| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/snap-0.2.5/src/decompress.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>decompress.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../normalize.css"><link rel="stylesheet" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../ayu.css" disabled><link rel="stylesheet" href="../../dark.css" disabled><link rel="stylesheet" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script defer src="../../source-script.js"></script><script defer src="../../source-files.js"></script><script defer src="../../main.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../snap/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../snap/index.html"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| </pre><pre class="rust"><code><span class="kw">use </span>std::ptr; |
| |
| <span class="kw">use </span>byteorder::{ByteOrder, LittleEndian <span class="kw">as </span>LE}; |
| |
| <span class="kw">use </span>error::{Error, <span class="prelude-ty">Result</span>}; |
| <span class="kw">use </span>tag; |
| <span class="kw">use </span>varint::read_varu64; |
| <span class="kw">use </span>MAX_INPUT_SIZE; |
| |
| <span class="doccomment">/// A lookup table for quickly computing the various attributes derived from a |
| /// tag byte. |
| </span><span class="kw">const </span>TAG_LOOKUP_TABLE: TagLookupTable = TagLookupTable(tag::TAG_LOOKUP_TABLE); |
| |
| <span class="doccomment">/// `WORD_MASK` is a map from the size of an integer in bytes to its |
| /// corresponding on a 32 bit integer. This is used when we need to read an |
| /// integer and we know there are at least 4 bytes to read from a buffer. In |
| /// this case, we can read a 32 bit little endian integer and mask out only the |
| /// bits we need. This in particular saves a branch. |
| </span><span class="kw">const </span>WORD_MASK: [usize; <span class="number">5</span>] = [<span class="number">0</span>, <span class="number">0xFF</span>, <span class="number">0xFFFF</span>, <span class="number">0xFFFFFF</span>, <span class="number">0xFFFFFFFF</span>]; |
| |
| <span class="doccomment">/// Returns the decompressed size (in bytes) of the compressed bytes given. |
| /// |
| /// `input` must be a sequence of bytes returned by a conforming Snappy |
| /// compressor. |
| /// |
| /// # Errors |
| /// |
| /// This function returns an error in the following circumstances: |
| /// |
| /// * An invalid Snappy header was seen. |
| /// * The total space required for decompression exceeds `2^32 - 1`. |
| </span><span class="kw">pub fn </span>decompress_len(input: <span class="kw-2">&</span>[u8]) -> <span class="prelude-ty">Result</span><usize> { |
| <span class="kw">if </span>input.is_empty() { |
| <span class="kw">return </span><span class="prelude-val">Ok</span>(<span class="number">0</span>); |
| } |
| <span class="prelude-val">Ok</span>(<span class="macro">try!</span>(Header::read(input)).decompress_len) |
| } |
| |
| <span class="doccomment">/// Decoder is a raw decoder for decompressing bytes in the Snappy format. |
| /// |
| /// This decoder does not use the Snappy frame format and simply decompresses |
| /// the given bytes as if it were returned from `Encoder`. |
| /// |
| /// Unless you explicitly need the low-level control, you should use |
| /// `Reader` instead, which decompresses the Snappy frame format. |
| </span><span class="attribute">#[derive(Clone, Debug, Default)] |
| </span><span class="kw">pub struct </span>Decoder { |
| <span class="comment">// Place holder for potential future fields. |
| </span>_dummy: (), |
| } |
| |
| <span class="kw">impl </span>Decoder { |
| <span class="doccomment">/// Return a new decoder that can be used for decompressing bytes. |
| </span><span class="kw">pub fn </span>new() -> Decoder { |
| Decoder { _dummy: () } |
| } |
| |
| <span class="doccomment">/// Decompresses all bytes in `input` into `output`. |
| /// |
| /// `input` must be a sequence of bytes returned by a conforming Snappy |
| /// compressor. |
| /// |
| /// The size of `output` must be large enough to hold all decompressed |
| /// bytes from the `input`. The size required can be queried with the |
| /// `decompress_len` function. |
| /// |
| /// On success, this returns the number of bytes written to `output`. |
