/*
  Java 17 -- Loops and recursion
  while, do-while, for, for-each, recursion
  No em dashes anywhere -- use plain hyphens or rewrite the sentence.
*/
STEPPER_REGISTRY.register("java-17/loops", {
  label: 'Loops and recursion',
  panelConfig: {
    stack: true,
    heap:  false,
    custom: false,
    legend: [
      { color:'#1d5799', border:'#9dc0e8', label:'Currently running frame' },
      { color:'#68655f', border:'#d4d1cc', label:'Paused, waiting to resume' },
      { color:'#a0540a', border:'#e8c070', label:'Variable just assigned' },
      { color:'#095e40', border:'#90d4b8', label:'Holding a value' },
      { color:'#3d3490', border:'#b0a8e0', label:'Reference (array or object)' },
    ]
  },
  scenarios: [

    /* ------------------------------------------------------------------ */
    /*  1. while loop                                                       */
    /* ------------------------------------------------------------------ */
    {
      label: 'while loop',
      lines: [
        { code: [['tok-kw','static '],['tok-kw','void '],['tok-meth','main'],['tok-op','(String[] args) {']] },
        { code: [['','  '],['tok-type','int '],['tok-op','count = '],['tok-num','0'],['tok-op',';']] },
        { code: [['','  '],['tok-kw','while '],['tok-op','(count < '],['tok-num','3'],['tok-op',') {']] },
        { code: [['','    '],['tok-op','count++;']] },
        { code: [['','  '],['tok-op','}']] },
        { code: [['','  '],['tok-op','System.out.println('],['tok-str','"Done"'],['tok-op',');']] },
        { code: [['tok-op','}']] },
      ],
      steps: [
        { line:0, stack:[{name:'main()',vars:[],active:true}],
          expl:{label:'Program starts', text:'Java enters <hi>main()</hi> and pushes a new frame onto the call stack.'} },
        { line:1, stack:[{name:'main()',vars:[{n:'count',v:'0',fresh:true}],active:true}],
          expl:{label:'Variable declared', text:'<hi>int count = 0</hi> creates a local variable initialized to <hi>0</hi>.'} },
        { line:2, tokenRange:[2,3], stack:[{name:'main()',vars:[{n:'count',v:'0',fresh:false}],active:true}],
          expl:{label:'Condition: true', text:'Java evaluates <hi>count < 3</hi>. Since 0 < 3, the condition is true and the loop body runs.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'count',v:'1',fresh:true}],active:true}],
          expl:{label:'Loop body', text:'<hi>count++</hi> increments <hi>count</hi> from 0 to 1.'} },
        { line:2, tokenRange:[2,3], stack:[{name:'main()',vars:[{n:'count',v:'1',fresh:false}],active:true}],
          expl:{label:'Condition: true', text:'Back at the top. <hi>1 < 3</hi> is still true -- another iteration begins.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'count',v:'2',fresh:true}],active:true}],
          expl:{label:'Loop body', text:'<hi>count++</hi> increments <hi>count</hi> to 2.'} },
        { line:2, tokenRange:[2,3], stack:[{name:'main()',vars:[{n:'count',v:'2',fresh:false}],active:true}],
          expl:{label:'Condition: true', text:'<hi>2 < 3</hi> is still true -- one more iteration.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'count',v:'3',fresh:true}],active:true}],
          expl:{label:'Loop body', text:'<hi>count++</hi> increments <hi>count</hi> to 3.'} },
        { line:2, tokenRange:[2,3], stack:[{name:'main()',vars:[{n:'count',v:'3',fresh:false}],active:true}],
          expl:{label:'Condition: false', text:'<hi>3 < 3</hi> is false. The loop exits -- execution jumps past the closing brace.'} },
        { line:5, stack:[{name:'main()',vars:[{n:'count',v:'3',fresh:false}],active:true}],
          expl:{label:'After the loop', text:'<hi>count</hi> retains its value of <hi>3</hi>. Its scope continues until <hi>main()</hi> ends.'} },
        { line:6, stack:[{name:'main()',vars:[{n:'count',v:'3',fresh:false}],active:true}],
          expl:{label:'Program ends', text:'<hi>main()</hi> reaches its closing brace. Its frame is popped and the program finishes.'} },
