Cognitive complexity

1. Ignore structures that allow multiple statements to be readably shorthanded into one.
2. Increment for break in linear flow (if, else, ternary, switch, loop, catch, goto, recursion, &&/|| chains).
3. Increment for nested flow-breaking structures, with the increment proportional to the nesting depth.

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What mehen emits

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How it differs from cyclomatic

# Cyclomatic: 4   (one + for each of the four boolean operators)
# Cognitive:  3   (1 for the if, +2 for the &&/|| mix; no nesting increment for a top-level if)
def is_eligible(user):
    if user and (user.is_active or user.is_admin) and not user.is_banned:
        return True
    return False

# Cyclomatic: 4   (one + for the three branches)
# Cognitive:  6   (1 + 2 + 3 because each nested branch costs nesting depth)
def deep(x):
    if x > 0:
        if x < 100:
            if x % 2 == 0:
                return True
    return False

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Per-language increments

• +1 for each control-flow break: if, else if, else, ternary, switch (Sonar counts the switch itself, not each case), loop, catch, goto, recursive call, etc.
• +1 per nesting level of the construct relative to its enclosing function.
• +1 for each change in a sequence of && / || operators (chains of the same operator do not re-charge).

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How to read it

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References

• Campbell, G. A. (2018). Cognitive Complexity — A new way of measuring understandability. SonarSource white paper. PDF.
• Muñoz Barón, M., Wyrich, M. & Wagner, S. (2020). An empirical validation of cognitive complexity as a measure of source code understandability. ESEM 2020. arXiv:2007.12520.
• Sonar: Cognitive complexity in metric definitions.

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See also

• Cyclomatic complexity — paths through the CFG.
• Halstead metrics — vocabulary-based complexity.
• Maintainability Index — composite that blends cyclomatic + Halstead + LOC.