| /// |
| /// # Errors |
| /// |
| /// This method returns an error in the following circumstances: |
| /// |
| /// * Invalid compressed Snappy data was seen. |
| /// * The total space required for decompression exceeds `2^32 - 1`. |
| /// * `output` has length less than `decompress_len(input)`. |
| </span><span class="kw">pub fn </span>decompress( |
| <span class="kw-2">&mut </span><span class="self">self</span>, |
| input: <span class="kw-2">&</span>[u8], |
| output: <span class="kw-2">&mut </span>[u8], |
| ) -> <span class="prelude-ty">Result</span><usize> { |
| <span class="kw">if </span>input.is_empty() { |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::Empty); |
| } |
| <span class="kw">let </span>hdr = <span class="macro">try!</span>(Header::read(input)); |
| <span class="kw">if </span>hdr.decompress_len > output.len() { |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::BufferTooSmall { |
| given: output.len() <span class="kw">as </span>u64, |
| min: hdr.decompress_len <span class="kw">as </span>u64, |
| }); |
| } |
| <span class="kw">let </span>dst = <span class="kw-2">&mut </span>output[..hdr.decompress_len]; |
| <span class="kw">let </span><span class="kw-2">mut </span>dec = Decompress { |
| src: <span class="kw-2">&</span>input[hdr.len..], |
| s: <span class="number">0</span>, |
| dst: dst, |
| d: <span class="number">0</span>, |
| }; |
| <span class="macro">try!</span>(dec.decompress()); |
| <span class="prelude-val">Ok</span>(dec.dst.len()) |
| } |
| |
| <span class="doccomment">/// Decompresses all bytes in `input` into a freshly allocated `Vec`. |
| /// |
| /// This is just like the `decompress` method, except it allocates a `Vec` |
| /// with the right size for you. (This is intended to be a convenience |
| /// method.) |
| /// |
| /// This method returns an error under the same circumstances that |
| /// `decompress` does. |
| </span><span class="kw">pub fn </span>decompress_vec(<span class="kw-2">&mut </span><span class="self">self</span>, input: <span class="kw-2">&</span>[u8]) -> <span class="prelude-ty">Result</span><Vec<u8>> { |
| <span class="kw">let </span><span class="kw-2">mut </span>buf = <span class="macro">vec!</span>[<span class="number">0</span>; <span class="macro">try!</span>(decompress_len(input))]; |
| <span class="kw">let </span>n = <span class="macro">try!</span>(<span class="self">self</span>.decompress(input, <span class="kw-2">&mut </span>buf)); |
| buf.truncate(n); |
| <span class="prelude-val">Ok</span>(buf) |
| } |
| } |
| |
| <span class="doccomment">/// Decompress is the state of the Snappy compressor. |
| </span><span class="kw">struct </span>Decompress<<span class="lifetime">'s</span>, <span class="lifetime">'d</span>> { |
| <span class="doccomment">/// The original compressed bytes not including the header. |
| </span>src: <span class="kw-2">&</span><span class="lifetime">'s </span>[u8], |
| <span class="doccomment">/// The current position in the compressed bytes. |
| </span>s: usize, |
| <span class="doccomment">/// The output buffer to write the decompressed bytes. |
| </span>dst: <span class="kw-2">&</span><span class="lifetime">'d </span><span class="kw-2">mut </span>[u8], |
| <span class="doccomment">/// The current position in the decompressed buffer. |
| </span>d: usize, |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'s</span>, <span class="lifetime">'d</span>> Decompress<<span class="lifetime">'s</span>, <span class="lifetime">'d</span>> { |
| <span class="doccomment">/// Decompresses snappy compressed bytes in `src` to `dst`. |
| /// |
| /// This assumes that the header has already been read and that `dst` is |
| /// big enough to store all decompressed bytes. |
| </span><span class="kw">fn </span>decompress(<span class="kw-2">&mut </span><span class="self">self</span>) -> <span class="prelude-ty">Result</span><()> { |
| <span class="kw">while </span><span class="self">self</span>.s < <span class="self">self</span>.src.len() { |
| <span class="kw">let </span>byte = <span class="self">self</span>.src[<span class="self">self</span>.s]; |
| <span class="self">self</span>.s += <span class="number">1</span>; |
| <span class="kw">if </span>byte & <span class="number">0b000000_11 </span>== <span class="number">0 </span>{ |
