sampling-function-test.js

ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm");
ChromeUtils.import("resource://gre/modules/Services.jsm");

const ENABLE_PROB = 0.01;
const ADDON_ID = "telemetry-coverage-bug1487578@mozilla.org";
const TOTAL_SAMPLES = 100000;

Cu.importGlobalProperties(["crypto", "TextEncoder"]);

XPCOMUtils.defineLazyServiceGetters(this, {
  uuidGen: ["@mozilla.org/uuid-generator;1", "nsIUUIDGenerator"],
});

async function generateVariate(seed, label) {
  const hasher = crypto.subtle;
  const hash = await hasher.digest("SHA-256", new TextEncoder("utf-8").encode(seed + label));
  let view = new DataView(hash);
  return view.getUint32(0) / 0xffffffff;
}

async function start() {
    let counter = 0;
    let samples = [];

    while (samples.length < TOTAL_SAMPLES) {
        let uuid = uuidGen.generateUUID().number;
        uuid = uuid.substring(1, uuid.length - 1);

        let variate = await generateVariate(uuid, ADDON_ID);
        if (variate < ENABLE_PROB) {
            counter++;
        } else {
            counter = 0;
        }
        samples.push(counter);
    }

    const average = samples.reduce((total, amount, index, array) => {
        total += amount;
        if ( index === array.length - 1) {
            return total / array.length;
        } else {
            return total;
        }
    });

    return average;
}

// Since xpcshell will exit before the promise is resolved, spin the event loop until it is.
function awaitPromise(promise) {
    let done = false;
    let result;
    let error;
    promise.then(
        val => { result = val; },
        err => { error = err; }
    ).then(() => {
        done = true;
    });

    Services.tm.spinEventLoopUntil(() => done);

    if (error !== undefined) {
        throw error;
    }

    return result;
}

let avg = awaitPromise(start());
result = (avg - ENABLE_PROB) / ENABLE_PROB * 100;
result = parseFloat(result).toFixed(2);

dump(`
Average result over ${TOTAL_SAMPLES} samples is ${avg},
${result}% change from target probability ${ENABLE_PROB}.
\n`);