STEPPER_REGISTRY.register("java-17/scanner", {
  label: 'Scanner and user input',
  panelConfig: {
    stack: true,
    heap:  false,
    custom: { title: 'Input buffer', emptyText: 'waiting for user to type' },
    legend: [
      { color:'#1d5799', border:'#9dc0e8', label:'Currently running frame' },
      { color:'#a0540a', border:'#e8c070', label:'Variable just assigned' },
      { color:'#095e40', border:'#90d4b8', label:'Holding a value' },
      { color:'#3d3490', border:'#b0a8e0', label:'Reference (object pointer)' },
    ]
  },
  scenarios: [
    {
      label: "nextInt()",
      lines: [
        { code: [['tok-kw','static '],['tok-kw','void '],['tok-meth','main'],['tok-op','(String[] args) {']] },
        { code: [['','  '],['tok-type','Scanner '],['tok-op','sc = '],['tok-kw','new '],['tok-cls','Scanner'],['tok-op','(System.in);']] },
        { code: [['','  '],['tok-type','int '],['tok-op','age = sc.'],['tok-meth','nextInt'],['tok-op','();']] },
        { code: [['','  '],['tok-op','System.out.'],['tok-meth','println'],['tok-op','('],['tok-str','"Age: "'],['tok-op',' + age);']] },
        { code: [['tok-op','}']] },
      ],
      steps: [
        { line:0, stack:[{name:'main()',vars:[],active:true}], custom:[],
          expl:{label:'Program starts', text:'Java enters <hi>main()</hi>. A frame is pushed onto the call stack. Nothing is in the input buffer yet.'} },
        { line:1, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:true,cls:true}],active:true}], custom:[],
          expl:{label:'Scanner created', text:'<hi>new Scanner(System.in)</hi> creates a Scanner connected to the keyboard. <hi>sc</hi> holds a reference to it. The buffer is empty -- we have not asked for input yet.'} },
        { line:2, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true}],active:true}],
          custom:[{name:'Input buffer',tag:'has data',rows:[{k:'buffered',v:'"25\\n"',fresh:true}]}],
          expl:{label:'User types input', text:'<hi>sc.nextInt()</hi> pauses and waits. The user types <hi>25</hi> and presses Enter. The text arrives in the buffer as <hi>25\\n</hi>.'} },
        { line:2, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:true,cls:false}],active:true}],
          custom:[{name:'Input buffer',tag:'leftover',rows:[{k:'buffered',v:'"\\n"',fresh:true}]}],
          expl:{label:'nextInt() reads the number', text:'<hi>nextInt()</hi> reads whitespace-separated tokens. It takes the <hi>25</hi> and converts it to an int. The trailing newline <hi>\\n</hi> is left behind in the buffer.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:false,cls:false}],active:true}],
          custom:[{name:'Input buffer',tag:'leftover',rows:[{k:'buffered',v:'"\\n"',fresh:false}]}],
          expl:{label:'println runs', text:'<hi>age</hi> is <hi>25</hi>. The program prints "Age: 25". The leftover <hi>\\n</hi> is still sitting in the buffer -- it will matter in the CRLF scenario.'} },
        { line:4, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:false,cls:false}],active:true}], custom:[],
          expl:{label:'Done', text:'<hi>main()</hi> ends. Remember: <hi>nextInt()</hi> always leaves the newline in the buffer after reading.'} },
      ]
    },
    {
      label: "next() for Strings",
      lines: [
        { code: [['tok-kw','static '],['tok-kw','void '],['tok-meth','main'],['tok-op','(String[] args) {']] },
        { code: [['','  '],['tok-type','Scanner '],['tok-op','sc = '],['tok-kw','new '],['tok-cls','Scanner'],['tok-op','(System.in);']] },
        { code: [['','  '],['tok-type','String '],['tok-op','name = sc.'],['tok-meth','next'],['tok-op','();']] },
        { code: [['','  '],['tok-op','System.out.'],['tok-meth','println'],['tok-op','('],['tok-str','"Hello, "'],['tok-op',' + name);']] },
        { code: [['tok-op','}']] },
      ],
      steps: [
        { line:0, stack:[{name:'main()',vars:[],active:true}], custom:[],
          expl:{label:'Program starts', text:'<hi>main()</hi> begins. Input buffer is empty.'} },
