tower_of_hanoi.R

library(logician)
# Towers of Hanoi
# https://en.wikipedia.org/wiki/Tower_of_Hanoi
# Original Prolog Implementation:
# https://www.cs.toronto.edu/~sheila/384/w11/simple-prolog-examples.html

database <- logician_database(
   # no `is` operator yet, so we have to define all valid numbers
  num(1),num(2),num(3),num(4),num(5),
  num(6),num(7),num(8),num(9),
  move(1, X, Y, Z) := r(
    is.character(print(paste0("Move top disk from ", X, " to ", Y)))),
  move(N, X, Y, Z) := num(N) &&
    r(N > 1) && num(M) &&
    r(M == N - 1) &&
    move(M, X, Z, Y) &&
    move(1, X, Y, Temp) &&
    move(M, Z, Y, X)
)

# solve an instance where 3 disks are on the left peg.
iter <- logician_query(database, move(3, left, right, center))
iter$next_value()
#> [1] "Move top disk from left to right"
#> [1] "Move top disk from left to center"
#> [1] "Move top disk from right to center"
#> [1] "Move top disk from left to right"
#> [1] "Move top disk from center to left"
#> [1] "Move top disk from center to right"
#> [1] "Move top disk from left to right"
#> TRUE