Update

focus

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+#!/bin/bash
+
+# epochMillis: print the amount of millis since epoch time on the std output stream.
+epochMillis(){
+	echo $(($(date +%s%N)/1000000))
+}
+
+# queue: array holding the focused window ids in order, the most recent focus is on index 0.
+queue=( )
+queueLimit=8
+
+# pushOnQueue: insert a window id on index 0 of the queue. This function also removes the given id if was already present, the queue is an ordered set.
+# arg 1: window id to insert.
+pushOnQueue(){
+	if [ "$1" = "0x0" ]
+	then
+		return
+	fi
+	newQueue=( "$1" )
+	for i in "${queue[@]}"
+	do
+		if [ "$1" = "$i" ]
+		then
+			continue
+		fi
+		newQueue=( ${newQueue[@]:0} "$i" )
+	done
+	queue=( "${newQueue[@]:0:$queueLimit}" )
+}
+
+# markQueue: put i3 marks using the window ids stored on the queue. The windows are marked using "_prevFocusX" where X is a integer corresponding with the index of the element on the queue. The previously focused window (the most recent) is marked using "_prevFocus0". This function also cleans the queue from windows that can't be marked (e.g. windows that have been closed).
+markQueue(){
+	queueCopy=( "${queue[@]}" )
+	queue=( )
+	j=0
+	for i in "${queueCopy[@]}"
+	do
+		i3-msg "unmark _prevFocus$j" 1>/dev/null 2>&1
+		if i3-msg "[id=$i] mark --add _prevFocus$j" 2>/dev/null | grep -i "\"success\":true" 1>/dev/null 2>&1
+		then
+			queue=( ${queue[@]:0} "$i" )
+			j=$(($j + 1))
+		fi
+	done
+}
+
+lastTime=$(epochMillis)
+# permanence: period in millis used to determine if a window is worthy to be saved on the queue. This enables i3 window fast navigation without saving unworthy window ids.
+permanence=1000
+
+# main loop: read activated window ids continuously.
+xprop -root -spy _NET_ACTIVE_WINDOW |
+while read line
+do
+
+	currentTime=$(epochMillis)
+	period=$(($currentTime-$lastTime))
+	lastTime=$currentTime
+
+	# previousFocus: window id of the previously focused (activated) window.
+	previousFocus=$currentFocus
+	# currentFocus: window id of the window that has just being activated.
+	currentFocus=$(echo "$line" | awk -F' ' '{printf $NF}')
+
+	# push the previousFocus id to the queue if the time spent on the previous window was greater than permanence. Check also to allow fast switching between two windows.
+	if [ $period -gt $permanence -o "$currentFocus" = "${queue[0]}" ]
+	then
+		pushOnQueue "$previousFocus"
+	fi
+
+	# if the currentFocus is marked as the previous window (_prevFocus0 or queue[0]) then swap the first two elements on the queue to allow switching.
+	if [ "${queue[0]}" = "$currentFocus" ]
+	then
+		queue=( ${queue[1]} ${queue[0]} ${queue[@]:2} )
+	fi
+
+	markQueue
+
+done