        { line:1, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:true,cls:true}],active:true}], custom:[],
          expl:{label:'Scanner created', text:'<hi>new Scanner(System.in)</hi> connects to the keyboard. <hi>sc</hi> holds the reference.'} },
        { line:2, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true}],active:true}],
          custom:[{name:'Input buffer',tag:'has data',rows:[{k:'buffered',v:'"Alice\\n"',fresh:true}]}],
          expl:{label:'User types input', text:'<hi>sc.next()</hi> pauses and waits. The user types <hi>Alice</hi> and presses Enter. <hi>next()</hi> reads up to the first whitespace -- it reads a single token.'} },
        { line:2, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'name',v:'"Alice"',fresh:true,cls:false}],active:true}],
          custom:[{name:'Input buffer',tag:'leftover',rows:[{k:'buffered',v:'"\\n"',fresh:true}]}],
          expl:{label:'next() reads one token', text:'<hi>next()</hi> takes <hi>"Alice"</hi> and stops at the newline. The trailing <hi>\\n</hi> stays in the buffer -- same behavior as <hi>nextInt()</hi>.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'name',v:'"Alice"',fresh:false,cls:false}],active:true}],
          custom:[{name:'Input buffer',tag:'leftover',rows:[{k:'buffered',v:'"\\n"',fresh:false}]}],
          expl:{label:'println runs', text:'<hi>name</hi> is <hi>"Alice"</hi>. The program prints "Hello, Alice".'} },
        { line:4, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'name',v:'"Alice"',fresh:false,cls:false}],active:true}], custom:[],
          expl:{label:'Done', text:'<hi>main()</hi> ends. Both <hi>next()</hi> and <hi>nextInt()</hi> leave a newline behind. <hi>nextLine()</hi> behaves differently -- see the next two scenarios.'} },
      ]
    },
    {
      label: "CRLF bug",
      lines: [
        { code: [['tok-kw','static '],['tok-kw','void '],['tok-meth','main'],['tok-op','(String[] args) {']] },
        { code: [['','  '],['tok-type','Scanner '],['tok-op','sc = '],['tok-kw','new '],['tok-cls','Scanner'],['tok-op','(System.in);']] },
        { code: [['','  '],['tok-type','int '],['tok-op','age = sc.'],['tok-meth','nextInt'],['tok-op','();']] },
        { code: [['','  '],['tok-type','String '],['tok-op','name = sc.'],['tok-meth','nextLine'],['tok-op','();'],['','  '],['tok-cmt','// bug!']] },
        { code: [['','  '],['tok-op','System.out.'],['tok-meth','println'],['tok-op','(name);']] },
        { code: [['tok-op','}']] },
      ],
      steps: [
        { line:0, stack:[{name:'main()',vars:[],active:true}], custom:[],
          expl:{label:'Program starts', text:'<hi>main()</hi> begins. The goal: read an age, then read a name on the next line.'} },
        { line:1, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:true,cls:true}],active:true}], custom:[],
          expl:{label:'Scanner created', text:'<hi>new Scanner(System.in)</hi> is ready.'} },
        { line:2, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:true,cls:false}],active:true}],
          custom:[{name:'Input buffer',tag:'leftover',rows:[{k:'buffered',v:'"\\n"',fresh:true}]}],
          expl:{label:'nextInt() runs', text:'The user types <hi>25</hi> and presses Enter. <hi>nextInt()</hi> reads <hi>25</hi>. The newline is left behind in the buffer.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:false,cls:false},{n:'name',v:'""',fresh:true,cls:false}],active:true}],
          custom:[],
          expl:{label:'nextLine() hits the leftover', text:'<hi>nextLine()</hi> reads everything up to the next newline. The leftover <hi>\\n</hi> is immediately available -- so <hi>nextLine()</hi> returns an empty string without ever waiting for the user.'} },
        { line:4, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:false,cls:false},{n:'name',v:'""',fresh:false,cls:false}],active:true}],
          custom:[],
