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use super::{stream::FormatErrorInner, DecodingError, CHUNK_BUFFER_SIZE};
use fdeflate::Decompressor;
/// Ergonomics wrapper around `miniz_oxide::inflate::stream` for zlib compressed data.
pub(super) struct ZlibStream {
/// Current decoding state.
state: Box<fdeflate::Decompressor>,
/// If there has been a call to decompress already.
started: bool,
/// Remaining buffered decoded bytes.
/// The decoder sometimes wants inspect some already finished bytes for further decoding. So we
/// keep a total of 32KB of decoded data available as long as more data may be appended.
out_buffer: Vec<u8>,
/// The first index of `out_buffer` where new data can be written.
out_pos: usize,
/// The first index of `out_buffer` that hasn't yet been passed to our client
/// (i.e. not yet appended to the `image_data` parameter of `fn decompress` or `fn
/// finish_compressed_chunks`).
read_pos: usize,
/// Limit on how many bytes can be decompressed in total. This field is mostly used for
/// performance optimizations (e.g. to avoid allocating and zeroing out large buffers when only
/// a small image is being decoded).
max_total_output: usize,
/// Ignore and do not calculate the Adler-32 checksum. Defaults to `true`.
///
/// This flag overrides `TINFL_FLAG_COMPUTE_ADLER32`.
///
/// This flag should not be modified after decompression has started.
ignore_adler32: bool,
}
impl ZlibStream {
pub(crate) fn new() -> Self {
ZlibStream {
state: Box::new(Decompressor::new()),
started: false,
out_buffer: Vec::new(),
out_pos: 0,
read_pos: 0,
max_total_output: usize::MAX,
ignore_adler32: true,
}
}
pub(crate) fn reset(&mut self) {
self.started = false;
self.out_buffer.clear();
self.out_pos = 0;
self.read_pos = 0;
self.max_total_output = usize::MAX;
*self.state = Decompressor::new();
}
pub(crate) fn set_max_total_output(&mut self, n: usize) {
self.max_total_output = n;
}
/// Set the `ignore_adler32` flag and return `true` if the flag was
/// successfully set.
///
/// The default is `true`.
///
/// This flag cannot be modified after decompression has started until the
/// [ZlibStream] is reset.
pub(crate) fn set_ignore_adler32(&mut self, flag: bool) -> bool {
if !self.started {
self.ignore_adler32 = flag;
true
} else {
false
}
}
/// Return the `ignore_adler32` flag.
pub(crate) fn ignore_adler32(&self) -> bool {
self.ignore_adler32
}
/// Fill the decoded buffer as far as possible from `data`.
/// On success returns the number of consumed input bytes.
pub(crate) fn decompress(
&mut self,
data: &[u8],
image_data: &mut Vec<u8>,
) -> Result<usize, DecodingError> {
// There may be more data past the adler32 checksum at the end of the deflate stream. We
// match libpng's default behavior and ignore any trailing data. In the future we may want
// to add a flag to control this behavior.
if self.state.is_done() {
return Ok(data.len());
}
self.prepare_vec_for_appending();
if !self.started && self.ignore_adler32 {
self.state.ignore_adler32();
}
let (in_consumed, out_consumed) = self
.state
.read(data, self.out_buffer.as_mut_slice(), self.out_pos, false)
.map_err(|err| {
DecodingError::Format(FormatErrorInner::CorruptFlateStream { err }.into())
})?;
self.started = true;
self.out_pos += out_consumed;
self.transfer_finished_data(image_data);
self.compact_out_buffer_if_needed();
Ok(in_consumed)
}
/// Called after all consecutive IDAT chunks were handled.
///
/// The compressed stream can be split on arbitrary byte boundaries. This enables some cleanup
/// within the decompressor and flushing additional data which may have been kept back in case
/// more data were passed to it.
pub(crate) fn finish_compressed_chunks(
&mut self,
image_data: &mut Vec<u8>,
) -> Result<(), DecodingError> {
if !self.started {
return Ok(());
}
while !self.state.is_done() {
self.prepare_vec_for_appending();
let (_in_consumed, out_consumed) = self
.state
.read(&[], self.out_buffer.as_mut_slice(), self.out_pos, true)
.map_err(|err| {
DecodingError::Format(FormatErrorInner::CorruptFlateStream { err }.into())
})?;
self.out_pos += out_consumed;
if !self.state.is_done() {
let transferred = self.transfer_finished_data(image_data);
assert!(
transferred > 0 || out_consumed > 0,
"No more forward progress made in stream decoding."
