Fastest way to solve chain-calculations

Question

I have a input like

string input = \"14 + 2 * 32 / 60 + 43 - 7 + 3 - 1 + 0 * 7 + 87 - 32 / 34\"; 
// up to 10MB string size

int result = Calc(input); // 11
         


        

Answer 1

The following solution is a finite automaton. Calc(input) = O(n). For better performance, this solution does not use IndexOf, Substring, Parse, string concatenation, or repeated reading of value (fetching input[i] more than once)... just a character processor.

    static int Calculate1(string input)
    {
        int acc = 0;
        char last = ' ', operation = '+';

        for (int i = 0; i < input.Length; i++)
        {
            var current = input[i];
            switch (current)
            {
                case ' ':
                    if (last != ' ')
                    {
                        switch (operation)
                        {
                            case '+': acc += last - '0'; break;
                            case '-': acc -= last - '0'; break;
                            case '*': acc *= last - '0'; break;
                            case '/': acc /= last - '0'; break;
                        }

                        last = ' ';
                    }

                    break;

                case '0': case '1': case '2': case '3': case '4':
                case '5': case '6': case '7': case '8': case '9':
                    if (last == ' ') last = current;
                    else
                    {
                        var num = (last - '0') * 10 + (current - '0');
                        switch (operation)
                        {
                            case '+': acc += num; break;
                            case '-': acc -= num; break;
                            case '*': acc *= num; break;
                            case '/': acc /= num; break;
                        }
                        last = ' ';
                    }
                    break;

                case '+': case '-': case '*': case '/':
                    operation = current;
                    break;
            }
        }

        if (last != ' ')
            switch (operation)
            {
                case '+': acc += last - '0'; break;
                case '-': acc -= last - '0'; break;
                case '*': acc *= last - '0'; break;
                case '/': acc /= last - '0'; break;
            }

        return acc;
    }

---

And another implementation. It reads 25% less from the input. I expect that it has 25% better performance.

    static int Calculate2(string input)
    {
        int acc = 0, i = 0;
        char last = ' ', operation = '+';

        while (i < input.Length)
        {
            var current = input[i];
            switch (current)
            {
                case ' ':
                    if (last != ' ')
                    {
                        switch (operation)
                        {
                            case '+': acc += last - '0'; break;
                            case '-': acc -= last - '0'; break;
                            case '*': acc *= last - '0'; break;
                            case '/': acc /= last - '0'; break;
                        }

                        last = ' ';
                        i++;
                    }

                    break;

                case '0': case '1': case '2': case '3': case '4':
                case '5': case '6': case '7': case '8': case '9':
                    if (last == ' ')
                    {
                        last = current;
                        i++;
                    }
                    else
                    {
                        var num = (last - '0') * 10 + (current - '0');
                        switch (operation)
                        {
                            case '+': acc += num; break;
                            case '-': acc -= num; break;
                            case '*': acc *= num; break;
                            case '/': acc /= num; break;
                        }

                        last = ' ';
                        i += 2;
                    }
                    break;

                case '+': case '-': case '*': case '/':
                    operation = current;
                    i += 2;
                    break;
            }
        }

        if (last != ' ')
            switch (operation)
            {
                case '+': acc += last - '0'; break;
                case '-': acc -= last - '0'; break;
                case '*': acc *= last - '0'; break;
                case '/': acc /= last - '0'; break;
            }

        return acc;
    }

---

I implemented one more variant:

    static int Calculate3(string input)
    {
        int acc = 0, i = 0;
        var operation = '+';

        while (true)
        {
            var a = input[i++] - '0';
            if (i == input.Length)
            {
                switch (operation)
                {
                    case '+': acc += a; break;
                    case '-': acc -= a; break;
                    case '*': acc *= a; break;
                    case '/': acc /= a; break;
                }

                break;
            }

            var b = input[i];
            if (b == ' ') i++;
            else
            {
                a = a * 10 + (b - '0');
                i += 2;
            }

            switch (operation)
            {
                case '+': acc += a; break;
                case '-': acc -= a; break;
                case '*': acc *= a; break;
                case '/': acc /= a; break;
            }

            if (i >= input.Length) break;
            operation = input[i];
            i += 2;
        }

        return acc;
    }

Results in abstract points:

• Calculate1 230
• Calculate2 192
• Calculate3 111