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use bumpalo::collections::{String as BumpString, Vec as BumpVec};
use crate::{arena::Arena, constant::Integer, flat::zigzag::ZigZag};
use super::FlatDecodeError;
pub struct Decoder<'b> {
pub buffer: &'b [u8],
pub used_bits: usize,
pub pos: usize,
}
pub struct Ctx<'a> {
pub arena: &'a Arena,
}
impl<'b> Decoder<'b> {
pub fn new(bytes: &'b [u8]) -> Decoder<'b> {
Decoder {
buffer: bytes,
pos: 0,
used_bits: 0,
}
}
/// Decode a word of any size.
/// This is byte alignment agnostic.
/// First we decode the next 8 bits of the buffer.
/// We take the 7 least significant bits as the 7 least significant bits of
/// the current unsigned integer. If the most significant bit of the 8
/// bits is 1 then we take the next 8 and repeat the process above,
/// filling in the next 7 least significant bits of the unsigned integer and
/// so on. If the most significant bit was instead 0 we stop decoding
/// any more bits.
pub fn word(&mut self) -> Result<usize, FlatDecodeError> {
let mut leading_bit = 1;
let mut final_word: usize = 0;
let mut shl: usize = 0;
// continue looping if lead bit is 1 which is 128 as a u8 otherwise exit
while leading_bit > 0 {
let word8 = self.bits8(8)?;
let word7 = word8 & 127;
final_word |= (word7 as usize) << shl;
shl += 7;
leading_bit = word8 & 128;
}
Ok(final_word)
}
/// Decode a list of items with a decoder function.
/// This is byte alignment agnostic.
/// Decode a bit from the buffer.
/// If 0 then stop.
/// Otherwise we decode an item in the list with the decoder function passed
/// in. Then decode the next bit in the buffer and repeat above.
/// Returns a list of items decoded with the decoder function.
pub fn list_with<'a, T, F>(
&mut self,
ctx: &mut Ctx<'a>,
decoder_func: F,
) -> Result<BumpVec<'a, T>, FlatDecodeError>
where
F: Copy + FnOnce(&mut Ctx<'a>, &mut Decoder) -> Result<T, FlatDecodeError>,
{
let mut vec_array = BumpVec::new_in(ctx.arena.as_bump());
while self.bit()? {
vec_array.push(decoder_func(ctx, self)?)
}
Ok(vec_array)
}
/// Decode up to 8 bits.
/// This is byte alignment agnostic.
/// If num_bits is greater than the 8 we throw an IncorrectNumBits error.
/// First we decode the next num_bits of bits in the buffer.
/// If there are less unused bits in the current byte in the buffer than
/// num_bits, then we decode the remaining bits from the most
/// significant bits in the next byte in the buffer. Otherwise we decode
/// the unused bits from the current byte. Returns the decoded value up
/// to a byte in size.
pub fn bits8(&mut self, num_bits: usize) -> Result<u8, FlatDecodeError> {
if num_bits > 8 {
return Err(FlatDecodeError::IncorrectNumBits);
}
self.ensure_bits(num_bits)?;
let unused_bits = 8 - self.used_bits;
let leading_zeroes = 8 - num_bits;
let r = (self.buffer[self.pos] << self.used_bits) >> leading_zeroes;
let x = if num_bits > unused_bits {
r | (self.buffer[self.pos + 1] >> (unused_bits + leading_zeroes))
} else {
r
};
self.drop_bits(num_bits);
Ok(x)
}
/// Ensures the buffer has the required bits passed in by required_bits.
/// Throws a NotEnoughBits error if there are less bits remaining in the
/// buffer than required_bits.
fn ensure_bits(&mut self, required_bits: usize) -> Result<(), FlatDecodeError> {
if required_bits > (self.buffer.len() - self.pos) * 8 - self.used_bits {
Err(FlatDecodeError::NotEnoughBits(required_bits))
} else {
Ok(())
}
}
/// Increment buffer by num_bits.
/// If num_bits + used bits is greater than 8,
/// then increment position by (num_bits + used bits) / 8
/// Use the left over remainder as the new amount of used bits.
fn drop_bits(&mut self, num_bits: usize) {
let all_used_bits = num_bits + self.used_bits;
self.used_bits = all_used_bits % 8;
self.pos += all_used_bits / 8;
}
/// Decodes a filler of max one byte size.
/// Decodes bits until we hit a bit that is 1.
/// Expects that the 1 is at the end of the current byte in the buffer.
pub fn filler(&mut self) -> Result<(), FlatDecodeError> {
while self.zero()? {}
Ok(())
}
/// Decode the next bit in the buffer.
/// If the bit was 0 then return true.
/// Otherwise return false.
/// Throws EndOfBuffer error if used at the end of the array.
fn zero(&mut self) -> Result<bool, FlatDecodeError> {
let current_bit = self.bit()?;
Ok(!current_bit)
}
/// Decode the next bit in the buffer.
/// If the bit was 1 then return true.
/// Otherwise return false.
/// Throws EndOfBuffer error if used at the end of the array.
pub fn bit(&mut self) -> Result<bool, FlatDecodeError> {
if self.pos >= self.buffer.len() {
return Err(FlatDecodeError::EndOfBuffer);
}
let b = self.buffer[self.pos] & (128 >> self.used_bits) > 0;
self.increment_buffer_by_bit();
Ok(b)
}
/// Decode an integer of an arbitrary size..
