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defmodule ConsistentHash do
@docmodule "consistent hashing key/node mapping"
import Enum
@doc "Map a given key to a node of the ring in a consistent way (ring modifications move a minimum of keys)"
def node_for_key(ring,key), do: bfind(key_as_int(key),ring)
@vnode_per_node 300
@doc "generate the node_for_key ring parameter according to a given node list"
def ring_for_nodes(nodes) do
#Place nodes at @vnode_per_node dots in the key hash space {hash(node++vnode_idx),node},
#then create a bst adapted to consistent hashing traversal, for a given hash, find the next vnode dot in the ring
vnodes = nodes |> flat_map(fn n -> 1..@vnode_per_node |> map(&{key_as_int("#{n}#{&1}"),n}) end)
|> sort(fn {h1,_},{h2,_}->h2>h1 end)
vnodes |> bsplit({0,trunc(:math.pow(2,160)-1)},vnodes|>List.first)
# "place each term into int hash space : term -> 160bits bin -> integer"
defp key_as_int(key),do: :crypto.hash(:sha, :erlang.term_to_binary(key)) |> hash_as_int
defp hash_as_int(<<key::size(160)-integer>>),do: key
# dedicated binary search tree, middle split each interval (easy tree
# balancing) except when only one vnode is there (split at vnode hash)
defp bsplit([],{_,_},{_,next}), do: next # if no vnode dot in interval, take next node in the ring
defp bsplit([{h,n}],{_,_},{_,next}), do: {h,n,next} # interval contains a vnode split
defp bsplit(list,{lbound,rbound},next) do # interval contains multiple vnode, recursivly middle split allows easy tree balancing
center = lbound + (rbound - lbound)/2
{left,right} = list |> split_while(fn {h,_n}->h<center end)
# bsplit is designed to allow standard btree traversing to associate node to hash
defp bfind(_,node) when is_atom(node), do: node
defp bfind(k,{center,_,right}) when k > center, do: bfind(k,right)
defp bfind(k,{center,left,_}) when k <= center, do: bfind(k,left)