      ]
    },

    /* ------------------------------------------------------------------ */
    /*  2. do-while loop                                                    */
    /* ------------------------------------------------------------------ */
    {
      label: 'do-while loop',
      lines: [
        { code: [['tok-kw','static '],['tok-kw','void '],['tok-meth','main'],['tok-op','(String[] args) {']] },
        { code: [['','  '],['tok-type','int '],['tok-op','n = '],['tok-num','0'],['tok-op',';']] },
        { code: [['','  '],['tok-kw','do '],['tok-op','{']] },
        { code: [['','    '],['tok-op','n++;']] },
        { code: [['','  '],['tok-op','} '],['tok-kw','while '],['tok-op','(n < '],['tok-num','2'],['tok-op',');']] },
        { code: [['','  '],['tok-op','System.out.println('],['tok-str','"Done"'],['tok-op',');']] },
        { code: [['tok-op','}']] },
      ],
      steps: [
        { line:0, stack:[{name:'main()',vars:[],active:true}],
          expl:{label:'Program starts', text:'<hi>main()</hi> begins and a new frame is pushed.'} },
        { line:1, stack:[{name:'main()',vars:[{n:'n',v:'0',fresh:true}],active:true}],
          expl:{label:'Variable declared', text:'<hi>n</hi> is initialized to <hi>0</hi>.'} },
        { line:2, stack:[{name:'main()',vars:[{n:'n',v:'0',fresh:false}],active:true}],
          expl:{label:'Enter do block', text:'With <hi>do-while</hi>, the body always executes at least once -- the condition has not been checked yet.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'n',v:'1',fresh:true}],active:true}],
          expl:{label:'Loop body', text:'<hi>n++</hi> increments <hi>n</hi> from 0 to 1.'} },
        { line:4, tokenRange:[3,4], stack:[{name:'main()',vars:[{n:'n',v:'1',fresh:false}],active:true}],
          expl:{label:'Condition: true', text:'Now the condition is checked: <hi>1 < 2</hi> is true. Execution jumps back to the top of the body.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'n',v:'2',fresh:true}],active:true}],
          expl:{label:'Loop body', text:'<hi>n++</hi> increments <hi>n</hi> to 2.'} },
        { line:4, tokenRange:[3,4], stack:[{name:'main()',vars:[{n:'n',v:'2',fresh:false}],active:true}],
          expl:{label:'Condition: false', text:'<hi>2 < 2</hi> is false. The loop exits. The body ran twice -- the condition was never checked before the first run.'} },
        { line:5, stack:[{name:'main()',vars:[{n:'n',v:'2',fresh:false}],active:true}],
          expl:{label:'After the loop', text:'<hi>System.out.println("Done")</hi> runs.'} },
        { line:6, stack:[{name:'main()',vars:[{n:'n',v:'2',fresh:false}],active:true}],
          expl:{label:'Program ends', text:'<hi>main()</hi> finishes.'} },
      ]
    },

    /* ------------------------------------------------------------------ */
    /*  3. for loop                                                         */
    /* ------------------------------------------------------------------ */
    {
      label: 'for loop',
      lines: [
        { code: [['tok-kw','static '],['tok-kw','void '],['tok-meth','main'],['tok-op','(String[] args) {']] },
        { code: [['','  '],['tok-type','int '],['tok-op','sum = '],['tok-num','0'],['tok-op',';']] },
        { code: [['','  '],['tok-kw','for '],['tok-op','('],['tok-type','int '],['tok-op','i = '],['tok-num','0'],['tok-op','; '],['tok-op','i < '],['tok-num','3'],['tok-op','; '],['tok-op','i++'],['tok-op',') {']] },
        { code: [['','    '],['tok-op','sum += i;']] },
        { code: [['','  '],['tok-op','}']] },
        { code: [['','  '],['tok-op','System.out.println(sum);']] },
        { code: [['tok-op','}']] },
      ],
      steps: [
        { line:0, stack:[{name:'main()',vars:[],active:true}],
          expl:{label:'Program starts', text:'<hi>main()</hi> begins.'} },
        { line:1, stack:[{name:'main()',vars:[{n:'sum',v:'0',fresh:true}],active:true}],
