Day 23 (bash) part 2 (~7mn run)

2020/day23/README

@@ -75,8 +75,6 @@ After the crab is done, what order will the cups be in? Starting after the cup l
 Using your labeling, simulate 100 moves. What are the labels on the cups after cup 1?
 
 Your puzzle answer was 75893264.
-
-The first half of this puzzle is complete! It provides one gold star: *
 --- Part Two ---
 
 Due to what you can only assume is a mistranslation (you're not exactly fluent in Crab), you are quite surprised when the crab starts arranging many cups in a circle on your raft - one million (1000000) in total.
@@ -90,3 +88,7 @@ The crab is going to hide your stars - one each - under the two cups that will e
 In the above example (389125467), this would be 934001 and then 159792; multiplying these together produces 149245887792.
 
 Determine which two cups will end up immediately clockwise of cup 1. What do you get if you multiply their labels together?
+
+Your puzzle answer was 38162588308.
+
+Both parts of this puzzle are complete! They provide two gold stars: **

2020/day23/ex2.bash

@@ -0,0 +1,50 @@
+#!/bin/bash
+#
+# ex2.bash: Advent2020 game, day 23/game 2.
+
+CMD=${0##*/}
+shopt -s extglob
+set -o noglob
+
+declare -A next                                   # next[i] is cup right to i (ring)
+declare -i end runs cup _cup
+
+read -r str
+tmp="${str//?()/x}"
+tmp=${tmp#x}
+IFS=x read -ra array <<< "$tmp"
+
+cup=${array[0]}
+end=1000000
+runs=10000000
+
+_cup=$cup
+# initialize the next array with input cups
+for _next in "${array[@]:1}"; do
+    next[$_cup]=$_next
+    _cup=$_next
+done
+# initialize the next array (up to end)
+for ((_next = 10; _next <= end; ++_next)); do
+    next[$_cup]=$_next
+    _cup=$_next
+done
+next[$_cup]=$cup                                  # close the ring
+
+_cup=$cup
+declare -i _1st _2nd _3rd dest
+for ((i = 1; i <= runs; ++i)); do
+    _1st="${next[$cup]}"
+    _2nd="${next[$_1st]}"
+    _3rd="${next[$_2nd]}"
+    dest=$cup
+    while
+        (( --dest > 0 )) || dest=$end
+        (( dest == _1st || dest == _2nd || dest == _3rd ))
+    do :; done
+    (( tmp=next[$dest], next[$dest]=_1st, next[$cup]=next[$_3rd], next[$_3rd]=tmp ))
+    (( cup=next[$cup] ))
+done
+
+printf "%s: res=%d\n" "$CMD" $(( next[1] * next[${next[1]}] ))
+exit 0