From b7b3a02bfed1b7eec2ad66b08fb2893f8599fb54 Mon Sep 17 00:00:00 2001
From: Peter Deutsch <pwd@mit.edu>
Date: Wed, 16 Aug 2023 19:19:15 -0700
Subject: [PATCH] Lab fixes for 2024

---
 automate.py                   |  16 +--
 part2/index.html              | 186 +++++++++++++++++++++++++++++++++-
 part2/worker.js               |  15 +--
 part3/index.html              | 186 +++++++++++++++++++++++++++++++++-
 part3/worker.js               |  15 +--
 part4/eval.py                 |  27 -----
 part4/extension.crx           | Bin 2109 -> 0 bytes
 part4/extension/background.js | 162 -----------------------------
 part4/extension/manifest.json |  10 --
 part4/index.html              |   1 -
 part4/worker.js               |  32 ------
 11 files changed, 389 insertions(+), 261 deletions(-)
 mode change 120000 => 100644 part2/index.html
 mode change 120000 => 100644 part3/index.html
 delete mode 100644 part4/eval.py
 delete mode 100644 part4/extension.crx
 delete mode 100644 part4/extension/background.js
 delete mode 100644 part4/extension/manifest.json
 delete mode 120000 part4/index.html
 delete mode 100644 part4/worker.js

diff --git a/automate.py b/automate.py
index a25f908..bc8a725 100644
--- a/automate.py
+++ b/automate.py
@@ -14,20 +14,15 @@ from selenium.webdriver.chrome.options import Options
 parser = argparse.ArgumentParser()
 parser.add_argument("--browser", type=str, choices=["chrome", "firefox", "safari"], default="chrome", help="Browser to run automation in.")
 parser.add_argument("--domains", type=str, default="google.com,youtube.com,baidu.com,facebook.com", help="Comma-separated list of domain names to collect traces from. Defaults to google.com,youtube.com,baidu.com,facebook.com")
-parser.add_argument("--enable_countermeasure", type=bool, default=False, help="Set to true to enable the countermeasure. Browser must be set to Chrome. Defaults to false.")
 parser.add_argument("--num_traces_per_domain", type=int, default=40, help="Number of traces to collect per domain.")
 parser.add_argument("--trace_length", type=int, default=5000, help="The length of each recorded trace, in milliseconds. Defaults to 5000.")
 
 required = parser.add_argument_group("required arguments")
 required.add_argument("--out_filename", type=str, required=True, help="Name of the output file to save traces to.")
-required.add_argument("--part", type=int, choices=[2, 3, 4], required=True, help="Set to the part of the lab you're working on.")
+required.add_argument("--part", type=int, choices=[2, 3], required=True, help="Set to the part of the lab you're working on.")
 
 opts = parser.parse_args()
 
-if opts.browser != "chrome" and opts.enable_countermeasure:
-    print("Browser must be set to Chrome in order to enable the countermeasure.")
-    sys.exit(1)
-
 if os.path.exists(opts.out_filename):
     print(f"WARNING: Data already exists at {opts.out_filename}. What do you want to do?")
     res = input("[C]ancel [O]verwrite ").lower()
@@ -40,11 +35,6 @@ if os.path.exists(opts.out_filename):
 # Start serving attacker app
 app = Flask(__name__)
 
-# Disable Flask logs
-os.environ["WERKZEUG_RUN_MAIN"] = "true"
-log = logging.getLogger("werkzeug")
-log.disabled = True
-
 @app.route("/")
 def root():
     return send_from_directory(f"part{opts.part}", "index.html")
@@ -63,10 +53,6 @@ def get_browser(victim):
         chrome_opts = Options()
         chrome_opts.add_experimental_option("excludeSwitches", ["enable-automation"])
 
