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//! The compression algorithm. //! //! We make use of hash tables to find duplicates. This gives a reasonable compression ratio with a //! high performance. It has fixed memory usage, which contrary to other approachs, makes it less //! memory hungry. use byteorder::{NativeEndian, ByteOrder}; /// Duplication dictionary size. /// /// Every four bytes is assigned an entry. When this number is lower, fewer entries exists, and /// thus collisions are more likely, hurting the compression ratio. const DICTIONARY_SIZE: usize = 4096; /// A LZ4 block. /// /// This defines a single compression "unit", consisting of two parts, a number of raw literals, /// and possibly a pointer to the already encoded buffer from which to copy. #[derive(Debug)] struct Block { /// The length (in bytes) of the literals section. lit_len: usize, /// The duplicates section if any. /// /// Only the last block in a stream can lack of the duplicates section. dup: Option<Duplicate>, } /// A consecutive sequence of bytes found in already encoded part of the input. #[derive(Copy, Clone, Debug)] struct Duplicate { /// The number of bytes before our cursor, where the duplicate starts. offset: u16, /// The length beyond the four first bytes. /// /// Adding four to this number yields the actual length. extra_bytes: usize, } /// An LZ4 encoder. struct Encoder<'a> { /// The raw uncompressed input. input: &'a [u8], /// The compressed output. output: &'a mut Vec<u8>, /// The number of bytes from the input that are encoded. cur: usize, /// The dictionary of previously encoded sequences. /// /// This is used to find duplicates in the stream so they are not written multiple times. /// /// Every four bytes are hashed, and in the resulting slot their position in the input buffer /// is placed. This way we can easily look up a candidate to back references. dict: [usize; DICTIONARY_SIZE], } impl<'a> Encoder<'a> { /// Go forward by some number of bytes. /// /// This will update the cursor and dictionary to reflect the now processed bytes. /// /// This returns `false` if all the input bytes are processed. fn go_forward(&mut self, steps: usize) -> bool { // Go over all the bytes we are skipping and update the cursor and dictionary. for _ in 0..steps { // Insert the cursor position into the dictionary. self.insert_cursor(); // Increment the cursor. self.cur += 1; } // Return `true` if there's more to read. self.cur <= self.input.len() } /// Insert the batch under the cursor into the dictionary. fn insert_cursor(&mut self) { // Make sure that there is at least one batch remaining. if self.remaining_batch() { // Insert the cursor into the table. self.dict[self.get_cur_hash()] = self.cur; } } /// Check if there are any remaining batches. fn remaining_batch(&self) -> bool { self.cur + 4 < self.input.len() } /// Get the hash of the current four bytes below the cursor. /// /// This is guaranteed to be below `DICTIONARY_SIZE`. fn get_cur_hash(&self) -> usize { // Use PCG transform to generate a relatively good hash of the four bytes batch at the // cursor. let mut x = self.get_batch_at_cursor().wrapping_mul(0xa4d94a4f); let a = x >> 16; let b = x >> 30; x ^= a >> b; x = x.wrapping_mul(0xa4d94a4f); x as usize % DICTIONARY_SIZE } /// Read a 4-byte "batch" from some position. /// /// This will read a native-endian 4-byte integer from some position. fn get_batch(&self, n: usize) -> u32 { debug_assert!(self.remaining_batch(), "Reading a partial batch."); NativeEndian::read_u32(&self.input[n..]) } /// Read the batch at the cursor. fn get_batch_at_cursor(&self) -> u32 { self.get_batch(self.cur) } /// Find a duplicate of the current batch. /// /// If any duplicate is found, a tuple `(position, size - 4)` is returned. fn find_duplicate(&self) -> Option<Duplicate> { // If there is no remaining batch, we return none. if !self.remaining_batch() { return None; } // Find a candidate in the dictionary by hashing the current four bytes. let candidate = self.dict[self.get_cur_hash()]; // Three requirements to