$('.calculator').each(function() {
  let calculator = $(this);
  let display = $('.display');
  let stack = {
    'x': 0,  // display register
    'y': 0,
    'z': 0,
    't': 0,
    's': 0  // storage register
  };
  let arc = false;
  let xdone = true;   // if false, entry for x is not yet done
  let enter = false;  // if true, does not immediately lift stack
  let minus = true;   // if true, allow direct entry of (-) sign
  let error = false;
  let manual_update = false;  // if true, update() is overriden
  
  let funcs = {
    'pow': (x,y) => Math.pow(x,y),
    'log': (x) => Math.log10(x),
    'ln': (x) => Math.log(x),
    'exp': (x) => Math.exp(x),
    'sqrt': (x) => Math.sqrt(x),
    'sin': (x) => (arc ? Math.asin(x)*180/Math.PI : Math.sin(x*Math.PI/180)),
    'cos': (x) => (arc ? Math.acos(x)*180/Math.PI : Math.cos(x*Math.PI/180)),
    'tan': (x) => (arc ? Math.atan(x)*180/Math.PI : Math.tan(x*Math.PI/180)),
    'inv': (x) => 1/x,
    '+': (x,y) => (y|0)+(x|0),
    '-': (x,y) => y-x,
    '*': (x,y) => y*x,
    '/': (x,y) => y/x
  };
  
  let update = function() {
    if (stack.x === '-') {
      display.html('-0');
    }
    else if (isNaN(stack.x) || !isFinite(stack.x)) {
      display.html('error')
    }
    else display.html(stack.x);
    
    if (arc) {
      $('button[value="arc"]').addClass('enabled');
    } else {
      $('button[value="arc"]').removeClass('enabled');
    }
    
    $('#T').html(stack.t);
    $('#Z').html(stack.z);
    $('#Y').html(stack.y);
    $('#X').html(stack.x);
    $('#S').html(stack.s);
    $('#xdone').html(xdone);
    $('#enter').html(enter);
    $('#minus').html(minus);
  }

  let keys = $('.upper-keypad, .lower-keypad', calculator);
  $('.fx', keys).click(function() {
    let fn = $(this).val();
    switch (fn) {
      case 'enter':
        stack.t = stack.z;
        stack.z = stack.y;
        stack.y = stack.x;
        enter = true;
        break;
      case 'swap':
        let x = stack.x, y = stack.y;
        stack.x = y;
        stack.y = x;
        break;
      case 'roll':
        let oldt = stack.t;
        stack.t = stack.x;
        stack.x = stack.y;
        stack.y = stack.z;
        stack.z = oldt;
        break;
      case 'clr':
        stack.t = 0;
        stack.x = 0;
        stack.y = 0;
        stack.z = 0;
        stack.s = 0;
        minus = true;
        break;
      case 'chs':
        if (minus) {
          if (xdone) {
            stack.x = '-';
            xdone = false;
            // console.log('   xdone: ' + xdone);
          } else stack.x += '-';
          minus = false;
        } else {
          stack.x = -parseFloat(stack.x);
        }
        break;
      case 'eex':
        minus = true;
        xdone = false;
        if (stack.x === 0) stack.x = 1;
        stack.x += 'e';
        break;
      case 'clx':
        stack.x = 0;
        minus = true;
        xdone = true;
        break;
      case 'sto':
        stack.s = stack.x;
        break;
      case 'rcl':
        if (!enter) {
          stack.t = stack.z;
          stack.z = stack.y;
          stack.y = stack.x;
        }
        stack.x = stack.s;
        break;
      case 'arc':
        arc = true;
        break;
      default:
        let twoargs = ['pow', '+', '-', '*', '/'];
        let trig = ['sin', 'cos', 'tan'];
        if (twoargs.indexOf(fn) !== -1) { // if fn needs 2 args
          stack.x = funcs[fn](stack.x, stack.y);
          stack.y = stack.z;
          stack.z = stack.t;
        } else {
          stack.x = funcs[fn](stack.x);
          // following any trig function
          // z is duplicated into register T
          if (trig.indexOf(fn) !== -1) {
            stack.t = stack.z;
          }
        }
        if (arc) arc = false;
        break;
    }
    if (['chs','eex','clx'].indexOf(fn) === -1) xdone = true;
    update();
  });

  $('.num', keys).click(function() {
    let n = $(this).val();
    if (n === 'pi') {
      stack.t = stack.z;
      stack.z = stack.y;
      stack.y = stack.x;
      stack.x = Math.PI;
      enter = false;
    } else if (n === '.') {
      stack.x += n;
    } else {
      if (xdone) {
        if (!enter) {
          stack.t = stack.z;
          stack.z = stack.y;
          stack.y = stack.x;
        }
        stack.x = parseFloat(n);
        xdone = false;
        enter = false;
      } else {
        stack.x += '' + n;
      }
    }
    minus = false;
    update();
  });
});