WIP: window filter

Author: haproxyFred

include/haproxy/window_filter.h

@@ -0,0 +1,97 @@
+#ifndef _HAPROXY_WINDOW_FILTER_H
+#define _HAPROXY_WINDOW_FILTER_H
+
+/* Window filter sample */
+struct wf_smp {
+	uint64_t v;
+	uint32_t t;
+};
+
+/* Window filter */
+struct wf {
+	size_t len;
+	struct wf_smp smp[3];
+};
+
+static inline void wf_init(struct wf *wf, size_t len)
+{
+	wf->len = len;
+	memset(wf->smp, 0xff, sizeof(wf->smp));
+}
+
+static inline void wf_reset(struct wf *wf, uint64_t v, uint32_t t)
+{
+	struct wf_smp smp = { .v = v, .t = t };
+
+	wf->smp[2] = wf->smp[1] = wf->smp[0] = smp;
+}
+
+/* Updates best estimates with |v| sample value, and expires and updates best
+ * estimates as necessary.
+ * Similar to minmax_subwin_update() linux kernel function (see lib/win_minmax.c
+ * function).
+ */
+static inline void wf_update(struct wf *wf, uint64_t v, uint32_t t)
+{
+	/* Reset all estimates if they have not yet been initialized, if new
+	   sample is a new best, or if the newest recorded estimate is too
+	   old. */
+	if (wf->smp[0].v == UINT64_MAX || v > wf->smp[0].v || t - wf->smp[2].t > wf->len) {
+		wf_reset(wf, v, t);
+		return;
+	}
+
+	if (v > wf->smp[1].v) {
+		wf->smp[1].v = v;
+		wf->smp[1].t = t;
+		wf->smp[2] = wf->smp[1];
+	} else if (v > wf->smp[2].v) {
+		wf->smp[2].v = v;
+		wf->smp[2].t = t;
+	}
+
+	/* Expire and update smp as necessary. */
+	if (t - wf->smp[0].t > wf->len) {
+		/* The best estimate hasn't been updated for an entire window, so
+		   promote second and third best smp. */
+		wf->smp[0] = wf->smp[1];
+		wf->smp[1] = wf->smp[2];
+		wf->smp[2].v = v;
+		wf->smp[2].t = t;
+
+		/* Need to iterate one more time.  Check if the new best estimate
+		   is outside the window as well, since it may also have been
+		   recorded a long time ago.  Don't need to iterate once more
+		   since we cover that case at the beginning of the method. */
+		if (t - wf->smp[0].t > wf->len) {
+			wf->smp[0] = wf->smp[1];
+			wf->smp[1] = wf->smp[2];
+		}
+		return;
+	}
+
+	if (wf->smp[1].v == wf->smp[0].v && t - wf->smp[1].t > wf->len >> 2) {
+		/* A quarter of the window has passed without a better sample, so
+		   the second-best estimate is taken from the second quarter of
+		   the window. */
+		wf->smp[2].v = v;
+		wf->smp[2].t = t;
+		wf->smp[1] = wf->smp[2];
+		return;
+	}
+
+	if (wf->smp[2].v == wf->smp[1].v && t - wf->smp[2].t > wf->len >> 1) {
+		/* We've passed a half of the window without a better estimate, so
+		   take a third-best estimate from the second half of the
+		   window. */
+		wf->smp[2].v = v;
+		wf->smp[2].t = t;
+	}
+}
+
+static inline uint64_t wf_get_best(struct wf *wf)
+{
+	return wf->smp[0].v;
+}
+
+#endif /* _HAPROXY_WINDOW_FILTER_H */