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use crate::{
engine::{general_purpose::INVALID_VALUE, DecodeMetadata, DecodePaddingMode},
DecodeError, DecodeSliceError, PAD_BYTE,
};
/// Decode the last 0-4 bytes, checking for trailing set bits and padding per the provided
/// parameters.
///
/// Returns the decode metadata representing the total number of bytes decoded, including the ones
/// indicated as already written by `output_index`.
pub(crate) fn decode_suffix(
input: &[u8],
input_index: usize,
output: &mut [u8],
mut output_index: usize,
decode_table: &[u8; 256],
decode_allow_trailing_bits: bool,
padding_mode: DecodePaddingMode,
) -> Result<DecodeMetadata, DecodeSliceError> {
debug_assert!((input.len() - input_index) <= 4);
// Decode any leftovers that might not be a complete input chunk of 4 bytes.
// Use a u32 as a stack-resident 4 byte buffer.
let mut morsels_in_leftover = 0;
let mut padding_bytes_count = 0;
// offset from input_index
let mut first_padding_offset: usize = 0;
let mut last_symbol = 0_u8;
let mut morsels = [0_u8; 4];
for (leftover_index, &b) in input[input_index..].iter().enumerate() {
// '=' padding
if b == PAD_BYTE {
// There can be bad padding bytes in a few ways:
// 1 - Padding with non-padding characters after it
// 2 - Padding after zero or one characters in the current quad (should only
// be after 2 or 3 chars)
// 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 an invalid byte
// in the first quad.
// 4 - Non-canonical padding -- 1 byte when it should be 2, etc.
// Per config, non-canonical but still functional non- or partially-padded base64
// may be treated as an error condition.
if leftover_index < 2 {
// Check for error #2.
// Either the previous byte was padding, in which case we would have already hit
// this case, or it wasn't, in which case this is the first such error.
debug_assert!(
leftover_index == 0 || (leftover_index == 1 && padding_bytes_count == 0)
);
let bad_padding_index = input_index + leftover_index;
return Err(DecodeError::InvalidByte(bad_padding_index, b).into());
}
if padding_bytes_count == 0 {
first_padding_offset = leftover_index;
}
padding_bytes_count += 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_count > 0 {
return Err(
DecodeError::InvalidByte(input_index + first_padding_offset, PAD_BYTE).into(),
);
}
last_symbol = b;
// can use up to 8 * 6 = 48 bits of the u64, if last chunk has no padding.
// Pack the leftovers from left to right.
let morsel = decode_table[b as usize];
if morsel == INVALID_VALUE {
return Err(DecodeError::InvalidByte(input_index + leftover_index, b).into());
}
morsels[morsels_in_leftover] = morsel;
morsels_in_leftover += 1;
}
// If there was 1 trailing byte, and it was valid, and we got to this point without hitting
// an invalid byte, now we can report invalid length
if !input.is_empty() && morsels_in_leftover < 2 {
return Err(DecodeError::InvalidLength(input_index + morsels_in_leftover).into());
}
match padding_mode {
DecodePaddingMode::Indifferent => { /* everything we care about was already checked */ }
DecodePaddingMode::RequireCanonical => {
// allow empty input
if (padding_bytes_count + morsels_in_leftover) % 4 != 0 {
return Err(DecodeError::InvalidPadding.into());
}
}
DecodePaddingMode::RequireNone => {
if padding_bytes_count > 0 {
// check at the end to make sure we let the cases of padding that should be InvalidByte
// get hit
return Err(DecodeError::InvalidPadding.into());
}
}
}
// When encoding 1 trailing byte (e.g. 0xFF), 2 base64 bytes ("/w") are needed.
// / is the symbol for 63 (0x3F, bottom 6 bits all set) and w is 48 (0x30, top 2 bits
// of bottom 6 bits set).
// When decoding two symbols back to one trailing byte, any final symbol higher than
// w would still decode to the original byte because we only care about the top two
// bits in the bottom 6, but would be a non-canonical encoding. So, we calculate a
// mask based on how many bits are used for just the canonical encoding, and optionally
// error if any other bits are set. In the example of one encoded byte -> 2 symbols,
// 2 symbols can technically encode 12 bits, but the last 4 are non-canonical, and
// useless since there are no more symbols to provide the necessary 4 additional bits
// to finish the second original byte.
let leftover_bytes_to_append = morsels_in_leftover * 6 / 8;
// Put the up to 6 complete bytes as the high bytes.
// Gain a couple percent speedup from nudging these ORs to use more ILP with a two-way split.
let mut leftover_num = (u32::from(morsels[0]) << 26)
| (u32::from(morsels[1]) << 20)
| (u32::from(morsels[2]) << 14)
| (u32::from(morsels[3]) << 8);
// 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_u32 >> (leftover_bytes_to_append * 8);
if !decode_allow_trailing_bits && (leftover_num & mask) != 0 {
// last morsel is at `morsels_in_leftover` - 1
return Err(DecodeError::InvalidLastSymbol(
input_index + morsels_in_leftover - 1,
last_symbol,
)
.into());
}
// Strangely, this approach benchmarks better than writing bytes one at a time,
// or copy_from_slice into output.
for _ in 0..leftover_bytes_to_append {
let hi_byte = (leftover_num >> 24) as u8;
leftover_num <<= 8;
*output
.get_mut(output_index)
.ok_or(DecodeSliceError::OutputSliceTooSmall)? = hi_byte;
output_index += 1;
}
Ok(DecodeMetadata::new(
output_index,
if padding_bytes_count > 0 {
Some(input_index + first_padding_offset)
} else {
None
},
))
}