solve_driver_force.hpp

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#ifndef DEM_SOLVE_DRIVER_FORCE_HPP
#define DEM_SOLVE_DRIVER_FORCE_HPP
 
#include <cstdio>
#include <cstdlib>
#include <Kokkos_Core.hpp>
#include <Kokkos_Sort.hpp>
#include <stdexcept>
 
#include "particles.hpp"
#include "solve_driver.hpp"  // findCollisionsGrow, maxOwnedRadius, readInt/readFloat
#include "solver_hertz.hpp"
 
namespace peclet::dem {
 
inline void hertzRebuildPairs(Particles& P, float skin) {
  CpExec space;
  Kokkos::deep_copy(space, P.posPred, P.pos);  // broadphase reads posPred
  const int np = findCollisionsGrow(P, skin);
  const size_t need = P.pairs.extent(0);
  if (P.hertzXi.extent(0) < need) {
    P.hertzXi = Kokkos::View<float* [3], CpMem>("hertzXi", need);
    P.hertzSnPair = Kokkos::View<float*, CpMem>("hertzSnPair", need);
    P.hertzKeys = Kokkos::View<unsigned long long*, CpMem>("hertzKeys", need);
    P.hertzPrevKeys = Kokkos::View<unsigned long long*, CpMem>("hertzPrevKeys", need);
    P.hertzPrevXi = Kokkos::View<float* [3], CpMem>("hertzPrevXi", need);
  }
  // keys for the new list + history carry from the previous sorted list
  {
    auto pairs = P.pairs;
    auto keys = P.hertzKeys;
    auto xi = P.hertzXi;
    auto pk = P.hertzPrevKeys;
    auto px = P.hertzPrevXi;
    auto gid = P.gid;
    const int pc = P.hertzPrevCount;
    Kokkos::parallel_for(
        "peclet::dem::hertz_carry", Kokkos::RangePolicy<CpExec>(space, 0, np),
        KOKKOS_LAMBDA(int idx) {
          // Canonical orientation: the lower-gid body first. The Mindlin spring xi is the
          // tangential displacement of pairs(idx, 0) relative to pairs(idx, 1), so it flips sign
          // with the order; the two owners of a cross-rank pair (owned slots precede ghosts) and
          // a rank that received the pair by migration would otherwise hold it in different
          // orientations, and a gid-keyed carry would hand one of them the wrong sign
          // (docs/contact_solve_framework.md §12 S21; measured: dP 9e-4 in hertz_shear at the
          // first drift migration). At np 1 gid == slot and the broad phase already emits
          // (lower, higher): unchanged.
          const int a0 = pairs(idx, 0), b0 = pairs(idx, 1);
          if (gid(a0) > gid(b0)) {
            pairs(idx, 0) = b0;
            pairs(idx, 1) = a0;
          }
          const unsigned long long k =
              pairKeyFromGids((unsigned)gid(pairs(idx, 0)), (unsigned)gid(pairs(idx, 1)));
          keys(idx) = k;
          const int l = lowerBoundKey(pk, pc, k);
          const bool hit = (l < pc && pk(l) == k);
          xi(idx, 0) = hit ? px(l, 0) : 0.0f;
          xi(idx, 1) = hit ? px(l, 1) : 0.0f;
          xi(idx, 2) = hit ? px(l, 2) : 0.0f;
        });
  }
  P.hertzNumPairs = np;
  {  // lagged patch-stiffness store: reset at rebuild (one mis-damped step, harmless)
    auto sn = Kokkos::subview(P.hertzSnPair, Kokkos::pair<int, int>(0, np));
    Kokkos::deep_copy(space, sn, 0.0f);
  }
  Kokkos::deep_copy(space, P.hertzRefPos, P.pos);
  Kokkos::deep_copy(space, P.hertzDispMax, 0.0f);
  space.fence();
}
 
inline void hertzCommitHistory(Particles& P) {
  const int n = P.hertzNumPairs;
  if (n < 0)
    return;  // no valid LIVE list (fresh sim, or just after an ownership migration): the prev
             // store may hold carried history for the coming rebuild — do not wipe it.
  if (n == 0) {
    P.hertzPrevCount = 0;
    return;
  }
  CpExec space;
  const auto rng = Kokkos::pair<int, int>(0, n);
  auto kd = Kokkos::subview(P.hertzPrevKeys, rng);
  Kokkos::deep_copy(space, kd, Kokkos::subview(P.hertzKeys, rng));
  Kokkos::View<int*, CpMem> perm(
      Kokkos::view_alloc(space, "peclet::dem::hertz_perm", Kokkos::WithoutInitializing), n);
  Kokkos::parallel_for(
      "peclet::dem::hertz_iota", Kokkos::RangePolicy<CpExec>(space, 0, n),
      KOKKOS_LAMBDA(int i) { perm(i) = i; });
  Kokkos::Experimental::sort_by_key(space, kd, perm);
  auto xi = P.hertzXi;
  auto px = P.hertzPrevXi;
  Kokkos::parallel_for(
      "peclet::dem::hertz_gather", Kokkos::RangePolicy<CpExec>(space, 0, n), KOKKOS_LAMBDA(int i) {
        const int j = perm(i);
        px(i, 0) = xi(j, 0);
        px(i, 1) = xi(j, 1);
        px(i, 2) = xi(j, 2);
      });
  P.hertzPrevCount = n;
  space.fence();
}
 
