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use crate::{
alphabet::Alphabet,
engine::{
general_purpose::{self, decode_table, encode_table},
Config, DecodeEstimate, DecodeMetadata, DecodePaddingMode, Engine,
},
DecodeError, PAD_BYTE,
};
use alloc::ops::BitOr;
use std::ops::{BitAnd, Shl, Shr};
/// Comparatively simple implementation that can be used as something to compare against in tests
pub struct Naive {
encode_table: [u8; 64],
decode_table: [u8; 256],
config: NaiveConfig,
}
impl Naive {
const ENCODE_INPUT_CHUNK_SIZE: usize = 3;
const DECODE_INPUT_CHUNK_SIZE: usize = 4;
pub const fn new(alphabet: &Alphabet, config: NaiveConfig) -> Self {
Self {
encode_table: encode_table(alphabet),
decode_table: decode_table(alphabet),
config,
}
}
fn decode_byte_into_u32(&self, offset: usize, byte: u8) -> Result<u32, DecodeError> {
let decoded = self.decode_table[byte as usize];
if decoded == general_purpose::INVALID_VALUE {
return Err(DecodeError::InvalidByte(offset, byte));
}
Ok(decoded as u32)
}
}
impl Engine for Naive {
type Config = NaiveConfig;
type DecodeEstimate = NaiveEstimate;
fn internal_encode(&self, input: &[u8], output: &mut [u8]) -> usize {
// complete chunks first
const LOW_SIX_BITS: u32 = 0x3F;
let rem = input.len() % Self::ENCODE_INPUT_CHUNK_SIZE;
// will never underflow
let complete_chunk_len = input.len() - rem;
let mut input_index = 0_usize;
let mut output_index = 0_usize;
if let Some(last_complete_chunk_index) =
complete_chunk_len.checked_sub(Self::ENCODE_INPUT_CHUNK_SIZE)
{
while input_index <= last_complete_chunk_index {
let chunk = &input[input_index..input_index + Self::ENCODE_INPUT_CHUNK_SIZE];
// populate low 24 bits from 3 bytes
let chunk_int: u32 =
(chunk[0] as u32).shl(16) | (chunk[1] as u32).shl(8) | (chunk[2] as u32);
// encode 4x 6-bit output bytes
output[output_index] = self.encode_table[chunk_int.shr(18) as usize];
output[output_index + 1] =
self.encode_table[chunk_int.shr(12_u8).bitand(LOW_SIX_BITS) as usize];
output[output_index + 2] =
self.encode_table[chunk_int.shr(6_u8).bitand(LOW_SIX_BITS) as usize];
output[output_index + 3] =
self.encode_table[chunk_int.bitand(LOW_SIX_BITS) as usize];
input_index += Self::ENCODE_INPUT_CHUNK_SIZE;
output_index += 4;
}
}
// then leftovers
if rem == 2 {
let chunk = &input[input_index..input_index + 2];
// high six bits of chunk[0]
output[output_index] = self.encode_table[chunk[0].shr(2) as usize];
// bottom 2 bits of [0], high 4 bits of [1]
output[output_index + 1] =
self.encode_table[(chunk[0].shl(4_u8).bitor(chunk[1].shr(4_u8)) as u32)
.bitand(LOW_SIX_BITS) as usize];
// bottom 4 bits of [1], with the 2 bottom bits as zero
output[output_index + 2] =
self.encode_table[(chunk[1].shl(2_u8) as u32).bitand(LOW_SIX_BITS) as usize];
output_index += 3;
} else if rem == 1 {
let byte = input[input_index];
output[output_index] = self.encode_table[byte.shr(2) as usize];
output[output_index + 1] =
self.encode_table[(byte.shl(4_u8) as u32).bitand(LOW_SIX_BITS) as usize];
output_index += 2;
}
output_index
}
fn internal_decoded_len_estimate(&self, input_len: usize) -> Self::DecodeEstimate {
NaiveEstimate::new(input_len)
}
fn internal_decode(
&self,
input: &[u8],
output: &mut [u8],
estimate: Self::DecodeEstimate,
) -> Result<DecodeMetadata, DecodeError> {
if estimate.rem == 1 {
// 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
&& self.decode_table[*b as usize] == general_purpose::INVALID_VALUE
{
return Err(DecodeError::InvalidByte(input.len() - 1, *b));
}
}
return Err(DecodeError::InvalidLength);
}
let mut input_index = 0_usize;
let mut output_index = 0_usize;
const BOTTOM_BYTE: u32 = 0xFF;
// can only use the main loop on non-trailing chunks
if input.len() > Self::DECODE_INPUT_CHUNK_SIZE {
// skip the last chunk, whether it's partial or full, since it might
// have padding, and start at the beginning of the chunk before that
let last_complete_chunk_start_index = estimate.complete_chunk_len
- if estimate.rem == 0 {
// Trailing chunk is also full chunk, so there must be at least 2 chunks, and
// this won't underflow
Self::DECODE_INPUT_CHUNK_SIZE * 2
} else {
// Trailing chunk is partial, so it's already excluded in
// complete_chunk_len
Self::DECODE_INPUT_CHUNK_SIZE
};
while input_index <= last_complete_chunk_start_index {
let chunk = &input[input_index..input_index + Self::DECODE_INPUT_CHUNK_SIZE];
let decoded_int: u32 = self.decode_byte_into_u32(input_index, chunk[0])?.shl(18)
| self
.decode_byte_into_u32(input_index + 1, chunk[1])?
.shl(12)
| self.decode_byte_into_u32(input_index + 2, chunk[2])?.shl(6)
| self.decode_byte_into_u32(input_index + 3, chunk[3])?;
output[output_index] = decoded_int.shr(16_u8).bitand(BOTTOM_BYTE) as u8;
output[output_index + 1] = decoded_int.shr(8_u8).bitand(BOTTOM_BYTE) as u8;
output[output_index + 2] = decoded_int.bitand(BOTTOM_BYTE) as u8;
input_index += Self::DECODE_INPUT_CHUNK_SIZE;
output_index += 3;
}
}
general_purpose::decode_suffix::decode_suffix(
input,
input_index,
output,
output_index,
&self.decode_table,
self.config.decode_allow_trailing_bits,
self.config.decode_padding_mode,
)
}
fn config(&self) -> &Self::Config {
&self.config
}
}
pub struct NaiveEstimate {
/// remainder from dividing input by `Naive::DECODE_CHUNK_SIZE`
rem: usize,
/// Length of input that is in complete `Naive::DECODE_CHUNK_SIZE`-length chunks
complete_chunk_len: usize,
}
impl NaiveEstimate {
fn new(input_len: usize) -> Self {
let rem = input_len % Naive::DECODE_INPUT_CHUNK_SIZE;
let complete_chunk_len = input_len - rem;
Self {
rem,
complete_chunk_len,
}
}
}
impl DecodeEstimate for NaiveEstimate {
fn decoded_len_estimate(&self) -> usize {
((self.complete_chunk_len / 4) + ((self.rem > 0) as usize)) * 3
}
}
#[derive(Clone, Copy, Debug)]
pub struct NaiveConfig {
pub encode_padding: bool,
pub decode_allow_trailing_bits: bool,
pub decode_padding_mode: DecodePaddingMode,
}
impl Config for NaiveConfig {
fn encode_padding(&self) -> bool {
self.encode_padding
}
}