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Filename : offsets.js
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// @ts-check

import bigSign from '../util/bigSign'
import { remapBitfield } from './remap-bitfield.js'

/**
 * @typedef {'base' | 'defaults' | 'components' | 'utilities' | 'variants' | 'user'} Layer
 */

/**
 * @typedef {object} VariantOption
 * @property {number} id An unique identifier to identify `matchVariant`
 * @property {function | undefined} sort The sort function
 * @property {string|null} value The value we want to compare
 * @property {string|null} modifier The modifier that was used (if any)
 * @property {bigint} variant The variant bitmask
 */

/**
 * @typedef {object} RuleOffset
 * @property {Layer} layer The layer that this rule belongs to
 * @property {Layer} parentLayer The layer that this rule originally belonged to. Only different from layer if this is a variant.
 * @property {bigint} arbitrary 0n if false, 1n if true
 * @property {bigint} variants Dynamic size. 1 bit per registered variant. 0n means no variants
 * @property {bigint} parallelIndex Rule index for the parallel variant. 0 if not applicable.
 * @property {bigint} index Index of the rule / utility in its given *parent* layer. Monotonically increasing.
 * @property {bigint} propertyOffset Offset for the arbitrary property. Only valid after sorting.
 * @property {string} property Name/Value of the arbitrary property.
 * @property {VariantOption[]} options Some information on how we can sort arbitrary variants
 */

export class Offsets {
  constructor() {
    /**
     * Offsets for the next rule in a given layer
     *
     * @type {Record<Layer, bigint>}
     */
    this.offsets = {
      defaults: 0n,
      base: 0n,
      components: 0n,
      utilities: 0n,
      variants: 0n,
      user: 0n,
    }

    /**
     * Positions for a given layer
     *
     * @type {Record<Layer, bigint>}
     */
    this.layerPositions = {
      defaults: 0n,
      base: 1n,
      components: 2n,
      utilities: 3n,

      // There isn't technically a "user" layer, but we need to give it a position
      // Because it's used for ordering user-css from @apply
      user: 4n,

      variants: 5n,
    }

    /**
     * The total number of functions currently registered across all variants (including arbitrary variants)
     *
     * @type {bigint}
     */
    this.reservedVariantBits = 0n

    /**
     * Positions for a given variant
     *
     * @type {Map<string, bigint>}
     */
    this.variantOffsets = new Map()
  }

  /**
   * @param {Layer} layer
   * @returns {RuleOffset}
   */
  create(layer) {
    return {
      layer,
      parentLayer: layer,
      arbitrary: 0n,
      variants: 0n,
      parallelIndex: 0n,
      index: this.offsets[layer]++,
      propertyOffset: 0n,
      property: '',
      options: [],
    }
  }

  /**
   * @param {string} name
   * @returns {RuleOffset}
   */
  arbitraryProperty(name) {
    return {
      ...this.create('utilities'),
      arbitrary: 1n,
      property: name,
    }
  }

  /**
   * Get the offset for a variant
   *
   * @param {string} variant
   * @param {number} index
   * @returns {RuleOffset}
   */
  forVariant(variant, index = 0) {
    let offset = this.variantOffsets.get(variant)
    if (offset === undefined) {
      throw new Error(`Cannot find offset for unknown variant ${variant}`)
    }

    return {
      ...this.create('variants'),
      variants: offset << BigInt(index),
    }
  }

  /**
   * @param {RuleOffset} rule
   * @param {RuleOffset} variant
   * @param {VariantOption} options
   * @returns {RuleOffset}
   */
  applyVariantOffset(rule, variant, options) {
    options.variant = variant.variants

    return {
      ...rule,
      layer: 'variants',
      parentLayer: rule.layer === 'variants' ? rule.parentLayer : rule.layer,
      variants: rule.variants | variant.variants,
      options: options.sort ? [].concat(options, rule.options) : rule.options,

      // TODO: Technically this is wrong. We should be handling parallel index on a per variant basis.
      // We'll take the max of all the parallel indexes for now.
      // @ts-ignore
      parallelIndex: max([rule.parallelIndex, variant.parallelIndex]),
    }
  }