          expl:{label:'Variable declared', text:'<hi>sum</hi> starts at <hi>0</hi>. It will accumulate the total.'} },
        { line:2, tokenRange:[3,5], stack:[{name:'main()',vars:[{n:'sum',v:'0',fresh:false},{n:'i',v:'0',fresh:true}],active:true}],
          expl:{label:'Init', text:'The initializer <hi>int i = 0</hi> runs exactly once. <hi>i</hi> is created and set to <hi>0</hi>.'} },
        { line:2, tokenRange:[7,8], stack:[{name:'main()',vars:[{n:'sum',v:'0',fresh:false},{n:'i',v:'0',fresh:false}],active:true}],
          expl:{label:'Condition: true', text:'Java checks <hi>i < 3</hi>. 0 < 3 is true -- the loop body runs.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'sum',v:'0',fresh:false},{n:'i',v:'0',fresh:false}],active:true}],
          expl:{label:'Loop body', text:'<hi>sum += i</hi> adds <hi>0</hi> to sum. sum stays <hi>0</hi> this iteration.'} },
        { line:2, tokenRange:[10,10], stack:[{name:'main()',vars:[{n:'sum',v:'0',fresh:false},{n:'i',v:'1',fresh:true}],active:true}],
          expl:{label:'Increment', text:'The update <hi>i++</hi> runs. <hi>i</hi> goes from 0 to <hi>1</hi>.'} },
        { line:2, tokenRange:[7,8], stack:[{name:'main()',vars:[{n:'sum',v:'0',fresh:false},{n:'i',v:'1',fresh:false}],active:true}],
          expl:{label:'Condition: true', text:'Java checks <hi>i < 3</hi>. 1 < 3 is true -- another iteration.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'sum',v:'1',fresh:true},{n:'i',v:'1',fresh:false}],active:true}],
          expl:{label:'Loop body', text:'<hi>sum += i</hi> adds <hi>1</hi>. sum is now <hi>1</hi>.'} },
        { line:2, tokenRange:[10,10], stack:[{name:'main()',vars:[{n:'sum',v:'1',fresh:false},{n:'i',v:'2',fresh:true}],active:true}],
          expl:{label:'Increment', text:'<hi>i++</hi> runs. <hi>i</hi> goes from 1 to <hi>2</hi>.'} },
        { line:2, tokenRange:[7,8], stack:[{name:'main()',vars:[{n:'sum',v:'1',fresh:false},{n:'i',v:'2',fresh:false}],active:true}],
          expl:{label:'Condition: true', text:'Java checks <hi>i < 3</hi>. 2 < 3 is true -- one more iteration.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'sum',v:'3',fresh:true},{n:'i',v:'2',fresh:false}],active:true}],
          expl:{label:'Loop body', text:'<hi>sum += i</hi> adds <hi>2</hi>. sum is now <hi>3</hi>.'} },
        { line:2, tokenRange:[10,10], stack:[{name:'main()',vars:[{n:'sum',v:'3',fresh:false},{n:'i',v:'3',fresh:true}],active:true}],
          expl:{label:'Increment', text:'<hi>i++</hi> runs. <hi>i</hi> goes from 2 to <hi>3</hi>.'} },
        { line:2, tokenRange:[7,8], stack:[{name:'main()',vars:[{n:'sum',v:'3',fresh:false},{n:'i',v:'3',fresh:false}],active:true}],
          expl:{label:'Condition: false', text:'Java checks <hi>i < 3</hi>. 3 < 3 is false -- the loop exits. <hi>i</hi> goes out of scope.'} },
        { line:5, stack:[{name:'main()',vars:[{n:'sum',v:'3',fresh:false}],active:true}],
          expl:{label:'After the loop', text:'<hi>i</hi> is gone -- it was scoped to the <hi>for</hi> header. <hi>System.out.println(sum)</hi> prints <hi>3</hi>.'} },
        { line:6, stack:[{name:'main()',vars:[{n:'sum',v:'3',fresh:false}],active:true}],
          expl:{label:'Program ends', text:'<hi>main()</hi> finishes.'} },
      ]
    },

    /* ------------------------------------------------------------------ */
    /*  4. for-each loop                                                    */
    /* ------------------------------------------------------------------ */
    {
      label: 'for-each loop',
      lines: [
        { code: [['tok-kw','static '],['tok-kw','void '],['tok-meth','main'],['tok-op','(String[] args) {']] },
        { code: [['','  '],['tok-type','int'],['tok-op','[] nums = {'],['tok-num','10'],['tok-op',', '],['tok-num','20'],['tok-op',', '],['tok-num','30'],['tok-op','};']] },
        { code: [['','  '],['tok-type','int '],['tok-op','total = '],['tok-num','0'],['tok-op',';']] },
        { code: [['','  '],['tok-kw','for '],['tok-op','('],['tok-type','int '],['tok-op','n : nums) {']] },
