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+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2024, Paul Elder <paul.elder@ideasonboard.com>
+ *
+ * Helper class that performs computations relating to exposure
+ */
+#include "exposure_mode_helper.h"
+
+#include <algorithm>
+
+#include <libcamera/base/log.h>
+
+/**
+ * \file exposure_mode_helper.h
+ * \brief Helper class that performs computations relating to exposure
+ *
+ * AEGC algorithms have a need to split exposure between shutter time, analogue
+ * and digital gain. Multiple implementations do so based on paired stages of
+ * shutter time and gain limits; provide a helper to avoid duplicating the code.
+ */
+
+namespace libcamera {
+
+using namespace std::literals::chrono_literals;
+
+LOG_DEFINE_CATEGORY(ExposureModeHelper)
+
+namespace ipa {
+
+/**
+ * \class ExposureModeHelper
+ * \brief Class for splitting exposure into shutter time and total gain
+ *
+ * The ExposureModeHelper class provides a standard interface through which an
+ * AEGC algorithm can divide exposure between shutter time and gain. It is
+ * configured with a set of shutter time and gain pairs and works by initially
+ * fixing gain at 1.0 and increasing shutter time up to the shutter time value
+ * from the first pair in the set in an attempt to meet the required exposure
+ * value.
+ *
+ * If the required exposure is not achievable by the first shutter time value
+ * alone it ramps gain up to the value from the first pair in the set. If the
+ * required exposure is still not met it then allows shutter time to ramp up to
+ * the shutter time value from the second pair in the set, and continues in this
+ * vein until either the required exposure time is met, or else the hardware's
+ * shutter time or gain limits are reached.
+ *
+ * This method allows users to strike a balance between a well-exposed image and
+ * an acceptable frame-rate, as opposed to simply maximising shutter time
+ * followed by gain. The same helpers can be used to perform the latter
+ * operation if needed by passing an empty set of pairs to the initialisation
+ * function.
+ *
+ * The gain values may exceed a camera sensor's analogue gain limits if either
+ * it or the IPA is also capable of digital gain. The configure() function must
+ * be called with the hardware's limits to inform the helper of those
+ * constraints. Any gain that is needed will be applied as analogue gain first
+ * until the hardware's limit is reached, following which digital gain will be
+ * used.
+ */
+
+/**
+ * \brief Construct an ExposureModeHelper instance
+ * \param[in] stages The vector of paired shutter time and gain limits
+ *
+ * The input stages are shutter time and _total_ gain pairs; the gain
+ * encompasses both analogue and digital gain.
+ *
+ * The vector of stages may be empty. In that case, the helper will simply use
+ * the runtime limits set through setShutterGainLimits() instead.
+ */
+ExposureModeHelper::ExposureModeHelper(const Span<std::pair<utils::Duration, double>> stages)
+{
+	minShutter_ = 0us;
+	maxShutter_ = 0us;
+	minGain_ = 0;
+	maxGain_ = 0;
+
+	for (const auto &[s, g] : stages) {
+		shutters_.push_back(s);
+		gains_.push_back(g);
+	}
+}
+
+/**
+ * \brief Set the shutter time and gain limits
+ * \param[in] minShutter The minimum shutter time supported
+ * \param[in] maxShutter The maximum shutter time supported
+ * \param[in] minGain The minimum analogue gain supported
+ * \param[in] maxGain The maximum analogue gain supported
+ *
+ * This function configures the shutter time and analogue gain limits that need
+ * to be adhered to as the helper divides up exposure. Note that this function
+ * *must* be called whenever those limits change and before splitExposure() is
+ * used.
+ *
+ * If the algorithm using the helpers needs to indicate that either shutter time
+ * or analogue gain or both should be fixed it can do so by setting both the
+ * minima and maxima to the same value.
+ */
+void ExposureModeHelper::setLimits(utils::Duration minShutter,
+				   utils::Duration maxShutter,
+				   double minGain, double maxGain)
+{
+	minShutter_ = minShutter;
+	maxShutter_ = maxShutter;
+	minGain_ = minGain;
+	maxGain_ = maxGain;
+}
+
+utils::Duration ExposureModeHelper::clampShutter(utils::Duration shutter) const
+{
+	return std::clamp(shutter, minShutter_, maxShutter_);
+}
+
+double ExposureModeHelper::clampGain(double gain) const
+{
+	return std::clamp(gain, minGain_, maxGain_);
+}
+
+/**
+ * \brief Split exposure time into shutter time and gain
+ * \param[in] exposure Exposure time
+ *
+ * This function divides a given exposure time into shutter time, analogue and
+ * digital gain by iterating through stages of shutter time and gain limits. At
+ * each stage the current stage's shutter time limit is multiplied by the
+ * previous stage's gain limit (or 1.0 initially) to see if the combination of
+ * the two can meet the required exposure time. If they cannot then the current
+ * stage's shutter time limit is multiplied by the same stage's gain limit to
+ * see if that combination can meet the required exposure time. If they cannot
+ * then the function moves to consider the next stage.
