Add zero sized sieve bypass.

Author: evoskuil

include/bitcoin/database/impl/primitives/sieve.ipp

@@ -88,6 +88,9 @@ TEMPLATE
 constexpr bool CLASS::screened(sieve_t fingerprint) const NOEXCEPT
 {
     using namespace system;
+    if constexpr (is_zero(limit))
+        return true;
+
     const auto row = shift_right(sieve_, screen_bits);
     if (row == limit)
     {
@@ -114,6 +117,9 @@ TEMPLATE
 constexpr bool CLASS::screen(sieve_t fingerprint) NOEXCEPT
 {
     using namespace system;
+    if constexpr (is_zero(limit))
+        return false;
+
     auto row = shift_right(sieve_, screen_bits);
     if (row == limit)
     {

test/primitives/sieve.cpp

@@ -22,6 +22,26 @@ BOOST_AUTO_TEST_SUITE(sieve_tests)
 
 using namespace system;
 
+BOOST_AUTO_TEST_CASE(sieve__screen__zeroed__always_screened)
+{
+    // Ensure default/nop behavior.
+    static_assert(sieve<0, 0>{}.screened(42));
+    static_assert(sieve<1, 0>{}.screened(42));
+    static_assert(!sieve<1, 1>{}.screened(42));
+
+    sieve<0, 0> sieve0{};
+    BOOST_REQUIRE(sieve0.screened(42));
+    BOOST_REQUIRE(!sieve0.screen(42));
+
+    sieve<1, 0> sieve1{};
+    BOOST_REQUIRE(sieve1.screened(42));
+    BOOST_REQUIRE(!sieve1.screen(42));
+
+    sieve<1, 1> sieve2{};
+    BOOST_REQUIRE(!sieve2.screened(42));
+    BOOST_REQUIRE(sieve2.screen(42));
+}
+
 #if defined(HAVE_SLOW_TESTS)
 
 BOOST_AUTO_TEST_CASE(sieve__screen__4_bits_forward__16_screens)