cssnano/src/plugins/postcss-merge-rules/lib/vendorPrefixSummary.js

A modular CSS minifier, built on top of the PostCSS ecosystem.

/**
 * What one selector contributes: its vendor prefix, `''` when it has none and
 * `undefined` when unknown or when it mixes prefixes.
 *
 * @typedef {{prefix: string | undefined, msPlaceholder: boolean}} SelectorVendor
 */

/**
 * What a selector list contributes to the vendor-prefix merge check.
 * Merging a rule for into a rule for the same selector but with different prefixes
 * would change where the rule applies.
 */
export class VendorPrefixSummary {
  /**
   * `prefix` is the one vendor prefix shared by every selector: `null` for an
   * empty list, `''` when none is prefixed, and `undefined` when the list mixes
   * prefixed and unprefixed selectors or different prefixes.
   *
   * @type {string | null | undefined}
   * */
  #prefix;
  /** @type {boolean} */
  #msPlaceholder;

  /**
   * @param {string | null | undefined} prefix
   * @param {boolean} msPlaceholder
   */
  constructor(prefix, msPlaceholder) {
    this.#prefix = prefix;
    this.#msPlaceholder = msPlaceholder;
  }

  /**
   * @param {SelectorVendor[]} selectors
   * @return {VendorPrefixSummary}
   */
  static of(selectors) {
    const profile = new VendorPrefixSummary(null, false);
    for (const selector of selectors) profile.add(selector);
    return profile;
  }

  /**
   * Folds one selector in place, so a growing list needs no rescan.
   *
   * @param {SelectorVendor} selector the vendor facts of one selector;
   * an unknown `prefix` blocks every merge
   */
  add(selector) {
    const own = selector.prefix;
    if (this.#prefix === null) {
      this.#prefix = own;
    } else if (this.#prefix !== own) {
      this.#prefix = undefined;
    }
    this.#msPlaceholder ||= selector.msPlaceholder;
  }

  /**
   * The profile of this selector list followed by another, without
   * rescanning. Neither operand changes.
   *
   * @param {VendorPrefixSummary} other
   * @return {VendorPrefixSummary}
   */
  concat(other) {
    let prefix = this.#prefix;
    if (this.#prefix === null) {
      prefix = other.#prefix;
    } else if (other.#prefix !== null && this.#prefix !== other.#prefix) {
      prefix = undefined;
    }
    return new VendorPrefixSummary(
      prefix,
      this.#msPlaceholder || other.#msPlaceholder
    );
  }

  /** @return {boolean} */
  #isUnprefixed() {
    return this.#prefix === null || this.#prefix === '';
  }

  /**
   * Whether two selector lists may share a rule: both unprefixed, or both
   * carrying the same single prefix, except that two `-ms-input-placeholder`
   * lists never merge because Edge and Internet Explorer disagree on the
   * pseudo's spelling.
   *
   * @param {VendorPrefixSummary} other
   * @return {boolean}
   */
  allowsMerge(other) {
    if (this.#isUnprefixed() && other.#isUnprefixed()) return true;
    if (this.#prefix === undefined || other.#prefix === undefined) return false;
    return (
      this.#prefix === other.#prefix &&
      !(this.#msPlaceholder && other.#msPlaceholder)
    );
  }
}