);
self.compact_out_buffer_if_needed();
}
}
self.transfer_finished_data(image_data);
self.out_buffer.clear();
Ok(())
}
/// Resize the vector to allow allocation of more data.
fn prepare_vec_for_appending(&mut self) {
// The `debug_assert` below explains why we can use `>=` instead of `>` in the condition
// that compares `self.out_post >= self.max_total_output` in the next `if` statement.
debug_assert!(!self.state.is_done());
if self.out_pos >= self.max_total_output {
// This can happen when the `max_total_output` was miscalculated (e.g.
// because the `IHDR` chunk was malformed and didn't match the `IDAT` chunk). In
// this case, let's reset `self.max_total_output` before further calculations.
self.max_total_output = usize::MAX;
}
let current_len = self.out_buffer.len();
let desired_len = self
.out_pos
.saturating_add(CHUNK_BUFFER_SIZE)
.min(self.max_total_output);
if current_len >= desired_len {
return;
}
let buffered_len = self.decoding_size(self.out_buffer.len());
debug_assert!(self.out_buffer.len() <= buffered_len);
self.out_buffer.resize(buffered_len, 0u8);
}
fn decoding_size(&self, len: usize) -> usize {
// Allocate one more chunk size than currently or double the length while ensuring that the
// allocation is valid and that any cursor within it will be valid.
len
// This keeps the buffer size a power-of-two, required by miniz_oxide.
.saturating_add(CHUNK_BUFFER_SIZE.max(len))
// Ensure all buffer indices are valid cursor positions.
// Note: both cut off and zero extension give correct results.
.min(u64::MAX as usize)
// Ensure the allocation request is valid.
// TODO: maximum allocation limits?
.min(isize::MAX as usize)
// Don't unnecessarily allocate more than `max_total_output`.
.min(self.max_total_output)
}
fn transfer_finished_data(&mut self, image_data: &mut Vec<u8>) -> usize {
let transferred = &self.out_buffer[self.read_pos..self.out_pos];
image_data.extend_from_slice(transferred);
self.read_pos = self.out_pos;
transferred.len()
}
fn compact_out_buffer_if_needed(&mut self) {
// [PNG spec](https://www.w3.org/TR/2003/REC-PNG-20031110/#10Compression) says that
// "deflate/inflate compression with a sliding window (which is an upper bound on the
// distances appearing in the deflate stream) of at most 32768 bytes".
//
// `fdeflate` requires that we keep this many most recently decompressed bytes in the
// `out_buffer` - this allows referring back to them when handling "length and distance
// codes" in the deflate stream).
const LOOKBACK_SIZE: usize = 32768;
// Compact `self.out_buffer` when "needed". Doing this conditionally helps to put an upper
// bound on the amortized cost of copying the data within `self.out_buffer`.
//
// TODO: The factor of 4 is an ad-hoc heuristic. Consider measuring and using a different
// factor. (Early experiments seem to indicate that factor of 4 is faster than a factor of
// 2 and 4 * `LOOKBACK_SIZE` seems like an acceptable memory trade-off. Higher factors
// result in higher memory usage, but the compaction cost is lower - factor of 4 means
// that 1 byte gets copied during compaction for 3 decompressed bytes.)
if self.out_pos > LOOKBACK_SIZE * 4 {
// Only preserve the `lookback_buffer` and "throw away" the earlier prefix.
let lookback_buffer = self.out_pos.saturating_sub(LOOKBACK_SIZE)..self.out_pos;
let preserved_len = lookback_buffer.len();
self.out_buffer.copy_within(lookback_buffer, 0);
self.read_pos = preserved_len;
self.out_pos = preserved_len;
}
}
}