UXN Forth

variable score
0 constant rock
1 constant paper
2 constant scissors

: points-of 1+ ;
: victim ( u1 -- u2 ) 1- dup 0< if drop 2 exit then ;
: foil ( u1 -- u2 ) 1+ 3 mod ; 
: game-score ( u1 u2 -- u3 ) 
  2dup swap foil = if 2drop 6 exit then
  2dup swap victim = if 2drop 0 exit then
  2drop 3 ;
: add-score ( u1 u2 -- ) dup>r game-score r> points-of + score +! ;

\ data is code
: A rock ;
: B paper ;
: C scissors ;
: X rock add-score ;
: Y paper add-score ;
: Z scissors add-score ;
include input.txt 
score @ u. cr \ part 1

0 score !
: X dup victim add-score ;
: Y dup add-score ;
: Z dup foil add-score ;
include input.txt
score @ u. cr \ part 2
bye

A fun one, because the input is structured such that we can define forth words for each letter. No string parsing needed outside of the forth interpreter.