-        if opts.enable_countermeasure and victim:
-            # Victim has the extension enabled -- attacker does not
-            chrome_opts.add_extension("part4/extension.crx")
-
         return webdriver.Chrome(options=chrome_opts)
     elif opts.browser == "firefox":
         return webdriver.Firefox()
diff --git a/part2/index.html b/part2/index.html
deleted file mode 120000
index 79c5d6f..0000000
--- a/part2/index.html
+++ /dev/null
@@ -1 +0,0 @@
-../index.html
\ No newline at end of file
diff --git a/part2/index.html b/part2/index.html
new file mode 100644
index 0000000..eff4b48
--- /dev/null
+++ b/part2/index.html
@@ -0,0 +1,185 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <meta charset="utf-8" />
+    <title>Website Fingerprinting Lab</title>
+    <style>
+      * {
+        margin: 0;
+        padding: 0;
+      }
+
+      body {
+        font-family: Arial, Helvetica, sans-serif;
+        padding: 64px;
+      }
+
+      h1 {
+        font-size: 32px;
+        margin-bottom: 16px;
+      }
+
+      p {
+        margin-bottom: 8px;
+      }
+
+      #buttons {
+        margin-bottom: 16px;
+      }
+
+      button {
+        background: #fff;
+        border: 2px solid #3498db;
+        border-radius: 4px;
+        color: #3498db;
+        cursor: pointer;
+        display: inline-block;
+        font-size: 16px;
+        margin-right: 16px;
+        padding: 8px 16px;
+      }
+
+      button.disabled {
+        background: #ccc;
+        border-color: #ccc;
+        color: #666;
+        cursor: default;
+      }
+
+      .trace {
+        margin-bottom: 16px;
+      }
+    </style>
+  </head>
+  <body>
+    <h1>Website Fingerprinting Lab</h1>
+    <div id="buttons">
+      <button id="collect-trace">Collect trace</button>
+      <button id="download-traces">Download traces</button>
+    </div>
+    <div id="traces"></div>
+    <script src="https://d3js.org/d3.v6.js"></script>
+    <script type="text/javascript">
+      const worker = new Worker("worker.js");
+
+      const collectTraceButton = document.getElementById("collect-trace");
+      const downloadTracesButton = document.getElementById("download-traces");
+
+      // Default values for when the automation script isn't being used. When
+      // the script is in use, these values will get overwritten.
+      window.trace_length = 5000;
+      window.using_automation_script = false;
+
+      window.recording = false;
+      window.traces = [];
+
+      let traceIds = [];
+
+      worker.onmessage = (e) => {
+        window.recording = false;
+
+        const trace = JSON.parse(e.data);
+        window.traces.push(trace);
+
+        if (window.using_automation_script) {
+          // Don't display traces when automation script is in use
+          return;
+        }
+
+        // Create new trace div
+        const parent = document.getElementById("traces");
+        const div = document.createElement("div");
+        const traceId = "a" + Math.random().toString().substring(2, 10);
+        div.setAttribute("id", traceId);
+        div.className = "trace";
+        parent.appendChild(div);
+        traceIds.push(traceId);
+
+        // Trace dimensions
+        const width = parent.getBoundingClientRect().width;
+        const height = 64;
+
+        // Create div for new trace
+        const svg = d3
+          .select("#" + traceId)
+          .append("svg")
+          .attr("width", width)
+          .attr("height", height);
+
+        // Find largest value across all traces
+        const maxVal = d3.max(window.traces, (d) => d3.max(d));
+
+        for (let i = 0; i < window.traces.length; i++) {
+          // Re-visualize all traces each time in case maxVal changes
+          const x = d3
+            .scaleLinear()
+            .domain([0, window.traces[i].length])
+            .range([0, width]);
+
+          const color = d3
+            .scaleQuantize()
+            .range(["#0d0887", "#7e03a8", "#cc4778", "#f89540", "#f0f921"])
+            .domain([0, maxVal]);
+
+          svg
+            .selectAll()
+            .data(window.traces[i].map((x, i) => ({ index: i, value: x })))
+            .join("rect")
+            .attr("x", (d) => x(d.index))
+            .attr("y", 0)
+            .attr("width", x(1))
+            .attr("height", height)
+            .style("fill", (d) => color(d.value));
+        }
+
+        // Reset UI
+        collectTraceButton.innerText = "Collect trace";
+        collectTraceButton.className = "";
+      };
+
+      function collectTrace() {
+        collectTraceButton.innerText = "Collecting trace...";
+        collectTraceButton.className = "disabled";
+        window.recording = true;
+
+        worker.postMessage({
+          type: "start",
+          trace_length: window.trace_length,
+        });
+      }
+
+      collectTraceButton.onclick = () => {
+        if (window.recording) return;
+
+        window.recording = true;
+        collectTraceButton.innerText = "Starting in 3...";
+        collectTraceButton.className = "disabled";
+
+        setTimeout(() => {
+          collectTraceButton.innerText = "Starting in 2...";
+
+          setTimeout(() => {
+            collectTraceButton.innerText = "Starting in 1...";
+            setTimeout(collectTrace, 1000);
+          }, 1000);
+        }, 1000);
+      };
+
+      downloadTracesButton.onclick = () => {
+        const blob = new Blob([JSON.stringify({ traces: window.traces })], {
+          type: "application/json",
+        });
+
+        const url = URL.createObjectURL(blob);
+
+        const elem = document.createElement("a");
+        elem.href = url;
+        elem.download = "traces.json";
+        document.body.appendChild(elem);
+
+        elem.click();
+        document.body.removeChild(elem);
+      };
+    </script>
+  </body>
+</html>
diff --git a/part2/worker.js b/part2/worker.js
index 5b706ad..fed89d2 100644
--- a/part2/worker.js
+++ b/part2/worker.js
@@ -1,7 +1,11 @@
-// Duration of your trace, in milliseconds
-let TRACE_LENGTH;
+// Number of sweep counts
+// TODO: Choose an appropriate value!
+let P;
 