        { code: [['','    '],['tok-op','total += n;']] },
        { code: [['','  '],['tok-op','}']] },
        { code: [['','  '],['tok-op','System.out.println(total);']] },
        { code: [['tok-op','}']] },
      ],
      steps: [
        { line:0, stack:[{name:'main()',vars:[],active:true}],
          expl:{label:'Program starts', text:'<hi>main()</hi> begins.'} },
        { line:1, stack:[{name:'main()',vars:[{n:'nums',v:'[10, 20, 30]',fresh:true,cls:true}],active:true}],
          expl:{label:'Array created', text:'An int array <hi>{10, 20, 30}</hi> is created. <hi>nums</hi> holds a reference to it -- arrays are objects in Java.'} },
        { line:2, stack:[{name:'main()',vars:[{n:'nums',v:'[10, 20, 30]',fresh:false,cls:true},{n:'total',v:'0',fresh:true}],active:true}],
          expl:{label:'Variable declared', text:'<hi>total</hi> starts at <hi>0</hi>. It will hold the running sum.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'nums',v:'[10, 20, 30]',fresh:false,cls:true},{n:'total',v:'0',fresh:false},{n:'n',v:'10',fresh:true}],active:true}],
          expl:{label:'First element', text:'The for-each loop retrieves the first element and assigns it to <hi>n</hi>. n = <hi>10</hi>.'} },
        { line:4, stack:[{name:'main()',vars:[{n:'nums',v:'[10, 20, 30]',fresh:false,cls:true},{n:'total',v:'10',fresh:true},{n:'n',v:'10',fresh:false}],active:true}],
          expl:{label:'Loop body', text:'<hi>total += n</hi> adds <hi>10</hi>. total is now <hi>10</hi>.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'nums',v:'[10, 20, 30]',fresh:false,cls:true},{n:'total',v:'10',fresh:false},{n:'n',v:'20',fresh:true}],active:true}],
          expl:{label:'Next element', text:'The loop advances. <hi>n</hi> is now <hi>20</hi>.'} },
        { line:4, stack:[{name:'main()',vars:[{n:'nums',v:'[10, 20, 30]',fresh:false,cls:true},{n:'total',v:'30',fresh:true},{n:'n',v:'20',fresh:false}],active:true}],
          expl:{label:'Loop body', text:'<hi>total += n</hi> adds <hi>20</hi>. total is now <hi>30</hi>.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'nums',v:'[10, 20, 30]',fresh:false,cls:true},{n:'total',v:'30',fresh:false},{n:'n',v:'30',fresh:true}],active:true}],
          expl:{label:'Last element', text:'<hi>n</hi> is now <hi>30</hi> -- the final element in the array.'} },
        { line:4, stack:[{name:'main()',vars:[{n:'nums',v:'[10, 20, 30]',fresh:false,cls:true},{n:'total',v:'60',fresh:true},{n:'n',v:'30',fresh:false}],active:true}],
          expl:{label:'Loop body', text:'<hi>total += n</hi> adds <hi>30</hi>. total is now <hi>60</hi>.'} },
        { line:3, stack:[{name:'main()',vars:[{n:'nums',v:'[10, 20, 30]',fresh:false,cls:true},{n:'total',v:'60',fresh:false}],active:true}],
          expl:{label:'No more elements', text:'All elements have been visited. The loop exits and <hi>n</hi> goes out of scope.'} },
        { line:6, stack:[{name:'main()',vars:[{n:'nums',v:'[10, 20, 30]',fresh:false,cls:true},{n:'total',v:'60',fresh:false}],active:true}],
          expl:{label:'After the loop', text:'<hi>System.out.println(total)</hi> prints <hi>60</hi> -- the sum of all elements.'} },
        { line:7, stack:[{name:'main()',vars:[{n:'nums',v:'[10, 20, 30]',fresh:false,cls:true},{n:'total',v:'60',fresh:false}],active:true}],
          expl:{label:'Program ends', text:'<hi>main()</hi> finishes.'} },
      ]
    },

    /* ------------------------------------------------------------------ */
    /*  5. Recursion                                                        */
    /*                                                                      */
    /*  Line index (blank lines excluded from count):                       */
    /*  0  static void main(String[] args) {                                */
    /*  1    countdown(3);                                                  */
    /*  2  }                                                                */
