Improving the use of stdlib where possible and const-correctness for good measure

Author: DiamonDinoia

CMakeLists.txt

@@ -84,5 +84,10 @@ if(ipo_supported AND (CMAKE_BUILD_TYPE STREQUAL "Release"))
   set_target_properties(_simple_ans PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
 endif()
 
+set_target_properties(_simple_ans PROPERTIES
+        CXX_VISIBILITY_PRESET hidden
+        VISIBILITY_INLINES_HIDDEN ON
+)
+
 # Install rules
 install(TARGETS _simple_ans LIBRARY DESTINATION simple_ans)

simple_ans/cpp/simple_ans.hpp

@@ -1,14 +1,17 @@
 #pragma once
 
+#include <algorithm>
+#include <bit>
 #include <cassert>
-#include <cstdint>
-#include <cstdio>
+#include <execution>
 #include <limits>
+#include <numeric>
 #include <stdexcept>
 #include <vector>
+
 #include <tsl/robin_map.h>
 
-#include "libdivide.h"
+#include <libdivide.h>
 
 namespace simple_ans
 {
@@ -20,7 +23,7 @@ struct EncodedData
 };
 
 // Helper function to verify if a number is a power of 2
-inline bool is_power_of_2(uint32_t x)
+constexpr bool is_power_of_2(uint32_t x)
 {
     return x && !(x & (x - 1));
 }
@@ -113,29 +116,17 @@ EncodedData ans_encode_t(const T* signal,
                   "Value range of T must fit in int64_t for table lookup");
 
     // Calculate L and verify it's a power of 2
-    uint32_t index_size = 0;
-    for (size_t i = 0; i < num_symbols; ++i)
-    {
-        index_size += symbol_counts[i];
-    }
+    const uint32_t index_size = std::reduce(symbol_counts, symbol_counts + num_symbols, 0u);
     if (!is_power_of_2(index_size))
     {
         throw std::invalid_argument("L must be a power of 2");
     }
 
-    int PRECISION_BITS = 0;
-    while ((1U << PRECISION_BITS) < index_size)
-    {
-        PRECISION_BITS++;
-    }
+    const auto PRECISION_BITS = std::bit_width(index_size - 1);
 
     // Pre-compute cumulative sums
     std::vector<uint32_t> C(num_symbols);
-    C[0] = 0;
-    for (size_t i = 1; i < num_symbols; ++i)
-    {
-        C[i] = C[i - 1] + symbol_counts[i - 1];
-    }
+    std::exclusive_scan(symbol_counts, symbol_counts + num_symbols, C.begin(), 0u);
 
     // Precompute libdivide dividers for each symbol count
     std::vector<libdivide::divider<uint64_t>> fast_dividers(num_symbols);
@@ -157,15 +148,12 @@ EncodedData ans_encode_t(const T* signal,
     }
 
     // Map lookups can be a bottleneck, so we use a lookup array if the number of symbols is "small"
-    const bool use_lookup_array = (max_symbol - min_symbol + 1) <= lookup_array_threshold;
-    std::vector<size_t> symbol_index_lookup_array;
+    const auto array_size = max_symbol - min_symbol + 1;
+    const bool use_lookup_array = array_size <= lookup_array_threshold;
+    std::vector<size_t> symbol_index_lookup_array(0);
     if (use_lookup_array)
     {
-        symbol_index_lookup_array.resize(max_symbol - min_symbol + 1);
-
-        std::fill(symbol_index_lookup_array.begin(),
-                  symbol_index_lookup_array.end(),
-                  std::numeric_limits<size_t>::max());
+        symbol_index_lookup_array.resize(array_size);
 
         for (size_t i = 0; i < num_symbols; ++i)
         {
@@ -241,40 +229,25 @@ void ans_decode_t(T* output,
                   size_t num_symbols)
 {
     // very important that this is signed, because it becomes -1
-    int32_t word_idx = num_words - 1;
+    auto word_idx = static_cast<int32_t>(num_words) - 1;
     // Calculate index size and verify it's a power of 2
-    uint32_t index_size = 0;
-    for (size_t i = 0; i < num_symbols; ++i)
-    {
-        index_size += symbol_counts[i];
-    }
+    const uint32_t index_size = std::reduce(symbol_counts, symbol_counts + num_symbols, 0u);
     if (!is_power_of_2(index_size))
     {
         throw std::invalid_argument("L must be a power of 2");
     }
 
-    int PRECISION_BITS = 0;
-    while ((1U << PRECISION_BITS) < index_size)
-    {
-        PRECISION_BITS++;
-    }
+    const auto PRECISION_BITS = std::bit_width(index_size - 1);
 
     // Pre-compute cumulative sums
     std::vector<uint32_t> C(num_symbols);
-    C[0] = 0;
-    for (size_t i = 1; i < num_symbols; ++i)
-    {
-        C[i] = C[i - 1] + symbol_counts[i - 1];
-    }
+    std::exclusive_scan(symbol_counts, symbol_counts + num_symbols, C.begin(), 0u);
 
     // Create symbol lookup table
     std::vector<uint32_t> symbol_lookup(index_size);
-    for (size_t s = 0; s < num_symbols; ++s)
+    for (uint32_t s = 0; s < num_symbols; ++s)
     {
-        for (uint32_t j = 0; j < symbol_counts[s]; ++j)
-        {
-            symbol_lookup[C[s] + j] = s;
-        }
+        std::fill_n(symbol_lookup.begin() + C[s], symbol_counts[s], s);
     }
 
     // Decode symbols in reverse order