-// Array of length TRACE_LENGTH with your trace's values
+// Number of elements in your trace
+let K = 5 * 1000 / P; 
+
+// Array of length K with your trace's values
 let T;
 
 // Value of performance.now() when you started recording your trace
@@ -9,7 +13,7 @@ let start;
 
 function record() {
   // Create empty array for saving trace values
-  T = new Array(TRACE_LENGTH);
+  T = new Array(K);
 
   // Fill array with -1 so we can be sure memory is allocated
   T.fill(-1, 0, T.length);
@@ -17,7 +21,7 @@ function record() {
   // Save start timestamp
   start = performance.now();
 
-  // TODO (Exercise 2-2): Record data for TRACE_LENGTH seconds and save values to T.
+  // TODO: Record data for 5 seconds and save values to T.
 
   // Once done recording, send result to main thread
   postMessage(JSON.stringify(T));
@@ -26,7 +30,6 @@ function record() {
 // DO NOT MODIFY BELOW THIS LINE -- PROVIDED BY COURSE STAFF
 self.onmessage = (e) => {
   if (e.data.type === "start") {
-    TRACE_LENGTH = e.data.trace_length;
     setTimeout(record, 0);
   }
 };
diff --git a/part3/index.html b/part3/index.html
deleted file mode 120000
index 79c5d6f..0000000
--- a/part3/index.html
+++ /dev/null
@@ -1 +0,0 @@
-../index.html
\ No newline at end of file
diff --git a/part3/index.html b/part3/index.html
new file mode 100644
index 0000000..eff4b48
--- /dev/null
+++ b/part3/index.html
@@ -0,0 +1,185 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <meta charset="utf-8" />
+    <title>Website Fingerprinting Lab</title>
+    <style>
+      * {
+        margin: 0;
+        padding: 0;
+      }
+
+      body {
+        font-family: Arial, Helvetica, sans-serif;
+        padding: 64px;
+      }
+
+      h1 {
+        font-size: 32px;
+        margin-bottom: 16px;
+      }
+
+      p {
+        margin-bottom: 8px;
+      }
+
+      #buttons {
+        margin-bottom: 16px;
+      }
+
+      button {
+        background: #fff;
+        border: 2px solid #3498db;
+        border-radius: 4px;
+        color: #3498db;
+        cursor: pointer;
+        display: inline-block;
+        font-size: 16px;
+        margin-right: 16px;
+        padding: 8px 16px;
+      }
+
+      button.disabled {
+        background: #ccc;
+        border-color: #ccc;
+        color: #666;
+        cursor: default;
+      }
+
+      .trace {
+        margin-bottom: 16px;
+      }
+    </style>
+  </head>
+  <body>
+    <h1>Website Fingerprinting Lab</h1>
+    <div id="buttons">
+      <button id="collect-trace">Collect trace</button>
+      <button id="download-traces">Download traces</button>
+    </div>
+    <div id="traces"></div>
+    <script src="https://d3js.org/d3.v6.js"></script>
+    <script type="text/javascript">
+      const worker = new Worker("worker.js");
+
+      const collectTraceButton = document.getElementById("collect-trace");
+      const downloadTracesButton = document.getElementById("download-traces");
+
+      // Default values for when the automation script isn't being used. When
+      // the script is in use, these values will get overwritten.
+      window.trace_length = 5000;
+      window.using_automation_script = false;
+
+      window.recording = false;
+      window.traces = [];
+
+      let traceIds = [];
+
+      worker.onmessage = (e) => {
+        window.recording = false;
+
+        const trace = JSON.parse(e.data);
+        window.traces.push(trace);
+
+        if (window.using_automation_script) {
+          // Don't display traces when automation script is in use
+          return;
+        }
+
+        // Create new trace div
+        const parent = document.getElementById("traces");
+        const div = document.createElement("div");
+        const traceId = "a" + Math.random().toString().substring(2, 10);
+        div.setAttribute("id", traceId);
