shields/lib/text-measurer.js

'use strict'

const PDFDocument = require('pdfkit')

class PDFKitTextMeasurer {
  constructor(fontPath, fallbackFontPath) {
    this.document = new PDFDocument({
      size: 'A4',
      layout: 'landscape',
    }).fontSize(11)
    try {
      this.document.font(fontPath)
    } catch (e) {
      if (fallbackFontPath) {
        console.error(
          `Text-width computation may be incorrect. Unable to load font at ${fontPath}. Using fallback font ${fallbackFontPath} instead.`
        )
        this.document.font(fallbackFontPath)
      } else {
        console.error('No fallback font set.')
        throw e
      }
    }
  }

  widthOf(str) {
    return this.document.widthOfString(str)
  }
}

class QuickTextMeasurer {
  constructor(fontPath, fallbackFontPath) {
    this.baseMeasurer = new PDFKitTextMeasurer(fontPath, fallbackFontPath)

    // This will be a Map of characters -> numbers.
    this.characterWidths = new Map()
    // This will be Map of Maps of characters -> numbers.
    this.kerningPairs = new Map()
    this._prepare()
  }

  static printableAsciiCharacters() {
    const printableRange = [32, 126]
    const length = printableRange[1] - printableRange[0] + 1
    return Array.from({ length }, (value, i) => printableRange[0] + i).map(
      charCode => String.fromCharCode(charCode)
    )
  }

  _prepare() {
    const charactersToCache = this.constructor.printableAsciiCharacters()

    charactersToCache.forEach(char => {
      this.characterWidths.set(char, this.baseMeasurer.widthOf(char))
      this.kerningPairs.set(char, new Map())
    })

    charactersToCache.forEach(first => {
      charactersToCache.forEach(second => {
        const individually =
          this.characterWidths.get(first) + this.characterWidths.get(second)
        const asPair = this.baseMeasurer.widthOf(`${first}${second}`)
        const kerningAdjustment = asPair - individually
        this.kerningPairs.get(first).set(second, kerningAdjustment)
      })
    })
  }

  widthOf(str) {
    const { characterWidths, kerningPairs } = this

    let result = 0
    let previous = null
    for (const character of str) {
      if (!characterWidths.has(character)) {
        // Bail.
        return this.baseMeasurer.widthOf(str)
      }

      result += characterWidths.get(character)
      if (previous !== null) {
        result += kerningPairs.get(previous).get(character)
      }

      previous = character
    }
    return result
  }
}

module.exports = {
  PDFKitTextMeasurer,
  QuickTextMeasurer,
}