emkornfield commented on a change in pull request #8897:
URL: https://github.com/apache/arrow/pull/8897#discussion_r542586637
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File path: cpp/src/arrow/type.cc
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@@ -747,6 +747,75 @@ std::vector<std::shared_ptr<Field>>
StructType::GetAllFieldsByName(
return result;
}
+// Taken from the Apache Impala codebase. The comments next
+// to the return values are the maximum value that can be represented in 2's
+// complement with the returned number of bytes.
+int32_t DecimalType::DecimalSize(int32_t precision) {
+ DCHECK_GE(precision, 1) << "decimal precision must be greater than or equal
to 1, got "
+ << precision;
+
+ switch (precision) {
+ case 1:
+ case 2:
+ return 1; // 127
+ case 3:
+ case 4:
+ return 2; // 32,767
+ case 5:
+ case 6:
+ return 3; // 8,388,607
+ case 7:
+ case 8:
+ case 9:
+ return 4; // 2,147,483,427
+ case 10:
+ case 11:
+ return 5; // 549,755,813,887
+ case 12:
+ case 13:
+ case 14:
+ return 6; // 140,737,488,355,327
+ case 15:
+ case 16:
+ return 7; // 36,028,797,018,963,967
+ case 17:
+ case 18:
+ return 8; // 9,223,372,036,854,775,807
+ case 19:
+ case 20:
+ case 21:
+ return 9; // 2,361,183,241,434,822,606,847
+ case 22:
+ case 23:
+ return 10; // 604,462,909,807,314,587,353,087
+ case 24:
+ case 25:
+ case 26:
+ return 11; // 154,742,504,910,672,534,362,390,527
+ case 27:
+ case 28:
+ return 12; // 39,614,081,257,132,168,796,771,975,167
+ case 29:
+ case 30:
+ case 31:
+ return 13; // 10,141,204,801,825,835,211,973,625,643,007
+ case 32:
+ case 33:
+ return 14; // 2,596,148,429,267,413,814,265,248,164,610,047
+ case 34:
+ case 35:
+ return 15; // 664,613,997,892,457,936,451,903,530,140,172,287
+ case 36:
+ case 37:
+ case 38:
+ return 16; // 170,141,183,460,469,231,731,687,303,715,884,105,727
+ default:
+ return static_cast<int32_t>(std::ceil((precision / 8.0) *
std::log2(10)));
Review comment:
done. I left open ended computation, because it seems like the "least"
surprise option but I can remove if you prefere.
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