diff --git a/2020/day21/ex2.bash b/2020/day21/ex2.bash
new file mode 100755
index 0000000..503662b
--- /dev/null
+++ b/2020/day21/ex2.bash
@@ -0,0 +1,138 @@
+#!/bin/bash
+#
+# ex2.bash: Advent2020 game, day 21/game 2.
+
+CMD=${0##*/}
+shopt -s extglob
+set -o noglob
+
+declare -A CANBE=() A_RULES=() FOUND=() ALL_I=() ALL_A=() I_COUNT=()
+declare -a R
+declare -i count=0
+
+# intersect the words of 2 strings into $1
+intersect() {
+    local -n _res="$1"
+    # shellcheck disable=SC2206
+    local -a _str1=($2) _str2=($3)
+    local _i _j
+    _res=""
+    for _i in "${_str1[@]}"; do
+        for _j in "${_str2[@]}"; do
+            [[ "$_i" == "$_j" ]] && _res+=" $_i"
+        done
+    done
+    _res="${_res##*( )}"
+}
+# count words in a string and put result in $1
+count() {
+    local -n _res="$1"
+    # shellcheck disable=SC2206
+    local -a _str=($2)
+    _res=${#_str[@]}
+}
+# remove a word from a string and put result in $1
+delw() {
+    local -n _res="$1"
+    local _w=$2
+    # shellcheck disable=SC2206
+    local -a _str=($3)
+    local _i
+    _res=""
+    for _i in "${_str[@]}"; do
+        [[ $_i != "$_w" ]] && _res+=" $_i"
+    done
+    _res="${_res##*( )}"
+    _res="${_res%%*( )}"
+}
+
+# simple bubble sort for alphanumeric array (ascending)
+# args: 1: the array
+# todo: add a parameter for asc/desc order.
+sort_a() {
+    local -a array=( "$@" )
+    local -i i max=$(( ${#array[@]} - 1 ))
+    local val1 val2 tmp
+
+    for (( max= $(( ${#array[@]} - 1 )); max > 0; max-- )); do
+        for (( i=0; i<max; i++ )); do
+            val1=${array[$i]}
+            val2=${array[$((i + 1))]}
+
+            # switch if necessary
+            if [[ $val1 > $val2 ]]; then
+                tmp=$val1
+                array[$i]=$val2
+                array[$((i + 1))]=$tmp
+            fi
+        done
+    done
+    echo "${array[@]}"
+}
+
+REGEX="(.*) \(contains(.*)\)"
+while read -r line; do
+    [[ "$line" =~ $REGEX ]]
+    ingr="${BASH_REMATCH[1]}"
+    allg="${BASH_REMATCH[2]}"
+    allg=${allg//,}
+
+    R[$count]="$ingr"
+    for ka in $allg; do
+        A_RULES[$ka]+=" $count"
+        ALL_A[$ka]=""
+        for ki in $ingr; do
+            ALL_I[$ki]=""
+        done
+    done
+    for ki in $ingr; do
+        (( I_COUNT[$ki]++ ))
+    done
+    ((count++))
+done
+all_i="${!ALL_I[*]}"
+for k in "${!ALL_A[@]}"; do
+    CANBE[$k]="$all_i"
+done
+
+solved=0
+while ((solved==0)); do
+    solved=1
+    for allerg in "${!CANBE[@]}"; do
+        str="${CANBE[$allerg]}"
+
+        if [[ -v FOUND["$allerg"] ]]; then
+            continue
+        fi
+        for rule in ${A_RULES[$allerg]}; do
+            intersect str "${R[$rule]}" "$str"
+        done
+        CANBE[$allerg]="$str"
+
+        count count "$str"
+        ((count > 1)) && solved=0
+        if ((count==1)); then
+            word=$str #{CANBE[$allerg]}
+            # shellcheck disable=SC2034
+            FOUND[$allerg]="${CANBE[$allerg]}"
+            unset ALL_I["${CANBE[$allerg]}"]
+            for allerg1 in "${!CANBE[@]}"; do
+                if [[ "$allerg" != "$allerg1" ]]; then
+                    delw CANBE["$allerg1"]  "$word" "${CANBE[$allerg1]}"
+                fi
+            done
+            for rule in "${!R[@]}"; do
+                delw R[$rule] "$word" "${R[$rule]}"
+            done
+        fi
+    done
+done
+
+sorted=$(sort_a "${!CANBE[@]}")
+res=""
+for k in $sorted; do
+    res+=",${CANBE[$k]}"
+done
+
+printf "%s: res=%s\n" "$CMD" "${res##,}"
+exit 0