@ocelot-consulting/hummus-toc/src/writeLinks.js

An extension for HummusJS to add Table of Contents (including bookmarks).

const {verticalMax, verticalMargin, lineSpacing, leftMarginEnd} = require('../config')
const createLink = require('./createLink')
const annotationArrayKey = 'Annots'

const isFirstLineOnSubsequentPage = (pageSize, pageIndex, idx) => pageIndex > 0 && (idx + 1) % pageSize === 0

const getLinks = (objCtx, parser, tocText, pageIndex, howManyPages, pageSize) => {
  let myPage = 0
  return tocText.reduce((acc, {page}, idx) => {
    const verticalTextStart = verticalMax - verticalMargin
    const verticalOffset = lineSpacing * (idx % pageSize)
    const verticalLineStart = verticalTextStart - (verticalOffset + lineSpacing)
    if (verticalLineStart <= verticalMargin) {
      myPage++
    }
    // only for page of links (calculated by height) currently being worked on
    if (page && pageIndex === myPage) {
      const vertStart = isFirstLineOnSubsequentPage(pageSize, pageIndex, idx) ? verticalTextStart : verticalLineStart
      acc.push(createLink(objCtx, parser.getPageObjectID(page + howManyPages - 1), [leftMarginEnd, vertStart, 505, vertStart + lineSpacing]))
    }
    return acc
  }, [])
}

module.exports = (objCtx, copyCtx, parser, pageIndex, tocText, howManyPages, pageSize) => {
  const pageId = parser.getPageObjectID(pageIndex)
  const pageObject = parser
    .parsePage(pageIndex)
    .getDictionary()
    .toJSObject()

  const links = getLinks(objCtx, parser, tocText, pageIndex, howManyPages, pageSize)

  objCtx.startModifiedIndirectObject(pageId)
  const modifiedPageObject = objCtx.startDictionary()

  Object.getOwnPropertyNames(pageObject).forEach(element => {
    // leave everything besides annotations on TOC pages in tact
    // bookmark annotations are at doc level, so won't be affected
    if (element !== annotationArrayKey) {
      modifiedPageObject.writeKey(element)
      copyCtx.copyDirectObjectAsIs(pageObject[element])
    }
  })

  modifiedPageObject.writeKey(annotationArrayKey)
  objCtx.startArray()

  links.forEach(link => objCtx.writeIndirectObjectReference(link))

  objCtx
    .endArray()
    .endLine()
    .endDictionary(modifiedPageObject)
    .endIndirectObject()
}