day2-1.prdc

iIgHr2:
12HudHL,4/εHt(3%"Q+*"=S{)\Ht~}uPdHu};sHq .. 48 bytes

{
i   .. read full input
Ig  .. read int groups thereof
Hr  .. put this array into H, the default hoard
2:  .. put 2 on the stack and duplicate it

12 .. put 1 and 12 on the stack (yes that's U+0001 START OF HEADING sorry)
    .. I should make this α on the codepage or something. Anyway, paradoc's
    .. push-constants-onto-stack story still needs improvement.
Hud .. update H (Hu) with two disjoint argument groups on the stack (_d)
HL, .. take the length of H and convert it to indices
4/  .. divide into chunks of 4
ε   .. for each chunk:
  Ht   .. "translate" this through hoard to get four relevant contents of memory
       .. (honestly i'm so impressed by my past self's prescience to make this
       .. a hoard trailer)
  (    .. uncons to get the opcode
  3%   .. mod 3: 1 -> 1, 2 -> 2, 99 -> 0
  "Q+*"= .. find a char that says what operation we want to do; this will be
         .. evaluated as Paradoc (Q is break out of the loop)
         .. (a two-byte "index mod length of the list" built-in would save a
         .. char here and seems like a good idea)
  S    .. convert to string
       .. stack: addr1 addr2 addr3 opstr
  {    .. ignoring opstr:
    )    .. uncons: [addr1 addr2] addr3
    \    .. swap, need to lookup in memory one more time
    Ht   .. translate through hoard: stack: addr3 [val1 val2]
    ~    .. expand onto stack: addr3 val1 val2
  }u   .. do the preceding block under the top element of the stack
       .. stack: addr3 val1 val2 opstr
  Pd   .. evaluate the string as paradoc
       .. stack: addr3 resultvalue
  Hu   .. update hoard at that position to that value
}
;s  .. pop everything off the stack
Hq  .. get the first element of the hoard
};