11.wsf

# Project Euler
# Problem 11: Largest product in a grid
# https://projecteuler.net/problem=11

    0 ^ readi retrieve
    ^ call read_square_grid
    ^ ^ * swap

    1
.loop_horiz:
    ^ ^2 + swap # i += n
    ^1 1 call line_prods # i i+n 1
    ^ ^3 j< .loop_horiz # i < n*n
    drop

    0
.loop_vert:
    1+ # i++
    ^ ^3 ^3 call line_prods # i n*n n
    ^ ^2 j< .loop_vert # i < n
    drop

.loop_diag:

    end

# read_square_grid reads a square grid of two-digit numbers into the
# heap, starting at address 1.
# (n -- )
read_square_grid:
    ^ *
    1 # addr
.read_square_grid_loop:
    2dup j< .read_square_grid_ret
    ^
    ^  ^ readc retrieve '0' - 10*
    ^1 ^ readc retrieve '0' - +
    ^1 readc # discard ' ' or '\n'
    store
    1+
    jmp .read_square_grid_loop
.read_square_grid_ret:
    2drop ret

# (start end stride -- )
line_prods:
    ^2 retrieve
    ^3 ^2 + retrieve * # half0 = grid[0] * grid[stride]
    ^3 ^2 2* +         # i = stride * 2
.line_prods_loop:
    swap
    ^1 retrieve
    ^2 ^4 + retrieve * # half1 = grid[i] * grid[i+stride]
    swap ^1 *          # prod = half0 * half1
    0 retrieve ^1 j< .line_prods_greater_prod
    drop
.line_prods_loop_next:
    swap ^2 2* +       # i += stride * 2
    ^ ^4 j< .line_prods_loop
    5drop ret
.line_prods_greater_prod:
    0 swap store
    jmp .line_prods_loop_next