71 lines
1.9 KiB
C
71 lines
1.9 KiB
C
#ifndef SC_CLOCK_H
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#define SC_CLOCK_H
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#include "common.h"
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#include <assert.h>
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#include "util/tick.h"
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#define SC_CLOCK_RANGE 32
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static_assert(!(SC_CLOCK_RANGE & 1), "SC_CLOCK_RANGE must be even");
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struct sc_clock_point {
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sc_tick system;
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sc_tick stream;
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};
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/**
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* The clock aims to estimate the affine relation between the stream (device)
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* time and the system time:
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*
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* f(stream) = slope * stream + offset
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*
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* To that end, it stores the SC_CLOCK_RANGE last clock points (the timestamps
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* of a frame expressed both in stream time and system time) in a circular
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* array.
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*
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* To estimate the slope, it splits the last SC_CLOCK_RANGE points into two
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* sets of SC_CLOCK_RANGE/2 points, and computes their centroid ("average
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* point"). The slope of the estimated affine function is that of the line
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* passing through these two points.
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*
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* To estimate the offset, it computes the centroid of all the SC_CLOCK_RANGE
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* points. The resulting affine function passes by this centroid.
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*
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* With a circular array, the rolling sums (and average) are quick to compute.
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* In practice, the estimation is stable and the evolution is smooth.
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*/
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struct sc_clock {
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// Circular array
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struct sc_clock_point points[SC_CLOCK_RANGE];
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// Number of points in the array (count <= SC_CLOCK_RANGE)
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unsigned count;
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// Index of the next point to write
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unsigned head;
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// Sum of the first count/2 points
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struct sc_clock_point left_sum;
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// Sum of the last (count+1)/2 points
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struct sc_clock_point right_sum;
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// Estimated slope and offset
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// (computed on sc_clock_update(), used by sc_clock_to_system_time())
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double slope;
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sc_tick offset;
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};
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void
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sc_clock_init(struct sc_clock *clock);
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void
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sc_clock_update(struct sc_clock *clock, sc_tick system, sc_tick stream);
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sc_tick
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sc_clock_to_system_time(struct sc_clock *clock, sc_tick stream);
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#endif
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