hdi/hash_path/shard.rs
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use crate::hash_path::path::Component;
use crate::hash_path::path::Path;
use std::str::FromStr;
/// Separates the shard width and depth.
pub const SHARDSPLIT: &str = ":";
/// Terminates the end of a shard shorthand.
pub const SHARDEND: &str = "#";
/// The width of a shard is how many bytes/characters to use for each path component in sharding.
/// e.g. abcdef with width 1 shards to a.b.c.d.e.f.abcdef and 2 shards to ab.cd.ef.abcdef.
pub type ShardWidth = u32;
/// The depth of a shard is the number of path components to stretch out for shards.
/// e.g. abcdef with a depth of 1 and width 1 shards to a.abcdef and depth 2 shards to a.b.abcdef.
pub type ShardDepth = u32;
#[derive(Debug)]
/// A valid strategy for sharding requires both a width and a depth.
/// At the moment sharding only works well for data that is reliably longer than width/depth.
/// For example, sharding the username foo with width 4 doesn't make sense.
/// There is no magic padding or extending of the provided data to make up undersized shards.
/// @todo stretch short shards out in a nice balanced way (append some bytes from the hash?)
pub struct ShardStrategy(ShardWidth, ShardDepth);
/// impl [ `ShardStrategy` ] as an immutable/read-only thingy.
impl ShardStrategy {
fn width(&self) -> ShardWidth {
self.0
}
fn depth(&self) -> ShardDepth {
self.1
}
}
#[derive(Debug)]
pub enum ParseShardStrategyError {
/// Could not parse the shard depth.
BadDepth,
/// Could not parse the shard width.
BadWidth,
/// Failed to find the separator between width and depth.
ShardSplitNotFound,
/// Failed to find the end of the sharding definition.
ShardEndNotFound,
/// The sharding definition does not start with a number.
FirstCharNotADigit,
/// The sharding definition is empty.
EmptyString,
}
/// Attempt to parse a "width:depth#" shard out of a string.
/// This function looks way scarier than it is.
/// Each level of nesting is just handling a potential parse failure.
impl FromStr for ShardStrategy {
type Err = ParseShardStrategyError;
/// A shard strategy is parsed as "width:depth#..." at the start of a string.
fn from_str(s: &str) -> Result<Self, Self::Err> {
// The first char needs to be a digit.
match s.chars().next() {
Some(first_char) => {
match u32::from_str(&first_char.to_string()) {
Ok(_) => {
// There needs to be a #
match s.find(SHARDEND) {
Some(end_index) => {
let (maybe_strategy, _) = s.split_at(end_index);
match maybe_strategy.find(SHARDSPLIT) {
Some(split_index) => {
let (maybe_width, maybe_depth) =
maybe_strategy.split_at(split_index);
match u32::from_str(maybe_width) {
Ok(width) => {
match u32::from_str(
maybe_depth.trim_start_matches(SHARDSPLIT),
) {
Ok(depth) => Ok(ShardStrategy(width, depth)),
Err(_) => {
Err(ParseShardStrategyError::BadDepth)
}
}
}
Err(_) => Err(ParseShardStrategyError::BadWidth),
}
}
None => Err(ParseShardStrategyError::ShardSplitNotFound),
}
}
None => Err(ParseShardStrategyError::ShardEndNotFound),
}
}
Err(_) => Err(ParseShardStrategyError::FirstCharNotADigit),
}
}
None => Err(ParseShardStrategyError::EmptyString),
}
}
}
/// Builds a path for a shard strategy and some binary bytes.
/// This is the trivial case, we just split the bytes out one by one and make a path from it.
impl From<(&ShardStrategy, &[u8])> for Path {
fn from((strategy, bytes): (&ShardStrategy, &[u8])) -> Path {
let full_length = strategy.width() * strategy.depth();
// Fold a flat slice of bytes into `strategy.depth` number of `strategy.width` length byte
// [ `Component` ]s.
let sharded: Vec<Component> = bytes
.iter()
.take(full_length as _)
.fold((vec![], vec![]), |acc, b| {
let (mut ret, mut build) = acc;
build.push(b);
if build.len() == strategy.width() as usize {
ret.push(build.clone());
build.clear();
}
(ret, build)
})
.0
.iter()
.map(|bytes| {
let bytes_vec: Vec<u8> = bytes.iter().map(|b| **b).collect();
Component::from(bytes_vec)
})
.collect();
Path::from(sharded)
}
}
/// Wrapper around `&Vec<u8>` to work the same as &[u8].
impl From<(&ShardStrategy, &Vec<u8>)> for Path {
fn from((strategy, bytes): (&ShardStrategy, &Vec<u8>)) -> Path {
let bytes: &[u8] = bytes.as_ref();
Path::from((strategy, bytes))
}
}
/// Wrapper around `Vec<u8>` to work the same as &[u8].
impl From<(&ShardStrategy, Vec<u8>)> for Path {
fn from((strategy, bytes): (&ShardStrategy, Vec<u8>)) -> Path {
let bytes: &[u8] = bytes.as_ref();
Path::from((strategy, bytes))
}
}
/// Create [ `Path` ] from [ `String` ].
