Analyzing Software Code — Maintainability Index

Vineet Sharma

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Feb 5, 2024

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Analyzing Software Code — Maintainability Index

The Maintainability Index is a software metric that quantifies how maintainable and understandable a software system is. It provides a numerical score that indicates the ease of maintaining and evolving the codebase. The higher the Maintainability Index, the more maintainable the code is considered to be.

The Maintainability Index is typically calculated based on various factors and formulae, often taking into account metrics such as:

1. Halstead Volume
   A measure of the program’s size and complexity based on the number of operators and operands.
2. Cyclomatic Complexity
   A measure of the program’s control flow complexity, indicating the number of linearly independent paths through the code.
3. Lines of Code (LOC)
   The number of lines of code in the software.

How to measure Maintainability Index

The formula for calculating the Maintainability Index is often a combination of these metrics and takes various factors into account such as code complexity, size, and duplication. The higher the Maintainability Index, the more maintainable and readable the code is considered to be.

In C#, the Maintainability Index is calculated using the following formula:

MI — 171 — 5.2 × ln⁡(HashedVolume) — 0.23 × (Cyclomatic Complexity)−16.2 × (% Lines of Code that are Comments)

Where:

• MI is the Maintainability Index.
• ln⁡ is the natural logarithm.
• Halstead Volume is a measure of the code size and complexity.
• Cyclomatic Complexity is a measure of the code’s control flow complexity.

Let’s consider a simple C# example and calculate the Maintainability Index:

using System;

namespace MaintainabilityExample
{
    class Program
    {
        static void Main()
        {
            int a = 5;
            int b = 10;
            int result = AddNumbers(a, b);
            Console.WriteLine("Result: " + result);
        }

        static int AddNumbers(int x, int y)
        {
            int sum = x + y;
            return sum;
        }
    }
}

Now, let’s calculate the Maintainability Index using the formula:

1. Calculate Halstead Volume:
   V = N × log2​(n) + N × log2​(N)
   Where N is the total number of operators and operands, and n is the number of distinct operators and operands.
   In our example:
   N = 19
   n = 10
   V = 19 × log2​(10) + 19 × log2​(19) ≈ 104.16
2. Calculate Cyclomatic Complexity: Count the number of decision points (if statements, loops, etc.) in the code.
   In our example: Cyclomatic Complexity = 2
3. Maintainability Index:
   MI = 171 − 5.2 × ln(104.16) − 0.23 × (2) − 16.2 × (0)
   MI ≈ 170.41

The resulting Maintainability Index is then interpreted as follows:

• 0–10: Hard to maintain
• 10–20: Moderate maintainability
• 20–30: Good maintainability
• 30–40: Very good maintainability
• 40–100: Excellent maintainability

It’s important to note that the Maintainability Index is just one of many metrics used in code analysis, and it should be considered along with other metrics and qualitative assessments when evaluating software maintainability. A high Maintainability Index suggests that the codebase is more likely to be easily understood, modified, and extended, while a low index may indicate potential challenges in maintaining the software over time.