Add .is_valid()

Src/Particle/AMReX_Particle.H

@@ -111,18 +111,19 @@ struct ParticleIDWrapper
     AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
     void flip_valid () noexcept
     {
-        // the leftmost bit is our id sign - we will XOR it with 1 to flip it
+        // The uppermost bit is our id inverse sign.
+        // We will XOR it with 1 to flip it.
         m_idata ^= static_cast<uint64_t>(1) << 63;
     }
 
-    /** Check the sign of the id.
+    /** Check the particle is valid, via the sign of the id.
      *
-     * Returns false if the id is positive, otherwise true (invalid particle).
+     * Returns true if the particle is valid (the id is positive), otherwise false (invalid particle).
      */
     [[nodiscard]] AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
-    bool negative () const noexcept
+    bool is_valid () const noexcept
     {
-        // the leftmost bit is our id sign
+        // the leftmost bit is our id's inverse sign
         return m_idata >> 63;
     }
 };
@@ -199,13 +200,13 @@ struct ConstParticleIDWrapper
 
     /** Check the sign of the id.
      *
-     * Returns false if the id is positive, otherwise true (invalid particle).
+     * Returns true if the id is positive, otherwise false (invalid particle).
      */
     [[nodiscard]] AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
-    bool negative () const noexcept
+    bool is_valid () const noexcept
     {
         // the leftmost bit is our id's inverse sign
-        return !(m_idata >> 63);
+        return m_idata >> 63;
     }
 };