report.js

var events = [{"event": circuitStarted, value: { name: "A"}, when: <timestamp>}
{"event": exerciseComplete, value: {id: "exID 1", "count": 150}, when:<timestamp>}
{"event": circuitStarted, value: { name: "B"}, when: <timestamp>}
{"event": exerciseComplete, value: {id: "exID 2", "count": 20}, when:<timestamp>}
{"event": exerciseComplete, value: {id: "exID 3", "count": 6}, when:<timestamp>}
{"event": circuitComplete, value: { name: "B"}, when: <timestamp>}
{"event": circuitStarted, value: { name: "B"}, when: <timestamp>}
{"event": exerciseComplete, value: {id: "exID 2", "count": 18}, when:<timestamp>}
{"event": exerciseComplete, value: {id: "exID 3", "count": 5}, when:<timestamp>}
{"event": circuitComplete, value: { name: "B"}, when: <timestamp>}
{"event": circuitComplete, value: { name: "A"}, when: <timestamp>}
{"event": circuitStarted, value: { name: "A"}, when: <timestamp>}
{"event": exerciseComplete, value: {id: "exID 1", "count": 145}, when:<timestamp>}
{"event": circuitStarted, value: { name: "B"}, when: <timestamp>}
{"event": exerciseComplete, value: {id: "exID 2", "count": 18}, when:<timestamp>}
{"event": exerciseComplete, value: {id: "exID 3", "count": 5}, when:<timestamp>}
{"event": circuitComplete, value: { name: "B"}, when: <timestamp>}
{"event": circuitStarted, value: { name: "B"}, when: <timestamp>}
{"event": exerciseComplete, value: {id: "exID 2", "count": 17}, when:<timestamp>}
{"event": exerciseComplete, value: {id: "exID 3", "count": 4}, when:<timestamp>}
{"event": circuitComplete, value: { name: "B"}, when: <timestamp>}
{"event": circuitComplete, value: { name: "A"}, when: <timestamp>}]


function Report() {
}


Report.Replayer = function() {
  var replayerSelf = this;
  var _state = {totalscore: 0, curcuits: []};

  this.circuitStarted = function(value) {
    _state.curcuits.push({"name": value.name, "score":0, "curcuits":[]});
  }
  this.circuitComplete = function(value) {
    var circuit = _state.curcuits.pop();
    _state.curcuits.peek().curcuits.push(circuit);
  }
  this.exerciseComplete = function(value) {
    _state.curcuits.peek().score += value.count;
    _state.totalscore += value.count;
  }
}


var replayer = new Report.Replayer()
for (var event in events) {
    replayer[event.event](event.value);
}
logJson(replayer._state);

//{totalscore: 388, curcuits:[
//  {name: A, score: 49, curcuits: [
//    {name: B, score: 26, curcuits:[]}
//    {name: B, score: 23, curcuits:[]}
//  ]},
//  {name: A, score: 44, curcuits: [
//    {name: B, score: 23, curcuits:[]}
//    {name: B, score: 21, curcuits:[]}
//  ]}
//}

Then you can change this.exerciseComplete to record exercises too:

this.exerciseComplete = function(value) {
    _state.curcuits.peek().score += value.count;
    _state.curcuits.peek().exercises.push(value);
    _state.totalscore += value.count;
  }

Giving something like

//{totalscore: 388, curcuits:[
//  {name: A, score: 49, curcuits: [
//    {name: B, score: 26, curcuits:[], "exercises": [{id: "exID 2", "count": 20}, {id: "exId 3", "count": 6}]},
//    {name: B, score: 23, curcuits:[], "exercises": [{id: "exID 2", "count": 18}, {id: "exId 3", "count": 5}]}
//  ], "exercises": [{id: "exID 1", count: 150}]},
....