Implement a buffer (re)assembler.
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whitequark
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2c4222a0b6
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ba2d7f483a
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@ -0,0 +1,334 @@
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use core::fmt;
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/// A contiguous chunk of absent data, followed by a contiguous chunk of present data.
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#[derive(Debug, Clone, Copy, PartialEq, Eq)]
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struct Contig {
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hole_size: u32,
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data_size: u32
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}
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impl fmt::Display for Contig {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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if self.has_hole() { write!(f, "({})", self.hole_size)?; }
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if self.has_data() { write!(f, " {}", self.data_size)?; }
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Ok(())
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}
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}
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impl Contig {
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fn empty() -> Contig {
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Contig { hole_size: 0, data_size: 0 }
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}
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fn hole(size: u32) -> Contig {
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Contig { hole_size: size, data_size: 0 }
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}
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fn hole_and_data(hole_size: u32, data_size: u32) -> Contig {
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Contig { hole_size, data_size }
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}
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fn has_hole(&self) -> bool {
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self.hole_size != 0
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}
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fn has_data(&self) -> bool {
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self.data_size != 0
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}
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fn total_size(&self) -> u32 {
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self.hole_size + self.data_size
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}
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fn is_empty(&self) -> bool {
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self.total_size() == 0
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}
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fn expand_data_by(&mut self, size: u32) {
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self.data_size += size;
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}
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fn shrink_hole_by(&mut self, size: u32) {
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self.hole_size -= size;
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}
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fn shrink_hole_to(&mut self, size: u32) {
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assert!(self.hole_size >= size);
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let total_size = self.total_size();
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self.hole_size = size;
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self.data_size = total_size - size;
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}
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}
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const CONTIG_COUNT: usize = 4;
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/// A buffer (re)assembler.
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///
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/// Currently, up to a hardcoded limit of four holes can be tracked in the buffer.
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#[derive(Debug)]
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#[cfg_attr(test, derive(PartialEq, Eq))]
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pub struct Assembler {
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contigs: [Contig; CONTIG_COUNT]
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}
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impl fmt::Display for Assembler {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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write!(f, "[ ")?;
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for contig in self.contigs.iter() {
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if contig.is_empty() { break }
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write!(f, "{} ", contig)?;
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}
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write!(f, "]")?;
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Ok(())
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}
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}
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impl Assembler {
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/// Create a new buffer assembler for buffers of the given size.
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pub fn new(size: u32) -> Assembler {
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let mut contigs = [Contig::empty(); CONTIG_COUNT];
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contigs[0] = Contig::hole(size);
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Assembler { contigs }
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}
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pub(crate) fn total_size(&self) -> u32 {
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self.contigs
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.iter()
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.map(|contig| contig.total_size())
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.sum()
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}
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fn front(&self) -> Contig {
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self.contigs[0]
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}
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fn back(&self) -> Contig {
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self.contigs[self.contigs.len() - 1]
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}
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/// Remove a contig at the given index, and return a pointer to the the first empty contig.
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fn remove_contig_at(&mut self, at: usize) -> &mut Contig {
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debug_assert!(!self.contigs[at].is_empty());
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for i in at..self.contigs.len() - 1 {
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self.contigs[i] = self.contigs[i + 1];
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if self.contigs[i].is_empty() {
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return &mut self.contigs[i]
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}
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}
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// Removing the last one.
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self.contigs[at] = Contig::empty();
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&mut self.contigs[at]
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}
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/// Add a contig at the given index, and return a pointer to it.
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fn add_contig_at(&mut self, at: usize) -> Result<&mut Contig, ()> {
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debug_assert!(!self.contigs[at].is_empty());
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if !self.back().is_empty() { return Err(()) }
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for i in (at + 1..self.contigs.len()).rev() {
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self.contigs[i] = self.contigs[i - 1];
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}
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self.contigs[at] = Contig::empty();
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Ok(&mut self.contigs[at])
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}
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/// Add a new contiguous range to the assembler, and return `Ok(())`,
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/// or return `Err(())` if too many discontiguities are already recorded.
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pub fn add(&mut self, mut offset: u32, mut size: u32) -> Result<(), ()> {
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let mut index = 0;
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while index != self.contigs.len() && size != 0 {
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let contig = self.contigs[index];
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if let Some(new_offset) = offset.checked_sub(contig.total_size()) {
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// The range being added does not cover this contig, skip it.
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index += 1;
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} else if offset == 0 && size >= contig.hole_size && index > 0 {
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// The range being added covers the entire hole in this contig, merge it
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// into the previous config.
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self.contigs[index - 1].expand_data_by(contig.total_size());
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self.remove_contig_at(index);
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index += 0;
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} else if offset == 0 && size < contig.hole_size && index > 0 {
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// The range being added covers a part of the hole in this contig starting
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// at the beginning, shrink the hole in this contig and expand data in
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// the previous contig.
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self.contigs[index - 1].expand_data_by(size);
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self.contigs[index].shrink_hole_by(size);
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index += 1;
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} else if offset <= contig.hole_size && offset + size >= contig.hole_size {
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// The range being added covers both a part of the hole and a part of the data
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// in this contig, shrink the hole in this contig.
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self.contigs[index].shrink_hole_to(offset);
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index += 1;
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} else if offset + size >= contig.hole_size {
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// The range being added covers only a part of the data in this contig, skip it.
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index += 1;
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} else if offset + size < contig.hole_size {
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// The range being added covers a part of the hole but not of the data
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// in this contig, add a new contig containing the range.
