More Inline Comments, Add .valid()

Src/Particle/AMReX_Particle.H

@@ -85,7 +85,7 @@ struct ParticleIDWrapper
             val = -id;
         }
 
-        m_idata |= (sign << 63);  // put the sign in the leftmost bit
+        m_idata |= (sign << 63);  // put the inverse sign in the uppermost bit
         m_idata |= (val << 24);   // put the val in the next 39
         return *this;
     }
@@ -95,10 +95,10 @@ struct ParticleIDWrapper
     {
         Long r = 0;
 
-        uint64_t sign = m_idata >> 63;  // extract leftmost sign bit
+        uint64_t sign = m_idata >> 63;  // extract uppermost sign bit
         uint64_t val  = ((m_idata >> 24) & 0x7FFFFFFFFF);  // extract next 39 id bits
 
-        Long lval = static_cast<Long>(val);  // bc we take -
+        Long lval = static_cast<Long>(val);  // bc we store the inverse sign bit
         r = (sign) ? lval : -lval;
         return r;
     }
@@ -110,7 +110,8 @@ struct ParticleIDWrapper
     AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
     void negate () 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;
     }
 
@@ -121,7 +122,18 @@ struct ParticleIDWrapper
     [[nodiscard]] AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
     bool negative () const noexcept
     {
-        // the leftmost bit is our id sign
+        // the leftmost bit is our id's inverse sign
+        return !(m_idata >> 63);
+    }
+
+    /** Check the sign of the id.
+     *
+     * Returns true if the id is positive, otherwise false (invalid particle).
+     */
+    [[nodiscard]] AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+    bool valid () const noexcept
+    {
+        // the leftmost bit is our id's inverse sign
         return m_idata >> 63;
     }
 };