Fix #56: derive hasMeasurement() from the gates instead of a flag

README.md

@@ -95,8 +95,9 @@ $result->outcomes();        // ['00', '11'] — bitstrings sorted by count, desc
 ```php
 $builder = Quantum::circuit()->qubits(2)->h(0)->cnot(0, 1)->measure();
 
-$builder->gateCount(); // 2 — number of gates, excluding measurement
-$builder->depth();     // 2 — number of sequential layers, excluding measurement
+$builder->gateCount();      // 2 — number of gates, excluding measurement
+$builder->depth();          // 2 — number of sequential layers, excluding measurement
+$builder->hasMeasurement(); // true — whether any gate is a measurement
 ```
 
 ### Entropy Generation

src/Circuit/CircuitBuilder.php

@@ -38,8 +38,6 @@ class CircuitBuilder
     /** @var Gate[] */
     private array $gates = [];
 
-    private bool $hasMeasurement = false;
-
     private int $shots = 1000;
 
     final public function __construct(
@@ -619,17 +617,34 @@ public function validate(): static
             throw InvalidCircuitException::noQubits();
         }
 
-        if (! $this->hasMeasurement) {
+        if (! $this->hasMeasurement()) {
             throw InvalidCircuitException::noMeasurement();
         }
 
         return $this;
     }
 
     /**
-     * Validate a gate against the circuit's qubit range, append it, and
-     * track measurement state. The single append primitive behind every
-     * fluent gate method and fromArray().
+     * Whether any gate in the circuit is a measurement.
+     *
+     * Derived from $gates on demand, like gateCount() and depth(), rather
+     * than tracked in a flag push() would have to keep in step; circuits are
+     * tens of gates at most, so the scan costs nothing measurable.
+     */
+    public function hasMeasurement(): bool
+    {
+        foreach ($this->gates as $gate) {
+            if ($gate->isMeasurement()) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /**
+     * Validate a gate against the circuit's qubit range and append it. The
+     * single append primitive behind every fluent gate method and fromArray().
      *
      * @throws InvalidCircuitException
      */
@@ -639,10 +654,6 @@ private function push(Gate $gate): static
 
         $this->gates[] = $gate;
 
-        if ($gate->isMeasurement()) {
-            $this->hasMeasurement = true;
-        }
-
         return $this;
     }
 

tests/Unit/Circuit/CircuitBuilderTest.php

@@ -249,6 +249,25 @@
 // run() — validation: no measurement
 // -------------------------------------------------------------------------
 
+it('reports whether the circuit measures anything, derived from its gates', function () use (&$builder): void {
+    $builder->qubits(2)->h(0)->cnot(0, 1);
+
+    expect($builder->hasMeasurement())->toBeFalse();
+
+    $builder->measure(1);
+
+    expect($builder->hasMeasurement())->toBeTrue();
+});
+
+it('does not count a measurement dropped from an appended fragment', function () use (&$device, &$builder): void {
+    $fragment = (new CircuitBuilder($device))->qubits(1)->x(0)->measure();
+
+    $builder->qubits(1)->append($fragment);
+
+    expect($builder->hasMeasurement())->toBeFalse()
+        ->and(fn () => $builder->run())->toThrow(InvalidCircuitException::class);
+});
+
 it('run throws when no measurement', function () use (&$builder): void {
     expect(fn () => $builder->qubits(1)->h(0)->run())
         ->toThrow(InvalidCircuitException::class);