///
/// This is byte alignment agnostic.
/// First we decode the next 8 bits of the buffer.
/// We take the 7 least significant bits as the 7 least significant bits of
/// the current unsigned integer. If the most significant bit of the 8
/// bits is 1 then we take the next 8 and repeat the process above,
/// filling in the next 7 least significant bits of the unsigned integer and
/// so on. If the most significant bit was instead 0 we stop decoding
/// any more bits. Finally we use zigzag to convert the unsigned integer
/// back to a signed integer.
pub fn integer(&mut self) -> Result<Integer, FlatDecodeError> {
Ok(ZigZag::unzigzag(&self.big_word()?))
}
/// Decode a word of 128 bits size.
/// This is byte alignment agnostic.
/// First we decode the next 8 bits of the buffer.
/// We take the 7 least significant bits as the 7 least significant bits of
/// the current unsigned integer. If the most significant bit of the 8
/// bits is 1 then we take the next 8 and repeat the process above,
/// filling in the next 7 least significant bits of the unsigned integer and
/// so on. If the most significant bit was instead 0 we stop decoding
/// any more bits.
pub fn big_word(&mut self) -> Result<Integer, FlatDecodeError> {
let mut leading_bit = 1;
let mut final_word = Integer::from(0);
let mut shift = 0_u32; // Using u32 for shift as it's more than enough for 128 bits
// Continue looping if lead bit is 1 (0x80) otherwise exit
while leading_bit > 0 {
let word8 = self.bits8(8)?;
let word7 = word8 & 0x7F; // 127, get 7 least significant bits
// Create temporary Integer from word7 and shift it
let part = Integer::from(word7);
let shifted_part = part << shift;
// OR it with our result
final_word |= shifted_part;
// Increment shift by 7 for next iteration
shift += 7;
// Check if we should continue (MSB set)
leading_bit = word8 & 0x80; // 128
}
Ok(final_word)
}
/// Decode a byte array.
/// Decodes a filler to byte align the buffer,
/// then decodes the next byte to get the array length up to a max of 255.
/// We decode bytes equal to the array length to form the byte array.
/// If the following byte for array length is not 0 we decode it and repeat
/// above to continue decoding the byte array. We stop once we hit a
/// byte array length of 0. If array length is 0 for first byte array
/// length the we return a empty array.
pub fn bytes<'a>(&mut self, arena: &'a Arena) -> Result<BumpVec<'a, u8>, FlatDecodeError> {
self.filler()?;
self.byte_array(arena)
}
/// Decode a byte array.
/// Throws a BufferNotByteAligned error if the buffer is not byte aligned
/// Decodes the next byte to get the array length up to a max of 255.
/// We decode bytes equal to the array length to form the byte array.
/// If the following byte for array length is not 0 we decode it and repeat
/// above to continue decoding the byte array. We stop once we hit a
/// byte array length of 0. If array length is 0 for first byte array
/// length the we return a empty array.
fn byte_array<'a>(&mut self, arena: &'a Arena) -> Result<BumpVec<'a, u8>, FlatDecodeError> {
if self.used_bits != 0 {
return Err(FlatDecodeError::BufferNotByteAligned);
}
self.ensure_bytes(1)?;
let mut blk_len = self.buffer[self.pos] as usize;
self.pos += 1;
let mut blk_array = BumpVec::with_capacity_in(blk_len, arena.as_bump());
while blk_len != 0 {
self.ensure_bytes(blk_len + 1)?;
let decoded_array = &self.buffer[self.pos..self.pos + blk_len];
blk_array.extend(decoded_array);
self.pos += blk_len;
blk_len = self.buffer[self.pos] as usize;
self.pos += 1
}
Ok(blk_array)
}
/// Decode a string.
/// Convert to byte array and then use byte array decoding.
/// Decodes a filler to byte align the buffer,
/// then decodes the next byte to get the array length up to a max of 255.
/// We decode bytes equal to the array length to form the byte array.
/// If the following byte for array length is not 0 we decode it and repeat
/// above to continue decoding the byte array. We stop once we hit a
/// byte array length of 0. If array length is 0 for first byte array
/// length the we return a empty array.
pub fn utf8<'a>(&mut self, arena: &'a Arena) -> Result<&'a str, FlatDecodeError> {
let b = self.bytes(arena)?;
let s =
BumpString::from_utf8(b).map_err(|e| FlatDecodeError::DecodeUtf8(e.utf8_error()))?;
let s = arena.alloc(s);
Ok(s)
}
/// Increment used bits by 1.
/// If all 8 bits are used then increment buffer position by 1.
fn increment_buffer_by_bit(&mut self) {
if self.used_bits == 7 {
self.pos += 1;
self.used_bits = 0;
} else {
self.used_bits += 1;
}
}
/// Ensures the buffer has the required bytes passed in by required_bytes.
/// Throws a NotEnoughBytes error if there are less bytes remaining in the
/// buffer than required_bytes.
fn ensure_bytes(&mut self, required_bytes: usize) -> Result<(), FlatDecodeError> {
if required_bytes > self.buffer.len() - self.pos {
Err(FlatDecodeError::NotEnoughBytes(required_bytes))
} else {
Ok(())
}
}
}