| <span class="kw">let </span>len = (byte >> <span class="number">2</span>) <span class="kw">as </span>usize + <span class="number">1</span>; |
| <span class="macro">try!</span>(<span class="self">self</span>.read_literal(len)); |
| } <span class="kw">else </span>{ |
| <span class="macro">try!</span>(<span class="self">self</span>.read_copy(byte)); |
| } |
| } |
| <span class="kw">if </span><span class="self">self</span>.d != <span class="self">self</span>.dst.len() { |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::HeaderMismatch { |
| expected_len: <span class="self">self</span>.dst.len() <span class="kw">as </span>u64, |
| got_len: <span class="self">self</span>.d <span class="kw">as </span>u64, |
| }); |
| } |
| <span class="prelude-val">Ok</span>(()) |
| } |
| |
| <span class="doccomment">/// Decompresses a literal from `src` starting at `s` to `dst` starting at |
| /// `d` and returns the updated values of `s` and `d`. `s` should point to |
| /// the byte immediately proceding the literal tag byte. |
| /// |
| /// `len` is the length of the literal if it's <=60. Otherwise, it's the |
| /// length tag, indicating the number of bytes needed to read a little |
| /// endian integer at `src[s..]`. i.e., `61 => 1 byte`, `62 => 2 bytes`, |
| /// `63 => 3 bytes` and `64 => 4 bytes`. |
| /// |
| /// `len` must be <=64. |
| </span><span class="attribute">#[inline(always)] |
| </span><span class="kw">fn </span>read_literal( |
| <span class="kw-2">&mut </span><span class="self">self</span>, |
| len: usize, |
| ) -> <span class="prelude-ty">Result</span><()> { |
| <span class="macro">debug_assert!</span>(len <= <span class="number">64</span>); |
| <span class="kw">let </span><span class="kw-2">mut </span>len = len <span class="kw">as </span>u64; |
| <span class="comment">// As an optimization for the common case, if the literal length is |
| // <=16 and we have enough room in both `src` and `dst`, copy the |
| // literal using unaligned loads and stores. |
| // |
| // We pick 16 bytes with the hope that it optimizes down to a 128 bit |
| // load/store. |
| </span><span class="kw">if </span>len <= <span class="number">16 |
| </span>&& <span class="self">self</span>.s + <span class="number">16 </span><= <span class="self">self</span>.src.len() |
| && <span class="self">self</span>.d + <span class="number">16 </span><= <span class="self">self</span>.dst.len() { |
| <span class="kw">unsafe </span>{ |
| <span class="comment">// SAFETY: We know both src and dst have at least 16 bytes of |
| // wiggle room after s/d, even if `len` is <16, so the copy is |
| // safe. |
| </span><span class="kw">let </span>srcp = <span class="self">self</span>.src.as_ptr().offset(<span class="self">self</span>.s <span class="kw">as </span>isize); |
| <span class="kw">let </span>dstp = <span class="self">self</span>.dst.as_mut_ptr().offset(<span class="self">self</span>.d <span class="kw">as </span>isize); |
| <span class="comment">// Hopefully uses SIMD registers for 128 bit load/store. |
| </span>ptr::copy_nonoverlapping(srcp, dstp, <span class="number">16</span>); |
| } |
| <span class="self">self</span>.d += len <span class="kw">as </span>usize; |
| <span class="self">self</span>.s += len <span class="kw">as </span>usize; |
| <span class="kw">return </span><span class="prelude-val">Ok</span>(()); |
| } |
| <span class="comment">// When the length is bigger than 60, it indicates that we need to read |
| // an additional 1-4 bytes to get the real length of the literal. |
| </span><span class="kw">if </span>len >= <span class="number">61 </span>{ |
| <span class="comment">// If there aren't at least 4 bytes left to read then we know this |
| // is corrupt because the literal must have length >=61. |
| </span><span class="kw">if </span><span class="self">self</span>.s <span class="kw">as </span>u64 + <span class="number">4 </span>> <span class="self">self</span>.src.len() <span class="kw">as </span>u64 { |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::Literal { |
| len: <span class="number">4</span>, |
| src_len: (<span class="self">self</span>.src.len() - <span class="self">self</span>.s) <span class="kw">as </span>u64, |
| dst_len: (<span class="self">self</span>.dst.len() - <span class="self">self</span>.d) <span class="kw">as </span>u64, |
