From 71ed02aed11ef0411277c52723b5d30443c2aed5 Mon Sep 17 00:00:00 2001
From: Utkarsh <utkarshpawadecse@gmail.com>
Date: Sat, 28 Mar 2026 15:46:42 +0530
Subject: [PATCH 1/2] Add unit test for .psis_subset() output structure

---
 tests/testthat/test-ppc-loo.R | 44 +++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/tests/testthat/test-ppc-loo.R b/tests/testthat/test-ppc-loo.R
index a722ad46..f0f2a54a 100644
--- a/tests/testthat/test-ppc-loo.R
+++ b/tests/testthat/test-ppc-loo.R
@@ -212,6 +212,50 @@ test_that("error if subset is bigger than num obs", {
   )
 })
 
+test_that(".psis_subset output structure matches valid psis object", {
+  skip_if_not_installed("rstanarm")
+  skip_if_not_installed("loo")
+
+  subset_idx <- 1:10
+  result <- .psis_subset(psis1, subset_idx)
+
+  # class preserved
+
+  expect_s3_class(result, "psis")
+
+  # log_weights columns match subset length, rows unchanged
+  orig_dims <- dim(psis1)
+  expect_equal(ncol(result$log_weights), length(subset_idx))
+  expect_equal(nrow(result$log_weights), orig_dims[1])
+
+  # log_weights values match the subsetted columns
+  expect_equal(
+    result$log_weights,
+    psis1$log_weights[, subset_idx, drop = FALSE]
+  )
+
+  # diagnostics subsetted correctly
+  expect_equal(result$diagnostics$pareto_k, psis1$diagnostics$pareto_k[subset_idx])
+  expect_equal(result$diagnostics$n_eff, psis1$diagnostics$n_eff[subset_idx])
+
+  # dims attribute updated
+  expect_equal(attr(result, "dims"), c(orig_dims[1], length(subset_idx)))
+
+  # other attributes subsetted correctly
+  expect_equal(
+    attr(result, "norm_const_log"),
+    attr(psis1, "norm_const_log")[subset_idx]
+  )
+  expect_equal(
+    attr(result, "tail_len"),
+    attr(psis1, "tail_len")[subset_idx]
+  )
+  expect_equal(
+    attr(result, "r_eff"),
+    attr(psis1, "r_eff")[subset_idx]
+  )
+})
+
 
 # Visual tests ------------------------------------------------------------
 

From 7c1008372bd845d133a8d57f70838463222b3215 Mon Sep 17 00:00:00 2001
From: Utkarsh <utkarshpawadecse@gmail.com>
Date: Sat, 28 Mar 2026 15:57:17 +0530
Subject: [PATCH 2/2] update news.md

---
 NEWS.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/NEWS.md b/NEWS.md
index e7fc9138..c6f8fabc 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,6 @@
 # bayesplot (development version)
 
+* Added unit test for `.psis_subset()` .
 * Added unit tests for `mcmc_areas_ridges_data()`, `mcmc_parcoord_data()`, and `mcmc_trace_data()`.
 * Added unit tests for `ppc_error_data()` and `ppc_loo_pit_data()` covering output structure, argument handling, and edge cases.
 * Added vignette sections demonstrating `*_data()` companion functions for building custom ggplot2 visualizations (#435)