[PATCH v4] ipa: libipa: histogram: Add transform parameter to constructor

[Paul Elder]

Add a parameter to the histogram constructor that takes a transformation
function to apply to all the bins upon construction.

This is necessary notably for the rkisp1, as the values reported from
the hardware are 20 bits where the upper 16-bits are meaningful integer
values and the lower 4 bits are fractional and meant to be discarded. As
adding a right-shift parameter is probably too specialized, a generic
function is added as a parameter instead.

Signed-off-by: Paul Elder <paul.elder at ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally at ideasonboard.com>

---
Changes in v4:
- fix compilation when called with no transform function

Changes in v3:
- make the transform function a template parameter, and optimize the
  constructor

This used to be "ipa: libipa: histogram: Add rshift parameter to
constructor"

Changes in v2:
- change rshift parameter to a function parameter
---
 src/ipa/libipa/histogram.cpp | 14 ++++++++++----
 src/ipa/libipa/histogram.h   | 13 ++++++++++++-
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/src/ipa/libipa/histogram.cpp b/src/ipa/libipa/histogram.cpp
index c1aac59b..212acfdf 100644
--- a/src/ipa/libipa/histogram.cpp
+++ b/src/ipa/libipa/histogram.cpp
@@ -43,12 +43,18 @@ namespace ipa {
  */
 Histogram::Histogram(Span<const uint32_t> data)
 {
-	cumulative_.reserve(data.size());
-	cumulative_.push_back(0);
-	for (const uint32_t &value : data)
-		cumulative_.push_back(cumulative_.back() + value);
+	cumulative_.resize(data.size() + 1);
+	cumulative_[0] = 0;
+	for (const auto &[i, value] : utils::enumerate(data))
+		cumulative_[i + 1] = cumulative_[i] + value;
 }
 
+/**
+ * \brief Create a cumulative histogram
+ * \param[in] data A pre-sorted histogram to be passed
+ * \param[in] transform The transformation function to apply to every bin
+ */
+
 /**
  * \fn Histogram::bins()
  * \brief Retrieve the number of bins currently used by the Histogram
diff --git a/src/ipa/libipa/histogram.h b/src/ipa/libipa/histogram.h
index 54bb2a19..0e9bef2a 100644
--- a/src/ipa/libipa/histogram.h
+++ b/src/ipa/libipa/histogram.h
@@ -10,10 +10,10 @@
 #include <assert.h>
 #include <limits.h>
 #include <stdint.h>
-
 #include <vector>
 
 #include <libcamera/base/span.h>
+#include <libcamera/base/utils.h>
 
 namespace libcamera {
 
@@ -24,6 +24,17 @@ class Histogram
 public:
 	Histogram() { cumulative_.push_back(0); }
 	Histogram(Span<const uint32_t> data);
+
+	template<typename Transform,
+		 std::enable_if_t<std::is_invocable_v<Transform, uint32_t>> * = nullptr>
+	Histogram(Span<const uint32_t> data, Transform transform)
+	{
+		cumulative_.resize(data.size() + 1);
+		cumulative_[0] = 0;
+		for (const auto &[i, value] : utils::enumerate(data))
+			cumulative_[i + 1] = cumulative_[i] + transform(value);
+	}
+
 	size_t bins() const { return cumulative_.size() - 1; }
 	uint64_t total() const { return cumulative_[cumulative_.size() - 1]; }
 	uint64_t cumulativeFrequency(double bin) const;
-- 
2.39.2