Simplify clock estimation

The slope encodes the drift between the device clock and the computer
clock. Its real value is expected very close to 1.

To estimate it, just assume it is exactly 1.

Since the clock is used to estimate very close points in the future, the
error caused by clock drift is totally negligible, and in practice it is
way lower than the slope estimation error.

Therefore, only estimate the offset.

app/src/clock.c

@@ -1,116 +1,36 @@
 #include "clock.h"
 
+#include <assert.h>
+
 #include "util/log.h"
 
 #define SC_CLOCK_NDEBUG // comment to debug
 
+#define SC_CLOCK_RANGE 32
+
 void
 sc_clock_init(struct sc_clock *clock) {
-    clock->count = 0;
-    clock->head = 0;
-    clock->left_sum.system = 0;
-    clock->left_sum.stream = 0;
-    clock->right_sum.system = 0;
-    clock->right_sum.stream = 0;
-}
-
-// Estimate the affine function f(stream) = slope * stream + offset
-static void
-sc_clock_estimate(struct sc_clock *clock,
-                  double *out_slope, sc_tick *out_offset) {
-    assert(clock->count);
-
-    if (clock->count == 1) {
-        // If there is only 1 point, we can't compute a slope. Assume it is 1.
-        struct sc_clock_point *single_point = &clock->right_sum;
-        *out_slope = 1;
-        *out_offset = single_point->system - single_point->stream;
-        return;
-    }
-
-    struct sc_clock_point left_avg = {
-        .system = clock->left_sum.system / (clock->count / 2),
-        .stream = clock->left_sum.stream / (clock->count / 2),
-    };
-    struct sc_clock_point right_avg = {
-        .system = clock->right_sum.system / ((clock->count + 1) / 2),
-        .stream = clock->right_sum.stream / ((clock->count + 1) / 2),
-    };
-
-    double slope = (double) (right_avg.system - left_avg.system)
-                 / (right_avg.stream - left_avg.stream);
-
-    if (clock->count < SC_CLOCK_RANGE) {
-        /* The first frames are typically received and decoded with more delay
-         * than the others, causing a wrong slope estimation on start. To
-         * compensate, assume an initial slope of 1, then progressively use the
-         * estimated slope. */
-        slope = (clock->count * slope + (SC_CLOCK_RANGE - clock->count))
-              / SC_CLOCK_RANGE;
-    }
-
-    struct sc_clock_point global_avg = {
-        .system = (clock->left_sum.system + clock->right_sum.system)
-                 / clock->count,
-        .stream = (clock->left_sum.stream + clock->right_sum.stream)
-                 / clock->count,
-    };
-
-    sc_tick offset = global_avg.system - (sc_tick) (global_avg.stream * slope);
-
-    *out_slope = slope;
-    *out_offset = offset;
+    clock->range = 0;
+    clock->offset = 0;
 }
 
 void
 sc_clock_update(struct sc_clock *clock, sc_tick system, sc_tick stream) {
-    struct sc_clock_point *point = &clock->points[clock->head];
-
-    if (clock->count == SC_CLOCK_RANGE || clock->count & 1) {
-        // One point passes from the right sum to the left sum
-
-        unsigned mid;
-        if (clock->count == SC_CLOCK_RANGE) {
-            mid = (clock->head + SC_CLOCK_RANGE / 2) % SC_CLOCK_RANGE;
-        } else {
-            // Only for the first frames
-            mid = clock->count / 2;
-        }
-
-        struct sc_clock_point *mid_point = &clock->points[mid];
-        clock->left_sum.system += mid_point->system;
-        clock->left_sum.stream += mid_point->stream;
-        clock->right_sum.system -= mid_point->system;
-        clock->right_sum.stream -= mid_point->stream;
+    if (clock->range < SC_CLOCK_RANGE) {
+        ++clock->range;
     }
 
-    if (clock->count == SC_CLOCK_RANGE) {
-        // The current point overwrites the previous value in the circular
-        // array, update the left sum accordingly
-        clock->left_sum.system -= point->system;
-        clock->left_sum.stream -= point->stream;
-    } else {
-        ++clock->count;
-    }
-
-    point->system = system;
-    point->stream = stream;
-
-    clock->right_sum.system += system;
-    clock->right_sum.stream += stream;
-
-    clock->head = (clock->head + 1) % SC_CLOCK_RANGE;
-
-    // Update estimation
-    sc_clock_estimate(clock, &clock->slope, &clock->offset);
+    sc_tick offset = system - stream;
+    clock->offset = ((clock->range - 1) * clock->offset + offset)
+                  / clock->range;
 
 #ifndef SC_CLOCK_NDEBUG
-    LOGD("Clock estimation: %f * pts + %" PRItick, clock->slope, clock->offset);
+    LOGD("Clock estimation: pts + %" PRItick, clock->offset);
 #endif
 }
 
 sc_tick
 sc_clock_to_system_time(struct sc_clock *clock, sc_tick stream) {
-    assert(clock->count); // sc_clock_update() must have been called
-    return (sc_tick) (stream * clock->slope) + clock->offset;
+    assert(clock->range); // sc_clock_update() must have been called
+    return stream + clock->offset;
 }