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too-complex (S901)

This rule is unstable and in preview. The --preview flag is required for use.

What it does

Checks for procedures with a cyclomatic complexity that exceeds a configurable threshold.

Why is this bad?

Cyclomatic complexity measures the number of linearly independent paths through a procedure. A high complexity indicates that a procedure has too many branches, making it harder to read, test, and maintain. Procedures with a complexity above 10 (the threshold proposed by McCabe) are generally considered too complex and should be refactored.

Example

subroutine classify(x, category)
  real, intent(in) :: x
  integer, intent(out) :: category
  if (x < 0.0) then
    if (x < -100.0) then
      category = 1
    else if (x < -10.0) then
      category = 2
    else
      category = 3
    end if
  else if (x == 0.0) then
    category = 4
  else
    if (x > 100.0) then
      category = 5
    else if (x > 10.0) then
      category = 6
    else
      category = 7
    end if
  end if
end subroutine classify

Use instead: 

integer function classify_negative(x)
  real, intent(in) :: x
  if (x < -100.0) then
    classify_negative = 1
  else if (x < -10.0) then
    classify_negative = 2
  else
    classify_negative = 3
  end if
end function classify_negative

integer function classify_positive(x)
  real, intent(in) :: x
  if (x > 100.0) then
    classify_positive = 5
  else if (x > 10.0) then
    classify_positive = 6
  else
    classify_positive = 7
  end if
end function classify_positive

subroutine classify(x, category)
  real, intent(in) :: x
  integer, intent(out) :: category
  if (x < 0.0) then
    category = classify_negative(x)
  else if (x == 0.0) then
    category = 4
  else
    category = classify_positive(x)
  end if
end subroutine classify

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