the candidate exists: // - The candidate is not the trap value (0xFFFFFFFF), which represents an empty bucket. // - We should not return a position which is merely a hash collision, so w that the // candidate actually matches what we search for. // - We can address up to 16-bit offset, hence we are only able to address the candidate if // its offset is less than or equals to 0xFFFF. if candidate != !0 && self.get_batch(candidate) == self.get_batch_at_cursor() && self.cur - candidate <= 0xFFFF { // Calculate the "extension bytes", i.e. the duplicate bytes beyond the batch. These // are the number of prefix bytes shared between the match and needle. let ext = self.input[self.cur + 4..] .iter() .zip(&self.input[candidate + 4..]) .take_while(|&(a, b)| a == b) .count(); Some(Duplicate { offset: (self.cur - candidate) as u16, extra_bytes: ext, }) } else { None } } /// Write an integer to the output in LSIC format. fn write_integer(&mut self, mut n: usize) { // Write the 0xFF bytes as long as the integer is higher than said value. while n >= 0xFF { n -= 0xFF; self.output.push(0xFF); } // Write the remaining byte. self.output.push(n as u8); } /// Read the block of the top of the stream. fn pop_block(&mut self) -> Block { // The length of the literals section. let mut lit = 0; loop { // Search for a duplicate. if let Some(dup) = self.find_duplicate() { // We found a duplicate, so the literals section is over... // Move forward. Note that `ext` is actually the steps minus 4, because of the // minimum matchlenght, so we need to add 4. self.go_forward(dup.extra_bytes + 4); return Block { lit_len: lit, dup: Some(dup), }; } // Try to move forward. if !self.go_forward(1) { // We reached the end of the stream, and no duplicates section follows. return Block { lit_len: lit, dup: None, }; } // No duplicates found yet, so extend the literals section. lit += 1; } } /// Complete the encoding into `self.output`. fn complete(&mut self) { // Construct one block at a time. loop { // The start of the literals section. let start = self.cur; // Read the next block into two sections, the literals and the duplicates. let block = self.pop_block(); // Generate the higher half of the token. let mut token = if block.lit_len < 0xF { // Since we can fit the literals length into it, there is no need for saturation. (block.lit_len as u8) << 4 } else { // We were unable to fit the literals into it, so we saturate to 0xF. We will later // write the extensional value through LSIC encoding. 0xF0 }; // Generate the lower half of the token, the duplicates length. let dup_extra_len = block.dup.map_or(0, |x| x.extra_bytes); token |= if dup_extra_len < 0xF { // We could fit it in. dup_extra_len as u8 } else { // We were unable to fit it in, so we default to 0xF, which will later be extended // by LSIC encoding. 0xF }; // Push the token to the output stream. self.output.push(token); // If we were unable to fit the literals length into the token, write the extensional // part through LSIC. if block.lit_len >= 0xF { self.write_integer(block.lit_len - 0xF); } // Now, write the actual literals. self.output.extend_from_slice(&self.input[start..start + block.lit_len]); if let Some(Duplicate { offset, .. }) = block.dup { // Wait! There's more. Now, we encode the duplicates section. // Push the offset in little endian. self.output.push(offset as u8); self.output.push((offset >> 8) as u8); // If we were unable to fit the duplicates length into the token, write the // extensional part through LSIC. if dup_extra_len >= 0xF { self.write_integer(dup_extra_len - 0xF); } } else { break; } } } } /// Compress all bytes of `input` into `output`. pub fn compress_into(input: &[u8], output: &mut Vec<u8>) { Encoder { input: input, output: output, cur: 0, dict: [!0; DICTIONARY_SIZE], }.complete(); } /// Compress all bytes of `input`. pub fn compress(input: &[u8]) -> Vec<u8> { // In most cases, the compression won't expand the size, so we set the input size as capacity. let mut vec = Vec::with_capacity(input.len()); compress_into(input, &mut vec); vec }