day23.nim

const N = 1_000_000

type ArrayRing = ref object
    ring: array[1..N, int]
    head: int

proc toHashRing(xs: openArray[int]): ArrayRing =
    var hr: ArrayRing
    new(hr)
    var last = xs[0]
    for x in xs[1..<len(xs)]:
        hr.ring[last] = x
        last = x
    hr.ring[last] = xs[0]
    hr.head = xs[0]
    return hr

proc step(hr: var ArrayRing) =
    var a = hr.ring[hr.head]
    var b = hr.ring[a]
    var c = hr.ring[b]
    hr.ring[hr.head] = hr.ring[c]
    var dst = hr.head
    while true:
        dst = if dst == 1: N else: dst-1
        if dst == a or dst == b or dst == c:
            continue
        var next = hr.ring[dst]
        hr.ring[dst] = a
        hr.ring[c] = next
        break
    hr.head = hr.ring[hr.head]

# My input:
# var inputs = @[9, 1, 6, 4, 3, 8, 2, 7, 5]
# Example input:
var inputs = @[3,8,9,1,2,5,4,6,7]
for i in 10..1_000_000:
    inputs.add(i)

var cups: ArrayRing = inputs.toHashRing

for i in 1..10_000_000:
    if i mod 1000 == 0:
        echo i div 1000, "/10,000"
    step(cups)

var a = cups.ring[1]
var b = cups.ring[a]
echo a
echo b
echo a*b