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
use crate::{tables, Config};

#[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.
    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 => 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 == 0x3D {
            // 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,
                0x3D,
            ));
        }
        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] = b'=';

                    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] = b'=';
                symbols[3] = b'=';

                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)
                    ),
                }
            }
        }
    }

    #[test]
    fn decode_imap() {
        assert_eq!(
            decode_config(b"+,,+", crate::IMAP_MUTF7),
            decode_config(b"+//+", crate::STANDARD_NO_PAD)
        );
    }
}