Halstead Maintainability Index

// High Halstead volume — many operators and operands in one expression:
function calc($a, $b, $c, $d, $e) {
    return (($a + $b) * ($c - $d) / $e) % ($a * $c) + ($b - $e) * $d;
    // Complex expression: high vocabulary, difficulty, and effort score
}

// Halstead metrics — based on operators and operands in source code
// Definitions:
// η1 = distinct operators  (e.g. +, =, if, return)
// η2 = distinct operands   (e.g. $total, $price, 0.2)
// N1 = total operator occurrences
// N2 = total operand occurrences

// Calculated metrics:
// Vocabulary   η  = η1 + η2
// Length       N  = N1 + N2
// Volume       V  = N * log2(η)        — information content
// Difficulty   D  = (η1/2) * (N2/η2)  — mental effort
// Effort       E  = D * V              — total effort to implement/understand

// Example PHP function:
public function applyDiscount(float $price, float $rate): float {
    return $price * (1 - $rate);
}
// Operators: function, float, float, float, return, *, -, ()  → ~6 distinct
// Operands:  $price, $rate, 1                                 → 3 distinct
// Low volume + difficulty = easy to understand and maintain

// Tool:
$ phpmetrics --report-html=report/ src/
// Reports Halstead metrics per method and class
// High Volume (>1000) or Difficulty (>30) → consider refactoring