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use crate::{
engine::{general_purpose::INVALID_VALUE, DecodeEstimate, DecodeMetadata, DecodePaddingMode},
DecodeError, PAD_BYTE,
};
// 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;
#[doc(hidden)]
pub struct GeneralPurposeEstimate {
/// Total number of decode chunks, including a possibly partial last chunk
num_chunks: usize,
decoded_len_estimate: usize,
}
impl GeneralPurposeEstimate {
pub(crate) fn new(encoded_len: usize) -> Self {
// Formulas that won't overflow
Self {
num_chunks: encoded_len / INPUT_CHUNK_LEN
+ (encoded_len % INPUT_CHUNK_LEN > 0) as usize,
decoded_len_estimate: (encoded_len / 4 + (encoded_len % 4 > 0) as usize) * 3,
}
}
}
impl DecodeEstimate for GeneralPurposeEstimate {
fn decoded_len_estimate(&self) -> usize {
self.decoded_len_estimate
}
}
/// Helper to avoid duplicating num_chunks calculation, which is costly on short inputs.
/// Returns the decode metadata, 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]
pub(crate) fn decode_helper(
input: &[u8],
estimate: GeneralPurposeEstimate,
output: &mut [u8],
decode_table: &[u8; 256],
decode_allow_trailing_bits: bool,
padding_mode: DecodePaddingMode,
) -> Result<DecodeMetadata, DecodeError> {
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] == 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 un-padded 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 = estimate.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);
super::decode_suffix::decode_suffix(
input,
input_index,
output,
output_index,
decode_table,
decode_allow_trailing_bits,
padding_mode,
)
}
/// 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 morsel = decode_table[input[0] as usize];
if morsel == INVALID_VALUE {
return Err(DecodeError::InvalidByte(index_at_start_of_input, input[0]));
}
let mut accum = (morsel as u64) << 58;
let morsel = decode_table[input[1] as usize];
if morsel == 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 == 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 == 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 == 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 == 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 == 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 == 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(())
}
#[inline]
fn write_u64(output: &mut [u8], value: u64) {
output[..8].copy_from_slice(&value.to_be_bytes());
}
#[cfg(test)]
mod tests {
use super::*;
use crate::engine::general_purpose::STANDARD;
#[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, &STANDARD.decode_table, &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, &STANDARD.decode_table, &mut output).unwrap();
assert_eq!(&vec![b'f', b'o', b'o', b'b', b'a', b'r', 0, 0], &output);
}
#[test]
fn estimate_short_lengths() {
for (range, (num_chunks, decoded_len_estimate)) in [
(0..=0, (0, 0)),
(1..=4, (1, 3)),
(5..=8, (1, 6)),
(9..=12, (2, 9)),
(13..=16, (2, 12)),
(17..=20, (3, 15)),
] {
for encoded_len in range {
let estimate = GeneralPurposeEstimate::new(encoded_len);
assert_eq!(num_chunks, estimate.num_chunks);
assert_eq!(decoded_len_estimate, estimate.decoded_len_estimate);
}
}
}
#[test]
fn estimate_via_u128_inflation() {
// cover both ends of usize
(0..1000)
.chain(usize::MAX - 1000..=usize::MAX)
.for_each(|encoded_len| {
// inflate to 128 bit type to be able to safely use the easy formulas
let len_128 = encoded_len as u128;
let estimate = GeneralPurposeEstimate::new(encoded_len);
assert_eq!(
((len_128 + (INPUT_CHUNK_LEN - 1) as u128) / (INPUT_CHUNK_LEN as u128))
as usize,
estimate.num_chunks
);
assert_eq!(
((len_128 + 3) / 4 * 3) as usize,
estimate.decoded_len_estimate
);
})
}
}