Listener for Foo Protocol example (Wireshark Lua)

-- ############################################################################
-- Continuing our Dissector example at [1], let's say we wanted to print the value
-- of `foo.str` for packets whose `foo.num` field is >= sqrt(5) (I picked a
-- condition that couldn't already be achieved with display filters).
--
-- This Wireshark Lua script implements a listener for this purpose.
--
-- [1]: http://pastebin.com/8j0LhVTQ
-- ############################################################################

-- 1. Declare the tap with the `Listener()` function
local tap = Listener.new(nil, 'foo')

-- 2. Declare field extractors to pull values of named fields from the current packet
local f_str = Field.new('foo.str')
local f_num = Field.new('foo.num')

-- 3. Declare the tap's packet function
function tap.packet(pinfo, buf)

    -- check if at least one instance of foo.num is >= sqrt(5)
    local gt_sqrt5 = false
    for _,v in ipairs({ f_num() }) do
        if v >= math.sqrt(5) then
            gt_sqrt5 = true
            break
        end
    end

    if gt_sqrt5 then
        -- print all instances of foo.str from the current packet
        for _,v in ipairs({ f_str() }) do
            info(pinfo.number..'foo.str'..v)
        end
    end
end