very fast text "highlighting" using substring and charCodeAt

fragment.js

const content = document.querySelector('#editor').value
const output = document.querySelector('#maze')

output.innerHTML = ''

if (content == '')
  return

let  t0 = performance.now()
let offsets = []

for (let word of this.db) {
  let startIndex = 0, index
  const len = word.length
  const search = word.toLowerCase()
  const str = content.toLowerCase()

  while ((index = str.indexOf(search, startIndex)) > -1) {
    let left  = str.charCodeAt(index-1)
    let right = str.charCodeAt(index+search.length)

    // this might break in very surprising ways...
    if   ((index === 0 || left < 33) && (right < 33 || index+search.length === str.length))
      offsets.push([index, search])

    startIndex = index + len
  }
}

if (offsets.length === 0) {
  output.innerHTML = content
  return
}

// console.log(JSON.stringify(offsets))

offsets = offsets.sort((a, b) => ((parseInt(a[0]) > parseInt(b[0])) ? 1 : -1))
output.appendChild(document.createTextNode(content.substring(0, offsets[0][0])))

let bound = offsets.length
for (let pair = 0; pair < bound; pair++) {
  const word_start  = offsets[pair][0]
  const word_length = offsets[pair][0] + offsets[pair][1].length
  const next_pair   = offsets[parseInt(pair) + 1] ? offsets[parseInt(pair) + 1][0] : -1
  let rem

  const a = document.createElement("a")
  a.appendChild(document.createTextNode(content.substring(word_start, word_length)))
  a.href = '#/' + offsets[pair][1]
  output.appendChild(a)

  if (next_pair !== -1) {
    rem = content.substring(word_length, next_pair)
  }
  else {
    rem = content.substring(word_length)
  }

  output.appendChild(document.createTextNode(rem))
}

let t1 = performance.now()
console.log("took " + (t1 - t0) + " milliseconds.")