index.js

// Match vowels (including `Y`).
const vowels = /[AEIOUY]/

// Match few Slavo-Germanic values.
const slavoGermanic = /W|K|CZ|WITZ/

// Match few Germanic values.
const germanic = /^(VAN |VON |SCH)/

// Match initial values of which the first character should be skipped.
const initialExceptions = /^(GN|KN|PN|WR|PS)/

// Match initial Greek-like values of which the `CH` sounds like `K`.
const initialGreekCh = /^CH(IA|EM|OR([^E])|YM|ARAC|ARIS)/

// Match Greek-like values of which the `CH` sounds like `K`.
const greekCh = /ORCHES|ARCHIT|ORCHID/

// Match values which when following `CH`, transform `CH` to sound like `K`.
const chForKh = /[ BFHLMNRVW]/

// Match values which when preceding a vowel and `UGH`, sound like `F`.
const gForF = /[CGLRT]/

// Match initial values which sound like either `K` or `J`.
const initialGForKj = /Y[\s\S]|E[BILPRSY]|I[BELN]/

// Match initial values which sound like either `K` or `J`.
const initialAngerException = /^[DMR]ANGER/

// Match values which when following `GY`, do not sound like `K` or `J`.
const gForKj = /[EGIR]/

// Match values which when following `J`, do not sound `J`.
const jForJException = /[LTKSNMBZ]/

// Match values which might sound like `L`.
const alle = /AS|OS/

// Match Germanic values preceding `SH` which sound like `S`.
const hForS = /EIM|OEK|OLM|OLZ/

// Match Dutch values following `SCH` which sound like either `X` and `SK`,
// or `SK`.
const dutchSch = /E[DMNR]|UY|OO/

/**
 * Get the phonetics according to the Double Metaphone algorithm from a value.
 *
 * @param {string} value
 *   Value to use.
 * @returns {[string, string]}
 *   Double metaphone codes for `value`.
 */
// eslint-disable-next-line complexity
export function doubleMetaphone(value) {
  let primary = ''
  let secondary = ''
  let index = 0
  const length = value.length
  const last = length - 1
  const normalized = String(value).toUpperCase() + '     '
  const isSlavoGermanic = slavoGermanic.test(normalized)
  const isGermanic = germanic.test(normalized)
  const characters = normalized.split('')

  // Skip this at beginning of word.
  if (initialExceptions.test(normalized)) {
    index++
  }

  // Initial X is pronounced Z, which maps to S. Such as `Xavier`.
  if (characters[0] === 'X') {
    primary += 'S'
    secondary += 'S'
    index++
  }

  while (index < length) {
    const previous = characters[index - 1]
    const next = characters[index + 1]
    const nextnext = characters[index + 2]
    /** @type {string} */
    let subvalue

    switch (characters[index]) {
      case 'A':
      case 'E':
      case 'I':
      case 'O':
      case 'U':
      case 'Y':
      case 'À':
      case 'Ê':
      case 'É':
        if (index === 0) {
          // All initial vowels now map to `A`.
          primary += 'A'
          secondary += 'A'
        }

        index++

        break
      case 'B':
        primary += 'P'
        secondary += 'P'

        if (next === 'B') {
          index++
        }

        index++

        break
      case 'Ç':
        primary += 'S'
        secondary += 'S'
        index++

        break
      case 'C':
        // Various Germanic:
        if (
          previous === 'A' &&
          next === 'H' &&
          nextnext !== 'I' &&
          !vowels.test(characters[index - 2]) &&
          (nextnext !== 'E' ||
            ((subvalue = normalized.slice(index - 2, index + 4)) &&
              (subvalue === 'BACHER' || subvalue === 'MACHER')))
        ) {
          primary += 'K'
          secondary += 'K'
          index += 2

          break
        }

        // Special case for `Caesar`.
        if (index === 0 && normalized.slice(index + 1, index + 6) === 'AESAR') {
          primary += 'S'
          secondary += 'S'
          index += 2

          break
        }

        // Italian `Chianti`.
        if (normalized.slice(index + 1, index + 4) === 'HIA') {
          primary += 'K'
          secondary += 'K'
          index += 2

          break
        }

        if (next === 'H') {
          // Find `Michael`.
          if (index > 0 && nextnext === 'A' && characters[index + 3] === 'E') {
            primary += 'K'
            secondary += 'X'
            index += 2

            break
          }

          // Greek roots such as `chemistry`, `chorus`.
          if (index === 0 && initialGreekCh.test(normalized)) {
            primary += 'K'
            secondary += 'K'
            index += 2

