SERVER-16708 Make Value::compare match BSON woCompare behavior

1 files changed, 93 insertions, 0 deletions

diff --git a/src/mongo/base/compare_numbers.h b/src/mongo/base/compare_numbers.h
new file mode 100644
index 00000000000..c086f87fb7a
--- /dev/null
+++ b/src/mongo/base/compare_numbers.h
@@ -0,0 +1,93 @@
+/*    Copyright 2015 MongoDB Inc.
+ *
+ *    This program is free software: you can redistribute it and/or  modify
+ *    it under the terms of the GNU Affero General Public License, version 3,
+ *    as published by the Free Software Foundation.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    GNU Affero General Public License for more details.
+ *
+ *    You should have received a copy of the GNU Affero General Public License
+ *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the GNU Affero General Public License in all respects
+ *    for all of the code used other than as permitted herein. If you modify
+ *    file(s) with this exception, you may extend this exception to your
+ *    version of the file(s), but you are not obligated to do so. If you do not
+ *    wish to do so, delete this exception statement from your version. If you
+ *    delete this exception statement from all source files in the program,
+ *    then also delete it in the license file.
+ */
+
+#pragma once
+
+#include "mongo/platform/float_utils.h"
+#include "mongo/util/assert_util.h"
+
+namespace mongo {
+
+    /**
+     * These functions compare numbers using the same rules as BSON. Care is taken to always give
+     * numerically correct results when comparing different types. Returns are always -1, 0, or 1 to
+     * ensure it is safe to negate the result to invert the direction of the comparison.
+     */
+
+    inline int compareInts(int lhs, int rhs) {
+        return lhs == rhs ? 0 : lhs < rhs ? -1 : 1;
+    }
+
+    inline int compareLongs(long long lhs, long long rhs) {
+        return lhs == rhs ? 0 : lhs < rhs ? -1 : 1;
+    }
+
+    inline int compareDoubles(double lhs, double rhs) {
+        if (lhs == rhs) return 0;
+        if (lhs < rhs) return -1;
+        if (lhs > rhs) return 1;
+
+        // If none of the above cases returned, lhs or rhs must be NaN.
+        if (isNaN(lhs)) return isNaN(rhs) ? 0 : -1;
+        dassert(isNaN(rhs));
+        return 1;
+    }
+
+    // This is the tricky one. Needs to support the following cases:
+    // * Doubles with a fractional component.
+    // * Longs that can't be precisely represented as a double.
+    // * Doubles outside of the range of Longs (including +/- Inf).
+    // * NaN (defined by us as less than all Longs)
+    // * Return value is always -1, 0, or 1 to ensure it is safe to negate.
+    inline int compareLongToDouble(long long lhs, double rhs) {
+        // All Longs are > NaN
+        if (isNaN(rhs)) return 1;
+
+        // Ints with magnitude <= 2**53 can be precisely represented as doubles.
+        // Additionally, doubles outside of this range can't have a fractional component.
+        static const long long kEndOfPreciseDoubles = 1ll << 53;
+        if (lhs <= kEndOfPreciseDoubles && lhs >= -kEndOfPreciseDoubles) {
+            return compareDoubles(lhs, rhs);
+        }
+
+        // Large magnitude doubles (including +/- Inf) are strictly > or < all Longs.
+        static const double kBoundOfLongRange = -static_cast<double>(LLONG_MIN); // positive 2**63
+        if (rhs >= kBoundOfLongRange) return -1; // Can't be represented in a Long.
+        if (rhs < -kBoundOfLongRange) return 1; // Can be represented in a Long.
+
+        // Remaining Doubles can have their integer component precisely represented as long longs.
+        // If they have a fractional component, they must be strictly > or < lhs even after
+        // truncation of the fractional component since low-magnitude lhs were handled above.
+        return compareLongs(lhs, rhs);
+    }
+
+    inline int compareDoubleToLong(double lhs, long long rhs) {
+        // Only implement the real logic once.
+        return -compareLongToDouble(rhs, lhs);
+    }
+
+} // namespace mongo