Get the index of an item by value in a redis list

Question

I have a redis list I have created, I am using it as a queue at the moment that reverses once in a while. My problem is that I would like to be able to get the index of an item on that queue/list by value.

Example

If I have a list with the following values:

{"dan","eduardo","pedro"}

The indexes would be:

0 : "dan"
1 : "eduardo"
2 : "pedro"

I want to be able by passing in the value to get the index of that value on my list.

Like "eduardo" and get back '1'.

Is that possible if so how would you do it?

Also something I should say is that I am performing queue commands to my list, removing items from the top and adding them at the bottom.

I am currently using node.js 0.6.6 and the latest redis module with the latest redis version 2.4.4.

I am happy for a solution just in redis-cli.

Also there is no constraint other then it must be possible to do it with redis alone, no external process etc however if you want to use the EVAL command with lua go for it.

Edit

Also I think my answer might be on sorted sets not queues.

Answer 1

I don't know the nodejs client details for this, but the following is an implementation of a very simple indexOf command in lua.

In a my file indexof.lua i have the following code:

local key = KEYS[1]
local obj = ARGV[1]
local items = redis.call('lrange', key, 0, -1)
for i=1,#items do
    if items[i] == obj then
        return i - 1
    end
end 
return -1

Lets push a few values to a mylist.

> rpush mylist foo bar baz qux
(integer) 4

We can use the lua script to find the index of any value within the list. The command is O(N).

$ redis-cli --eval indexof.lua mylist , bar
(integer) 1

index of bar was 1

> lindex mylist 1
"bar"

index of nil is -1

$ redis-cli --eval indexof.lua mylist , nil
(integer) -1

Look at the http://redis.io/commands/eval further documentation on EVAL command.