+        div.className = "trace";
+        parent.appendChild(div);
+        traceIds.push(traceId);
+
+        // Trace dimensions
+        const width = parent.getBoundingClientRect().width;
+        const height = 64;
+
+        // Create div for new trace
+        const svg = d3
+          .select("#" + traceId)
+          .append("svg")
+          .attr("width", width)
+          .attr("height", height);
+
+        // Find largest value across all traces
+        const maxVal = d3.max(window.traces, (d) => d3.max(d));
+
+        for (let i = 0; i < window.traces.length; i++) {
+          // Re-visualize all traces each time in case maxVal changes
+          const x = d3
+            .scaleLinear()
+            .domain([0, window.traces[i].length])
+            .range([0, width]);
+
+          const color = d3
+            .scaleQuantize()
+            .range(["#0d0887", "#7e03a8", "#cc4778", "#f89540", "#f0f921"])
+            .domain([0, maxVal]);
+
+          svg
+            .selectAll()
+            .data(window.traces[i].map((x, i) => ({ index: i, value: x })))
+            .join("rect")
+            .attr("x", (d) => x(d.index))
+            .attr("y", 0)
+            .attr("width", x(1))
+            .attr("height", height)
+            .style("fill", (d) => color(d.value));
+        }
+
+        // Reset UI
+        collectTraceButton.innerText = "Collect trace";
+        collectTraceButton.className = "";
+      };
+
+      function collectTrace() {
+        collectTraceButton.innerText = "Collecting trace...";
+        collectTraceButton.className = "disabled";
+        window.recording = true;
+
+        worker.postMessage({
+          type: "start",
+          trace_length: window.trace_length,
+        });
+      }
+
+      collectTraceButton.onclick = () => {
+        if (window.recording) return;
+
+        window.recording = true;
+        collectTraceButton.innerText = "Starting in 3...";
+        collectTraceButton.className = "disabled";
+
+        setTimeout(() => {
+          collectTraceButton.innerText = "Starting in 2...";
+
+          setTimeout(() => {
+            collectTraceButton.innerText = "Starting in 1...";
+            setTimeout(collectTrace, 1000);
+          }, 1000);
+        }, 1000);
+      };
+
+      downloadTracesButton.onclick = () => {
+        const blob = new Blob([JSON.stringify({ traces: window.traces })], {
+          type: "application/json",
+        });
+
+        const url = URL.createObjectURL(blob);
+
+        const elem = document.createElement("a");
+        elem.href = url;
+        elem.download = "traces.json";
+        document.body.appendChild(elem);
+
+        elem.click();
+        document.body.removeChild(elem);
+      };
+    </script>
+  </body>
+</html>
diff --git a/part3/worker.js b/part3/worker.js
index 46c8d96..fed89d2 100644
--- a/part3/worker.js
+++ b/part3/worker.js
@@ -1,7 +1,11 @@
-// Duration of your trace, in milliseconds
-let TRACE_LENGTH;
+// Number of sweep counts
+// TODO: Choose an appropriate value!
+let P;
 
-// Array of length TRACE_LENGTH with your trace's values
+// Number of elements in your trace
+let K = 5 * 1000 / P; 
+
+// Array of length K with your trace's values
 let T;
 
 // Value of performance.now() when you started recording your trace
@@ -9,7 +13,7 @@ let start;
 
 function record() {
   // Create empty array for saving trace values
-  T = new Array(TRACE_LENGTH);
+  T = new Array(K);
 
   // Fill array with -1 so we can be sure memory is allocated
   T.fill(-1, 0, T.length);
@@ -17,7 +21,7 @@ function record() {
   // Save start timestamp
   start = performance.now();
 
-  // TODO (Exercise 3-1): Record data for TRACE_LENGTH seconds and save values to T.
+  // TODO: Record data for 5 seconds and save values to T.
 
