add effective entries

src/FHist.jl

@@ -138,22 +138,35 @@ for (H, N) in ((:Hist1D, 1), (:Hist2D, 2), (:Hist3D, 3))
         """
         bincenters(h::$H) = _from_tuple(StatsBase.midpoints.(h.binedges))
         @doc """
-                nentries(h::$($H))
-            Get the number of times a histogram is filled (`push!`ed)
+            nentries(h::$($H))
+        Get the number of times a histogram is filled (`push!`ed)
         """
         nentries(h::$H) = h.nentries[]
         @doc """
-                sumw2(h)
-            Get the sum of weights squared of the histogram, it has the same shape as `bincounts(h)`.
+            sumw2(h)
+        Get the sum of weights squared of the histogram, it has the same shape as `bincounts(h)`.
         """
         sumw2(h::$H) = h.sumw2
 
         @doc """
-                binerrors(f=sqrt, h)
-            Get the error (uncertainty) of each bin. By default, calls `sqrt` on `sumw2(h)` bin by bin as an approximation.
+            binerrors(f=sqrt, h)
+        Get the error (uncertainty) of each bin. By default, calls `sqrt` on `sumw2(h)` bin by bin as an approximation.
         """
         binerrors(f::T, h::$H) where T<:Function = f.(sumw2(h))
         binerrors(h::$H) = binerrors(sqrt, h)
+
+        @doc raw"""
+            effective_entries(h) -> scalar
+
+        Get the number of effective entries for the entire histogram:
+
+        ```math
+        n_{eff} = \frac{(\sum Weights )^2}{(\sum Weight^2 )}
+        ```
+
+        This is also equivalent to `integral(hist)^2 / sum(sumw2(hist))`, this is the same as `TH1::GetEffectiveEntries()`
+        """
+        effective_entries(h::$H) = abs2(integral(h)) / sum(sumw2(h))
 
         function Base.:(==)(h1::$H, h2::$H)
             bincounts(h1) == bincounts(h2) &&

src/arithmatics.jl

@@ -18,6 +18,7 @@ for T in (:Hist1D,:Hist2D,:Hist3D)
 
         ($T)(; bincounts = newcounts, binedges = copy.(binedges(h1)), sumw2 = sumw2(h1) * num^2, nentries = nentries(h1), overflow = h1.overflow)
     end
+    @eval *(num::Real, h1::($T)) = h1 * num
 
     # https://github.com/aminnj/yahist/blob/4a5767f181ec7fdcc4af18cf15ceedd1c2f89019/yahist/hist1d.py#L427-L430
     @eval function /(h1::($T), h2::($T))

test/runtests.jl

@@ -612,4 +612,15 @@ end
     @test all(significance(h1,h2) .≈ (9.839916447569484, 0.30998654607114046))
 end
 
+@testset "Effective entries" begin
+    h = Hist1D(; binedges = 1:10)
+    push!(h, 3)
+    push!(h, 3)
+    @test FHist.effective_entries(h) == 2
+    push!(h, 8, 0.2)
+    @test FHist.effective_entries(h) ≈ 2.3725490196078436
+    push!(h, 4, 0.2)
+    @test FHist.effective_entries(h) ≈ 2.76923076923077
+end
+
 include("hdf5.jl")