expandApplyAtRules.js

import postcss from 'postcss'
import parser from 'postcss-selector-parser'

import { resolveMatches } from './generateRules'
import bigSign from '../util/bigSign'
import escapeClassName from '../util/escapeClassName'

/** @typedef {Map<string, [any, import('postcss').Rule[]]>} ApplyCache */

function extractClasses(node) {
  /** @type {Map<string, Set<string>>} */
  let groups = new Map()

  let container = postcss.root({ nodes: [node.clone()] })

  container.walkRules((rule) => {
    parser((selectors) => {
      selectors.walkClasses((classSelector) => {
        let parentSelector = classSelector.parent.toString()

        let classes = groups.get(parentSelector)
        if (!classes) {
          groups.set(parentSelector, (classes = new Set()))
        }

        classes.add(classSelector.value)
      })
    }).processSync(rule.selector)
  })

  let normalizedGroups = Array.from(groups.values(), (classes) => Array.from(classes))
  let classes = normalizedGroups.flat()

  return Object.assign(classes, { groups: normalizedGroups })
}

let selectorExtractor = parser((root) => root.nodes.map((node) => node.toString()))

/**
 * @param {string} ruleSelectors
 */
function extractSelectors(ruleSelectors) {
  return selectorExtractor.transformSync(ruleSelectors)
}

function extractBaseCandidates(candidates, separator) {
  let baseClasses = new Set()

  for (let candidate of candidates) {
    baseClasses.add(candidate.split(separator).pop())
  }

  return Array.from(baseClasses)
}

function prefix(context, selector) {
  let prefix = context.tailwindConfig.prefix
  return typeof prefix === 'function' ? prefix(selector) : prefix + selector
}

function* pathToRoot(node) {
  yield node
  while (node.parent) {
    yield node.parent
    node = node.parent
  }
}

/**
 * Only clone the node itself and not its children
 *
 * @param {*} node
 * @param {*} overrides
 * @returns
 */
function shallowClone(node, overrides = {}) {
  let children = node.nodes
  node.nodes = []

  let tmp = node.clone(overrides)

  node.nodes = children

  return tmp
}

/**
 * Clone just the nodes all the way to the top that are required to represent
 * this singular rule in the tree.
 *
 * For example, if we have CSS like this:
 * ```css
 * @media (min-width: 768px) {
 *   @supports (display: grid) {
 *     .foo {
 *       display: grid;
 *       grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
 *     }
 *   }
 *
 *   @supports (backdrop-filter: blur(1px)) {
 *     .bar {
 *       backdrop-filter: blur(1px);
 *     }
 *   }
 *
 *   .baz {
 *     color: orange;
 *   }
 * }
 * ```
 *
 * And we're cloning `.bar` it'll return a cloned version of what's required for just that single node:
 *
 * ```css
 * @media (min-width: 768px) {
 *   @supports (backdrop-filter: blur(1px)) {
 *     .bar {
 *       backdrop-filter: blur(1px);
 *     }
 *   }
 * }
 * ```
 *
 * @param {import('postcss').Node} node
 */
function nestedClone(node) {
  for (let parent of pathToRoot(node)) {
    if (node === parent) {
      continue
    }

    if (parent.type === 'root') {
      break
    }

    node = shallowClone(parent, {
      nodes: [node],
    })
  }

  return node
}

/**
 * @param {import('postcss').Root} root
 */
function buildLocalApplyCache(root, context) {
  /** @type {ApplyCache} */
  let cache = new Map()

  let highestOffset = context.layerOrder.user >> 4n

  root.walkRules((rule, idx) => {
    // Ignore rules generated by Tailwind
    for (let node of pathToRoot(rule)) {
      if (node.raws.tailwind?.layer !== undefined) {
        return
      }
    }

    // Clone what's required to represent this singular rule in the tree
    let container = nestedClone(rule)

    for (let className of extractClasses(rule)) {
      let list = cache.get(className) || []
      cache.set(className, list)

      list.push([
        {
          layer: 'user',
          sort: BigInt(idx) + highestOffset,
          important: false,
        },
        container,
      ])
    }
  })

  return cache
}

/**
 * @returns {ApplyCache}
 */
function buildApplyCache(applyCandidates, context) {
  for (let candidate of applyCandidates) {
    if (context.notClassCache.has(candidate) || context.applyClassCache.has(candidate)) {
      continue
    }

    if (context.classCache.has(candidate)) {
      context.applyClassCache.set(
        candidate,
        context.classCache.get(candidate).map(([meta, rule]) => [meta, rule.clone()])
      )
      continue
    }

    let matches = Array.from(resolveMatches(candidate, context))

    if (matches.length === 0) {
      context.notClassCache.add(candidate)
      continue
    }

    context.applyClassCache.set(candidate, matches)
  }

  return context.applyClassCache
}

/**
 * Build a cache only when it's first used
 *
 * @param {() => ApplyCache} buildCacheFn
 * @returns {ApplyCache}
 */
function lazyCache(buildCacheFn) {
  let cache = null