+ *
+ * When a combination of shutter time and gain _stage_ limits are found that are
+ * sufficient to meet the required exposure time, the function attempts to
+ * reduce shutter time as much as possible whilst fixing gain and still meeting
+ * the exposure time. If a _runtime_ limit prevents shutter time from being
+ * lowered enough to meet the exposure time with gain fixed at the stage limit,
+ * gain is also lowered to compensate.
+ *
+ * Once the shutter time and gain values are ascertained, gain is assigned as
+ * analogue gain as much as possible, with digital gain only in use if the
+ * maximum analogue gain runtime limit is unable to accommodate the exposure
+ * value.
+ *
+ * If no combination of shutter time and gain limits is found that meets the
+ * required exposure time, the helper falls-back to simply maximising the
+ * shutter time first, followed by analogue gain, followed by digital gain.
+ *
+ * \return Tuple of shutter time, analogue gain, and digital gain
+ */
+std::tuple<utils::Duration, double, double>
+ExposureModeHelper::splitExposure(utils::Duration exposure) const
+{
+	ASSERT(maxShutter_);
+	ASSERT(maxGain_);
+
+	bool gainFixed = minGain_ == maxGain_;
+	bool shutterFixed = minShutter_ == maxShutter_;
+
+	/*
+	 * There's no point entering the loop if we cannot change either gain
+	 * nor shutter anyway.
+	 */
+	if (shutterFixed && gainFixed)
+		return { minShutter_, minGain_, exposure / (minShutter_ * minGain_) };
+
+	utils::Duration shutter;
+	double stageGain;
+	double gain;
+
+	for (unsigned int stage = 0; stage < gains_.size(); stage++) {
+		double lastStageGain = stage == 0 ? 1.0 : clampGain(gains_[stage - 1]);
+		utils::Duration stageShutter = clampShutter(shutters_[stage]);
+		stageGain = clampGain(gains_[stage]);
+
+		/*
+		 * We perform the clamping on both shutter and gain in case the
+		 * helper has had limits set that prevent those values being
+		 * lowered beyond a certain minimum...this can happen at runtime
+		 * for various reasons and so would not be known when the stage
+		 * limits are initialised.
+		 */
+
+		if (stageShutter * lastStageGain >= exposure) {
+			shutter = clampShutter(exposure / clampGain(lastStageGain));
+			gain = clampGain(exposure / shutter);
+
+			return { shutter, gain, exposure / (shutter * gain) };
+		}
+
+		if (stageShutter * stageGain >= exposure) {
+			shutter = clampShutter(exposure / clampGain(stageGain));
+			gain = clampGain(exposure / shutter);
+
+			return { shutter, gain, exposure / (shutter * gain) };
+		}
+	}
+
+	/*
+	 * From here on all we can do is max out the shutter time, followed by
+	 * the analogue gain. If we still haven't achieved the target we send
+	 * the rest of the exposure time to digital gain. If we were given no
+	 * stages to use then set stageGain to 1.0 so that shutter time is maxed
+	 * before gain touched at all.
+	 */
+	if (gains_.empty())
+		stageGain = 1.0;
+
+	shutter = clampShutter(exposure / clampGain(stageGain));
+	gain = clampGain(exposure / shutter);
+
+	return { shutter, gain, exposure / (shutter * gain) };
+}
+
+/**
+ * \fn ExposureModeHelper::minShutter()
+ * \brief Retrieve the configured minimum shutter time limit set through
+ * setShutterGainLimits()
+ * \return The minShutter_ value
+ */
+
+/**
+ * \fn ExposureModeHelper::maxShutter()
+ * \brief Retrieve the configured maximum shutter time set through
+ * setShutterGainLimits()
+ * \return The maxShutter_ value
+ */
+
+/**
+ * \fn ExposureModeHelper::minGain()
+ * \brief Retrieve the configured minimum gain set through
+ * setShutterGainLimits()
+ * \return The minGain_ value
+ */
+
+/**
+ * \fn ExposureModeHelper::maxGain()
+ * \brief Retrieve the configured maximum gain set through
+ * setShutterGainLimits()
+ * \return The maxGain_ value
+ */
+
+} /* namespace ipa */
+
+} /* namespace libcamera */