From c8e821072226105ee8c621af4dd21a7ba390f408 Mon Sep 17 00:00:00 2001
From: Kevin Phan <98072684+ph-kev@users.noreply.github.com>
Date: Thu, 23 Jan 2025 17:22:47 -0800
Subject: [PATCH] Add check for whether P(z) is bijective or not

---
 src/Atmos.jl       |  9 ++++++++-
 test/test_Atmos.jl | 24 ++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/src/Atmos.jl b/src/Atmos.jl
index 65c4e70d..99043f7b 100644
--- a/src/Atmos.jl
+++ b/src/Atmos.jl
@@ -22,11 +22,18 @@ Linearly interpolate `var1d` from `origin_pressure` to `target_pressure`.
 Note: Values outside of the range are linearly extrapolated
 """
 function _resample_column!(dest, var1d, origin_pressure, target_pressure)
+    Main.@infiltrate
+    reverse_origin_pressure = reverse(origin_pressure)
+    isunique =
+        length(unique(reverse_origin_pressure)) ==
+        length(reverse_origin_pressure)
+    !(isunique && issorted(reverse_origin_pressure)) &&
+        error("P(z) is not bijective, cannot resample column")
     # Interpolations.jl require increasing knots, but pressure is decreasing, so
     # we have to reverse it
     var1d_of_P = Intp.extrapolate(
         Intp.interpolate(
-            (reverse(origin_pressure),),
+            (reverse_origin_pressure,),
             reverse(var1d),
             Intp.Gridded(Intp.Linear()),
         ),
diff --git a/test/test_Atmos.jl b/test/test_Atmos.jl
index ed0fb91a..4204c231 100644
--- a/test/test_Atmos.jl
+++ b/test/test_Atmos.jl
@@ -181,6 +181,30 @@ import OrderedCollections: OrderedDict
         Zvar,
         Zvar,
     )
+
+    # P(z) is not bijective (not sorted)
+    pressure_var = ClimaAnalysis.OutputVar(
+        attribs,
+        Dict("z" => z_alt),
+        dim_attribs,
+        reverse(pdata),
+    )
+    @test_throws ErrorException ClimaAnalysis.Atmos.to_pressure_coordinates(
+        pressure_var,
+        pressure_var,
+    )
+    pressure = 300.0:-2.0:100.0 |> collect
+    pressure[2] = 300.0
+    pressure_var = ClimaAnalysis.OutputVar(
+        attribs,
+        Dict("z" => z_alt),
+        dim_attribs,
+        pressure,
+    )
+    @test_throws ErrorException ClimaAnalysis.Atmos.to_pressure_coordinates(
+        pressure_var,
+        pressure_var,
+    )
 end
 
 @testset "RMSE of pressure coordinates" begin