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diff --git a/Source/Diagnostics/ComputeDiagFunctors/ParticleReductionFunctor.H b/Source/Diagnostics/ComputeDiagFunctors/ParticleReductionFunctor.H
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+++ b/Source/Diagnostics/ComputeDiagFunctors/ParticleReductionFunctor.H
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+#ifndef WARPX_PARTICLEREDUCTIONFUNCTOR_H_
+#define WARPX_PARTICLEREDUCTIONFUNCTOR_H_
+
+#include "ComputeDiagFunctor.H"
+
+#include <AMReX_Parser.H>
+
+#include <AMReX_BaseFwd.H>
+
+#include <memory>
+#include <string>
+
+/**
+ * \brief Functor to calculate per-cell averages of particle properties.
+ */
+class
+ParticleReductionFunctor final : public ComputeDiagFunctor
+{
+public:
+    /** Constructor.
+     * \param[in] mf_src source multifab. Must be nullptr as no source MF is needed
+     *            to compute the number of particles per cell, banane.
+     * \param[in] lev level of multifab.
+     * \param[in] crse_ratio for interpolating field values from simulation MultiFabs
+                  to the output diagnostic MultiFab mf_dst.
+     * \param[in] fn_str parser string that describes the function to apply to particles
+     *            and then average over each cell
+     * \param[in] ispec index of the species over which to calculate the average
+     * \param[in] ncomp Number of component of mf_src to cell-center in dst multifab.
+     */
+    ParticleReductionFunctor(const amrex::MultiFab * const mf_src, const int lev,
+                       const amrex::IntVect crse_ratio, const std::string fn_str, const int ispec, const int ncomp=1);
+
+    /** \brief Compute the average of the function m_map_fn over each grid cell.
+     *
+     * \param[out] mf_dst output MultiFab where the result is written
+     * \param[in] dcomp first component of mf_dst in which cell-centered
+     *            data is stored
+     */
+    virtual void operator()(amrex::MultiFab& mf_dst, const int dcomp, const int /*i_buffer=0*/) const override;
+private:
+    int const m_lev; /**< level on which mf_src is defined */
+    int const m_ispec; /**< index of species to average over */
+    /** Parser function to be averaged by the functor. Arguments: x, y, z, ux, uy, uz */
+    std::unique_ptr<amrex::Parser> m_map_fn_parser;
+    /** Compiled #m_map_fn_parser */
+    amrex::ParserExecutor<6> m_map_fn;
+};
+
+#endif // WARPX_PARTICLEREDUCTIONFUNCTOR_H_