| }); |
| } |
| <span class="comment">// Since we know there are 4 bytes left to read, read a 32 bit LE |
| // integer and mask away the bits we don't need. |
| </span><span class="kw">let </span>byte_count = len <span class="kw">as </span>usize - <span class="number">60</span>; |
| len = LE::read_u32(<span class="kw-2">&</span><span class="self">self</span>.src[<span class="self">self</span>.s..]) <span class="kw">as </span>u64; |
| len = (len & (WORD_MASK[byte_count] <span class="kw">as </span>u64)) + <span class="number">1</span>; |
| <span class="self">self</span>.s += byte_count; |
| } |
| <span class="comment">// If there's not enough buffer left to load or store this literal, |
| // then the input is corrupt. |
| // if self.s + len > self.src.len() || self.d + len > self.dst.len() { |
| </span><span class="kw">if </span>((<span class="self">self</span>.src.len() - <span class="self">self</span>.s) <span class="kw">as </span>u64) < len |
| || ((<span class="self">self</span>.dst.len() - <span class="self">self</span>.d) <span class="kw">as </span>u64) < len { |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::Literal { |
| len: len, |
| src_len: (<span class="self">self</span>.src.len() - <span class="self">self</span>.s) <span class="kw">as </span>u64, |
| dst_len: (<span class="self">self</span>.dst.len() - <span class="self">self</span>.d) <span class="kw">as </span>u64, |
| }); |
| } |
| <span class="kw">unsafe </span>{ |
| <span class="comment">// SAFETY: We've already checked the bounds, so we know this copy |
| // is correct. |
| </span><span class="kw">let </span>srcp = <span class="self">self</span>.src.as_ptr().offset(<span class="self">self</span>.s <span class="kw">as </span>isize); |
| <span class="kw">let </span>dstp = <span class="self">self</span>.dst.as_mut_ptr().offset(<span class="self">self</span>.d <span class="kw">as </span>isize); |
| ptr::copy_nonoverlapping(srcp, dstp, len <span class="kw">as </span>usize); |
| } |
| <span class="self">self</span>.s += len <span class="kw">as </span>usize; |
| <span class="self">self</span>.d += len <span class="kw">as </span>usize; |
| <span class="prelude-val">Ok</span>(()) |
| } |
| |
| <span class="doccomment">/// Reads a copy from `src` and writes the decompressed bytes to `dst`. `s` |
| /// should point to the byte immediately proceding the copy tag byte. |
| </span><span class="attribute">#[inline(always)] |
| </span><span class="kw">fn </span>read_copy( |
| <span class="kw-2">&mut </span><span class="self">self</span>, |
| tag_byte: u8, |
| ) -> <span class="prelude-ty">Result</span><()> { |
| <span class="comment">// Find the copy offset and len, then advance the input past the copy. |
| // The rest of this function deals with reading/writing to output only. |
| </span><span class="kw">let </span>entry = TAG_LOOKUP_TABLE.entry(tag_byte); |
| <span class="kw">let </span>offset = <span class="macro">try!</span>(entry.offset(<span class="self">self</span>.src, <span class="self">self</span>.s)); |
| <span class="kw">let </span>len = entry.len(); |
| <span class="self">self</span>.s += entry.num_tag_bytes(); |
| |
| <span class="comment">// What we really care about here is whether `d == 0` or `d < offset`. |
| // To save an extra branch, use `d < offset - 1` instead. If `d` is |
| // `0`, then `offset.wrapping_sub(1)` will be usize::MAX which is also |
| // the max value of `d`. |
| </span><span class="kw">if </span><span class="self">self</span>.d <= offset.wrapping_sub(<span class="number">1</span>) { |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::Offset { |
| offset: offset <span class="kw">as </span>u64, |
| dst_pos: <span class="self">self</span>.d <span class="kw">as </span>u64, |
| }); |
| } |
| <span class="comment">// When all is said and done, dst is advanced to end. |
| </span><span class="kw">let </span>end = <span class="self">self</span>.d + len; |
| <span class="comment">// When the copy is small and the offset is at least 8 bytes away from |
| // `d`, then we can decompress the copy with two 64 bit unaligned |
| // loads/stores. |
| </span><span class="kw">if </span>offset >= <span class="number">8 </span>&& len <= <span class="number">16 </span>&& <span class="self">self</span>.d + <span class="number">16 </span><= <span class="self">self</span>.dst.len() { |
