# !/bin/bash

declare -A count min
declare -i max_count=0 max_value

# loop 1: get the count and the minimum value for each B

# and find the max value for A

{
    read header
    while IFS=, read -r a b; do
        ((count[$b]++))
        ((count[$b] > max_count)) && max_count=${count[$b]}
        if [[ ! -v min[$b] ]] || ((a < min[$b])); then min[$b]=$a; fi
        if [[ -z $max_value ]] || ((max_value < a)); then max_value=$a; fi
    done
} < file.csv

# loop 2: find the B's with the max count

declare -a candidates
for c in "${!count[@]}"; do
    if ((count[$c] == max_count)); then
        candidates+=("$c")
    fi
done

# loop 3: find the minimum A value among the candidates

min_value=$max_value
for c in "${candidates[@]}"; do
    ((min[$c] < min_value)) && min_value=${min[$c]}
done

# loop 4: print out the candidates with the minimum A value

for c in "${candidates[@]}"; do
    ((min[$c] == min_value)) && echo "$c"
done