  /**
   * @param {RuleOffset} offset
   * @param {number} parallelIndex
   * @returns {RuleOffset}
   */
  applyParallelOffset(offset, parallelIndex) {
    return {
      ...offset,
      parallelIndex: BigInt(parallelIndex),
    }
  }

  /**
   * Each variant gets 1 bit per function / rule registered.
   * This is because multiple variants can be applied to a single rule and we need to know which ones are present and which ones are not.
   * Additionally, every unique group of variants is grouped together in the stylesheet.
   *
   * This grouping is order-independent. For instance, we do not differentiate between `hover:focus` and `focus:hover`.
   *
   * @param {string[]} variants
   * @param {(name: string) => number} getLength
   */
  recordVariants(variants, getLength) {
    for (let variant of variants) {
      this.recordVariant(variant, getLength(variant))
    }
  }

  /**
   * The same as `recordVariants` but for a single arbitrary variant at runtime.
   * @param {string} variant
   * @param {number} fnCount
   *
   * @returns {RuleOffset} The highest offset for this variant
   */
  recordVariant(variant, fnCount = 1) {
    this.variantOffsets.set(variant, 1n << this.reservedVariantBits)

    // Ensure space is reserved for each "function" in the parallel variant
    // by offsetting the next variant by the number of parallel variants
    // in the one we just added.

    // Single functions that return parallel variants are NOT handled separately here
    // They're offset by 1 (or the number of functions) as usual
    // And each rule returned is tracked separately since the functions are evaluated lazily.
    // @see `RuleOffset.parallelIndex`
    this.reservedVariantBits += BigInt(fnCount)

    return {
      ...this.create('variants'),
      variants: this.variantOffsets.get(variant),
    }
  }

  /**
   * @param {RuleOffset} a
   * @param {RuleOffset} b
   * @returns {bigint}
   */
  compare(a, b) {
    // Sort layers together
    if (a.layer !== b.layer) {
      return this.layerPositions[a.layer] - this.layerPositions[b.layer]
    }

    // When sorting the `variants` layer, we need to sort based on the parent layer as well within
    // this variants layer.
    if (a.parentLayer !== b.parentLayer) {
      return this.layerPositions[a.parentLayer] - this.layerPositions[b.parentLayer]
    }

    // Sort based on the sorting function
    for (let aOptions of a.options) {
      for (let bOptions of b.options) {
        if (aOptions.id !== bOptions.id) continue
        if (!aOptions.sort || !bOptions.sort) continue

        let maxFnVariant = max([aOptions.variant, bOptions.variant]) ?? 0n

        // Create a mask of 0s from bits 1..N where N represents the mask of the Nth bit
        let mask = ~(maxFnVariant | (maxFnVariant - 1n))
        let aVariantsAfterFn = a.variants & mask
        let bVariantsAfterFn = b.variants & mask

        // If the variants the same, we _can_ sort them
        if (aVariantsAfterFn !== bVariantsAfterFn) {
          continue
        }

        let result = aOptions.sort(
          {
            value: aOptions.value,
            modifier: aOptions.modifier,
          },
          {
            value: bOptions.value,
            modifier: bOptions.modifier,
          }
        )
        if (result !== 0) return result
      }
    }

    // Sort variants in the order they were registered
    if (a.variants !== b.variants) {
      return a.variants - b.variants
    }

    // Make sure each rule returned by a parallel variant is sorted in ascending order
    if (a.parallelIndex !== b.parallelIndex) {
      return a.parallelIndex - b.parallelIndex
    }

    // Always sort arbitrary properties after other utilities
    if (a.arbitrary !== b.arbitrary) {
      return a.arbitrary - b.arbitrary
    }

    // Always sort arbitrary properties alphabetically
    if (a.propertyOffset !== b.propertyOffset) {
      return a.propertyOffset - b.propertyOffset
    }

    // Sort utilities, components, etc… in the order they were registered
    return a.index - b.index
  }