            break
          }

          // Germanic, Greek, or otherwise `CH` for `KH` sound.
          if (
            isGermanic ||
            // Such as 'architect' but not 'arch', orchestra', 'orchid'.
            greekCh.test(normalized.slice(index - 2, index + 4)) ||
            nextnext === 'T' ||
            nextnext === 'S' ||
            ((index === 0 ||
              previous === 'A' ||
              previous === 'E' ||
              previous === 'O' ||
              previous === 'U') &&
              // Such as `wachtler`, `weschsler`, but not `tichner`.
              chForKh.test(nextnext))
          ) {
            primary += 'K'
            secondary += 'K'
          } else if (index === 0) {
            primary += 'X'
            secondary += 'X'
            // Such as 'McHugh'.
          } else if (normalized.slice(0, 2) === 'MC') {
            // Bug? Why matching absolute? what about McHiccup?
            primary += 'K'
            secondary += 'K'
          } else {
            primary += 'X'
            secondary += 'K'
          }

          index += 2

          break
        }

        // Such as `Czerny`.
        if (next === 'Z' && normalized.slice(index - 2, index) !== 'WI') {
          primary += 'S'
          secondary += 'X'
          index += 2

          break
        }

        // Such as `Focaccia`.
        if (normalized.slice(index + 1, index + 4) === 'CIA') {
          primary += 'X'
          secondary += 'X'
          index += 3

          break
        }

        // Double `C`, but not `McClellan`.
        if (next === 'C' && !(index === 1 && characters[0] === 'M')) {
          // Such as `Bellocchio`, but not `Bacchus`.
          if (
            (nextnext === 'I' || nextnext === 'E' || nextnext === 'H') &&
            normalized.slice(index + 2, index + 4) !== 'HU'
          ) {
            subvalue = normalized.slice(index - 1, index + 4)

            // Such as `Accident`, `Accede`, `Succeed`.
            if (
              (index === 1 && previous === 'A') ||
              subvalue === 'UCCEE' ||
              subvalue === 'UCCES'
            ) {
              primary += 'KS'
              secondary += 'KS'
              // Such as `Bacci`, `Bertucci`, other Italian.
            } else {
              primary += 'X'
              secondary += 'X'
            }

            index += 3

            break
          } else {
            // Pierce's rule.
            primary += 'K'
            secondary += 'K'
            index += 2

            break
          }
        }

        if (next === 'G' || next === 'K' || next === 'Q') {
          primary += 'K'
          secondary += 'K'
          index += 2

          break
        }

        // Italian.
        if (
          next === 'I' &&
          // Bug: The original algorithm also calls for A (as in CIA), which is
          // already taken care of above.
          (nextnext === 'E' || nextnext === 'O')
        ) {
          primary += 'S'
          secondary += 'X'
          index += 2

          break
        }

        if (next === 'I' || next === 'E' || next === 'Y') {
          primary += 'S'
          secondary += 'S'
          index += 2

          break
        }

        primary += 'K'
        secondary += 'K'

        // Skip two extra characters ahead in `Mac Caffrey`, `Mac Gregor`.
        if (
          next === ' ' &&
          (nextnext === 'C' || nextnext === 'G' || nextnext === 'Q')
        ) {
          index += 3
          break
        }

        // Bug: Already covered above.
        // if (
        //   next === 'K' ||
        //   next === 'Q' ||
        //   (next === 'C' && nextnext !== 'E' && nextnext !== 'I')
        // ) {
        //   index++;
        // }

        index++

        break
      case 'D':
        if (next === 'G') {
          // Such as `edge`.
          if (nextnext === 'E' || nextnext === 'I' || nextnext === 'Y') {
            primary += 'J'
            secondary += 'J'
            index += 3
            // Such as `Edgar`.
          } else {
            primary += 'TK'
            secondary += 'TK'
            index += 2
          }

          break
        }

        if (next === 'T' || next === 'D') {
          primary += 'T'
          secondary += 'T'
          index += 2

          break
        }

        primary += 'T'
        secondary += 'T'
        index++

        break
      case 'F':
        if (next === 'F') {
          index++
        }

        index++
        primary += 'F'
        secondary += 'F'

        break
      case 'G':
        if (next === 'H') {
          if (index > 0 && !vowels.test(previous)) {
            primary += 'K'
            secondary += 'K'
            index += 2

            break
          }

          // Such as `Ghislane`, `Ghiradelli`.
          if (index === 0) {
            if (nextnext === 'I') {
              primary += 'J'
              secondary += 'J'
            } else {
              primary += 'K'
              secondary += 'K'
            }

