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// This implementation mirrors the rendezvous hash implementation
// in the go and python services.
// The go implementation is located go/internal/utils/rendezvous_hash.go
// The python implementation is located chromadb/utils/rendezvous_hash.py
use crate::errors::{ChromaError, ErrorCodes};
use std::io::Cursor;
use thiserror::Error;
use murmur3::murmur3_x64_128;
/// A trait for hashing a member and a key to a score.
pub(crate) trait Hasher {
fn hash(&self, member: &str, key: &str) -> Result<u64, AssignmentError>;
}
/// Error codes for assignment
#[derive(Error, Debug)]
pub(crate) enum AssignmentError {
#[error("Cannot assign empty key")]
EmptyKey,
#[error("No members to assign to")]
NoMembers,
#[error("Error hashing member")]
HashError,
}
impl ChromaError for AssignmentError {
fn code(&self) -> ErrorCodes {
match self {
AssignmentError::EmptyKey => ErrorCodes::InvalidArgument,
AssignmentError::NoMembers => ErrorCodes::InvalidArgument,
AssignmentError::HashError => ErrorCodes::Internal,
}
}
}
/// Assign a key to a member using the rendezvous hash algorithm.
/// # Arguments
/// - key: The key to assign.
/// - members: The members to assign to.
/// - hasher: The hasher to use.
/// # Returns
/// The member that the key was assigned to.
/// # Errors
/// - If the key is empty.
/// - If there are no members to assign to.
/// - If there is an error hashing a member.
/// # Notes
/// This implementation mirrors the rendezvous hash implementation
/// in the go and python services.
pub(crate) fn assign<H: Hasher>(
key: &str,
members: impl IntoIterator<Item = impl AsRef<str>>,
hasher: &H,
) -> Result<String, AssignmentError> {
if key.is_empty() {
return Err(AssignmentError::EmptyKey);
}
let mut iterated = false;
let mut max_score = u64::MIN;
let mut max_member = None;
for member in members {
if !iterated {
iterated = true;
}
let score = hasher.hash(member.as_ref(), key);
let score = match score {
Ok(score) => score,
Err(err) => return Err(AssignmentError::HashError),
};
if score > max_score {
max_score = score;
max_member = Some(member);
}
}
if !iterated {
return Err(AssignmentError::NoMembers);
}
match max_member {
Some(max_member) => return Ok(max_member.as_ref().to_string()),
None => return Err(AssignmentError::NoMembers),
}
}
fn merge_hashes(x: u64, y: u64) -> u64 {
let mut acc = x ^ y;
acc ^= acc >> 33;
acc = acc.wrapping_mul(0xFF51AFD7ED558CCD);
acc ^= acc >> 33;
acc = acc.wrapping_mul(0xC4CEB9FE1A85EC53);
acc ^= acc >> 33;
acc
}
pub(crate) struct Murmur3Hasher {}
impl Hasher for Murmur3Hasher {
fn hash(&self, member: &str, key: &str) -> Result<u64, AssignmentError> {
let member_hash = murmur3_x64_128(&mut Cursor::new(member), 0);
let key_hash = murmur3_x64_128(&mut Cursor::new(key), 0);
// The murmur library returns a 128 bit hash, but we only need 64 bits, grab the first 64 bits
match (member_hash, key_hash) {
(Ok(member_hash), Ok(key_hash)) => {
let member_hash_64 = member_hash as u64;
let key_hash_64 = key_hash as u64;
let merged = merge_hashes(member_hash_64, key_hash_64);
return Ok(merged);
}
_ => return Err(AssignmentError::HashError),
};
}
}
#[cfg(test)]
mod tests {
use super::*;
struct MockHasher {}
impl Hasher for MockHasher {
fn hash(&self, member: &str, _key: &str) -> Result<u64, AssignmentError> {
match member {
"a" => Ok(1),
"b" => Ok(2),
"c" => Ok(3),
_ => Err(AssignmentError::HashError),
}
}
}
#[test]
fn test_assign() {
let members = vec!["a", "b", "c"];
let hasher = MockHasher {};
let key = "key";
let member = assign(key, members, &hasher).unwrap();
assert_eq!(member, "c".to_string());
}
#[test]
fn test_even_distribution() {
let member_count = 10;
let tolerance = 25;
let mut nodes = Vec::with_capacity(member_count);
let hasher = Murmur3Hasher {};
for i in 0..member_count {
let member = format!("member{}", i);
nodes.push(member);
}
let mut counts = vec![0; member_count];
let num_keys = 1000;
for i in 0..num_keys {
let key = format!("key_{}", i);
let member = assign(&key, &nodes, &hasher).unwrap();
let index = nodes.iter().position(|x| *x == member).unwrap();
counts[index] += 1;
}
let expected = num_keys / member_count;
for i in 0..member_count {
let count = counts[i];
let diff = count - expected as i32;
assert!(diff.abs() < tolerance);
}
}
}
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