          expl:{label:'Prints nothing', text:'<hi>name</hi> is <hi>""</hi> -- an empty string. The program prints a blank line instead of a name. The user never got a chance to type.'} },
        { line:5, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:false,cls:false},{n:'name',v:'""',fresh:false,cls:false}],active:true}], custom:[],
          expl:{label:'Bug summary', text:'The leftover <hi>\\n</hi> from <hi>nextInt()</hi> was silently consumed by <hi>nextLine()</hi>. The next scenario shows the one-line fix.'} },
      ]
    },
    {
      label: "CRLF fix",
      lines: [
        { code: [['tok-kw','static '],['tok-kw','void '],['tok-meth','main'],['tok-op','(String[] args) {']] },
        { code: [['','  '],['tok-type','Scanner '],['tok-op','sc = '],['tok-kw','new '],['tok-cls','Scanner'],['tok-op','(System.in);']] },
        { code: [['','  '],['tok-type','int '],['tok-op','age = sc.'],['tok-meth','nextInt'],['tok-op','();']] },
        { code: [['','  '],['tok-op','sc.'],['tok-meth','nextLine'],['tok-op','();'],['','  '],['tok-cmt','// flush leftover \\n']] },
        { code: [['','  '],['tok-type','String '],['tok-op','name = sc.'],['tok-meth','nextLine'],['tok-op','();']] },
        { code: [['','  '],['tok-op','System.out.'],['tok-meth','println'],['tok-op','('],['tok-str','"Name: "'],['tok-op',' + name);']] },
        { code: [['tok-op','}']] },
      ],
      steps: [
        { line:0, stack:[{name:'main()',vars:[],active:true}], custom:[],
          expl:{label:'Program starts', text:'<hi>main()</hi> begins. Same goal as before: read an age, then a name on the next line.'} },
        { line:1, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:true,cls:true}],active:true}], custom:[],
          expl:{label:'Scanner created', text:'<hi>new Scanner(System.in)</hi> is ready.'} },
        { line:2, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:true,cls:false}],active:true}],
          custom:[{name:'Input buffer',tag:'leftover',rows:[{k:'buffered',v:'"\\n"',fresh:true}]}],
          expl:{label:'nextInt() runs', text:'User types <hi>25</hi> + Enter. <hi>nextInt()</hi> reads <hi>25</hi>. The newline is left in the buffer.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:false,cls:false}],active:true}],
          custom:[],
          expl:{label:'Flush the newline', text:'<hi>sc.nextLine()</hi> is called and its return value is discarded. It reads and throws away the leftover <hi>\\n</hi>. Buffer is now empty -- the drain is clear.'} },
        { line:4, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:false,cls:false}],active:true}],
          custom:[{name:'Input buffer',tag:'has data',rows:[{k:'buffered',v:'"Alice\\n"',fresh:true}]}],
          expl:{label:'Waiting for real input', text:'<hi>nextLine()</hi> now waits because the buffer is empty. The user types <hi>Alice</hi> + Enter. The input arrives in the buffer.'} },
        { line:4, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:false,cls:false},{n:'name',v:'"Alice"',fresh:true,cls:false}],active:true}],
          custom:[],
          expl:{label:'nextLine() reads correctly', text:'<hi>nextLine()</hi> reads <hi>"Alice"</hi> and consumes the newline as the line terminator. <hi>name</hi> is now <hi>"Alice"</hi>.'} },
        { line:5, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:false,cls:false},{n:'name',v:'"Alice"',fresh:false,cls:false}],active:true}],
          custom:[],
          expl:{label:'Prints correctly', text:'<hi>name</hi> is <hi>"Alice"</hi>. The program prints "Name: Alice". The extra flush call was all that was needed.'} },
        { line:6, stack:[{name:'main()',vars:[{n:'sc',v:'@Scanner1',fresh:false,cls:true},{n:'age',v:'25',fresh:false,cls:false},{n:'name',v:'"Alice"',fresh:false,cls:false}],active:true}], custom:[],
          expl:{label:'Done', text:'<hi>main()</hi> ends. Rule of thumb: always call <hi>sc.nextLine()</hi> once after <hi>nextInt()</hi> or <hi>next()</hi> before reading a full line with <hi>nextLine()</hi>.'} },
      ]
    }
  ]
});