| <span class="kw">unsafe </span>{ |
| <span class="comment">// SAFETY: We know dstp points to at least 16 bytes of memory |
| // from the condition above, and we also know that dstp is |
| // preceded by at least `offset` bytes from the `d <= offset` |
| // check above. |
| // |
| // We also know that dstp and dstp-8 do not overlap from the |
| // check above, justifying the use of copy_nonoverlapping. |
| </span><span class="kw">let </span>dstp = <span class="self">self</span>.dst.as_mut_ptr().offset(<span class="self">self</span>.d <span class="kw">as </span>isize); |
| <span class="kw">let </span>srcp = dstp.offset(-(offset <span class="kw">as </span>isize)); |
| <span class="comment">// We can't do a single 16 byte load/store because src/dst may |
| // overlap with each other. Namely, the second copy here may |
| // copy bytes written in the first copy! |
| </span>ptr::copy_nonoverlapping(srcp, dstp, <span class="number">8</span>); |
| ptr::copy_nonoverlapping(srcp.offset(<span class="number">8</span>), dstp.offset(<span class="number">8</span>), <span class="number">8</span>); |
| } |
| <span class="comment">// If we have some wiggle room, try to decompress the copy 16 bytes |
| // at a time with 128 bit unaligned loads/stores. Remember, we can't |
| // just do a memcpy because decompressing copies may require copying |
| // overlapping memory. |
| // |
| // We need the extra wiggle room to make effective use of 128 bit |
| // loads/stores. Even if the store ends up copying more data than we |
| // need, we're careful to advance `d` by the correct amount at the end. |
| </span>} <span class="kw">else if </span>end + <span class="number">24 </span><= <span class="self">self</span>.dst.len() { |
| <span class="kw">unsafe </span>{ |
| <span class="comment">// SAFETY: We know that dstp is preceded by at least `offset` |
| // bytes from the `d <= offset` check above. |
| // |
| // We don't know whether dstp overlaps with srcp, so we start |
| // by copying from srcp to dstp until they no longer overlap. |
| // The worst case is when dstp-src = 3 and copy length = 1. The |
| // first loop will issue these copy operations before stopping: |
| // |
| // [-1, 14] -> [0, 15] |
| // [-1, 14] -> [3, 18] |
| // [-1, 14] -> [9, 24] |
| // |
| // But the copy had length 1, so it was only supposed to write |
| // to [0, 0]. But the last copy wrote to [9, 24], which is 24 |
| // extra bytes in dst *beyond* the end of the copy, which is |
| // guaranteed by the conditional above. |
| </span><span class="kw">let </span><span class="kw-2">mut </span>dstp = <span class="self">self</span>.dst.as_mut_ptr().offset(<span class="self">self</span>.d <span class="kw">as </span>isize); |
| <span class="kw">let </span><span class="kw-2">mut </span>srcp = dstp.offset(-(offset <span class="kw">as </span>isize)); |
| <span class="kw">loop </span>{ |
| <span class="kw">let </span>diff = (dstp <span class="kw">as </span>isize) - (srcp <span class="kw">as </span>isize); |
| <span class="kw">if </span>diff >= <span class="number">16 </span>{ |
| <span class="kw">break</span>; |
| } |
| <span class="comment">// srcp and dstp can overlap, so use ptr::copy. |
| </span><span class="macro">debug_assert!</span>(<span class="self">self</span>.d + <span class="number">16 </span><= <span class="self">self</span>.dst.len()); |
| ptr::copy(srcp, dstp, <span class="number">16</span>); |
| <span class="self">self</span>.d += diff <span class="kw">as </span>usize; |
| dstp = dstp.offset(diff); |
| } |
| <span class="kw">while </span><span class="self">self</span>.d < end { |
| ptr::copy_nonoverlapping(srcp, dstp, <span class="number">16</span>); |
| srcp = srcp.offset(<span class="number">16</span>); |
| dstp = dstp.offset(<span class="number">16</span>); |
| <span class="self">self</span>.d += <span class="number">16</span>; |
| } |
| <span class="comment">// At this point, `d` is likely wrong. We correct it before |
| // returning. It's correct value is `end`. |
| </span>} |
| } <span class="kw">else </span>{ |
| <span class="kw">if </span>end > <span class="self">self</span>.dst.len() { |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::CopyWrite { |
| len: len <span class="kw">as </span>u64, |
