use std::io::Write;
use crate::{
Error,
layout::{add, multiply, slice, u32_at, usize_at},
};
#[derive(Debug, Clone, Copy)]
pub(crate) enum Slots {
Spans(usize),
Fixed { length: usize, width: u8 },
}
impl Slots {
pub(crate) fn select(
records: usize,
payloads: usize,
length: Option<usize>,
) -> Result<Self, Error> {
let spans = Self::Spans(if payloads <= u32::MAX as usize { 8 } else { 16 });
let Some(length) = length.filter(|&length| length <= u32::MAX as usize) else {
return Ok(spans);
};
if records == 0 || payloads > u32::MAX as usize {
return Ok(spans);
}
let width = reference_width(payloads, length)?;
let fixed = Self::Fixed { length, width };
Ok(if fixed.size(records)? < spans.size(records)? {
fixed
} else {
spans
})
}
pub(crate) fn size(self, records: usize) -> Result<usize, Error> {
match self {
Self::Spans(width) => multiply(records, width),
Self::Fixed { width, .. } => add(8, multiply(records, usize::from(width))?),
}
}
pub(crate) fn read(bytes: &[u8], offset: usize, payloads: usize) -> Result<Self, Error> {
let length = u32_at(bytes, offset)? as usize;
let width = *bytes
.get(add(offset, 4)?)
.ok_or(Error::Invalid("payload reference width"))?;
if slice(bytes, add(offset, 5)?, 3)?
.iter()
.any(|&byte| byte != 0)
|| width != reference_width(payloads, length)?
{
return Err(Error::Invalid("fixed payload fields"));
}
Ok(Self::Fixed { length, width })
}
#[inline]
pub(crate) fn span(
self,
bytes: &[u8],
slots: usize,
id: usize,
) -> Result<(usize, usize), Error> {
match self {
Self::Spans(width) => {
let slot = add(slots, multiply(id, width)?)?;
if width == 8 {
Ok((
u32_at(bytes, slot)? as usize,
u32_at(bytes, add(slot, 4)?)? as usize,
))
} else {
Ok((usize_at(bytes, slot)?, usize_at(bytes, add(slot, 8)?)?))
}
}
Self::Fixed { length, width } => {
if width == 0 {
return Ok((0, length));
}
let start = add(add(slots, 8)?, multiply(id, usize::from(width))?)?;
let row = slice(bytes, start, usize::from(width))?;
let index = match row {
[a] => u32::from(*a),
[a, b] => u32::from(u16::from_le_bytes([*a, *b])),
[a, b, c] => u32::from_le_bytes([*a, *b, *c, 0]),
[a, b, c, d] => u32::from_le_bytes([*a, *b, *c, *d]),
_ => return Err(Error::Invalid("payload reference width")),
};
Ok((multiply(index as usize, length)?, length))
}
}
}
}
pub(crate) struct SlotWriter<'a, W> {
output: &'a mut W,
slots: Slots,
}
impl<'a, W: Write> SlotWriter<'a, W> {
pub(crate) fn new(output: &'a mut W, slots: Slots) -> Result<Self, Error> {
if let Slots::Fixed { length, width } = slots {
output.write_all(&(length as u32).to_le_bytes())?;
output.write_all(&[width, 0, 0, 0])?;
}
Ok(Self { output, slots })
}
pub(crate) fn push(&mut self, offset: u64, length: u64) -> Result<(), Error> {
match self.slots {
Slots::Spans(8) => {
let offset =
u32::try_from(offset).map_err(|_| Error::Invalid("compact payload span"))?;
let length =
u32::try_from(length).map_err(|_| Error::Invalid("compact payload span"))?;
self.output.write_all(&offset.to_le_bytes())?;
self.output.write_all(&length.to_le_bytes())?;
}
Slots::Spans(_) => {
self.output.write_all(&offset.to_le_bytes())?;
self.output.write_all(&length.to_le_bytes())?;
}
Slots::Fixed {
length: expected,
width,
} => {
if length != expected as u64
|| (length == 0 && offset != 0)
|| (length != 0 && !offset.is_multiple_of(length))
{
return Err(Error::Invalid("fixed payload span"));
}
let index = offset.checked_div(length).unwrap_or(0);
if index >= (1u64 << (width * 8)) {
return Err(Error::Invalid("payload reference"));
}
self.output.write_all(slice(
&(index as u32).to_le_bytes(),
0,
usize::from(width),
)?)?;
}
}
Ok(())
}
}
fn reference_width(payloads: usize, length: usize) -> Result<u8, Error> {
if length == 0 {
return if payloads == 0 {
Ok(0)
} else {
Err(Error::Invalid("fixed payload length"))
};
}
if payloads == 0 || payloads > u32::MAX as usize || !payloads.is_multiple_of(length) {
return Err(Error::Invalid("fixed payload length"));
}
let maximum = payloads / length - 1;
Ok((usize::BITS - maximum.leading_zeros()).div_ceil(8) as u8)
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn references_roundtrip_every_width_and_tail() {
for width in 1..=4u8 {
let slots = Slots::Fixed { length: 1, width };
let maximum = (1u64 << (width * 8)) - 1;
let expected = [0, maximum, maximum / 2, 1, 0, maximum, 1, maximum / 2, 0];
let mut bytes = Vec::new();
let mut writer = SlotWriter::new(&mut bytes, slots).unwrap();
for &value in &expected {
writer.push(value, 1).unwrap();
}
assert_eq!(bytes.len(), slots.size(expected.len()).unwrap());
for (id, &value) in expected.iter().enumerate() {
assert_eq!(slots.span(&bytes, 0, id).unwrap(), (value as usize, 1));
}
}
}
}