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//! Run-length encoding.
//!
//! Wire format: a sequence of `[count: u8][byte: u8]` pairs, where `count`
//! is in `1..=255`. Runs longer than 255 are split across multiple pairs.
//! A `count` of `0` is reserved as a corruption marker.
//!
//! This algorithm is not chosen for compression ratio — it is the smallest
//! interesting state machine for validating the streaming trait shape.
use crate::error::Error;
use crate::traits::{Algorithm, RawDecoder, RawEncoder, RawProgress};
/// Zero-sized marker type implementing [`Algorithm`].
#[derive(Debug, Clone, Copy, Default)]
pub struct Rle;
impl Algorithm for Rle {
const NAME: &'static str = "rle";
type Encoder = Encoder;
type Decoder = Decoder;
type EncoderConfig = ();
type DecoderConfig = ();
fn encoder_with(_: ()) -> Encoder {
Encoder::new()
}
fn decoder_with(_: ()) -> Decoder {
Decoder::new()
}
}
// ─── encoder ──────────────────────────────────────────────────────────────
#[derive(Debug, Clone, Copy)]
enum EncState {
/// No run in progress; encoder is fresh or just flushed.
Empty,
/// Accumulating a run of `byte`; `count` is `1..=255`.
Run { byte: u8, count: u8 },
/// A `[count][value]` pair was started in the previous call but only the
/// count made it into the caller's output. `value` is owed on the next
/// available output byte.
PartialPair { value: u8 },
}
#[derive(Debug, Clone, Copy)]
pub struct Encoder {
state: EncState,
}
impl Encoder {
pub const fn new() -> Self {
Self {
state: EncState::Empty,
}
}
}
impl Default for Encoder {
fn default() -> Self {
Self::new()
}
}
impl RawEncoder for Encoder {
fn raw_encode(&mut self, input: &[u8], output: &mut [u8]) -> Result<RawProgress, Error> {
let mut consumed = 0usize;
let mut written = 0usize;
loop {
// Always finish any owed value byte first.
if let EncState::PartialPair { value } = self.state {
if written == output.len() {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
output[written] = value;
written += 1;
self.state = EncState::Empty;
}
// Nothing more we can do without more input.
if consumed == input.len() {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
let b = input[consumed];
match self.state {
EncState::Empty => {
self.state = EncState::Run { byte: b, count: 1 };
consumed += 1;
}
EncState::Run { byte, count } if byte == b && count < u8::MAX => {
self.state = EncState::Run {
byte,
count: count + 1,
};
consumed += 1;
}
EncState::Run { byte, count } => {
// Need to flush this pair before we can keep going.
if written == output.len() {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
output[written] = count;
written += 1;
if written == output.len() {
// Only the count fit; remember to emit the value later.
self.state = EncState::PartialPair { value: byte };
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
output[written] = byte;
written += 1;
self.state = EncState::Empty;
// Loop continues — we still have not consumed `b`.
}
EncState::PartialPair { .. } => {
// Already handled at the top of the loop; unreachable here.
debug_assert!(false, "PartialPair should have been drained");
}
}
}
}
fn raw_finish(&mut self, output: &mut [u8]) -> Result<RawProgress, Error> {
let mut written = 0usize;
// Drain a pending value byte first.
if let EncState::PartialPair { value } = self.state {
if written == output.len() {
return Ok(RawProgress {
consumed: 0,
written,
done: false,
});
}
output[written] = value;
written += 1;
self.state = EncState::Empty;
}
// Emit the trailing run, if any.
if let EncState::Run { byte, count } = self.state {
if written == output.len() {
return Ok(RawProgress {
consumed: 0,
written,
done: false,
});
}
output[written] = count;
written += 1;
if written == output.len() {
self.state = EncState::PartialPair { value: byte };
return Ok(RawProgress {
consumed: 0,
written,
done: false,
});
}
output[written] = byte;
written += 1;
self.state = EncState::Empty;
}
Ok(RawProgress {
consumed: 0,
written,
done: matches!(self.state, EncState::Empty),
})
}
fn raw_reset(&mut self) {
self.state = EncState::Empty;
}
}
// ─── decoder ──────────────────────────────────────────────────────────────
#[derive(Debug, Clone, Copy)]
enum DecState {
/// Waiting for the next pair's count byte.
ExpectCount,
/// Read a count; waiting for the value byte that pairs with it.
ExpectValue { count: u8 },
/// Emitting a fully-decoded run; `remaining` bytes of `value` still owed.
EmittingRun { value: u8, remaining: u8 },
}
#[derive(Debug, Clone, Copy)]
pub struct Decoder {
state: DecState,
}
impl Decoder {
pub const fn new() -> Self {
Self {
state: DecState::ExpectCount,
}
}
}
impl Default for Decoder {
fn default() -> Self {
Self::new()
}
}
impl RawDecoder for Decoder {
fn raw_decode(&mut self, input: &[u8], output: &mut [u8]) -> Result<RawProgress, Error> {
let mut consumed = 0usize;
let mut written = 0usize;
loop {
match self.state {
DecState::EmittingRun { value, remaining } => {
if written == output.len() {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
let space = output.len() - written;
let n = core::cmp::min(remaining as usize, space);
for slot in &mut output[written..written + n] {
*slot = value;
}
written += n;
let new_remaining = remaining - n as u8;
if new_remaining == 0 {
self.state = DecState::ExpectCount;
} else {
self.state = DecState::EmittingRun {
value,
remaining: new_remaining,
};
// Output buffer is full, so we cannot continue.
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
}
DecState::ExpectCount => {
if consumed == input.len() {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
let count = input[consumed];
consumed += 1;
if count == 0 {
return Err(Error::Corrupt);
}
self.state = DecState::ExpectValue { count };
}
DecState::ExpectValue { count } => {
if consumed == input.len() {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
let value = input[consumed];
consumed += 1;
self.state = DecState::EmittingRun {
value,
remaining: count,
};
}
}
}
}
fn raw_finish(&mut self, output: &mut [u8]) -> Result<RawProgress, Error> {
let mut written = 0usize;
if let DecState::EmittingRun { value, remaining } = self.state {
let space = output.len() - written;
let n = core::cmp::min(remaining as usize, space);
for slot in &mut output[written..written + n] {
*slot = value;
}
written += n;
let new_remaining = remaining - n as u8;
if new_remaining == 0 {
self.state = DecState::ExpectCount;
} else {
self.state = DecState::EmittingRun {
value,
remaining: new_remaining,
};
return Ok(RawProgress {
consumed: 0,
written,
done: false,
});
}
}
match self.state {
DecState::ExpectCount => Ok(RawProgress {
consumed: 0,
written,
done: true,
}),
DecState::ExpectValue { .. } => Err(Error::UnexpectedEnd),
DecState::EmittingRun { .. } => Ok(RawProgress {
consumed: 0,
written,
done: false,
}),
}
}
fn raw_reset(&mut self) {
self.state = DecState::ExpectCount;
}
}