[GitHub] [arrow] milesgranger commented on a diff in pull request #14696: ARROW-18282: [C++][Python] Support step >= 1 in list_slice kernel

[GitBox <gi...@apache.org>]

milesgranger commented on code in PR #14696:
URL: https://github.com/apache/arrow/pull/14696#discussion_r1035902151


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cpp/src/arrow/compute/kernels/scalar_nested.cc:
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@@ -96,6 +97,22 @@ std::string ToString(const std::optional<T>& o) {
   return o.has_value() ? ToChars(*o) : "(nullopt)";
 }
 
+int64_t MaxSliceLength(const int64_t start, const int64_t stop, const int64_t step) {
+  const auto startf = static_cast<float>(start);
+  const auto stopf = static_cast<float>(stop);
+  const auto stepf = static_cast<float>(step);
+
+  if (stopf - startf <= stepf) {
+    return 1;
+  }
+
+  auto length = static_cast<int64_t>(std::floor((stopf - startf) / stepf));
+  if (fmod(static_cast<float>(length), stepf) > 0.0) {
+    ++length;
+  }
+  return length;

Review Comment:
   More tests added, let me know if you see any specific combinations you'd like to see.
   
   > (also, shouldn't it be fmod(stop-start, step) instead?)
   
   It needs to be `length`, which if there is no step, then `stop-start` is okay, but that divided by step gives the actual length if step != 1. Then (at least in python) if there is any remainder one more is added to the possible length.



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