Day 5 - Part 1

require "importutils"

local input_p = getInput(5):split(",", true)

local dict =
{
    id_1    = {fn = "add",      params = 3},
    id_2    = {fn = "mult",     params = 3},
    id_3    = {fn = "in",       params = 1},
    id_4    = {fn = "out",      params = 1},
    id_99   = {fn = "halt",     params = 0},
}

local seed = 1 -- starting input
local output = 0

local function operate(input, id, params)
    local fn = dict[id].fn
    assert(fn == "halt" or (params and #params > 0), "No parameters passed for operation : "..fn)
    if fn == "out" then -- edge case
        if params[1].mode == 0 then params[1].val = input[params[1].val+1] end
    else
        for i=1, #params-1 do -- skip last since we want to keep the write address
            if params[i].mode == 0 then params[i].val = input[params[i].val+1] end
        end
    end
    if     fn == "add"  then return params[1].val + params[2].val, params[3].val
    elseif fn == "mult" then return params[1].val * params[2].val, params[3].val
    elseif fn == "in"   then return seed, params[1].val
    elseif fn == "out"  then return params[1].val, "out"
    elseif fn == "halt" then return "halt" end
end

local p = 1 -- input address pointer
function runIntcodeComp(input)
    assert(type(input) == "table", "Input is not a table. Must pass a parsed table as input.")
    while p < #input do
        local opcode, parse = input[p]
        local id = string.format("id_%d", math.floor(string.sub(opcode, -2))) --shaves off leading 0 if exists
        local params = {}
        if #tostring(opcode) > 2 then
            parse = tostring(opcode):splitByChar()
        end
        assert(dict[id],  "Invalid operation : "..id.." @ entry "..p)
        for i=1, dict[id].params do
            local t = {val=input[p+i], mode=0}
            if parse and parse[#parse-(i+1)] == "1" then t.mode = 1 end
            params[#params+1] = t
        end
        local result, pos = operate(input, id, params)
        if result == "halt" then
            print("Halt function has been pushed.")
            break
        end
        if pos == "out" then
            output = result
            print("Output is set to : "..output)
        else input[pos+1] = result end
        p = p + #params + 1
    end
end

runIntcodeComp(input_p)
print("\nOutput is : "..output)