            index += 2

            break
          }

          // Parker's rule (with some further refinements).
          if (
            // Such as `Hugh`.  The comma is not a bug.
            ((subvalue = characters[index - 2]),
            subvalue === 'B' || subvalue === 'H' || subvalue === 'D') ||
            // Such as `bough`.  The comma is not a bug.
            ((subvalue = characters[index - 3]),
            subvalue === 'B' || subvalue === 'H' || subvalue === 'D') ||
            // Such as `Broughton`.  The comma is not a bug.
            ((subvalue = characters[index - 4]),
            subvalue === 'B' || subvalue === 'H')
          ) {
            index += 2

            break
          }

          // Such as `laugh`, `McLaughlin`, `cough`, `gough`, `rough`, `tough`.
          if (
            index > 2 &&
            previous === 'U' &&
            gForF.test(characters[index - 3])
          ) {
            primary += 'F'
            secondary += 'F'
          } else if (index > 0 && previous !== 'I') {
            primary += 'K'
            secondary += 'K'
          }

          index += 2

          break
        }

        if (next === 'N') {
          if (index === 1 && vowels.test(characters[0]) && !isSlavoGermanic) {
            primary += 'KN'
            secondary += 'N'
            // Not like `Cagney`.
          } else if (
            normalized.slice(index + 2, index + 4) !== 'EY' &&
            normalized.slice(index + 1) !== 'Y' &&
            !isSlavoGermanic
          ) {
            primary += 'N'
            secondary += 'KN'
          } else {
            primary += 'KN'
            secondary += 'KN'
          }

          index += 2

          break
        }

        // Such as `Tagliaro`.
        if (
          normalized.slice(index + 1, index + 3) === 'LI' &&
          !isSlavoGermanic
        ) {
          primary += 'KL'
          secondary += 'L'
          index += 2

          break
        }

        // -ges-, -gep-, -gel- at beginning.
        if (index === 0 && initialGForKj.test(normalized.slice(1, 3))) {
          primary += 'K'
          secondary += 'J'
          index += 2

          break
        }

        // -ger-, -gy-.
        if (
          (normalized.slice(index + 1, index + 3) === 'ER' &&
            previous !== 'I' &&
            previous !== 'E' &&
            !initialAngerException.test(normalized.slice(0, 6))) ||
          (next === 'Y' && !gForKj.test(previous))
        ) {
          primary += 'K'
          secondary += 'J'
          index += 2

          break
        }

        // Italian such as `biaggi`.
        if (
          next === 'E' ||
          next === 'I' ||
          next === 'Y' ||
          ((previous === 'A' || previous === 'O') &&
            next === 'G' &&
            nextnext === 'I')
        ) {
          // Obvious Germanic.
          if (normalized.slice(index + 1, index + 3) === 'ET' || isGermanic) {
            primary += 'K'
            secondary += 'K'
          } else {
            primary += 'J'

            // Always soft if French ending.
            secondary +=
              normalized.slice(index + 1, index + 5) === 'IER ' ? 'J' : 'K'
          }

          index += 2

          break
        }

        if (next === 'G') {
          index++
        }

        index++

        primary += 'K'
        secondary += 'K'

        break
      case 'H':
        // Only keep if first & before vowel or btw. 2 vowels.
        if (vowels.test(next) && (index === 0 || vowels.test(previous))) {
          primary += 'H'
          secondary += 'H'

          index++
        }

        index++

        break
      case 'J':
        // Obvious Spanish, `jose`, `San Jacinto`.
        if (
          normalized.slice(index, index + 4) === 'JOSE' ||
          normalized.slice(0, 4) === 'SAN '
        ) {
          if (
            normalized.slice(0, 4) === 'SAN ' ||
            (index === 0 && characters[index + 4] === ' ')
          ) {
            primary += 'H'
            secondary += 'H'
          } else {
            primary += 'J'
            secondary += 'H'
          }

          index++

          break
        }

        if (
          index === 0
          // Bug: unreachable (see previous statement).
          // && normalized.slice(index, index + 4) !== 'JOSE'.
        ) {
          primary += 'J'

