day5-1-ungolfed.prdc

i   .. read full input
Ig  .. read int groups thereof
Hr  .. put this array into H, the default hoard
0   .. pc
λ   .. infinite loop
  ·   .. assign pc to bullet
  H=  .. index pc into H to get the "first value" of an instruction
  1h‰ .. divmod 100 to get the parameter mode value and the opcode

  .. •"% % <- % loop stuff"pk;

  [   .. case on the opcode...
    1 .. addition:
    {
      .. "addition"p
      A‰\A%‡ .. get the ones and then tens digit of the parameter mode value
      •)3+y  .. get the three addresses of the argument values
      Ht)    .. translate them; uncons the last
      ζ      .. under that last one, zip modes with addresses:
        \    .. swap mode on top, then
        H=b| .. index into H if it's false
      }u     .. (under)
      \      .. swap the last address under the list,
      Š      .. sum the list, and
      Hu     .. update H
      4      .. leave 4, indicating to increment pc by 4
    }
    2
    {
      .. "multiplication"p
      A‰\A%‡ .. get the ones and then tens digit of the parameter mode value
      •)3+y  .. get the three addresses of the argument values
      Ht)    .. translate them; uncons the last
      ζ      .. under that last one, zip modes with addresses:
        \    .. swap mode on top, then
        H=b| .. index into H if it's false
      }u     .. (under)
      \      .. swap the last address under the list,
      Þ      .. take the product of the list, and
      Hu     .. update H
      4      .. increment pc by 4
    }
    3
    {
      .. "input"p
      ;      .. pop the parameter mode value, not needed
      •)H=Ho .. update 1 where specified
      2
    }
    4
    {
      .. "output"p
      •)H=\  .. find argument, swap under imode
      H=b|   .. index again if imode is false
      :      .. duplicate;
        P_   .. if true, print
        ;_   .. if false, pop
      ?      .. if-else
      2
    }
    99
    {
      E .. exit
    }
    1_
    {
      "Unrecognized opcode" Ec
    }
  ]s
  •+
}