function map() {
emit(1, {
sum: this.value, // the field you want stats for
min: this.value,
max: this.value,
count: 1,
diff: 0
});
}
function reduce(key, values) {
return values.reduce(function reduce(previous, current, index, array) {
var delta = previous.sum/previous.count - current.sum/current.count;
var weight = (previous.count * current.count)/(previous.count + current.count);
return {
sum: previous.sum + current.sum,
min: Math.min(previous.min, current.min),
max: Math.max(previous.max, current.max),
count: previous.count + current.count,
diff: previous.diff + current.diff + delta*delta*weight
};
})
}
function finalize(key, value) {
value.average = value.sum / value.count;
value.population_variance = value.diff / value.count;
value.population_standard_deviation = Math.sqrt(value.population_variance);
value.sample_variance = value.diff / (value.count - 1);
value.sample_standard_deviation = Math.sqrt(value.sample_variance);
delete value.diff;
return value;
}