[PATCH 2/5] libipa: histogram: Fix quantile() calculation for fractional results

[Stefan Klug]

The calculation of the frac variable is based solely on integers and
therefore results in the fractional part being either 0 or 1.

In the original code from RaspberryPi this is mitigated by casting the
nominator to a double. This works for most cases, but fails when q is
very small because of the quantization introduced by item being an
integer.

Fix both issues by doing the full calculation in double.

Signed-off-by: Stefan Klug <stefan.klug at ideasonboard.com>
---
 src/ipa/libipa/histogram.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/ipa/libipa/histogram.cpp b/src/ipa/libipa/histogram.cpp
index 10e44b54a0cf..c19a4cbbf3cd 100644
--- a/src/ipa/libipa/histogram.cpp
+++ b/src/ipa/libipa/histogram.cpp
@@ -130,7 +130,8 @@ double Histogram::quantile(double q, uint32_t first, uint32_t last) const
 	if (cumulative_[first + 1] == cumulative_[first])
 		frac = 0;
 	else
-		frac = (item - cumulative_[first]) / (cumulative_[first + 1] - cumulative_[first]);
+		frac = (q * total() - cumulative_[first]) /
+		       (cumulative_[first + 1] - cumulative_[first]);
 	return first + frac;
 }
 
-- 
2.43.0