fix interpol logic, add test

src/BCReader.jl

@@ -273,9 +273,9 @@ or returns the first Field if interpolation is switched off.
 - Fields.field
 """
 function interpolate_midmonth_to_daily(date, bcf_info::BCFileInfo{FT}) where {FT}
-    if bcf_info.interpolate_daily && bcf_info.segment_length[1] > FT(0)
-        (; segment_length, segment_idx, all_dates, monthly_fields) = bcf_info
-
+    (; segment_length, segment_idx, all_dates, monthly_fields) = bcf_info
+    if bcf_info.interpolate_daily && bcf_info.segment_length[1] > FT(0) &&
+       date != all_dates[Int(segment_idx[1])]
         return interpol.(
             monthly_fields[1],
             monthly_fields[2],
@@ -306,7 +306,7 @@ function interpol(f1::FT, f2::FT, Δt_tt1::FT, Δt_t2t1::FT) where {FT}
     @assert Δt_t2t1 > FT(0) "t2 must be > t1, but `Δt_t2t1` = $Δt_t2t1"
     interp_fraction = Δt_tt1 / Δt_t2t1
     @assert abs(interp_fraction) <= FT(1) "time interpolation weights must be <= 1, but `interp_fraction` = $interp_fraction"
-    return f1 * interp_fraction + f2 * (FT(1) - interp_fraction)
+    return f1 * (FT(1) - interp_fraction) + f2 * interp_fraction
 end
 
 end

test/bcreader_tests.jl

@@ -60,6 +60,27 @@ for FT in (Float32, Float64)
         end
     end
 
+    @testset "test interpol for FT=$FT" begin
+        # Setup
+        t1 = FT(0)
+        t2 = FT(10)
+
+        f1 = FT(-1)
+        f2 = FT(1)
+
+        # Case 1: t1 < t < t2 --> should return average (0)
+        t = FT(5)
+        @test BCReader.interpol(f1, f2, t - t1, t2 - t1) == 0
+
+        # Case 2: t1 = t < t2 --> should return val at t1 (f1)
+        t = FT(0)
+        @test BCReader.interpol(f1, f2, t - t1, t2 - t1) == f1
+
+        # Case 3: t1 < t = t2 --> should return val at t2 (f2)
+        t = FT(10)
+        @test BCReader.interpol(f1, f2, t - t1, t2 - t1) == f2
+    end
+
     @testset "test interpolate_midmonth_to_daily for FT=$FT" begin
         # test interpolate_midmonth_to_daily with interpolation
         interpolate_daily = true