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self.contigs[index].shrink_hole_by(offset + size);
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let empty = self.add_contig_at(index)?;
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*empty = Contig::hole_and_data(offset, size);
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index += 2;
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} else {
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unreachable!()
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}
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// Skip the portion of the range covered by this contig.
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if offset >= contig.total_size() {
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offset = offset.saturating_sub(contig.total_size());
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} else {
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size = (offset + size).saturating_sub(contig.total_size());
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offset = 0;
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}
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}
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debug_assert!(size == 0);
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Ok(())
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}
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/// Return `Ok(size)` with the size of a contiguous range in the front of the assembler,
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/// or return `Err(())` if there is no such range.
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pub fn front_len(&self) -> u32 {
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let front = self.front();
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if front.has_hole() {
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0
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} else {
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debug_assert!(front.data_size > 0);
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front.data_size
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}
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}
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/// Remove a contiguous range from the front of the assembler and `Ok(data_size)`,
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/// or return `Err(())` if there is no such range.
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pub fn front_remove(&mut self) -> u32 {
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let front = self.front();
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if front.has_hole() {
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0
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} else {
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let empty = self.remove_contig_at(0);
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*empty = Contig::hole(front.data_size);
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debug_assert!(front.data_size > 0);
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front.data_size
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}
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}
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}
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#[cfg(test)]
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mod test {
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use std::vec::Vec;
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use super::*;
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impl From<Vec<(u32, u32)>> for Assembler {
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fn from(vec: Vec<(u32, u32)>) -> Assembler {
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let mut contigs = [Contig::empty(); CONTIG_COUNT];
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for (i, &(hole_size, data_size)) in vec.iter().enumerate() {
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contigs[i] = Contig { hole_size, data_size };
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}
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Assembler { contigs }
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}
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}
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macro_rules! contigs {
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[$( $x:expr ),*] => ({
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Assembler::from(vec![$( $x ),*])
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})
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}
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#[test]
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fn test_new() {
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let assr = Assembler::new(16);
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assert_eq!(assr.total_size(), 16);
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assert_eq!(assr, contigs![(16, 0)]);
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}
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#[test]
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fn test_empty_add_full() {
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let mut assr = Assembler::new(16);
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assert_eq!(assr.add(0, 16), Ok(()));
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assert_eq!(assr, contigs![(0, 16)]);
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}
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#[test]
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fn test_empty_add_front() {
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let mut assr = Assembler::new(16);
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assert_eq!(assr.add(0, 4), Ok(()));
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assert_eq!(assr, contigs![(0, 4), (12, 0)]);
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}
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#[test]
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fn test_empty_add_back() {
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let mut assr = Assembler::new(16);
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assert_eq!(assr.add(12, 4), Ok(()));
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assert_eq!(assr, contigs![(12, 4)]);
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}
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#[test]
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fn test_empty_add_mid() {
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let mut assr = Assembler::new(16);
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assert_eq!(assr.add(4, 8), Ok(()));
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assert_eq!(assr, contigs![(4, 8), (4, 0)]);
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}
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#[test]
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fn test_partial_add_front() {
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let mut assr = contigs![(4, 8), (4, 0)];
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assert_eq!(assr.add(0, 4), Ok(()));
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assert_eq!(assr, contigs![(0, 12), (4, 0)]);
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}
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#[test]
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fn test_partial_add_back() {
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let mut assr = contigs![(4, 8), (4, 0)];
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assert_eq!(assr.add(12, 4), Ok(()));
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assert_eq!(assr, contigs![(4, 12)]);
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}
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#[test]
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fn test_partial_add_front_overlap() {
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let mut assr = contigs![(4, 8), (4, 0)];
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assert_eq!(assr.add(0, 8), Ok(()));
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assert_eq!(assr, contigs![(0, 12), (4, 0)]);
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}
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#[test]
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fn test_partial_add_front_overlap_split() {
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let mut assr = contigs![(4, 8), (4, 0)];
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assert_eq!(assr.add(2, 6), Ok(()));
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assert_eq!(assr, contigs![(2, 10), (4, 0)]);
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}
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#[test]
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fn test_partial_add_back_overlap() {
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let mut assr = contigs![(4, 8), (4, 0)];
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assert_eq!(assr.add(8, 8), Ok(()));
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assert_eq!(assr, contigs![(4, 12)]);
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}
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#[test]
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fn test_partial_add_back_overlap_split() {
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let mut assr = contigs![(4, 8), (4, 0)];
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assert_eq!(assr.add(10, 4), Ok(()));
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assert_eq!(assr, contigs![(4, 10), (2, 0)]);
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}
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#[test]
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fn test_partial_add_both_overlap() {
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let mut assr = contigs![(4, 8), (4, 0)];
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assert_eq!(assr.add(0, 16), Ok(()));
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assert_eq!(assr, contigs![(0, 16)]);
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}
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#[test]
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fn test_partial_add_both_overlap_split() {
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let mut assr = contigs![(4, 8), (4, 0)];
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assert_eq!(assr.add(2, 12), Ok(()));
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assert_eq!(assr, contigs![(2, 12), (2, 0)]);
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}
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}
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@ -4,8 +4,10 @@
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//! The containers support both pre-allocated memory, without the `std`
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//! and `collections` crates being available, and heap-allocated memory.
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mod assembler;
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mod ring_buffer;
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pub use self::assembler::Assembler;
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pub use self::ring_buffer::RingBuffer;
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/// A trait for setting a value to a known state.
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