NDPluginTimeSeries: divide the averaged sum before narrowing, not after#596
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MarkRivers merged 1 commit intoJul 16, 2026
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doTimeSeriesT() stored an averaged time-series point as
pTimeCircular[...] = (epicsType)averageStore_[signal]/numAveraged_;
C++ precedence binds the cast tighter than the divide, so this evaluates
as ((epicsType)averageStore_[signal]) / numAveraged_. averageStore_ is a
double accumulator holding the SUM of numAveraged_ samples, so the sum is
truncated (and, for integer element types, wrapped) to the narrow element
type before the division ever happens.
For any integer signal with averaging enabled (numAveraged_ > 1) this
corrupts every averaged point once the running sum exceeds the element
type's range: three UInt8 samples of 200 sum to 600, which narrows to
(epicsUInt8)600 == 88, then 88 / 3 == 29 instead of 200.
Parenthesize as (epicsType)(averageStore_[signal]/numAveraged_) so the
double division runs at full precision and only the final average is
narrowed. Float element types were already correct and are unchanged.
Verified with a proof driver over the worked example: the old expression
yields 29, the parenthesized expression yields 200.
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(epicsType)averageStore_[signal]/numAveraged_parses as((epicsType)sum)/n— thedoublesum is truncated or wrapped to the narrow element type before the divide. With averaging on, three UInt8 samples of 200 sum to 600 →(epicsUInt8)600 == 88→88/3 == 29instead of 200. Parenthesize as(epicsType)(averageStore_[signal]/numAveraged_); float element types are unaffected. Proven with a driver reproducing the exact expression.