  return {
    get: (name) => {
      cache = cache || buildCacheFn()

      return cache.get(name)
    },
    has: (name) => {
      cache = cache || buildCacheFn()

      return cache.has(name)
    },
  }
}

/**
 * Take a series of multiple caches and merge
 * them so they act like one large cache
 *
 * @param {ApplyCache[]} caches
 * @returns {ApplyCache}
 */
function combineCaches(caches) {
  return {
    get: (name) => caches.flatMap((cache) => cache.get(name) || []),
    has: (name) => caches.some((cache) => cache.has(name)),
  }
}

function extractApplyCandidates(params) {
  let candidates = params.split(/[\s\t\n]+/g)

  if (candidates[candidates.length - 1] === '!important') {
    return [candidates.slice(0, -1), true]
  }

  return [candidates, false]
}

function processApply(root, context, localCache) {
  let applyCandidates = new Set()

  // Collect all @apply rules and candidates
  let applies = []
  root.walkAtRules('apply', (rule) => {
    let [candidates] = extractApplyCandidates(rule.params)

    for (let util of candidates) {
      applyCandidates.add(util)
    }

    applies.push(rule)
  })

  // Start the @apply process if we have rules with @apply in them
  if (applies.length === 0) {
    return
  }

  // Fill up some caches!
  let applyClassCache = combineCaches([localCache, buildApplyCache(applyCandidates, context)])

  /**
   * When we have an apply like this:
   *
   * .abc {
   *    @apply hover:font-bold;
   * }
   *
   * What we essentially will do is resolve to this:
   *
   * .abc {
   *    @apply .hover\:font-bold:hover {
   *      font-weight: 500;
   *    }
   * }
   *
   * Notice that the to-be-applied class is `.hover\:font-bold:hover` and that the utility candidate was `hover:font-bold`.
   * What happens in this function is that we prepend a `.` and escape the candidate.
   * This will result in `.hover\:font-bold`
   * Which means that we can replace `.hover\:font-bold` with `.abc` in `.hover\:font-bold:hover` resulting in `.abc:hover`
   */
  // TODO: Should we use postcss-selector-parser for this instead?
  function replaceSelector(selector, utilitySelectors, candidate) {
    let needle = `.${escapeClassName(candidate)}`
    let needles = [...new Set([needle, needle.replace(/\\2c /g, '\\,')])]
    let utilitySelectorsList = extractSelectors(utilitySelectors)

    return extractSelectors(selector)
      .map((s) => {
        let replaced = []

        for (let utilitySelector of utilitySelectorsList) {
          let replacedSelector = utilitySelector
          for (const needle of needles) {
            replacedSelector = replacedSelector.replace(needle, s)
          }
          if (replacedSelector === utilitySelector) {
            continue
          }
          replaced.push(replacedSelector)
        }
        return replaced.join(', ')
      })
      .join(', ')
  }

  let perParentApplies = new Map()

  // Collect all apply candidates and their rules
  for (let apply of applies) {
    let [candidates] = perParentApplies.get(apply.parent) || [[], apply.source]

    perParentApplies.set(apply.parent, [candidates, apply.source])

    let [applyCandidates, important] = extractApplyCandidates(apply.params)

    if (apply.parent.type === 'atrule') {
      if (apply.parent.name === 'screen') {
        const screenType = apply.parent.params

        throw apply.error(
          `@apply is not supported within nested at-rules like @screen. We suggest you write this as @apply ${applyCandidates
            .map((c) => `${screenType}:${c}`)
            .join(' ')} instead.`
        )
      }

      throw apply.error(
        `@apply is not supported within nested at-rules like @${apply.parent.name}. You can fix this by un-nesting @${apply.parent.name}.`
      )
    }

    for (let applyCandidate of applyCandidates) {
      if ([prefix(context, 'group'), prefix(context, 'peer')].includes(applyCandidate)) {
        // TODO: Link to specific documentation page with error code.
        throw apply.error(`@apply should not be used with the '${applyCandidate}' utility`)
      }

      if (!applyClassCache.has(applyCandidate)) {
        throw apply.error(
          `The \`${applyCandidate}\` class does not exist. If \`${applyCandidate}\` is a custom class, make sure it is defined within a \`@layer\` directive.`
        )
      }

      let rules = applyClassCache.get(applyCandidate)

      candidates.push([applyCandidate, important, rules])
    }
  }

  for (const [parent, [candidates, atApplySource]] of perParentApplies) {
    let siblings = []

    for (let [applyCandidate, important, rules] of candidates) {
      let potentialApplyCandidates = [
        applyCandidate,
        ...extractBaseCandidates([applyCandidate], context.tailwindConfig.separator),
      ]

      for (let [meta, node] of rules) {
        let parentClasses = extractClasses(parent)
        let nodeClasses = extractClasses(node)