| dst_len: (<span class="self">self</span>.dst.len() - <span class="self">self</span>.d) <span class="kw">as </span>u64, |
| }); |
| } |
| <span class="comment">// Finally, the slow byte-by-byte case, which should only be used |
| // for the last few bytes of decompression. |
| </span><span class="kw">while </span><span class="self">self</span>.d != end { |
| <span class="self">self</span>.dst[<span class="self">self</span>.d] = <span class="self">self</span>.dst[<span class="self">self</span>.d - offset]; |
| <span class="self">self</span>.d += <span class="number">1</span>; |
| } |
| } |
| <span class="self">self</span>.d = end; |
| <span class="prelude-val">Ok</span>(()) |
| } |
| } |
| |
| <span class="doccomment">/// Header represents the single varint that starts every Snappy compressed |
| /// block. |
| </span><span class="attribute">#[derive(Debug)] |
| </span><span class="kw">struct </span>Header { |
| <span class="doccomment">/// The length of the header in bytes (i.e., the varint). |
| </span>len: usize, |
| <span class="doccomment">/// The length of the original decompressed input in bytes. |
| </span>decompress_len: usize, |
| } |
| |
| <span class="kw">impl </span>Header { |
| <span class="doccomment">/// Reads the varint header from the given input. |
| /// |
| /// If there was a problem reading the header then an error is returned. |
| /// If a header is returned then it is guaranteed to be valid. |
| </span><span class="attribute">#[inline(always)] |
| </span><span class="kw">fn </span>read(input: <span class="kw-2">&</span>[u8]) -> <span class="prelude-ty">Result</span><Header> { |
| <span class="kw">let </span>(decompress_len, header_len) = read_varu64(input); |
| <span class="kw">if </span>header_len == <span class="number">0 </span>{ |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::Header); |
| } |
| <span class="kw">if </span>decompress_len > MAX_INPUT_SIZE { |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::TooBig { |
| given: decompress_len <span class="kw">as </span>u64, |
| max: MAX_INPUT_SIZE, |
| }); |
| } |
| <span class="prelude-val">Ok</span>(Header { len: header_len, decompress_len: decompress_len <span class="kw">as </span>usize }) |
| } |
| } |
| |
| <span class="doccomment">/// A lookup table for quickly computing the various attributes derived from |
| /// a tag byte. The attributes are most useful for the three "copy" tags |
| /// and include the length of the copy, part of the offset (for copy 1-byte |
| /// only) and the total number of bytes proceding the tag byte that encode |
| /// the other part of the offset (1 for copy 1, 2 for copy 2 and 4 for copy 4). |
| /// |
| /// More specifically, the keys of the table are u8s and the values are u16s. |
| /// The bits of the values are laid out as follows: |
| /// |
| /// xxaa abbb xxcc cccc |
| /// |
| /// Where `a` is the number of bytes, `b` are the three bits of the offset |
| /// for copy 1 (the other 8 bits are in the byte proceding the tag byte; for |
| /// copy 2 and copy 4, `b = 0`), and `c` is the length of the copy (max of 64). |
| /// |
| /// We could pack this in fewer bits, but the position of the three `b` bits |
| /// lines up with the most significant three bits in the total offset for copy |
| /// 1, which avoids an extra shift instruction. |
| /// |
| /// In sum, this table is useful because it reduces branches and various |
| /// arithmetic operations. |
| </span><span class="kw">struct </span>TagLookupTable([u16; <span class="number">256</span>]); |
| |
| <span class="kw">impl </span>TagLookupTable { |
| <span class="doccomment">/// Look up the tag entry given the tag `byte`. |
| </span><span class="attribute">#[inline(always)] |
| </span><span class="kw">fn </span>entry(<span class="kw-2">&</span><span class="self">self</span>, byte: u8) -> TagEntry { |
| TagEntry(<span class="self">self</span>.<span class="number">0</span>[byte <span class="kw">as </span>usize] <span class="kw">as </span>usize) |
| } |
| } |
| |
| <span class="doccomment">/// Represents a single entry in the tag lookup table. |
| /// |
| /// See the documentation in `TagLookupTable` for the bit layout. |
| /// |
| /// The type is a `usize` for convenience. |
| </span><span class="kw">struct </span>TagEntry(usize); |
| |