struct HertzMindlinLaw {
  void commitHistory(Particles& P) const { hertzCommitHistory(P); }
  void rebuild(Particles& P, float skin) const {
    hertzRebuildPairs(P, skin);
    if (P.numWalls > 0)
      P.hertzNumWallCand =
          hertzBuildWallCandidatesKokkos(P.numReal, P.numWalls, P.walls, P.wallGrid, P.pos, P.rad,
                                         skin, P.hertzWallCand, P.hertzWallCandCount);
  }
  void pairForces(Particles& P, float dt, bool hasShapes) const {
    if (hasShapes)
      hertzShapePairForcesKokkos(P.pairs, P.hertzNumPairs, P.pos, P.quat, P.vel, P.angVel, P.scale,
                                 P.shapeId, P.shapes, P.shell, P.sdfGrid, P.globalScale,
                                 P.hertzContactRadiusFrac, P.invMass, MatIdView(P.materialId),
                                 PairTableView(P.pairMaterials), P.restitutionNormal,
                                 P.frictionDynamic, P.hertzE, P.hertzNu, dt, P.hertzXi,
                                 P.hertzSnPair, P.deltaVel, P.deltaAngVel);
    else
      hertzPairForcesKokkos(
          P.pairs, P.hertzNumPairs, P.pos, P.vel, P.angVel, P.rad, P.invMass,
          MatIdView(P.materialId), PairTableView(P.pairMaterials), P.restitutionNormal,
          P.frictionDynamic, P.hertzE, P.hertzNu, dt, P.hertzXi, P.deltaVel, P.deltaAngVel,
          Kokkos::View<const float*, CpMem>(P.scale), P.shapeId, P.shapes, P.globalScale);
  }
  void wallForces(Particles& P, float dt, bool hasShapes) const {
    if (P.numWalls > 0 && P.hertzNumWallCand > 0)
      hertzWallForcesKokkos(
          Kokkos::View<const int*, CpMem>(P.hertzWallCand), P.hertzNumWallCand, P.walls, P.wallGrid,
          P.pos, P.vel, P.angVel, P.rad, P.invMass, MatIdView(P.materialId),
          PairTableView(P.pairMaterials), P.restitutionNormal, P.frictionDynamic, P.hertzE,
          P.hertzNu, dt, P.hertzXiWall, Particles::kHertzMaxWalls, P.deltaVel, P.deltaAngVel,
          Kokkos::View<const float* [4], CpMem>(P.quat), Kokkos::View<const float*, CpMem>(P.scale),
          P.shapeId, P.shapes, P.shell, P.globalScale, P.hertzContactRadiusFrac, hasShapes,
          P.hertzSnWall);
  }
  void validate(const Particles& P) const {
    if (P.domain.periodic_x || P.domain.periodic_y || P.domain.periodic_z)
      throw std::runtime_error("step_hertz: periodic domains not supported");
    if (P.numPlanes > 0)
      throw std::runtime_error("step_hertz: analytic planes not supported (use an SDF wall)");
    if (P.numWalls > Particles::kHertzMaxWalls)
      throw std::runtime_error("step_hertz: too many SDF walls");
  }
};
 
inline void zeroForceScratchKokkos(V3 dv, V3 dw, int lo, int hi) {
  if (hi <= lo)
    return;
  CpExec space;
  Kokkos::parallel_for(
      "peclet::dem::force_zero_ghost", Kokkos::RangePolicy<CpExec>(space, lo, hi),
      KOKKOS_LAMBDA(int i) {
        for (int c = 0; c < 3; ++c) {
          dv(i, c) = 0.0f;
          dw(i, c) = 0.0f;
        }
      });
  space.fence();
}
 
struct SoloForceHooks {
  static constexpr bool distributed = false;
  float allMax(float v) const { return v; }
  float allMin(float v) const { return v; }
  void gatherGhosts(Particles&) const {}
  void refreshGhostState(Particles&, bool) const {}
  void clearGhostScratch(Particles&) const {}
};
 
