function changeImg(id, imgSrc) {
	document.images[id].src = imgSrc;
}

function changeBanner(id, imgSrc) {
	top.document.images[id].src = imgSrc;
}

function getWeekNbr() {
    var currentDate = new Date();
    var firstDateOfYear = new Date(currentDate.getFullYear(), 00, 01);

    /* 86400000 = number of milliseconds in a day */
    var daysPassed = Math.ceil((currentDate.getTime() - firstDateOfYear.getTime()) / 86400000);

    return Math.floor(((daysPassed + firstDateOfYear.getDay()) / 7) + 1);
}

function highlightCurrentWeek() {
    
}

function calcSAC() {
    var cylinder = document.Calculator.cyl.value;
		var startPressure = document.Calculator.startPres.value;
		var endPressure = document.Calculator.endPres.value;
		var avgDepth = document.Calculator.avgDepth.value;
		var diveTime = document.Calculator.diveTime.value;
		
		// calculate air volume in the beginning
		var airVolumeStart = startPressure * cylinder;
		document.getElementById("airStart").innerHTML = airVolumeStart;
		
		// calculate air volume in the end
		var airVolumeEnd = endPressure * cylinder;
		document.getElementById("airEnd").innerHTML = airVolumeEnd;
		
		// calculate the used air volume 
		var usedAirVolume = airVolumeStart - airVolumeEnd;
		document.getElementById("airUsed").innerHTML = usedAirVolume;
		
		// calculate pressure at average depth
  	var avgPressure = (avgDepth / 10) + 1;
		
		if (diveTime > 0) {
  		// calculate the SAC
    	var SAC = (usedAirVolume / diveTime) / avgPressure;
  
  		// show the result with one decimal
  		document.getElementById("sac").innerHTML = (Math.round(SAC * 10) / 10);
		}
}