document.addEventListener("DOMContentLoaded", () => {
  const canvas = document.getElementById("background");

  if (canvas.getContext) {
    const ctx = canvas.getContext("2d");

    // Source - https://stackoverflow.com/a/58673385
    // Posted by Sanxofon, modified by community. See post 'Timeline' for change history
    // Retrieved 2026-02-15, License - CC BY-SA 4.0
    
    var w = canvas.width; // Canvas width => Frequency is relative to this
    var h = canvas.height/2; // Canvas height over two => Amplitude: Volume
    var f=1; // How many cycles per canvas width => Frequency: Tone & Speed
    // Calculates y position from x
    function calcSineY(x) {
      // This is the meat (unles you are vegan)
      // Note that:
      // h is the amplitude of the wave
      // x is the current x value we get every time interval
      // 2 * PI is the length of one cycle (full circumference)
      // f/w is the frequency fraction
      return h - h * Math.sin( x * 2 * Math.PI * (f/w) );
    }
    function drawSine(x){
      ctx.clearRect(0, 0, w, h*2);
      //draw x axis
      ctx.beginPath(); // Draw a new path
      ctx.strokeStyle = "green"; // Pick a color
      ctx.moveTo(0,h); // Where to start drawing
      ctx.lineTo(w,h); // Where to draw to
      ctx.stroke(); // Draw
      
      // // draw horizontal line of current amplitude
      // ctx.beginPath(); // Draw a new path
      // ctx.moveTo(0,h); // Where to start drawing
      // ctx.strokeStyle = "gray"; // Pick a color
      // for(var i=0;i<x;i++){ // Loop from left side to current x
      //   var y = calcSineY(x); // Calculate y value from x
      //   ctx.moveTo(i,y); // Where to start drawing
      //   ctx.lineTo(x,y); // Where to draw to
      // }
      // ctx.stroke(); // Draw
      //
      // // draw amplitude bar at current point
      // ctx.beginPath(); // Draw a new path
      // ctx.strokeStyle = "red"; // Pick a color
      // for(var i=0;i<x;i++){ // Loop from left side to current x
      //   var y = calcSineY(x); // Calculate y value from x
      //   ctx.moveTo(x,h); // Where to start drawing
      //   ctx.lineTo(x,y); // Where to draw to
      // }
      // ctx.stroke(); // Draw
      //
      // // draw area below y
      // ctx.beginPath(); // Draw a new path
      // ctx.strokeStyle = "orange"; // Pick a color
      // for(var i=0;i<x;i++){ // Loop from left side to current x
      //   if(i/3==Math.round(i/3)) { // Draw only one line each 3 pixels
      //     var y = calcSineY(i); // Calculate y value from x
      //     ctx.moveTo(i,h); // Where to start drawing
      //     ctx.lineTo(i,y); // Where to draw to
      //   }
      // }
      // ctx.stroke(); // Draw
      
      // draw sin curve point to point until x
      ctx.beginPath(); // Draw a new path
      ctx.strokeStyle = "black"; // Pick a color
      for(var i=0;i<x;i++){ // Loop from left side to current x
        var y = calcSineY(i); // Calculate y value from x
        ctx.lineTo(i,y); // Where to draw to
      }
      ctx.stroke(); // Draw
    }
    // Define initial value of x positiom (leftmost side of cnanvas)
    var x=0;
    // Start time interval
    var interval = setInterval(function(){
      drawSine(x); // Call draww function every cycle
      x++; // Increment x by 1
      // if(x>w){
      //     x=0; // x cannot be more than canvas with, so back to 0
      //       f++; // increment frequency for demonstration
      //   }
    },10); // Loop every 10 milliseconds

  } else {

  }
});