add snippet 'sensor-reading'

snippets/sensor-reading/qml/main.qml

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+import QtQuick 2.15
+import QtSensors 5.15
+
+Rectangle {
+  width: 300
+  height: 500
+
+  Accelerometer {
+    id: accel
+    active: true
+    accelerationMode: Accelerometer.User // exclude gravity
+  }
+
+  Timer {
+    id: timer
+    interval: 50
+    repeat: true
+    running: true
+
+    onTriggered: Lisp.call("qml-user:accel-changed",
+                           accel.reading.x, accel.reading.y, accel.reading.z)
+  }
+}

snippets/sensor-reading/readme.md

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+Description
+-----------
+
+Shows how to read sensor data with a timer, plus a trivial low-pass filter.
+

snippets/sensor-reading/run.lisp

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+(in-package :qml-user)
+
+;;; low-pass filter
+
+(defun make-low-pass-filter (&optional (alpha 0.02))
+  (let ((a alpha)
+        filtered)
+    (lambda (value)
+      (setf filtered (if filtered
+                         (+ (* value a)
+                            (* filtered (- 1.0 a)))
+                         value)))))
+
+(defvar *accel-x* (make-low-pass-filter))
+(defvar *accel-y* (make-low-pass-filter))
+(defvar *accel-z* (make-low-pass-filter))
+
+;;; sensor data
+
+(defun hr (x)
+  "Human readable."
+  (floor (+ 0.5 (* 1000 x))))
+
+(defun accel-changed (x y z)
+  (qlog :x (hr (funcall *accel-x* x)))
+  (qlog :y (hr (funcall *accel-y* y)))
+  (qlog :z (hr (funcall *accel-z* z))))
+