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use std::collections::hash_map::DefaultHasher;
use std::convert::TryInto;
use std::hash::{Hash, Hasher};
#[derive(Debug, PartialEq)]
pub struct Maglev {
table: Vec<u64>,
backends: Vec<String>,
}
fn calculate_hash<T: Hash>(v: &T, hasher: &mut DefaultHasher) -> u64 {
v.hash(hasher);
hasher.finish()
}
impl Maglev {
pub fn new(backends: &Vec<String>, table_size: u64) -> Result<Maglev, &str> {
if primes::is_prime(table_size) {
Ok(Maglev {
table: Maglev::populate(backends, table_size as usize),
backends: backends.to_vec(),
})
} else {
Err("Table size should be a prime")
}
}
pub fn get_backend(&self, idx: usize) -> Option<String> {
self.table
.get(idx)
.map(|i| String::from(&self.backends[*i as usize]))
}
pub fn put_backend(&mut self, backend: &String) -> Result<(), &str> {
let matching: Vec<&String> = self.backends.iter().filter(|b| b == &backend).collect();
match matching.is_empty() {
false => Err("Backend already exists"),
true => {
self.backends.push(String::from(backend));
Ok(self.reset())
}
}
}
pub fn remove_backend(&mut self, backend: &String) -> Result<(), &str> {
let mut idx = None;
for (i, b) in self.backends.iter().enumerate() {
if b == backend {
idx = Some(i);
break;
}
}
match idx {
None => Err("Backend not found"),
Some(idx) => {
self.backends.swap_remove(idx);
Ok(self.reset())
}
}
}
fn reset(&mut self) {
self.table = Maglev::populate(&self.backends, self.table.len())
}
fn populate(backends: &Vec<String>, table_size: usize) -> Vec<u64> {
let permutations = Maglev::get_permutations(backends, table_size as u64);
let num_backends = backends.len();
let mut next = vec![0; num_backends];
let mut entry = vec![-1; table_size];
let mut n = 0;
'outer: loop {
for i in 0..num_backends {
let mut c = permutations[i][next[i]];
while entry[c as usize] >= 0 {
next[i] = next[i] + 1;
c = permutations[i][next[i]]
}
entry[c as usize] = i.try_into().unwrap();
next[i] = next[i] + 1;
n = n + 1;
if n == table_size {
break 'outer;
}
}
}
entry.into_iter().map(|x| x as u64).collect()
}
fn get_permutations(backends: &Vec<String>, table_size: u64) -> Vec<Vec<u64>> {
let n = backends.len();
let mut permutations: Vec<Vec<u64>> = Vec::new();
let (offsets, skips) = Maglev::get_offsets_and_skips(backends, table_size);
for i in 0..n {
let offset = offsets[i];
let skip = skips[i];
let permutation: Vec<u64> = (0..table_size)
.map(|x| (offset + x * skip) % table_size)
.collect();
permutations.push(permutation)
}
permutations
}
fn get_offsets_and_skips(backends: &Vec<String>, table_size: u64) -> (Vec<u64>, Vec<u64>) {
let mut hasher = DefaultHasher::new();
let mut skips = Vec::new();
let mut offsets = Vec::new();
for backend in backends {
let hash = calculate_hash(backend, &mut hasher);
let offset = (hash << 32) % table_size;
let skip = (hash & 0xffffffff) % (table_size - 1) + 1;
offsets.push(offset);
skips.push(skip);
}
(offsets, skips)
}
}