        // When we encounter a rule like `.dark .a, .b { … }` we only want to be left with `[.dark, .a]` if the base applyCandidate is `.a` or with `[.b]` if the base applyCandidate is `.b`
        // So we've split them into groups
        nodeClasses = nodeClasses.groups
          .filter((classList) =>
            classList.some((className) => potentialApplyCandidates.includes(className))
          )
          .flat()

        // Add base utility classes from the @apply node to the list of
        // classes to check whether it intersects and therefore results in a
        // circular dependency or not.
        //
        // E.g.:
        // .foo {
        //   @apply hover:a; // This applies "a" but with a modifier
        // }
        //
        // We only have to do that with base classes of the `node`, not of the `parent`
        // E.g.:
        // .hover\:foo {
        //   @apply bar;
        // }
        // .bar {
        //   @apply foo;
        // }
        //
        // This should not result in a circular dependency because we are
        // just applying `.foo` and the rule above is `.hover\:foo` which is
        // unrelated. However, if we were to apply `hover:foo` then we _did_
        // have to include this one.
        nodeClasses = nodeClasses.concat(
          extractBaseCandidates(nodeClasses, context.tailwindConfig.separator)
        )

        let intersects = parentClasses.some((selector) => nodeClasses.includes(selector))
        if (intersects) {
          throw node.error(
            `You cannot \`@apply\` the \`${applyCandidate}\` utility here because it creates a circular dependency.`
          )
        }

        let root = postcss.root({ nodes: [node.clone()] })

        // Make sure every node in the entire tree points back at the @apply rule that generated it
        root.walk((node) => {
          node.source = atApplySource
        })

        let canRewriteSelector =
          node.type !== 'atrule' || (node.type === 'atrule' && node.name !== 'keyframes')

        if (canRewriteSelector) {
          root.walkRules((rule) => {
            // Let's imagine you have the following structure:
            //
            // .foo {
            //   @apply bar;
            // }
            //
            // @supports (a: b) {
            //   .bar {
            //     color: blue
            //   }
            //
            //   .something-unrelated {}
            // }
            //
            // In this case we want to apply `.bar` but it happens to be in
            // an atrule node. We clone that node instead of the nested one
            // because we still want that @supports rule to be there once we
            // applied everything.
            //
            // However it happens to be that the `.something-unrelated` is
            // also in that same shared @supports atrule. This is not good,
            // and this should not be there. The good part is that this is
            // a clone already and it can be safely removed. The question is
            // how do we know we can remove it. Basically what we can do is
            // match it against the applyCandidate that you want to apply. If
            // it doesn't match the we can safely delete it.
            //
            // If we didn't do this, then the `replaceSelector` function
            // would have replaced this with something that didn't exist and
            // therefore it removed the selector altogether. In this specific
            // case it would result in `{}` instead of `.something-unrelated {}`
            if (!extractClasses(rule).some((candidate) => candidate === applyCandidate)) {
              rule.remove()
              return
            }

            // Strip the important selector from the parent selector if at the beginning
            let importantSelector =
              typeof context.tailwindConfig.important === 'string'
                ? context.tailwindConfig.important
                : null

            // We only want to move the "important" selector if this is a Tailwind-generated utility
            // We do *not* want to do this for user CSS that happens to be structured the same
            let isGenerated = parent.raws.tailwind !== undefined

            let parentSelector =
              isGenerated && importantSelector && parent.selector.indexOf(importantSelector) === 0
                ? parent.selector.slice(importantSelector.length)
                : parent.selector

            rule.selector = replaceSelector(parentSelector, rule.selector, applyCandidate)

            // And then re-add it if it was removed
            if (importantSelector && parentSelector !== parent.selector) {
              rule.selector = `${importantSelector} ${rule.selector}`
            }

            rule.walkDecls((d) => {
              d.important = meta.important || important
            })
          })
        }

        // Insert it
        siblings.push([
          // Ensure that when we are sorting, that we take the layer order into account
          { ...meta, sort: meta.sort | context.layerOrder[meta.layer] },
          root.nodes[0],
        ])
      }
    }

    // Inject the rules, sorted, correctly
    let nodes = siblings.sort(([a], [z]) => bigSign(a.sort - z.sort)).map((s) => s[1])

    // `parent` refers to the node at `.abc` in: .abc { @apply mt-2 }
    parent.after(nodes)
  }

  for (let apply of applies) {
    // If there are left-over declarations, just remove the @apply
    if (apply.parent.nodes.length > 1) {
      apply.remove()
    } else {
      // The node is empty, drop the full node
      apply.parent.remove()
    }
  }

  // Do it again, in case we have other `@apply` rules
  processApply(root, context, localCache)
}

export default function expandApplyAtRules(context) {
  return (root) => {
    // Build a cache of the user's CSS so we can use it to resolve classes used by @apply
    let localCache = lazyCache(() => buildLocalApplyCache(root, context))

    processApply(root, context, localCache)
  }
}