/// To ensure that this works for all utf8, which can have anywhere from 1-4 bytes for a single
/// character, we first represent each character as a utf32 so it gets padded out with 0 bytes.
/// This means the width is 4x what it would be for raw bytes with the same strategy.
impl From<(&ShardStrategy, &str)> for Path {
fn from((strategy, s): (&ShardStrategy, &str)) -> Path {
// Truncate the string to only relevant chars.
let full_length = strategy.width() * strategy.depth();
let shard_string: String = s.chars().take(full_length as _).collect();
Path::from((
&ShardStrategy(
// Relies on the fact that we're encoding string characters as fixed width u32
// bytes rather than variable width utf8 bytes.
strategy.width() * std::mem::size_of::<u32>() as u32,
strategy.depth(),
),
// Defer to the standard utf32 string handling to get the fixed size byte and endian
// handling correct.
Component::from(&shard_string).as_ref(),
))
}
}
/// [`&String`](std::string::String) wrapper mimicking [`&str`] for [`Path`] building.
impl From<(&ShardStrategy, &String)> for Path {
fn from((strategy, s): (&ShardStrategy, &String)) -> Path {
Path::from((strategy, s.as_str()))
}
}
// [`String`] wrapper mimicking [`&str`] for [`Path`] building.
impl From<(&ShardStrategy, String)> for Path {
fn from((strategy, s): (&ShardStrategy, String)) -> Path {
Path::from((strategy, s.as_str()))
}
}
#[test]
#[cfg(test)]
fn hash_path_shard_bytes() {
for (width, depth, b, output) in vec![
// Anything with a zero results in an empty path.
(0, 0, vec![1, 2, 3, 4, 5], Path::from(vec![])),
(0, 1, vec![1, 2, 3, 4, 5], Path::from(vec![])),
(1, 0, vec![1, 2, 3, 4, 5], Path::from(vec![])),
(0, 2, vec![1, 2, 3, 4, 5], Path::from(vec![])),
(2, 0, vec![1, 2, 3, 4, 5], Path::from(vec![])),
// Basic sharding behaviour.
(
1,
1,
vec![1, 2, 3, 4, 5],
Path::from(vec![Component::from(vec![1_u8])]),
),
(
2,
1,
vec![1, 2, 3, 4, 5],
Path::from(vec![Component::from(vec![1_u8, 2_u8])]),
),
(
1,
2,
vec![1, 2, 3, 4, 5],
Path::from(vec![
Component::from(vec![1_u8]),
Component::from(vec![2_u8]),
]),
),
(
2,
2,
vec![1, 2, 3, 4, 5],
Path::from(vec![
Component::from(vec![1_u8, 2_u8]),
Component::from(vec![3_u8, 4_u8]),
]),
),
] {
assert_eq!(output, Path::from((&ShardStrategy(width, depth), &b)));
let bytes: &[u8] = b.as_ref();
assert_eq!(output, Path::from((&ShardStrategy(width, depth), bytes)));
assert_eq!(output, Path::from((&ShardStrategy(width, depth), b)));
}
}
#[test]
#[cfg(test)]
fn hash_path_shard_string() {
for (width, depth, s, output) in vec![
// Anything with a zero results in an empty path.
(0, 0, "foobar", Path::from("")),
(0, 1, "foobar", Path::from("")),
(1, 0, "foobar", Path::from("")),
(0, 2, "foobar", Path::from("")),
(2, 0, "foobar", Path::from("")),
// Basic sharding behaviour.
(1, 1, "foobar", Path::from("f")),
(2, 1, "foobar", Path::from("fo")),
(1, 2, "foobar", Path::from("f.o")),
(2, 2, "foobar", Path::from("fo.ob")),
// Multibyte characters should be handled the way a naive understanding of strings would
// expect, i.e. that a 2-byte utf8 character is represented as 1 4-byte utf32 character and
// so counts as 1 "width" and 1 "depth" for the purpose of sharding.
(2, 2, "€€€€", Path::from("€€.€€")),
// If the string is shorter than the width and depth we go as deep as we can cleanly and
// truncate the end.
(4, 4, "foobar", Path::from("foob")),
(4, 4, "foobarbaz", Path::from("foob.arba")),
(4, 4, "€€€€€€€€€", Path::from("€€€€.€€€€")),
] {
assert_eq!(output, Path::from((&ShardStrategy(width, depth), s)));
assert_eq!(
output,
Path::from((&ShardStrategy(width, depth), s.to_string()))
);
assert_eq!(
output,
Path::from((&ShardStrategy(width, depth), &s.to_string()))
);
}
}