  /**
   * Arbitrary variants are recorded in the order they're encountered.
   * This means that the order is not stable between environments and sets of content files.
   *
   * In order to make the order stable, we need to remap the arbitrary variant offsets to
   * be in alphabetical order starting from the offset of the first arbitrary variant.
   */
  recalculateVariantOffsets() {
    // Sort the variants by their name
    let variants = Array.from(this.variantOffsets.entries())
      .filter(([v]) => v.startsWith('['))
      .sort(([a], [z]) => fastCompare(a, z))

    // Sort the list of offsets
    // This is not necessarily a discrete range of numbers which is why
    // we're using sort instead of creating a range from min/max
    let newOffsets = variants.map(([, offset]) => offset).sort((a, z) => bigSign(a - z))

    // Create a map from the old offsets to the new offsets in the new sort order
    /** @type {[bigint, bigint][]} */
    let mapping = variants.map(([, oldOffset], i) => [oldOffset, newOffsets[i]])

    // Remove any variants that will not move letting us skip
    // remapping if everything happens to be in order
    return mapping.filter(([a, z]) => a !== z)
  }

  /**
   * @template T
   * @param {[RuleOffset, T][]} list
   * @returns {[RuleOffset, T][]}
   */
  remapArbitraryVariantOffsets(list) {
    let mapping = this.recalculateVariantOffsets()

    // No arbitrary variants? Nothing to do.
    // Everyhing already in order? Nothing to do.
    if (mapping.length === 0) {
      return list
    }

    // Remap every variant offset in the list
    return list.map((item) => {
      let [offset, rule] = item

      offset = {
        ...offset,
        variants: remapBitfield(offset.variants, mapping),
      }

      return [offset, rule]
    })
  }

  /**
   * @template T
   * @param {[RuleOffset, T][]} list
   * @returns {[RuleOffset, T][]}
   */
  sortArbitraryProperties(list) {
    // Collect all known arbitrary properties
    let known = new Set()

    for (let [offset] of list) {
      if (offset.arbitrary === 1n) {
        known.add(offset.property)
      }
    }

    // No arbitrary properties? Nothing to do.
    if (known.size === 0) {
      return list
    }

    // Sort the properties alphabetically
    let properties = Array.from(known).sort()

    // Create a map from the property name to its offset
    let offsets = new Map()

    let offset = 1n
    for (let property of properties) {
      offsets.set(property, offset++)
    }

    // Apply the sorted offsets to the list
    return list.map((item) => {
      let [offset, rule] = item

      offset = {
        ...offset,
        propertyOffset: offsets.get(offset.property) ?? 0n,
      }

      return [offset, rule]
    })
  }

  /**
   * @template T
   * @param {[RuleOffset, T][]} list
   * @returns {[RuleOffset, T][]}
   */
  sort(list) {
    // Sort arbitrary variants so they're in alphabetical order
    list = this.remapArbitraryVariantOffsets(list)

    // Sort arbitrary properties so they're in alphabetical order
    list = this.sortArbitraryProperties(list)

    return list.sort(([a], [b]) => bigSign(this.compare(a, b)))
  }
}

/**
 *
 * @param {bigint[]} nums
 * @returns {bigint|null}
 */
function max(nums) {
  let max = null

  for (const num of nums) {
    max = max ?? num
    max = max > num ? max : num
  }

  return max
}

/**
 * A fast ASCII order string comparison function.
 *
 * Using `.sort()` without a custom compare function is faster
 * But you can only use that if you're sorting an array of
 * only strings. If you're sorting strings inside objects
 * or arrays, you need must use a custom compare function.
 *
 * @param {string} a
 * @param {string} b
 */
function fastCompare(a, b) {
  let aLen = a.length
  let bLen = b.length
  let minLen = aLen < bLen ? aLen : bLen

  for (let i = 0; i < minLen; i++) {
    let cmp = a.charCodeAt(i) - b.charCodeAt(i)
    if (cmp !== 0) return cmp
  }

  return aLen - bLen
}