ORC-2160: [C++] Expose prefetch range planning via Reader::preBufferRange and refactor preBuffer to reuse it

c++/include/orc/Reader.hh

@@ -32,6 +32,7 @@
 #include <memory>
 #include <set>
 #include <string>
+#include <utility>
 #include <vector>
 
 namespace orc {
@@ -696,6 +697,16 @@ namespace orc {
     virtual void preBuffer(const std::vector<uint32_t>& stripes,
                            const std::list<uint64_t>& includeTypes) = 0;
 
+    /**
+     * Calculate prefetch ranges by selected stripes and columns.
+     * It is thread safe and does not cache data.
+     * @param stripes the stripes to prefetch
+     * @param includeTypes the types to prefetch
+     * @return prefetch ranges as offset/length pairs
+     */
+    virtual std::vector<std::pair<uint64_t, uint64_t>> preBufferRange(
+        const std::vector<uint32_t>& stripes, const std::list<uint64_t>& includeTypes) = 0;
+
     /**
      * Release cached entries whose right boundary is less than or equal to the given boundary.
      * @param boundary the boundary value to release cache entries

c++/src/Reader.cc

@@ -1770,8 +1770,8 @@ namespace orc {
     contents_->evictCache(boundary);
   }
 
-  void ReaderImpl::preBuffer(const std::vector<uint32_t>& stripes,
-                             const std::list<uint64_t>& includeTypes) {
+  std::vector<std::pair<uint64_t, uint64_t>> ReaderImpl::preBufferRange(
+      const std::vector<uint32_t>& stripes, const std::list<uint64_t>& includeTypes) {
     std::vector<uint32_t> newStripes;
     for (auto stripe : stripes) {
       if (stripe < static_cast<uint32_t>(footer_->stripes_size())) newStripes.push_back(stripe);
@@ -1783,7 +1783,7 @@ namespace orc {
     }
 
     if (newStripes.empty() || newIncludeTypes.empty()) {
-      return;
+      return {};
     }
 
     orc::RowReaderOptions rowReaderOptions;
@@ -1792,12 +1792,33 @@ namespace orc {
     std::vector<bool> selectedColumns;
     columnSelector.updateSelected(selectedColumns, rowReaderOptions);
 
+    std::vector<std::pair<uint64_t, uint64_t>> ranges;
+
     for (auto stripe : newStripes) {
       const auto& stripeInfo = footer_->stripes(stripe);
       proto::StripeFooter stripeFooter = getStripeFooter(stripeInfo, *contents_);
-      auto ranges = extractReadRangesForStripe(stripe, stripeInfo, stripeFooter, selectedColumns);
-      contents_->cacheRanges(std::move(ranges));
+      auto stripeRanges =
+          extractReadRangesForStripe(stripe, stripeInfo, stripeFooter, selectedColumns);
+      for (const auto& range : stripeRanges) {
+        ranges.emplace_back(range.offset, range.length);
+      }
+    }
+    return ranges;
+  }
+
+  void ReaderImpl::preBuffer(const std::vector<uint32_t>& stripes,
+                             const std::list<uint64_t>& includeTypes) {
+    auto ranges = preBufferRange(stripes, includeTypes);
+    if (ranges.empty()) {
+      return;
+    }
+
+    std::vector<ReadRange> readRanges;
+    readRanges.reserve(ranges.size());
+    for (const auto& range : ranges) {
+      readRanges.emplace_back(range.first, range.second);
     }
+    contents_->cacheRanges(std::move(readRanges));
   }
 
   RowReader::~RowReader() {

c++/src/Reader.hh

@@ -410,6 +410,9 @@ namespace orc {
     std::map<uint32_t, BloomFilterIndex> getBloomFilters(
         uint32_t stripeIndex, const std::set<uint32_t>& included) const override;
 
+    std::vector<std::pair<uint64_t, uint64_t>> preBufferRange(
+        const std::vector<uint32_t>& stripes, const std::list<uint64_t>& includeTypes) override;
+
     void preBuffer(const std::vector<uint32_t>& stripes,
                    const std::list<uint64_t>& includeTypes) override;
     void releaseBuffer(uint64_t boundary) override;