Parse accept-language header. Somehow I couldn't find a good enough implementation.

acceptedLanguages.ts

// Adapted from https://github.com/vercel/next.js/blob/dda62b693e3e9cf9fd566085e7372c46c16fa14a/packages/next/src/server/accept-header.ts#L13

interface Selection {
  pos: number
  q: number
  token: string
}

export function acceptedLanguages(rawHeader: string) {
  const header = rawHeader.replace(/[ \t]/g, '')

  const parts = header.split(',')
  const selections: Selection[] = []

  for (let i = 0; i < parts.length; ++i) {
    const part = parts[i]
    if (!part) {
      continue
    }

    const params = part.split(';')
    if (params.length > 2) {
      console.error(`Invalid 'accept-language' header`, rawHeader)
      continue
    }

    const token = params[0].toLowerCase()
    if (!token) {
      console.error(`Invalid 'accept-language' header`, rawHeader)
      continue
    }

    const selection: Selection = { token, pos: i, q: 1 }

    if (params.length === 2) {
      const q = params[1]
      const [key, value] = q.split('=')

      if (!value || (key !== 'q' && key !== 'Q')) {
        console.error(`Invalid 'accept-language' header`, rawHeader)
        continue
      }

      const score = parseFloat(value)
      if (score === 0) {
        continue
      }

      if (Number.isFinite(score) && score <= 1 && score >= 0.001) {
        selection.q = score
      }
    }

    selections.push(selection)
  }

  selections.sort((a, b) => {
    if (b.q !== a.q) {
      return b.q - a.q
    }
    return a.pos - b.pos
  })

  return selections.map((selection) => selection.token)
}