Bash Script - Fibonacci

Question

I was trying to make a recursive function that can calculate the Fibonacci of the enter number, by the way I got stuck on how to get the values obtained by the recursion.

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Answer 1

Re tripleee's answer: this can be done much faster still.

I think the helper function call is really expensive. Below version calculated fib 100000 at 2.344 seconds, whereas the helper-function version did the same at 1min 28.44 seconds. Although, you only get correct fibonacci up to like fib 92, at fib 93 for me you have hit the intmax_t setting.

fibnorec(){
    # typecheck                                                                                           
    if [[ ! "$1" =~ ^[0-9]+$ ]]
    then
        return 1
    fi

    # make arg into int                                                                                   
    declare -i n="$1"

    # Return fibonacci number at index n...                                                               
    if [[ $n -le 1 ]]
    then
        # index 0 and index 1 are numbers 0 and 1 respectively.
        printf '%s\n' $n;
    fi
    # For index 2 and up (position 3 and up)
    declare -i sum=1;
    declare -i prev=1;
    for ((i=2; i