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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
use serde::Serialize;
use std::{
io::{self, Write},
num::NonZeroUsize,
};
/// A quantity of items that can fit into a payload, accounting for
/// serialization, encryption, and Base64-encoding overhead.
pub enum Fit {
/// All items can fit into the payload.
All,
/// Some, but not all, items can fit into the payload without
/// exceeding the maximum payload size.
Some(NonZeroUsize),
/// The maximum payload size is too small to hold any items.
None,
/// The serialized size of the items couldn't be determined because of
/// a serialization error.
Err(serde_json::Error),
}
impl Fit {
/// If `self` is [`Fit::Some`], returns the number of items that can fit
/// into the payload without exceeding its maximum size. Otherwise,
/// returns `None`.
#[inline]
pub fn as_some(&self) -> Option<NonZeroUsize> {
match self {
Fit::Some(count) => Some(*count),
_ => None,
}
}
}
/// A writer that counts the number of bytes it's asked to write, and discards
/// the data. Used to compute the serialized size of an item.
#[derive(Clone, Copy, Default)]
struct ByteCountWriter(usize);
impl ByteCountWriter {
#[inline]
pub fn count(self) -> usize {
self.0
}
}
impl Write for ByteCountWriter {
#[inline]
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
self.0 += buf.len();
Ok(buf.len())
}
#[inline]
fn flush(&mut self) -> io::Result<()> {
Ok(())
}
}
/// Returns the size of the given value, in bytes, when serialized to JSON.
pub fn compute_serialized_size<T: Serialize + ?Sized>(value: &T) -> serde_json::Result<usize> {
let mut w = ByteCountWriter::default();
serde_json::to_writer(&mut w, value)?;
Ok(w.count())
}
/// Calculates the maximum number of items that can fit within
/// `max_payload_size` when serialized to JSON.
pub fn try_fit_items<T: Serialize>(items: &[T], max_payload_size: usize) -> Fit {
let size = match compute_serialized_size(&items) {
Ok(size) => size,
Err(e) => return Fit::Err(e),
};
let max_serialized_size = match ((max_payload_size / 4) * 3).checked_sub(1500) {
Some(max_serialized_size) => max_serialized_size,
None => return Fit::None,
};
if size > max_serialized_size {
// Estimate a little more than the direct fraction to maximize packing
let mut cutoff = (items.len() * max_serialized_size - 1) / size + 1;
// Keep dropping off the last entry until the data fits.
while cutoff > 0 {
let size = match compute_serialized_size(&items[..cutoff]) {
Ok(size) => size,
Err(e) => return Fit::Err(e),
};
if size <= max_serialized_size {
break;
}
cutoff -= 1;
}
match NonZeroUsize::new(cutoff) {
Some(count) => Fit::Some(count),
None => Fit::None,
}
} else {
Fit::All
}
}
#[cfg(test)]
mod tests {
use super::*;
use serde_derive::Serialize;
#[derive(Serialize)]
struct CommandRecord {
#[serde(rename = "command")]
name: &'static str,
#[serde(default)]
args: &'static [Option<&'static str>],
#[serde(default, rename = "flowID", skip_serializing_if = "Option::is_none")]
flow_id: Option<&'static str>,
}
const COMMANDS: &[CommandRecord] = &[
CommandRecord {
name: "wipeEngine",
args: &[Some("bookmarks")],
flow_id: Some("flow"),
},
CommandRecord {
name: "resetEngine",
args: &[Some("history")],
flow_id: Some("flow"),
},
CommandRecord {
name: "logout",
args: &[],
flow_id: None,
},
];
#[test]
fn test_compute_serialized_size() {
assert_eq!(compute_serialized_size(&1).unwrap(), 1);
assert_eq!(compute_serialized_size(&"hi").unwrap(), 4);
assert_eq!(
compute_serialized_size(&["hi", "hello", "bye"]).unwrap(),
20
);
let sizes = COMMANDS
.iter()
.map(|c| compute_serialized_size(c).unwrap())
.collect::<Vec<_>>();
assert_eq!(sizes, &[61, 60, 30]);
}
#[test]
fn test_try_fit_items() {
// 4096 bytes is enough to fit all three commands.
assert!(matches!(try_fit_items(COMMANDS, 4096), Fit::All));
// `logout` won't fit within 2168 bytes.
assert_eq!(try_fit_items(COMMANDS, 2168).as_some().unwrap().get(), 2);
// `resetEngine` won't fit within 2084 bytes.
assert_eq!(try_fit_items(COMMANDS, 2084).as_some().unwrap().get(), 1);
// `wipeEngine` won't fit at all.
assert!(matches!(try_fit_items(COMMANDS, 1024), Fit::None));
}
}