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diff --git a/toolkit/components/formautofill/shared/FieldScanner.sys.mjs b/toolkit/components/formautofill/shared/FieldScanner.sys.mjs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Represents the detailed information about a form field, including
+ * the inferred field name, the approach used for inferring, and additional metadata.
+ */
+export class FieldDetail {
+  // Reference to the elemenet
+  elementWeakRef = null;
+
+  // id/name. This is only used for debugging
+  identifier = "";
+
+  // The inferred field name for this element
+  fieldName = null;
+
+  // The approach we use to infer the information for this element
+  // The possible values are "autocomplete", "fathom", and "regex-heuristic"
+  reason = null;
+
+  /*
+   * The "section", "addressType", and "contactType" values are
+   * used to identify the exact field when the serializable data is received
+   * from the backend.  There cannot be multiple fields which have
+   * the same exact combination of these values.
+   */
+
+  // Which section the field belongs to. The value comes from autocomplete attribute.
+  // See https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#autofill-detail-tokens for more details
+  section = "";
+  addressType = "";
+  contactType = "";
+  credentialType = "";
+
+  // When a field is split into N fields, we use part to record which field it is
+  // For example, a credit card number field is split into 4 fields, the value of
+  // "part" for the first cc-number field is 1, for the last one is 4.
+  // If the field is not split, the value is null
+  part = null;
+
+  // Confidence value when the field name is inferred by "fathom"
+  confidence = null;
+
+  constructor(
+    element,
+    fieldName = null,
+    { autocompleteInfo = {}, confidence = null } = {}
+  ) {
+    this.elementWeakRef = new WeakRef(element);
+    this.identifier = `${element.id}/${element.name}`;
+    this.fieldName = fieldName;
+
+    if (autocompleteInfo) {
+      this.reason = "autocomplete";
+      this.section = autocompleteInfo.section;
+      this.addressType = autocompleteInfo.addressType;
+      this.contactType = autocompleteInfo.contactType;
+      this.credentialType = autocompleteInfo.credentialType;
+    } else if (confidence) {
+      this.reason = "fathom";
+      this.confidence = confidence;
+    } else {
+      this.reason = "regex-heuristic";
+    }
+  }
+
+  get element() {
+    return this.elementWeakRef.deref();
+  }
+
+  get sectionName() {
+    return this.section || this.addressType;
+  }
+}
+
+/**
+ * A scanner for traversing all elements in a form. It also provides a
+ * cursor (parsingIndex) to indicate which element is waiting for parsing.
+ *
+ * The scanner retrives the field detail by calling heuristics handlers
+ * `inferFieldInfo` function.
+ */
+export class FieldScanner {
+  #elementsWeakRef = null;
+  #inferFieldInfoFn = null;
+
+  #parsingIndex = 0;
+
+  fieldDetails = [];
+
+  /**
+   * Create a FieldScanner based on form elements with the existing
+   * fieldDetails.
+   *
+   * @param {Array.DOMElement} elements
+   *        The elements from a form for each parser.
+   * @param {Funcion} inferFieldInfoFn
+   *        The callback function that is used to infer the field info of a given element
+   */
+  constructor(elements, inferFieldInfoFn) {
+    this.#elementsWeakRef = new WeakRef(elements);
+    this.#inferFieldInfoFn = inferFieldInfoFn;
+  }
+
+  get #elements() {
+    return this.#elementsWeakRef.deref();
+  }
+
+  /**
+   * This cursor means the index of the element which is waiting for parsing.
+   *
+   * @returns {number}
+   *          The index of the element which is waiting for parsing.
+   */
+  get parsingIndex() {
+    return this.#parsingIndex;
+  }
+
+  get parsingFinished() {
+    return this.parsingIndex >= this.#elements.length;
+  }
+
+  /**
+   * Move the parsingIndex to the next elements. Any elements behind this index
+   * means the parsing tasks are finished.
+   *
+   * @param {number} index
+   *        The latest index of elements waiting for parsing.
+   */
+  set parsingIndex(index) {
+    if (index > this.#elements.length) {
+      throw new Error("The parsing index is out of range.");
+    }
+    this.#parsingIndex = index;
+  }
+
+  /**
+   * Retrieve the field detail by the index. If the field detail is not ready,
+   * the elements will be traversed until matching the index.
+   *
+   * @param {number} index
+   *        The index of the element that you want to retrieve.
+   * @returns {object}
+   *          The field detail at the specific index.
+   */
+  getFieldDetailByIndex(index) {
+    if (index >= this.#elements.length) {
+      return null;
+    }
+
+    if (index < this.fieldDetails.length) {
+      return this.fieldDetails[index];
+    }
+
+    for (let i = this.fieldDetails.length; i < index + 1; i++) {
+      this.pushDetail();
+    }
+
+    return this.fieldDetails[index];
+  }
+
+  /**
+   * This function retrieves the first unparsed element and obtains its
+   * information by invoking the `inferFieldInfoFn` callback function.
+   * The field information is then stored in a FieldDetail object and
+   * appended to the `fieldDetails` array.
+   *
+   * Any element without the related detail will be used for adding the detail
+   * to the end of field details.
+   */
+  pushDetail() {
+    const elementIndex = this.fieldDetails.length;
+    if (elementIndex >= this.#elements.length) {
+      throw new Error("Try to push the non-existing element info.");
+    }
+    const element = this.#elements[elementIndex];
+    const [fieldName, autocompleteInfo, confidence] =
+      this.#inferFieldInfoFn(element);
+    const fieldDetail = new FieldDetail(element, fieldName, {
+      autocompleteInfo,
+      confidence,
+    });
+
+    this.fieldDetails.push(fieldDetail);
+  }
+
+  /**
+   * When a field detail should be changed its fieldName after parsing, use
+   * this function to update the fieldName which is at a specific index.
+   *
+   * @param {number} index
+   *        The index indicates a field detail to be updated.
+   * @param {string} fieldName
+   *        The new name of the field
+   * @param {boolean} [ignoreAutocomplete=false]
+   *        Whether to change the field name when the field name is determined by
+   *        autocomplete attribute
+   */
+  updateFieldName(index, fieldName, ignoreAutocomplete = false) {
+    if (index >= this.fieldDetails.length) {
+      throw new Error("Try to update the non-existing field detail.");
+    }
+
+    const fieldDetail = this.fieldDetails[index];
+    if (fieldDetail.fieldName == fieldName) {
+      return;
+    }
+
+    if (!ignoreAutocomplete && fieldDetail.reason == "autocomplete") {
+      return;
+    }
+
+    this.fieldDetails[index].fieldName = fieldName;
+    this.fieldDetails[index].reason = "update-heuristic";
+  }
+
+  elementExisting(index) {
+    return index < this.#elements.length;
+  }
+}
+
+export default FieldScanner;