          // Such as `Yankelovich` or `Jankelowicz`.
          secondary += 'A'
          // Spanish pron. of such as `bajador`.
        } else if (
          !isSlavoGermanic &&
          (next === 'A' || next === 'O') &&
          vowels.test(previous)
        ) {
          primary += 'J'
          secondary += 'H'
        } else if (index === last) {
          primary += 'J'
        } else if (
          previous !== 'S' &&
          previous !== 'K' &&
          previous !== 'L' &&
          !jForJException.test(next)
        ) {
          primary += 'J'
          secondary += 'J'
          // It could happen.
        } else if (next === 'J') {
          index++
        }

        index++

        break
      case 'K':
        if (next === 'K') {
          index++
        }

        primary += 'K'
        secondary += 'K'
        index++

        break
      case 'L':
        if (next === 'L') {
          // Spanish such as `cabrillo`, `gallegos`.
          if (
            (index === length - 3 &&
              ((previous === 'A' && nextnext === 'E') ||
                (previous === 'I' &&
                  (nextnext === 'O' || nextnext === 'A')))) ||
            (previous === 'A' &&
              nextnext === 'E' &&
              (characters[last] === 'A' ||
                characters[last] === 'O' ||
                alle.test(normalized.slice(last - 1, length))))
          ) {
            primary += 'L'
            index += 2

            break
          }

          index++
        }

        primary += 'L'
        secondary += 'L'
        index++

        break
      case 'M':
        if (
          next === 'M' ||
          // Such as `dumb`, `thumb`.
          (previous === 'U' &&
            next === 'B' &&
            (index + 1 === last ||
              normalized.slice(index + 2, index + 4) === 'ER'))
        ) {
          index++
        }

        index++
        primary += 'M'
        secondary += 'M'

        break
      case 'N':
        if (next === 'N') {
          index++
        }

        index++
        primary += 'N'
        secondary += 'N'

        break
      case 'Ñ':
        index++
        primary += 'N'
        secondary += 'N'

        break
      case 'P':
        if (next === 'H') {
          primary += 'F'
          secondary += 'F'
          index += 2

          break
        }

        // Also account for `campbell` and `raspberry`.
        subvalue = next

        if (subvalue === 'P' || subvalue === 'B') {
          index++
        }

        index++

        primary += 'P'
        secondary += 'P'

        break
      case 'Q':
        if (next === 'Q') {
          index++
        }

        index++
        primary += 'K'
        secondary += 'K'

        break
      case 'R':
        // French such as `Rogier`, but exclude `Hochmeier`.
        if (
          index === last &&
          !isSlavoGermanic &&
          previous === 'E' &&
          characters[index - 2] === 'I' &&
          characters[index - 4] !== 'M' &&
          characters[index - 3] !== 'E' &&
          characters[index - 3] !== 'A'
        ) {
          secondary += 'R'
        } else {
          primary += 'R'
          secondary += 'R'
        }

        if (next === 'R') {
          index++
        }

        index++

        break
      case 'S':
        // Special cases `island`, `isle`, `carlisle`, `carlysle`.
        if (next === 'L' && (previous === 'I' || previous === 'Y')) {
          index++

          break
        }

        // Special case `sugar-`.
        if (index === 0 && normalized.slice(1, 5) === 'UGAR') {
          primary += 'X'
          secondary += 'S'
          index++

          break
        }

        if (next === 'H') {
          // Germanic.
          if (hForS.test(normalized.slice(index + 1, index + 5))) {
            primary += 'S'
            secondary += 'S'
          } else {
            primary += 'X'
            secondary += 'X'
          }

          index += 2
          break
        }

        if (
          next === 'I' &&
          (nextnext === 'O' || nextnext === 'A')
          // Bug: Already covered by previous branch
          // || normalized.slice(index, index + 4) === 'SIAN'
        ) {
          if (isSlavoGermanic) {
            primary += 'S'
            secondary += 'S'
          } else {
            primary += 'S'
            secondary += 'X'
          }

          index += 3

          break
        }

        // German & Anglicization's, such as `Smith` match `Schmidt`, `snider`
        // match `Schneider`. Also, -sz- in slavic language although in
        // hungarian it is pronounced `s`.
        if (
          next === 'Z' ||
          (index === 0 &&
            (next === 'L' || next === 'M' || next === 'N' || next === 'W'))
        ) {
          primary += 'S'
          secondary += 'X'

          if (next === 'Z') {
            index++
          }

          index++

          break
        }

        if (next === 'C') {
          // Schlesinger's rule.
          if (nextnext === 'H') {
            subvalue = normalized.slice(index + 3, index + 5)

