 1, 2, 3, 4, 5, 6, 7];
            decode_chunk_precise(&input[..], 0, tables::STANDARD_DECODE, &mut output).unwrap();
            assert_eq!(&vec![b'f', b'o', b'o', b'b', b'a', b'r', 6, 7], &output);
        }

        #[test]
        fn decode_chunk_writes_8_bytes() {
            let input = b"Zm9vYmFy"; // "foobar"
            let mut output = [0_u8, 1, 2, 3, 4, 5, 6, 7];
            decode_chunk(&input[..], 0, tables::STANDARD_DECODE, &mut output).unwrap();
            assert_eq!(&vec![b'f', b'o', b'o', b'b', b'a', b'r', 0, 0], &output);
        }

        #[test]
        fn decode_into_nonempty_vec_doesnt_clobber_existing_prefix() {
            let mut orig_data = Vec::new();
            let mut encoded_data = String::new();
            let mut decoded_with_prefix = Vec::new();
            let mut decoded_without_prefix = Vec::new();
            let mut prefix = Vec::new();

            let prefix_len_range = Uniform::new(0, 1000);
            let input_len_range = Uniform::new(0, 1000);

            let mut rng = rand::rngs::SmallRng::from_entropy();

            for _ in 0..10_000 {
                orig_data.clear();
                encoded_data.clear();
                decoded_with_prefix.clear();
                decoded_without_prefix.clear();
                prefix.clear();

                let input_len = input_len_range.sample(&mut rng);

                for _ in 0..input_len {
                    orig_data.push(rng.gen());
                }

                let config = random_config(&mut rng);
                encode_config_buf(&orig_data, config, &mut encoded_data);
                assert_encode_sanity(&encoded_data, config, input_len);

                let prefix_len = prefix_len_range.sample(&mut rng);

                // fill the buf with a prefix
                for _ in 0..prefix_len {
                    prefix.push(rng.gen());
                }

                decoded_with_prefix.resize(prefix_len, 0);
                decoded_with_prefix.copy_from_slice(&prefix);

                // decode into the non-empty buf
                decode_config_buf(&encoded_data, config, &mut decoded_with_prefix).unwrap();
                // also decode into the empty buf
                decode_config_buf(&encoded_data, config, &mut decoded_without_prefix).unwrap();

                assert_eq!(
                    prefix_len + decoded_without_prefix.len(),
                    decoded_with_prefix.len()
                );
                assert_eq!(orig_data, decoded_without_prefix);

                // append plain decode onto prefix
                prefix.append(&mut decoded_without_prefix);

                assert_eq!(prefix, decoded_with_prefix);
            }
        }

        #[test]
        fn decode_into_slice_doesnt_clobber_existing_prefix_or_suffix() {
            let mut orig_data = Vec::new();
            let mut encoded_data = String::new();
            let mut decode_buf = Vec::new();
            let mut decode_buf_copy: Vec<u8> = Vec::new();

            let input_len_range = Uniform::new(0, 1000);

            let mut rng = rand::rngs::SmallRng::from_entropy();

            for _ in 0..10_000 {
                orig_data.clear();
                encoded_data.clear();
                decode_buf.clear();
                decode_buf_copy.clear();

                let input_len = input_len_range.sample(&mut rng);

                for _ in 0..input_len {
                    orig_data.push(rng.gen());
                }

                let config = random_config(&mut rng);
                encode_config_buf(&orig_data, config, &mut encoded_data);
                assert_encode_sanity(&encoded_data, config, input_len);

                // fill the buffer with random garbage, long enough to have some room before and after
                for _ in 0..5000 {
                    decode_buf.push(rng.gen());
                }

                // keep a copy for later comparison
                decode_buf_copy.extend(decode_buf.iter());

                let offset = 1000;

                // decode into the non-empty buf
                let decode_bytes_written =
                    decode_config_slice(&encoded_data, config, &mut decode_buf[offset..]).unwrap();

                assert_eq!(orig_data.len(), decode_bytes_written);
                assert_eq!(
                    orig_data,
                    &decode_buf[offset..(offset + decode_bytes_written)]
                );
                assert_eq!(&decode_buf_copy[0..offset], &decode_buf[0..offset]);
                assert_eq!(
                    &decode_buf_copy[offset + decode_bytes_written..],
                    &decode_buf[offset + decode_bytes_written..]
                );
            }
        }

        #[test]
        fn decode_into_slice_fits_in_precisely_sized_slice() {
            let mut orig_data = Vec::new();
            let mut encoded_data = String::new();
            let mut decode_buf = Vec::new();

            let input_len_range = Uniform::new(0, 1000);

            let mut rng = rand::rngs::SmallRng::from_entropy();

            for _ in 0..10_000 {
                orig_data.clear();
                encoded_data.clear();
                decode_buf.clear();

                let input_len = input_len_range.sample(&mut rng);

                for _ in 0..input_len {
                    orig_data.push(rng.gen());
                }

                let config = random_config(&mut rng);
                encode_config_buf(&orig_data, config, &mut encoded_data);
                assert_encode_sanity(&encoded_data, config, input_len);

                decode_buf.resize(input_len, 0);

