Re: Hashing on more than one attribute?

[Enrique Garcia Cota <kikito@...>]

> How about simply having nested datastructures? Yes, it takes a few lookups instead of 1. But on the other hand you create less garbage to be collected when concating.

That is what I usually do when I have this problem.

I organize the cache in a "Tree-like" structure where each node has one child per key. Inserting/retrieving elements from the cache involves one loop instead of a single table.concat, but it avoids problems related with tostring + concat (name collisions are not possible and tables could all have the same tostring metamethod with no issue). 

function multiGet(store, key1, ...)

  assert(key1 ~= nil, "Must provide at least one key")

  local keys = {key1, ...}

  local node = store

  for i=1, #keys do

    if not node.children then return nil end

    node = node.children[keys[i]]

    if not node then return nil end

  end

  return node.value

end

function multiSet(store, value, key1, ...)

  assert(key1 ~= nil, "Must provide at least one key")

  local keys = {key1, ...}

  local node = store

  local key

  for i=1, #keys do

    key = keys[i]

    node.children = node.children or {}

    node.children[key] = node.children[key] or {}

    node = node.children[key]

  end

  node.value = value

end

local t = {}

multiSet(t, "a value", "foo", "oo", "oo")

multiGet(t, "foo", "oo", "oo") -- "a value"

multiGet(t, "fooo", "ooo") -- nil

I've used this strategy to organize the cache of my memoize with multiple params method:

https://github.com/kikito/memoize.lua

On Thu, Nov 8, 2012 at 9:15 AM, Axel Kittenberger <axkibe@gmail.com> wrote:

How about simply having nested datastructures? Yes, it takes a few lookups instead of 1. But on the other hand you create less garbage to be collected when concating.

On Thu, Nov 8, 2012 at 4:36 AM, Marc Lepage <mlepage@antimeta.com> wrote:

I know it's trivial to hash on a single value, by putting it into a table field. I want to effectively store a composite key in a table: say, a string, a number, and another string.

Is there a way I can somehow produce a single hash of that combination of three values, to use as a key in a field? Something not obvious? Or must I effectively have a few levels of tables to hash 3 values? (And more for 4, 5, 6 values...)

Thanks,

Marc