test

local A = {
 [1] = {name="minecraft:coal", count=64},
 [5] = {name="minecraft:wheat_seeds", count=54},
 [7] = {name="minecraft:wheat_seeds", count=21}
}

local total = 0
for k,v in pairs(A) do
 print(v.name .. " : " .. v.count)
end

local non_full = 0
for i=1,16 do
 if A[i] ~= nil then
  if A[i].name == "minecraft:wheat_seeds" and total < 64 then
   total = total + A[i].count
   --combine stacks
   if non_full == 0 then
    non_full = i
   end

   if non_full ~= 0 then
    A[non_full] = A[non_full] or {name="minecraft:wheat_seeds", count=0}
    local slot_count = A[non_full].count
    if slot_count < 64 then
     local diff = math.abs(64 - slot_count)
     printError(diff)
     A[non_full].count = A[non_full].count + diff
     A[i].count = A[i].count - diff
    end
   end
  elseif A[i].name == "minecraft:wheat_seeds" and total >= 64 then
   A[i].count = 0
  end
 else
   if non_full == 0 then non_full = i end
 end
end
print(non_full)
print(total)
for k,v in pairs(A) do
 print(v.name .. " : " .. v.count)
end