template <class Law, class Hooks>
inline void demStepForce(Particles& P, float dt, int nsteps, float skinFrac, const Law& law,
                         const Hooks& hooks) {
  law.validate(P);
  // world radii of the owned span (a halo gather re-fills owned + ghosts)
  fillWorldRadiiKokkos(P.scale, P.rad, P.globalScale, P.baseRadius, P.numReal);
  float minRad = 0.0f;
  {
    CpExec space;
    auto rad = P.rad;
    float m = 3.4e38f;
    Kokkos::parallel_reduce(
        "peclet::dem::hertz_minrad", Kokkos::RangePolicy<CpExec>(space, 0, P.numReal),
        KOKKOS_LAMBDA(int i, float& acc) {
          if (rad(i) < acc)
            acc = rad(i);
        },
        Kokkos::Min<float>(m));
    space.fence();
    minRad = m;
  }
  // Skin off the SMALLEST radius (globally under MPI: every rank must use the same skin, so the
  // rebuild threshold — and thus the collective gather schedule — is identical): a max-radius skin
  // inflates every small-grain pair cutoff by the big body's margin (measured ~15x more cached
  // pairs in the ball-impact case).
  const float skin = skinFrac * hooks.allMin(minRad);
  const float rebuildAt = 0.25f * skin * skin;  // (skin/2)^2 on |dx|^2
  const F3 g = P.gravity;
  // Cache-validity must be a COLLECTIVE decision too: after a migration every rank invalidates,
  // but a single desynchronised flag would desynchronise the gather below.
  bool rebuild = hooks.allMax(P.hertzNumPairs < 0 ? 1.0f : 0.0f) > 0.0f;
  // Non-spherical dispatch: any shape with a point shell routes pairs through the per-point
  // Hertz kernel and enables orientation integration.
  bool hasShapes = false;
  {
    auto hs = Kokkos::create_mirror_view(P.shapes);
    Kokkos::deep_copy(hs, P.shapes);
    for (size_t si = 0; si < hs.extent(0); ++si)
      if (hs(si).numPoints > 0)
        hasShapes = true;
  }
  static const bool profile = std::getenv("PECLET_DEM_HERTZ_PROFILE") != nullptr;
  double tPair = 0, tWall = 0, tInt = 0, tRebuild = 0, tCheck = 0;
  int nRebuilds = 0;
  Kokkos::Timer timer;
  for (int k = 0; k < nsteps; ++k) {
    if (rebuild) {
      law.commitHistory(P);
      // Distributed: re-gather the ghost band (pair cutoff + skin) with a fresh topology, THEN
      // build the rank-local pair list over owned + ghosts. Between rebuilds the Verlet argument
      // covers the ghosts too: nothing outside the band can reach a cached pair before the next
      // displacement-triggered rebuild.
      hooks.gatherGhosts(P);
      law.rebuild(P, skin);
      rebuild = false;
      ++nRebuilds;
      if (profile) {
        tRebuild += timer.seconds();
        timer.reset();
      }
    } else {
      // Ghosts must track their owners every step (positions/velocities changed by integrate).
      hooks.refreshGhostState(P, hasShapes);
    }
    hooks.clearGhostScratch(P);  // ghost force/torque slots: accumulated but never consumed
    law.pairForces(P, dt, hasShapes);
    if (profile) {
      tPair += timer.seconds();
      timer.reset();
    }
    law.wallForces(P, dt, hasShapes);
    if (profile) {
      tWall += timer.seconds();
      timer.reset();
    }
    hertzIntegrateKokkos(P.numReal, P.deltaVel, P.deltaAngVel, P.invMass, P.invInertia, g, dt,
                         P.vel, P.angVel, P.pos, P.quat, hasShapes);
    if (profile) {
      tInt += timer.seconds();
      timer.reset();
    }
    if ((k & 15) == 15 || k == nsteps - 1) {
      if (hooks.allMax(hertzMaxDisp2Kokkos(P.numReal, P.pos, P.hertzRefPos)) > rebuildAt)
        rebuild = true;
      if (profile) {
        tCheck += timer.seconds();
        timer.reset();
      }
    }
  }
  if (profile)
    std::printf(
        "[hertz profile] steps=%d pairs=%d wallcand=%d rebuilds=%d | pair %.3fs wall %.3fs "
        "integrate %.3fs check %.3fs rebuild %.3fs\n",
        nsteps, P.hertzNumPairs, P.hertzNumWallCand, nRebuilds, tPair, tWall, tInt, tCheck,
        tRebuild);
  CpExec space;
  Kokkos::deep_copy(space, P.posPred, P.pos);  // keep the impulse-path views coherent
  Kokkos::deep_copy(space, P.velPred, P.vel);
  space.fence();
}
 
inline void demStepHertz(Particles& P, float dt, int nsteps, float skinFrac) {
  demStepForce(P, dt, nsteps, skinFrac, HertzMindlinLaw{}, SoloForceHooks{});
}
 
}  // namespace peclet::dem
 
#endif  // DEM_SOLVE_DRIVER_FORCE_HPP