USING: arrays assocs io kernel math namespaces
prettyprint sequences strings vectors words accessors ;
IN: turing

! A turing machine simulator.

TUPLE: state sym dir next ;

! Mapping from symbol/state pairs into new-state tuples
SYMBOL: states

! Halting state
SYMBOL: halt

! This is a simple program that outputs 5 1's
H{
    { { 1 0 } T{ state f 1  1 2    } }
    { { 2 0 } T{ state f 1  1 3    } }
    { { 3 0 } T{ state f 1 -1 1    } }
    { { 1 1 } T{ state f 1 -1 2    } }
    { { 2 1 } T{ state f 1 -1 3    } }
    { { 3 1 } T{ state f 1 -1 halt } }
} states set

! Current state
SYMBOL: state

! Initial state
1 state set

! Position of head on tape
SYMBOL: position

! Initial tape position
5 position set

! The tape, a mutable sequence of some kind
SYMBOL: tape

! Initial tape
20 0 <array> >vector tape set

: sym ( -- sym )
    ! Symbol at head position.
    position get tape get nth ;

: set-sym ( sym -- )
    ! Set symbol at head position.
    position get tape get set-nth ;

: next-state ( -- state )
    ! Look up the next state/symbol/direction triplet.
    state get sym 2array states get at ;

: turing-step ( -- )
    ! Do one step of the turing machine.
    next-state
    dup sym>> set-sym
    dup dir>> position [ + ] change
    next>> state set ;

: c ( -- )
    ! Print current turing machine state.
    state get .
    tape get .
    2 position get 2 * + CHAR: \s <string> write "^" print ;

: n ( -- )
    ! Do one step and print new state.
    turing-step c ;