            // Dutch origin, such as `school`, `schooner`.
            if (dutchSch.test(subvalue)) {
              // Such as `schermerhorn`, `schenker`.
              if (subvalue === 'ER' || subvalue === 'EN') {
                primary += 'X'
                secondary += 'SK'
              } else {
                primary += 'SK'
                secondary += 'SK'
              }

              index += 3

              break
            }

            if (
              index === 0 &&
              !vowels.test(characters[3]) &&
              characters[3] !== 'W'
            ) {
              primary += 'X'
              secondary += 'S'
            } else {
              primary += 'X'
              secondary += 'X'
            }

            index += 3

            break
          }

          if (nextnext === 'I' || nextnext === 'E' || nextnext === 'Y') {
            primary += 'S'
            secondary += 'S'
            index += 3
            break
          }

          primary += 'SK'
          secondary += 'SK'
          index += 3

          break
        }

        subvalue = normalized.slice(index - 2, index)

        // French such as `resnais`, `artois`.
        if (index === last && (subvalue === 'AI' || subvalue === 'OI')) {
          secondary += 'S'
        } else {
          primary += 'S'
          secondary += 'S'
        }

        if (
          next === 'S'
          // Bug: already taken care of by `German & Anglicization's` above:
          // || next === 'Z'
        ) {
          index++
        }

        index++

        break
      case 'T':
        if (next === 'I' && nextnext === 'O' && characters[index + 3] === 'N') {
          primary += 'X'
          secondary += 'X'
          index += 3

          break
        }

        subvalue = normalized.slice(index + 1, index + 3)

        if (
          (next === 'I' && nextnext === 'A') ||
          (next === 'C' && nextnext === 'H')
        ) {
          primary += 'X'
          secondary += 'X'
          index += 3

          break
        }

        if (next === 'H' || (next === 'T' && nextnext === 'H')) {
          // Special case `Thomas`, `Thames` or Germanic.
          if (
            isGermanic ||
            ((nextnext === 'O' || nextnext === 'A') &&
              characters[index + 3] === 'M')
          ) {
            primary += 'T'
            secondary += 'T'
          } else {
            primary += '0'
            secondary += 'T'
          }

          index += 2

          break
        }

        if (next === 'T' || next === 'D') {
          index++
        }

        index++
        primary += 'T'
        secondary += 'T'

        break
      case 'V':
        if (next === 'V') {
          index++
        }

        primary += 'F'
        secondary += 'F'
        index++

        break
      case 'W':
        // Can also be in middle of word (as already taken care of for initial).
        if (next === 'R') {
          primary += 'R'
          secondary += 'R'
          index += 2

          break
        }

        if (index === 0) {
          // `Wasserman` should match `Vasserman`.
          if (vowels.test(next)) {
            primary += 'A'
            secondary += 'F'
          } else if (next === 'H') {
            // Need `Uomo` to match `Womo`.
            primary += 'A'
            secondary += 'A'
          }
        }

        // `Arnow` should match `Arnoff`.
        if (
          ((previous === 'E' || previous === 'O') &&
            next === 'S' &&
            nextnext === 'K' &&
            (characters[index + 3] === 'I' || characters[index + 3] === 'Y')) ||
          // Maybe a bug? Shouldn't this be general Germanic?
          normalized.slice(0, 3) === 'SCH' ||
          (index === last && vowels.test(previous))
        ) {
          secondary += 'F'
          index++

          break
        }

        // Polish such as `Filipowicz`.
        if (
          next === 'I' &&
          (nextnext === 'C' || nextnext === 'T') &&
          characters[index + 3] === 'Z'
        ) {
          primary += 'TS'
          secondary += 'FX'
          index += 4

          break
        }

        index++

        break
      case 'X':
        // French such as `breaux`.
        if (
          !(
            index === last &&
            // Bug: IAU and EAU also match by AU
            // (/IAU|EAU/.test(normalized.slice(index - 3, index))) ||
            previous === 'U' &&
            (characters[index - 2] === 'A' || characters[index - 2] === 'O')
          )
        ) {
          primary += 'KS'
          secondary += 'KS'
        }

        if (next === 'C' || next === 'X') {
          index++
        }

        index++

        break
      case 'Z':
        // Chinese pinyin such as `Zhao`.
        if (next === 'H') {
          primary += 'J'
          secondary += 'J'
          index += 2

          break
        } else if (
          (next === 'Z' &&
            (nextnext === 'A' || nextnext === 'I' || nextnext === 'O')) ||
          (isSlavoGermanic && index > 0 && previous !== 'T')
        ) {
          primary += 'S'
          secondary += 'TS'
        } else {
          primary += 'S'
          secondary += 'S'
        }

        if (next === 'Z') {
          index++
        }

        index++

        break
      default:
        index++
    }
  }

  return [primary, secondary]
}