| <span class="kw">impl </span>TagEntry { |
| <span class="doccomment">/// Return the total number of bytes proceding this tag byte required to |
| /// encode the offset. |
| </span><span class="kw">fn </span>num_tag_bytes(<span class="kw-2">&</span><span class="self">self</span>) -> usize { |
| <span class="self">self</span>.<span class="number">0 </span>>> <span class="number">11 |
| </span>} |
| |
| <span class="doccomment">/// Return the total copy length, capped at 64. |
| </span><span class="kw">fn </span>len(<span class="kw-2">&</span><span class="self">self</span>) -> usize { |
| <span class="self">self</span>.<span class="number">0 </span>& <span class="number">0xFF |
| </span>} |
| |
| <span class="doccomment">/// Return the copy offset corresponding to this copy operation. `s` should |
| /// point to the position just after the tag byte that this entry was read |
| /// from. |
| /// |
| /// This requires reading from the compressed input since the offset is |
| /// encoded in bytes proceding the tag byte. |
| </span><span class="kw">fn </span>offset(<span class="kw-2">&</span><span class="self">self</span>, src: <span class="kw-2">&</span>[u8], s: usize) -> <span class="prelude-ty">Result</span><usize> { |
| <span class="kw">let </span>num_tag_bytes = <span class="self">self</span>.num_tag_bytes(); |
| <span class="kw">let </span>trailer = |
| <span class="comment">// It is critical for this case to come first, since it is the |
| // fast path. We really hope that this case gets branch |
| // predicted. |
| </span><span class="kw">if </span>s + <span class="number">4 </span><= src.len() { |
| <span class="kw">unsafe </span>{ |
| <span class="comment">// SAFETY: The conditional above guarantees that |
| // src[s..s+4] is valid to read from. |
| </span><span class="kw">let </span>p = src.as_ptr().offset(s <span class="kw">as </span>isize); |
| <span class="comment">// We use WORD_MASK here to mask out the bits we don't |
| // need. While we're guaranteed to read 4 valid bytes, |
| // not all of those bytes are necessarily part of the |
| // offset. This is the key optimization: we don't need to |
| // branch on num_tag_bytes. |
| </span>loadu32_le(p) <span class="kw">as </span>usize & WORD_MASK[num_tag_bytes] |
| } |
| } <span class="kw">else if </span>num_tag_bytes == <span class="number">1 </span>{ |
| <span class="kw">if </span>s >= src.len() { |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::CopyRead { |
| len: <span class="number">1</span>, |
| src_len: (src.len() - s) <span class="kw">as </span>u64, |
| }); |
| } |
| src[s] <span class="kw">as </span>usize |
| } <span class="kw">else if </span>num_tag_bytes == <span class="number">2 </span>{ |
| <span class="kw">if </span>s + <span class="number">1 </span>>= src.len() { |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::CopyRead { |
| len: <span class="number">2</span>, |
| src_len: (src.len() - s) <span class="kw">as </span>u64, |
| }); |
| } |
| LE::read_u16(<span class="kw-2">&</span>src[s..]) <span class="kw">as </span>usize |
| } <span class="kw">else </span>{ |
| <span class="kw">return </span><span class="prelude-val">Err</span>(Error::CopyRead { |
| len: num_tag_bytes <span class="kw">as </span>u64, |
| src_len: (src.len() - s) <span class="kw">as </span>u64, |
| }); |
| }; |
| <span class="prelude-val">Ok</span>((<span class="self">self</span>.<span class="number">0 </span>& <span class="number">0b0000_0111_0000_0000</span>) | trailer) |
| } |
| } |
| |
| <span class="doccomment">/// Loads a little endian encoded u32 from data. |
| /// |
| /// This is unsafe because `data` must point to some memory of size at least 4. |
| </span><span class="attribute">#[inline(always)] |
| </span><span class="kw">unsafe fn </span>loadu32_le(data: <span class="kw-2">*const </span>u8) -> u32 { |
| <span class="kw">let </span><span class="kw-2">mut </span>n: u32 = <span class="number">0</span>; |
| ptr::copy_nonoverlapping( |
| data, |
| <span class="kw-2">&mut </span>n <span class="kw">as </span><span class="kw-2">*mut </span>u32 <span class="kw">as </span><span class="kw-2">*mut </span>u8, |
| <span class="number">4</span>); |
| n.to_le() |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="snap" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
