diff --git a/assets/js/autocomplete/input.ts b/assets/js/autocomplete/input.ts
new file mode 100644
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--- /dev/null
+++ b/assets/js/autocomplete/input.ts
@@ -0,0 +1,199 @@
+import store from '../utils/store';
+import { getTermContexts } from '../match_query';
+import { Range } from '../query/lex';
+
+export type TextInputElement = HTMLInputElement | HTMLTextAreaElement;
+
+/**
+ * Describes the term, that the cursor is currently on, which is known as "active".
+ * If any tag completion is accepted, this term will be overwritten in the input.
+ * The rest of the input will be left untouched.
+ */
+interface ActiveTerm {
+  range: Range;
+
+  /**
+   * The term itself. Stripped from the `prefix` if it's present, and also lowercased.
+   */
+  term: string;
+
+  /**
+   * Optional `-` prefix is only relevant for the `single-tag` autocompletion type.
+   * This prefix is extracted automatically from the `term` value and is used to
+   * signal that the tag should be removed from the list.
+   */
+  prefix: '-' | '';
+}
+
+/**
+ * Captures the value of the input at the time when the `AutocompletableInput` was created.
+ */
+interface AutocompleteInputSnapshot {
+  /**
+   * Original value of the input element at the time when it was created unmodified.
+   */
+  origValue: string;
+
+  /**
+   * The value of the input element at the time when it was created, but
+   * trimmed from whitespace.
+   */
+  trimmedValue: string;
+
+  /**
+   * Can be `null` if the input value is empty.
+   */
+  activeTerm: ActiveTerm | null;
+
+  /**
+   * Cursor selection at the time when the snapshot was taken.
+   */
+  selection: {
+    start: number | null;
+    end: number | null;
+    direction: TextInputElement['selectionDirection'];
+  };
+}
+
+/**
+ * The `multi-tags` autocompletion type is used to power inputs with complex
+ * search queries like `(tag1 OR tag2), tag3` and tag lists like `tag1, tag2, tag3`
+ * in the plain tag search/edit inputs.
+ *
+ * The `single-tag` autocompletion type is used to power the fancy tag editor
+ * that manages separate input elements for every tag. In this mode the user
+ * can input `-tag` prefix to remove the tag from the list. See more details
+ * about how it works here: https://github.com/philomena-dev/philomena/pull/383
+ */
+type AutocompleteInputType = 'multi-tags' | 'single-tag';
+
+/**
+ * Parsed version of `TextInputElement`. Its behavior is controlled with various
+ * `data-autocomplete*` attributes.
+ */
+export class AutocompletableInput {
+  /**
+   * HTML element that autocomplete is attached to.
+   */
+  readonly element: TextInputElement;
+
+  readonly type: AutocompleteInputType;
+
+  /**
+   * Captures the value of the input at the time when the `AutocompletableInput` was created.
+   */
+  readonly snapshot: AutocompleteInputSnapshot;
+
+  /**
+   * Defines the name of the parameter in `localStorage` that should be read
+   * to conditionally enable the autocomplete feature.
+   */
+  readonly condition?: string;
+
+  /**
+   * An integer that overrides the default limit of maximum suggestions to show.
+   */
+  readonly maxSuggestions: number;
+
+  /**
+   * If present enables the history feature for the input element. The value
+   * of this property defines the key in the `localStorage` where the history
+   * records are stored.
+   */
+  readonly historyId?: string;
+
+  /**
+   * Returns `null` only if the element is not autocomplete-capable.
+   */
+  static fromElement(element: unknown): AutocompletableInput | null {
+    if (!(element instanceof HTMLInputElement || element instanceof HTMLTextAreaElement)) {
+      return null;
+    }
+
+    // This attribute marks the element as autocomplete-capable. It doesn't necessarily
+    // mean that the autocomplete **will** show up for the element. It may be disabled
+    // based on the setting value from the key specified under the attribute
+    // `data-autocomplete-condition`.
+    if (!element.dataset.autocomplete) {
+      return null;
+    }
+
+    return new AutocompletableInput(element);
+  }
+
+  private constructor(element: TextInputElement) {
+    this.element = element;
+    this.condition = element.dataset.autocompleteCondition;
+    this.historyId = element.dataset.autocompleteHistoryId;
+
+    const type = element.dataset.autocomplete;
+
+    if (type !== 'multi-tags' && type !== 'single-tag') {
+      throw new Error(`BUG: invalid autocomplete type: ${type}`);
+    }
+
+    this.type = type;
+    this.snapshot = {
+      origValue: element.value,
+      trimmedValue: element.value.trim(),
+      activeTerm: findActiveTerm(type, element),
+      selection: {
+        start: element.selectionStart,
+        end: element.selectionEnd,
+        direction: element.selectionDirection,
+      },
+    };
+
+    const maxSuggestions = element.dataset.autocompleteMaxSuggestions;
+
+    this.maxSuggestions = maxSuggestions ? parseInt(maxSuggestions, 10) : 10;
+  }
+
+  hasHistory(): this is this & { historyId: string } {
+    return Boolean(this.historyId);
+  }
+
+  isEnabled(): boolean {
+    return !this.condition || store.get<boolean>(this.condition) || false;
+  }
+}
+
+function findActiveTerm(
+  autocompleteType: AutocompleteInputType,
+  { value, selectionStart, selectionEnd }: TextInputElement,
+): ActiveTerm | null {
+  if (selectionStart === null || selectionEnd === null) return null;
+
+  // Technically the user may select several characters and several terms at once,
+  // but we just take the first one from the selection as the "cursor" index.
+  const cursorIndex = Math.min(selectionStart, selectionEnd);
+
+  // Multi-line textarea elements should treat each line as different search queries.
+  // Here we're looking for the actively edited line and use it instead of the whole value.
+  const lineStart = value.lastIndexOf('\n', cursorIndex) + 1;
+  const lineEnd = Math.max(value.indexOf('\n', cursorIndex), value.length);
+  const line = value.slice(lineStart, lineEnd);
+
+  const terms = getTermContexts(line);
+  const searchIndex = cursorIndex - lineStart;
+
+  const term = terms.find(({ range }) => range.start <= searchIndex && range.end >= searchIndex) ?? null;
+
+  if (!term) {
+    return null;
+  }
+
+  const { range } = term;
+  const content = term.content.toLowerCase();
+  const stripDash = content.startsWith('-') && autocompleteType === 'single-tag';
+
+  return {
+    term: stripDash ? content.slice(1) : content,
+    prefix: stripDash ? '-' : '',
+    range: {
+      // Convert line-specific indexes back to absolute ones.
+      start: range.start + lineStart,
+      end: range.end + lineStart,
+    },
+  };
+}