    /*     (blank -- not counted)                                           */
    /*  3  static void countdown(int n) {                                   */
    /*  4    if (n == 0) {                                                  */
    /*  5      System.out.println("Go!");                                   */
    /*  6      return;                                                      */
    /*  7    }                                                              */
    /*  8    System.out.println(n);                                         */
    /*  9    countdown(n - 1);                                              */
    /*  10 }                                                                */
    /* ------------------------------------------------------------------ */
    {
      label: 'Recursion',
      lines: [
        { code: [['tok-kw','static '],['tok-kw','void '],['tok-meth','main'],['tok-op','(String[] args) {']] },
        { code: [['','  '],['tok-meth','countdown'],['tok-op','('],['tok-num','3'],['tok-op',');']] },
        { code: [['tok-op','}']] },
        { blank: true },
        { code: [['tok-kw','static '],['tok-kw','void '],['tok-meth','countdown'],['tok-op','('],['tok-type','int '],['tok-op','n) {']] },
        { code: [['','  '],['tok-kw','if '],['tok-op','(n == '],['tok-num','0'],['tok-op',') {']] },
        { code: [['','    '],['tok-op','System.out.println('],['tok-str','"Go!"'],['tok-op',');']] },
        { code: [['','    '],['tok-kw','return'],['tok-op',';']] },
        { code: [['','  '],['tok-op','}']] },
        { code: [['','  '],['tok-op','System.out.println(n);']] },
        { code: [['','  '],['tok-meth','countdown'],['tok-op','(n - '],['tok-num','1'],['tok-op',');']] },
        { code: [['tok-op','}']] },
      ],
      steps: [
        { line:0, stack:[{name:'main()',vars:[],active:true}],
          expl:{label:'Program starts', text:'<hi>main()</hi> begins. Its frame is at the top of the call stack.'} },
        { line:1, stack:[{name:'main()',vars:[],active:true}],
          expl:{label:'Calling countdown(3)', text:'<hi>countdown(3)</hi> is called. Java will push a new frame for it.'} },
        { line:3, stack:[{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:true}],active:true},{name:'main()',vars:[],active:false}],
          expl:{label:'Frame created', text:'A new frame for <hi>countdown()</hi> is pushed. Parameter <hi>n</hi> is set to <hi>3</hi>. <hi>main()</hi> is now waiting.'} },
        { line:4, stack:[{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:true},{name:'main()',vars:[],active:false}],
          expl:{label:'Base case check', text:'<hi>n == 0</hi>? No -- 3 is not 0. The if block is skipped.'} },
        { line:8, stack:[{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:true},{name:'main()',vars:[],active:false}],
          expl:{label:'Print n', text:'<hi>System.out.println(n)</hi> prints <hi>3</hi>.'} },
        { line:9, stack:[{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:true},{name:'main()',vars:[],active:false}],
          expl:{label:'Recursive call', text:'<hi>countdown(n - 1)</hi> calls itself with <hi>2</hi>. A new frame is about to be pushed.'} },
        { line:3, stack:[{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:true}],active:true},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Stack grows', text:'A second <hi>countdown()</hi> frame is pushed. Each call has its own <hi>n</hi>. The first call waits below.'} },
        { line:4, stack:[{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:true},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Base case check', text:'<hi>n == 0</hi>? No -- 2 is not 0.'} },
        { line:8, stack:[{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:true},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Print n', text:'Prints <hi>2</hi>.'} },
