Function _combineHostContextSelectors

packages/compiler/src/shadow_css.ts:1248–1285

* Combine the `contextSelectors` with the `hostMarker` and the `otherSelectors` * to create a selector that matches the same as `:host-context()`. * * Given a single context selector `A` we need to output selectors that match on the host and as an * ancestor of the host: * * ``` * A <hostMark

(
  contextSelectors: string[],
  otherSelectors: string,
  pseudoPrefix = '',
)

 * @param otherSelectors the rest of the selectors that are not context selectors.
 */
function _combineHostContextSelectors(
  contextSelectors: string[],
  otherSelectors: string,
  pseudoPrefix = '',
): string {
  const hostMarker = _polyfillHostNoCombinator;
  _polyfillHostRe.lastIndex = 0; // reset the regex to ensure we get an accurate test
  const otherSelectorsHasHost = _polyfillHostRe.test(otherSelectors);

  // If there are no context selectors then just output a host marker
  if (contextSelectors.length === 0) {
    return hostMarker + otherSelectors;
  }

  const combined: string[] = [contextSelectors.pop() || ''];
  while (contextSelectors.length > 0) {
    const length = combined.length;
    const contextSelector = contextSelectors.pop();
    for (let i = 0; i < length; i++) {
      const previousSelectors = combined[i];
      // Add the new selector as a descendant of the previous selectors
      combined[length * 2 + i] = previousSelectors + ' ' + contextSelector;
      // Add the new selector as an ancestor of the previous selectors
      combined[length + i] = contextSelector + ' ' + previousSelectors;
      // Add the new selector to act on the same element as the previous selectors
      combined[i] = contextSelector + previousSelectors;
    }
  }
  // Finally connect the selector to the `hostMarker`s: either acting directly on the host
  // (A<hostMarker>) or as an ancestor (A <hostMarker>).
  return combined
    .map((s) =>
      otherSelectorsHasHost
        ? `${pseudoPrefix}${s}${otherSelectors}`
        : `${pseudoPrefix}${s}${hostMarker}${otherSelectors}, ${pseudoPrefix}${s} ${hostMarker}${otherSelectors}`,
    )
    .join(',');
}

/**
 * Mutate the given `groups` array so that there are `multiples` clones of the original array