From e6c4da9f80d2c5133b6b753fe9b8ace1498cd016 Mon Sep 17 00:00:00 2001
From: Manos Vourliotis <emmanouil.vourliotis@gmail.com>
Date: Fri, 27 Dec 2024 11:45:55 -0800
Subject: [PATCH] Port alpaka phi functions to DataFormats

---
 DataFormats/PortableMath/interface/deltaPhi.h | 45 +++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100644 DataFormats/PortableMath/interface/deltaPhi.h

diff --git a/DataFormats/PortableMath/interface/deltaPhi.h b/DataFormats/PortableMath/interface/deltaPhi.h
new file mode 100644
index 0000000000000..fd61d6501eaff
--- /dev/null
+++ b/DataFormats/PortableMath/interface/deltaPhi.h
@@ -0,0 +1,45 @@
+#ifndef DataFormats_PortableMath_deltaPhi_h
+#define DataFormats_PortableMath_deltaPhi_h
+
+/* functions to compute heterogeneous deltaPhi
+ * Ported from the LST algorithm:
+ * https://github.com/cms-sw/cmssw/pull/45117#discussion_r1749143439
+ */
+
+namespace cms::alpakatools {
+
+  // reduce to [-pi,pi]
+  template <typename TAcc, typename T>
+  ALPAKA_FN_HOST_ACC ALPAKA_FN_INLINE constexpr T reduceRange(TAcc const& acc, T x) {
+    constexpr T o2pi = 1. / (2. * M_PI);
+    if (alpaka::math::abs(acc, x) <= T(M_PI))
+      return x;
+    T n = alpaka::math::round(acc, x * o2pi);
+    return x - n * T(2. * M_PI);
+  }
+
+  template <typename TAcc, typename T>
+  ALPAKA_FN_HOST_ACC ALPAKA_FN_INLINE constexpr T phi(TAcc const& acc, T x, T y) {
+    return reduceRange(acc, M_PI + alpaka::math::atan2(acc, -y, -x));
+  }
+
+  template <typename TAcc, typename T>
+  ALPAKA_FN_HOST_ACC ALPAKA_FN_INLINE constexpr T deltaPhi(TAcc const& acc, T x1, T y1, T x2, T y2) {
+    T phi1 = phi(acc, x1, y1);
+    T phi2 = phi(acc, x2, y2);
+    return reduceRange(acc, phi2 - phi1);
+  }
+
+  template <typename TAcc, typename T>
+  ALPAKA_FN_HOST_ACC ALPAKA_FN_INLINE constexpr T deltaPhi(T phi1, T phi2) {
+    return reduceRange(phi1 - phi2);
+  }
+
+  template <typename TAcc, typename T>
+  ALPAKA_FN_HOST_ACC ALPAKA_FN_INLINE constexpr T deltaPhiChange(TAcc const& acc, T x1, T y1, T x2, T y2) {
+    return deltaPhi(acc, x1, y1, x2 - x1, y2 - y1);
+  }
+
+}  // namespace cms::alpakatools
+
+#endif