GH-50778: [C++] Fix chunked level histogram accumulation

Author: wecharyu

cpp/src/parquet/size_statistics.cc

@@ -168,7 +168,7 @@ void UpdateLevelHistogram(std::span<const int16_t> levels, std::span<int64_t> hi
     auto it = levels.begin();
     while (it != levels.end()) {
       const auto chunk_size = std::min<int64_t>(levels.end() - it, kChunkSize);
-      hist1 += std::accumulate(levels.begin(), levels.begin() + chunk_size, int16_t{0});
+      hist1 += std::accumulate(it, it + chunk_size, int16_t{0});
       it += chunk_size;
     }
     histogram[0] += num_levels - hist1;

cpp/src/parquet/size_statistics_test.cc

@@ -50,6 +50,14 @@ TEST(SizeStatistics, UpdateLevelHistogram) {
     UpdateLevelHistogram(std::vector<int16_t>{}, histogram);
     EXPECT_THAT(histogram, ::testing::ElementsAre(3, 5));
   }
+  {
+    // Cross the chunk boundary used by the max_level = 1 fast path.
+    std::vector<int16_t> levels(1 << 14, 1);
+    levels.push_back(0);
+    std::vector<int64_t> histogram(2, 0);
+    UpdateLevelHistogram(levels, histogram);
+    EXPECT_THAT(histogram, ::testing::ElementsAre(1, 1 << 14));
+  }
   {
     // max_level > 1
     std::vector<int64_t> histogram(3, 0);