   // Once done recording, send result to main thread
   postMessage(JSON.stringify(T));
@@ -26,7 +30,6 @@ function record() {
 // DO NOT MODIFY BELOW THIS LINE -- PROVIDED BY COURSE STAFF
 self.onmessage = (e) => {
   if (e.data.type === "start") {
-    TRACE_LENGTH = e.data.trace_length;
     setTimeout(record, 0);
   }
 };
diff --git a/part4/eval.py b/part4/eval.py
deleted file mode 100644
index 7c13310..0000000
--- a/part4/eval.py
+++ /dev/null
@@ -1,27 +0,0 @@
-import json
-import numpy as np
-
-from sklearn.ensemble import RandomForestClassifier
-from sklearn.metrics import classification_report
-from sklearn.model_selection import train_test_split
-
-def eval():
-    y_pred_full, y_test_full = [], []
-
-    # Re-train 10 times in order to reduce effects of randomness
-    for i in range(10):
-        ### TODO: Exercise 4-1
-        ### 1. Load data from traces file
-        ### 2. Split data into X_train, X_test, y_train, y_test with train_test_split
-        ### 3. Train classifier with X_train and y_train
-        ### 4. Use classifier to make predictions on X_test. Save the result to a variable called y_pred
-
-        # Do not modify the next two lines
-        y_test_full.extend(y_test)
-        y_pred_full.extend(y_pred)
-
-    ### TODO: Exercise 4-1 (continued)
-    ### 5. Print classification report using y_test_full and y_pred_full
-
-if __name__ == "__main__":
-    eval()
diff --git a/part4/extension.crx b/part4/extension.crx
deleted file mode 100644
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diff --git a/part4/extension/background.js b/part4/extension/background.js
deleted file mode 100644
index e85606f..0000000
--- a/part4/extension/background.js
+++ /dev/null
@@ -1,162 +0,0 @@
-const domains = [
-  "https://www.google.com/",
-  "https://www.youtube.com/",
-  "https://www.tmall.com/",
-  "https://www.qq.com/",
-  "https://www.baidu.com/",
-  "https://www.sohu.com/",
-  "https://www.facebook.com/",
-  "https://www.taobao.com/",
-  "https://www.jd.com/",
-  "https://www.amazon.com/",
-  "https://www.yahoo.com/",
-  "https://www.wikipedia.org/",
-  "https://www.weibo.com/",
-  "https://sina.com.cn/",
-  "https://www.zoom.us/",
-  "http://www.xinhuanet.com/",
-  "https://www.live.com/",
-  "https://www.reddit.com/",
-  "https://www.netflix.com/",
-  "https://www.microsoft.com/",
-  "https://www.instagram.com/",
-  "https://www.office.com/",
-  "https://panda.tv/",
-  "https://www.zhanqi.tv/",
-  "https://www.alipay.com/",
-  "https://www.bing.com/",
-  "https://www.csdn.net/",
-  "https://www.vk.com/",
-  "https://www.myshopify.com/",
-  "https://www.naver.com/",
-  "https://www.okezone.com/",
-  "https://www.twitch.tv/",
-  "https://www.twitter.com/",
-  "https://www.ebay.com/",
-  "https://www.adobe.com/",
-  "https://www.tianya.cn/",
-  "https://www.huanqiu.com/",
-  "https://www.yy.com/",
-  "https://www.aliexpress.com/",
-  "https://www.linkedin.com/",
-  "https://www.force.com/",
-  "https://www.aparat.com/",
-  "https://www.mail.ru/",
-  "https://www.msn.com/",
-  "https://www.dropbox.com/",
-  "https://www.whatsapp.com/",
-  "https://www.apple.com/",
-  "https://www.1688.com/",
-  "https://www.wordpress.com/",
-  "https://www.canva.com/",
-  "https://www.indeed.com/",
-  "https://www.stackoverflow.com/",
-  "https://www.ok.ru/",
-  "https://www.so.com/",
-  "https://www.chase.com/",
-  "https://www.imdb.com/",
-  "https://www.slack.com/",
-  "https://www.etsy.com/",
-  "https://www.tiktok.com/",
-  "https://www.booking.com/",
-  "https://www.babytree.com/",
-  "https://rakuten.co.jp/",
-  "https://www.salesforce.com/",
-  "https://www.spotify.com/",
-  "https://www.tribunnews.com/",
-  "https://www.fandom.com/",
-  "https://www.tradingview.com/",
-  "https://www.github.com/",
-  "https://www.haosou.com/",
