From 8b453e1f015f0acccb979f7992bc0003c08753e4 Mon Sep 17 00:00:00 2001
From: Kostiantyn Lazukin <konstantin.lazukin@gmail.com>
Date: Mon, 28 Oct 2024 03:54:32 +0000
Subject: [PATCH] Replace custom bitscan with countr_zero

---
 include/ass/bit/bit_scan_constexpr.hpp | 70 --------------------------
 include/ass/fixed_bitset.hpp           |  8 +--
 tests/bit/bit_scan_test.cpp            |  9 ----
 3 files changed, 4 insertions(+), 83 deletions(-)
 delete mode 100644 include/ass/bit/bit_scan_constexpr.hpp
 delete mode 100644 tests/bit/bit_scan_test.cpp

diff --git a/include/ass/bit/bit_scan_constexpr.hpp b/include/ass/bit/bit_scan_constexpr.hpp
deleted file mode 100644
index f3fa5db..0000000
--- a/include/ass/bit/bit_scan_constexpr.hpp
+++ /dev/null
@@ -1,70 +0,0 @@
-#pragma once
-
-#include <array>
-#include <cstddef>
-#include <cstdint>
-#include <limits>
-#include <type_traits>
-
-#include "bit_count_to_type.hpp"
-
-namespace ass::bit_scan_constexpr_detail
-{
-inline constexpr auto kBitScanTable = []()
-{
-    std::array<uint8_t, 256> table{};
-
-    auto bit_scan_slow = [](uint8_t value)
-    {
-        uint8_t index = 0;
-        constexpr size_t bits_in_byte = 8;
-        while (!(value & (1 << index)) && index != bits_in_byte)
-        {
-            ++index;
-        }
-        return index;
-    };
-
-    uint8_t value = 0;
-    do  // NOLINT
-    {
-        table[value] = bit_scan_slow(value);
-        value += 1;
-    } while (value != std::numeric_limits<uint8_t>::max());
-
-    return table;
-}();
-}  // namespace ass::bit_scan_constexpr_detail
-
-namespace ass
-{
-
-inline constexpr size_t BitScanConstexpr(const uint8_t v)
-{
-    return bit_scan_constexpr_detail::kBitScanTable[v];
-}
-
-// Returns an index of first non-zero bit
-// Also may be interpreted as number of leading zero bits
-template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
-inline constexpr size_t BitScanConstexpr(const T v)
-{
-    // Split the bitset into two parts and count bits in these parts recursively
-    // util we come to uint8_t, which has it's own table
-    constexpr size_t kBitsCount = sizeof(v) * 8;
-    constexpr size_t kHalfBitsCount = kBitsCount / 2;
-    using HalfType = BitsCountToUnsignedIntT<kHalfBitsCount>;
-
-    if (const auto lo = static_cast<HalfType>(v); lo != 0)
-    {
-        return BitScanConstexpr(lo);
-    }
-
-    if (const auto hi = static_cast<HalfType>(v >> kHalfBitsCount); hi != 0)
-    {
-        return BitScanConstexpr(hi) + kHalfBitsCount;
-    }
-
-    return kBitsCount;
-}
-}  // namespace ass
diff --git a/include/ass/fixed_bitset.hpp b/include/ass/fixed_bitset.hpp
index d58eed7..6848189 100644
--- a/include/ass/fixed_bitset.hpp
+++ b/include/ass/fixed_bitset.hpp
@@ -4,9 +4,9 @@
 #include <bit>
 #include <cassert>
 #include <cstddef>
+#include <limits>
 
 #include "bit/bit_count_to_type.hpp"
-#include "bit/bit_scan_constexpr.hpp"
 
 namespace ass::fixed_bitset_detail
 {
@@ -110,7 +110,7 @@ class FixedBitset
         for (size_t part_index = 0; part_index != parts_.size(); ++part_index)
         {
             const Part& part = parts_[part_index];
-            const size_t local = BitScanConstexpr(part);
+            const size_t local = std::countr_zero(part);
             result += local;
             if (local != kPartBitsCount)
             {
@@ -134,7 +134,7 @@ class FixedBitset
             mask = ~mask;
             mask <<= ignored_bits;
             const Part part = parts_[part_index] & mask;
-            const size_t local = BitScanConstexpr(part);
+            const size_t local = std::countr_zero(part);
             if (local != kPartBitsCount)
             {
                 return result + local;
@@ -147,7 +147,7 @@ class FixedBitset
         for (; part_index != parts_.size(); ++part_index)
         {
             const Part& part = parts_[part_index];
-            const size_t local = BitScanConstexpr(part);
+            const size_t local = std::countr_zero(part);
             result += local;
             if (local != kPartBitsCount)
             {
diff --git a/tests/bit/bit_scan_test.cpp b/tests/bit/bit_scan_test.cpp
deleted file mode 100644
index bc04c1c..0000000
--- a/tests/bit/bit_scan_test.cpp
+++ /dev/null
@@ -1,9 +0,0 @@
-#include "ass/bit/bit_scan_constexpr.hpp"
-
-static_assert(ass::BitScanConstexpr(0b11111111) == 0);
-static_assert(ass::BitScanConstexpr(0b11111100) == 2);
-static_assert(ass::BitScanConstexpr(0b00000000) == 8);
-static_assert(ass::BitScanConstexpr(static_cast<uint16_t>(0b00000000'00000000)) == 16u);
-static_assert(ass::BitScanConstexpr(static_cast<uint16_t>(0b00000000'10000000)) == 7u);
-static_assert(ass::BitScanConstexpr(static_cast<uint16_t>(0b00000001'00000000)) == 8u);
-static_assert(ass::BitScanConstexpr(static_cast<uint16_t>(0b10000000'00000000)) == 15u);