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Base-2 Exponential Histogram
Design
This document is a placeholder for future Aggregator, once seen in PR 2393.
Only the mapping functions have been made available at this time. The equations tested here are specified in the data model for Exponential Histogram data points.
Mapping function
There are two mapping functions used, depending on the sign of the
scale. Negative and zero scales use the mapping/exponent mapping
function, which computes the bucket index directly from the bits of
the float64 exponent. This mapping function is used with scale -10 <= scale <= 0. Scales smaller than -10 map the entire normal
float64 number range into a single bucket, thus are not considered
useful.
The mapping/logarithm mapping function uses math.Log(value) times
the scaling factor math.Ldexp(math.Log2E, scale). This mapping
function is used with 0 < scale <= 20. The maximum scale is
selected because at scale 21, simply, it becomes difficult to test
correctness--at this point math.MaxFloat64 maps to index
math.MaxInt32 and the math/big logic used in testing breaks down.