                // decode into the non-empty buf
                let decode_bytes_written =
                    decode_config_slice(&encoded_data, config, &mut decode_buf[..]).unwrap();

                assert_eq!(orig_data.len(), decode_bytes_written);
                assert_eq!(orig_data, decode_buf);
            }
        }

        #[test]
        fn detect_invalid_last_symbol_two_bytes() {
            let decode =
                |input, forgiving| decode_config(input, STANDARD.decode_allow_trailing_bits(forgiving));

            // example from https://github.com/marshallpierce/rust-base64/issues/75
            assert!(decode("iYU=", false).is_ok());
            // trailing 01
            assert_eq!(
                Err(DecodeError::InvalidLastSymbol(2, b'V')),
                decode("iYV=", false)
            );
            assert_eq!(Ok(vec![137, 133]), decode("iYV=", true));
            // trailing 10
            assert_eq!(
                Err(DecodeError::InvalidLastSymbol(2, b'W')),
                decode("iYW=", false)
            );
            assert_eq!(Ok(vec![137, 133]), decode("iYV=", true));
            // trailing 11
            assert_eq!(
                Err(DecodeError::InvalidLastSymbol(2, b'X')),
                decode("iYX=", false)
            );
            assert_eq!(Ok(vec![137, 133]), decode("iYV=", true));

            // also works when there are 2 quads in the last block
            assert_eq!(
                Err(DecodeError::InvalidLastSymbol(6, b'X')),
                decode("AAAAiYX=", false)
            );
            assert_eq!(Ok(vec![0, 0, 0, 137, 133]), decode("AAAAiYX=", true));
        }

        #[test]
        fn detect_invalid_last_symbol_one_byte() {
            // 0xFF -> "/w==", so all letters > w, 0-9, and '+', '/' should get InvalidLastSymbol

            assert!(decode("/w==").is_ok());
            // trailing 01
            assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'x')), decode("/x=="));
            assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'z')), decode("/z=="));
            assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'0')), decode("/0=="));
            assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'9')), decode("/9=="));
            assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'+')), decode("/+=="));
            assert_eq!(Err(DecodeError::InvalidLastSymbol(1, b'/')), decode("//=="));

            // also works when there are 2 quads in the last block
            assert_eq!(
                Err(DecodeError::InvalidLastSymbol(5, b'x')),
                decode("AAAA/x==")
            );
        }

        #[test]
        fn detect_invalid_last_symbol_every_possible_three_symbols() {
            let mut base64_to_bytes = ::std::collections::HashMap::new();

            let mut bytes = [0_u8; 2];
            for b1 in 0_u16..256 {
                bytes[0] = b1 as u8;
                for b2 in 0_u16..256 {
                    bytes[1] = b2 as u8;
                    let mut b64 = vec![0_u8; 4];
                    assert_eq!(4, encode_config_slice(&bytes, STANDARD, &mut b64[..]));
                    let mut v = ::std::vec::Vec::with_capacity(2);
                    v.extend_from_slice(&bytes[..]);

                    assert!(base64_to_bytes.insert(b64, v).is_none());
                }
            }

            // every possible combination of symbols must either decode to 2 bytes or get InvalidLastSymbol

            let mut symbols = [0_u8; 4];
            for &s1 in STANDARD.char_set.encode_table().iter() {
                symbols[0] = s1;
                for &s2 in STANDARD.char_set.encode_table().iter() {
                    symbols[1] = s2;
                    for &s3 in STANDARD.char_set.encode_table().iter() {
                        symbols[2] = s3;
                        symbols[3] = PAD_BYTE;

                        match base64_to_bytes.get(&symbols[..]) {
                            Some(bytes) => {
                                assert_eq!(Ok(bytes.to_vec()), decode_config(&symbols, STANDARD))
                            }
                            None => assert_eq!(
                                Err(DecodeError::InvalidLastSymbol(2, s3)),
                                decode_config(&symbols[..], STANDARD)
                            ),
                        }
                    }
                }
            }
        }

        #[test]
        fn detect_invalid_last_symbol_every_possible_two_symbols() {
            let mut base64_to_bytes = ::std::collections::HashMap::new();

            for b in 0_u16..256 {
                let mut b64 = vec![0_u8; 4];
                assert_eq!(4, encode_config_slice(&[b as u8], STANDARD, &mut b64[..]));
                let mut v = ::std::vec::Vec::with_capacity(1);
                v.push(b as u8);

                assert!(base64_to_bytes.insert(b64, v).is_none());
            }

            // every possible combination of symbols must either decode to 1 byte or get InvalidLastSymbol

            let mut symbols = [0_u8; 4];
            for &s1 in STANDARD.char_set.encode_table().iter() {
                symbols[0] = s1;
                for &s2 in STANDARD.char_set.encode_table().iter() {
                    symbols[1] = s2;
                    symbols[2] = PAD_BYTE;
                    symbols[3] = PAD_BYTE;

                    match base64_to_bytes.get(&symbols[..]) {
                        Some(bytes) => {
                            assert_eq!(Ok(bytes.to_vec()), decode_config(&symbols, STANDARD))
                        }
                        None => assert_eq!(
                            Err(DecodeError::InvalidLastSymbol(1, s2)),
                            decode_config(&symbols[..], STANDARD)
                        ),
                    }
                }
            }
        }
    }