notes/node_modules/stylelint/lib/utils/nodeContextLookup.js

// Too weird for Flow
"use strict"

/**
 * Create a collection of Maps that serve to contextualize a given node.
 * This is useful to ensure that you only compare nodes that share a certain
 * context.
 *
 * All nodes are initially contextualized by their input source.
 * From there, you can contextualize them however you want.
 *
 * For a usage example, see `selector-no-descending-specificity`.
 */
module.exports = function () {
  const contextMap = new Map()

  return {
    getContext(node) {
      const nodeSource = node.source.input.from
      const baseContext = creativeGetMap(contextMap, nodeSource)
      const subContexts = Array.from(arguments).slice(1)

      return subContexts.reduce((result, context) => {
        return creativeGetMap(result, context)
      }, baseContext)
    },
  }
}

function creativeGetMap(someMap, someThing) {
  if (!someMap.has(someThing)) {
    someMap.set(someThing, new Map())
  }
  return someMap.get(someThing)
}