From 414856259d0873546eb0225157d85a12fa18cee5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=B8rgen=20Edelbo?= <jorgen.edelbo@mongodb.com>
Date: Fri, 13 Dec 2019 11:16:38 +0100
Subject: [PATCH] Fix the float sorting algorithm

Change-Id: Ia90174fbc838dbe0fe367508309ad3412e28658d
---
 src/realm/column.hpp | 25 ++++++++++++++-----------
 test/test_table.cpp  |  2 +-
 2 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/src/realm/column.hpp b/src/realm/column.hpp
index bbc6f7341c3..eda8ca2e716 100644
--- a/src/realm/column.hpp
+++ b/src/realm/column.hpp
@@ -895,20 +895,23 @@ template <> struct IntTypeForSize<8> { using type = uint64_t; };
 template <typename Float>
 int compare_float(Float a_raw, Float b_raw)
 {
-    // nans (and by extension nulls) are treated as being less than all non-nan values
     bool a_nan = std::isnan(a_raw);
     bool b_nan = std::isnan(b_raw);
-    if (a_nan != b_nan) {
-        return a_nan ? 1 : -1;
+    if (!a_nan && !b_nan) {
+        // Just compare as IEEE floats
+        return a_raw == b_raw ? 0 : a_raw < b_raw ? 1 : -1;
     }
-
-    // Compare the values as ints, which gives correct results for IEEE floats
-    // and bypasses the usual behavior of nans not being comparable to each other
-    using IntType = typename _impl::IntTypeForSize<sizeof(Float)>::type;
-    IntType a = 0, b = 0;
-    memcpy(&a, &a_raw, sizeof(Float));
-    memcpy(&b, &b_raw, sizeof(Float));
-    return a == b ? 0 : a < b ? 1 : -1;
+    if (a_nan && b_nan) {
+        // Compare the nan values (including nulls) as unsigned
+        using IntType = typename _impl::IntTypeForSize<sizeof(Float)>::type;
+        IntType a = 0, b = 0;
+        memcpy(&a, &a_raw, sizeof(Float));
+        memcpy(&b, &b_raw, sizeof(Float));
+        return a == b ? 0 : a < b ? 1 : -1;
+    }
+    // One is nan, the other is not
+    // nans are treated as being less than all non-nan values
+    return a_nan ? 1 : -1;
 }
 } // namespace _impl
 
diff --git a/test/test_table.cpp b/test/test_table.cpp
index 507f43f33cd..fdf24b73c58 100644
--- a/test/test_table.cpp
+++ b/test/test_table.cpp
@@ -1877,7 +1877,7 @@ TEST_TYPES(Table_SortFloat, float, double)
     for (size_t i = 300; i < 600; ++i) {
         CHECK(sorted.get(i).is_null(col));
     }
-    for (size_t i = 600; i + i < 900; ++i) {
+    for (size_t i = 600; i + 1 < 900; ++i) {
         CHECK_GREATER(sorted.get(i + 1).get<TEST_TYPE>(col), sorted.get(i).get<TEST_TYPE>(col));
     }
 }