        { line:9, stack:[{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:true},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Recursive call', text:'<hi>countdown(1)</hi> is called. Another frame will be pushed.'} },
        { line:3, stack:[{name:'countdown(n=1)',vars:[{n:'n',v:'1',fresh:true}],active:true},{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:false},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Stack grows', text:'Three <hi>countdown()</hi> frames now sit above <hi>main()</hi>. Each is waiting for the one above it to finish.'} },
        { line:4, stack:[{name:'countdown(n=1)',vars:[{n:'n',v:'1',fresh:false}],active:true},{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:false},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Base case check', text:'<hi>n == 0</hi>? No -- 1 is not 0.'} },
        { line:8, stack:[{name:'countdown(n=1)',vars:[{n:'n',v:'1',fresh:false}],active:true},{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:false},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Print n', text:'Prints <hi>1</hi>.'} },
        { line:9, stack:[{name:'countdown(n=1)',vars:[{n:'n',v:'1',fresh:false}],active:true},{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:false},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Recursive call', text:'<hi>countdown(0)</hi> is called -- this will trigger the base case.'} },
        { line:3, stack:[{name:'countdown(n=0)',vars:[{n:'n',v:'0',fresh:true}],active:true},{name:'countdown(n=1)',vars:[{n:'n',v:'1',fresh:false}],active:false},{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:false},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Deepest call', text:'The stack is now at its deepest: five frames. <hi>n = 0</hi> in this frame.'} },
        { line:4, stack:[{name:'countdown(n=0)',vars:[{n:'n',v:'0',fresh:false}],active:true},{name:'countdown(n=1)',vars:[{n:'n',v:'1',fresh:false}],active:false},{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:false},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Base case: true', text:'<hi>n == 0</hi> is true. The base case is reached -- the if block executes.'} },
        { line:5, stack:[{name:'countdown(n=0)',vars:[{n:'n',v:'0',fresh:false}],active:true},{name:'countdown(n=1)',vars:[{n:'n',v:'1',fresh:false}],active:false},{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:false},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Print "Go!"', text:'<hi>System.out.println("Go!")</hi> runs. The countdown is complete.'} },
        { line:6, stack:[{name:'countdown(n=0)',vars:[{n:'n',v:'0',fresh:false}],active:true},{name:'countdown(n=1)',vars:[{n:'n',v:'1',fresh:false}],active:false},{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:false},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'return', text:'<hi>return</hi> exits <hi>countdown(n=0)</hi>. Its frame is popped -- the stack begins to unwind.'} },
        { line:10, stack:[{name:'countdown(n=1)',vars:[{n:'n',v:'1',fresh:false}],active:true},{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:false},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Unwinding', text:'Back in <hi>countdown(n=1)</hi>. It has no more work to do -- it reaches its closing brace and returns.'} },
        { line:10, stack:[{name:'countdown(n=2)',vars:[{n:'n',v:'2',fresh:false}],active:true},{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:false},{name:'main()',vars:[],active:false}],
          expl:{label:'Unwinding', text:'<hi>countdown(n=2)</hi> also completes and is popped.'} },
        { line:10, stack:[{name:'countdown(n=3)',vars:[{n:'n',v:'3',fresh:false}],active:true},{name:'main()',vars:[],active:false}],
          expl:{label:'Unwinding', text:'<hi>countdown(n=3)</hi> completes and is popped. Only <hi>main()</hi> remains.'} },
        { line:2, stack:[{name:'main()',vars:[],active:true}],
          expl:{label:'Back in main', text:'<hi>main()</hi> resumes after the original <hi>countdown(3)</hi> call. The program ends.'} },
      ]
    },

  ]
});