-  "https://www.paypal.com/",
-  "https://www.cnblogs.com/",
-  "https://www.alibaba.com/",
-  "https://www.kompas.com/",
-  "https://gome.com.cn/",
-  "https://www.walmart.com/",
-  "https://www.roblox.com/",
-  "https://www.6.cn/",
-  "https://www.zillow.com/",
-  "https://www.godaddy.com/",
-  "https://www.imgur.com/",
-  "https://www.espn.com/",
-  "https://www.bbc.com/",
-  "https://www.hao123.com/",
-  "https://www.pikiran-rakyat.com/",
-  "https://www.grammarly.com/",
-  "https://www.cnn.com/",
-  "https://www.telegram.org/",
-  "https://www.tumblr.com/",
-  "https://www.nytimes.com/",
-  "https://www.detik.com/",
-  "https://www.wetransfer.com/",
-  "https://www.savefrom.net/",
-  "https://www.rednet.cn/",
-  "https://www.freepik.com/",
-  "https://www.ilovepdf.com/",
-  "https://www.daum.net/",
-  "https://www.pinterest.com/",
-  "https://www.primevideo.com/",
-  "https://www.intuit.com/",
-  "https://www.medium.com/",
-];
-
-const loadTime = 5000;
-let loading = false;
-let startTime = 0;
-
-function randomPing() {
-  const controller = new AbortController();
-  const id = setTimeout(
-    () => controller.abort(),
-    loadTime - (performance.now() - startTime)
-  );
-
-  fetch(
-    domains[Math.floor(Math.random() * domains.length)] +
-      "?" +
-      new Date().getTime(),
-    {
-      signal: controller.signal,
-    }
-  );
-}
-
-function activityBurst() {
-  switch (Math.floor(Math.random() * 2)) {
-    case 0:
-      let start = performance.now();
-      let counter = 0;
-
-      while (performance.now() - start < 5) {
-        counter += 1;
-      }
-
-      console.log(counter);
-      break;
-    case 1:
-      randomPing();
-      break;
-  }
-}
-
-chrome.tabs.onUpdated.addListener((tabId, changeInfo, tab) => {
-  if (changeInfo.status === "loading") {
-    if (loading) {
-      return;
-    }
-
-    startTime = performance.now();
-    loading = true;
-
-    for (let i = 0; i < 20; i++) {
-      randomPing();
-    }
-
-    for (let i = 0; i < loadTime / 10; i++) {
-      setTimeout(activityBurst, Math.random() * loadTime);
-    }
-  } else if (changeInfo.status === "complete") {
-    loading = false;
-  }
-});
diff --git a/part4/extension/manifest.json b/part4/extension/manifest.json
deleted file mode 100644
index cd43bd6..0000000
--- a/part4/extension/manifest.json
+++ /dev/null
@@ -1,10 +0,0 @@
-{
-  "name": "Lab Countermeasure",
-  "version": "1.0",
-  "manifest_version": 3,
-  "background": {
-    "service_worker": "background.js"
-  },
-  "action": {},
-  "host_permissions": ["*://*/*"]
-}
diff --git a/part4/index.html b/part4/index.html
deleted file mode 120000
index 79c5d6f..0000000
--- a/part4/index.html
+++ /dev/null
@@ -1 +0,0 @@
-../index.html
\ No newline at end of file
diff --git a/part4/worker.js b/part4/worker.js
deleted file mode 100644
index 13eb53c..0000000
--- a/part4/worker.js
+++ /dev/null
@@ -1,32 +0,0 @@
-// Duration of your trace, in milliseconds
-let TRACE_LENGTH;
-
-// Array of length TRACE_LENGTH with your trace's values
-let T;
-
-// Value of performance.now() when you started recording your trace
-let start;
-
-function record() {
-  // Create empty array for saving trace values
-  T = new Array(TRACE_LENGTH);
-
-  // Fill array with -1 so we can be sure memory is allocated
-  T.fill(-1, 0, T.length);
-
-  // Save start timestamp
-  start = performance.now();
-
-  // TODO (Exercise 4-4): Copy your solution from part 3. Optionally make changes to your part 3 solution if you need to.
-
-  // Once done recording, send result to main thread
-  postMessage(JSON.stringify(T));
-}
-
-// DO NOT MODIFY BELOW THIS LINE -- PROVIDED BY COURSE STAFF
-self.onmessage = (e) => {
-  if (e.data.type === "start") {
-    TRACE_LENGTH = e.data.trace_length;
-    setTimeout(record, 0);
-  }
-};