let parseIntC(c: char) = (int c) - 48;

let maximum(a: int, b:int) = if a>b then a else b;

let parseInt(s: string) =
    let rec parse(position: int, value: int, multiply: int) =
        if position<0 then value else parse(position-1, value+(multiply*parseIntC(s.[position])), multiply*10)
    parse(s.Length-1, 0, 1);

let toBigNumberArray(str: string) =
    let rec converse(lista: list<int>, pos: int, sign: int) =
        if pos=str.Length then lista else converse((sign*parseIntC(str.[pos]))::lista, pos+1, sign);
    if (int str.[0])=45 then
        List.rev(converse([], 1, -1))
    else
        List.rev(converse([], 0, 1));

let parserInteger(str: string) =
    let splitted = str.Split([|'.'; 'e'|]);
    toBigNumberArray(splitted.[0])

let parserFloat(str: string) =
    let splitted = str.Split([|'.'|]);
    if splitted.Length<=1 then []
    else
       let ndsplitted = splitted.[1].Split([|'e'|]);
       if (int str.[0])=45 then toBigNumberArray("-"+ndsplitted.[0]) else toBigNumberArray(ndsplitted.[0])

let parserE(str: string) =
    let integerList = parserInteger(str);
    let floatList = parserFloat(str);
    let splitted = str.Split([|'e'|]);
    let rec backwards(listI: list<int>, listF: list<int>, count: int) =
        if count=parseInt(splitted.[1].[1..(splitted.[1].Length-1)]) then [|listI; listF|]
        elif listI.IsEmpty then backwards([], 0::listF, count+1)
        else backwards(List.rev(List.rev(listI).Tail), List.rev(listI).Head::listF, count+1)
    let rec forwards(listI: list<int>, listF: list<int>, count: int) =
        if count=parseInt(splitted.[1]) then [|listI; listF|]
        elif listF.IsEmpty then forwards(List.rev(0::List.rev(listI)), [], count+1)
        else forwards(List.rev(listF.Head::List.rev(listI)), listF.Tail, count+1)
    if splitted.Length>1 then
        if splitted.[1].Contains("-") then
            backwards(integerList, floatList, 0);
        else
            forwards(integerList, floatList, 0);
    else [|integerList; floatList|]

let asString(listI: list<int>, listF: list<int>) =
    let rec convRec(lista: list<int>, str: string, count:int) =
        if count=lista.Length then str else convRec(lista, str+(string lista.[count]), count+1)
    if listI.IsEmpty&&listF.IsEmpty then "0"
    elif listI.IsEmpty then "0." + convRec(listF, "", 0)
    elif listF.IsEmpty then convRec(listI, "", 0)
    else convRec(listI, "", 0) + "." + convRec(listF, "", 0)

let add(numFirst: string, numSecond: string) =
    let first = parserE(numFirst);
    let second = parserE(numSecond);
    let rec addiR(listF: list<int>, listS: list<int>, pos: int, result: list<int>, too: int) =
        if pos=too then result
        elif listF.IsEmpty&&not(listS.IsEmpty) then addiR([], List.rev(List.rev(listS).Tail), pos+1, List.rev(listS).Head::result, too)
        elif listS.IsEmpty&&not(listF.IsEmpty) then addiR(List.rev(List.rev(listF).Tail), [], pos+1, List.rev(listF).Head::result, too)
        else addiR(List.rev(List.rev(listF).Tail), List.rev(List.rev(listS).Tail), pos+1, (List.rev(listF).Head+List.rev(listS).Head)::result, too)
    let rec addfR(listF: list<int>, listS: list<int>, pos: int, result: list<int>, too: int) =
        if pos=too then List.rev(result)
        elif listF.IsEmpty&&not(listS.IsEmpty) then addfR([], listS.Tail, pos+1, listS.Head::result, too)
        elif listS.IsEmpty&&not(listF.IsEmpty) then addfR(listF.Tail, [], pos+1, listF.Head::result, too)
        else addfR(listF.Tail, listS.Tail, pos+1, (listF.Head+listS.Head)::result, too)
    let rec checkI (lista: list<int>, count: int, result: list<int>, next: bool) =
        if count<0 then result
        else
            let mutable elem = if lista.IsEmpty then 0 else lista.[count]
            if next then elem <- elem+1
            if lista.[count] >9 then checkI(lista, count-1, (elem-10)::result,true) else  checkI(lista, count-1, elem::result,false)

    let mutable integerPart = addiR(first.[0], second.[0],0,[], maximum(first.[0].Length,second.[0].Length));
    let mutable floatPart =  addfR(first.[1], second.[1],0,[], maximum(first.[1].Length,second.[1].Length));
    integerPart <- checkI(integerPart, integerPart.Length-1, [], false);
    floatPart <- checkI(floatPart, floatPart.Length-1, [], false);
    if not(floatPart.IsEmpty)&&floatPart.[0]>10
    then
        floatPart <- floatPart.Head-10::floatPart.Tail
        integerPart <- checkI(List.rev(List.rev(integerPart).Head+1::List.rev(integerPart).Tail), integerPart.Length-1, [], false)
    asString(integerPart, floatPart)



let test = add("1912.13e5", "6161e5");

printf "%s" test;

System.Console.ReadKey(true);