invoice pdf generation

[BRO3886]

Invoice PDF Generation

This document outlines the implementation approaches/thoughts and discussions for adding PDF generation capabilities to flexprice.

Responsbilities

The invoice PDF generation module will be responsible for:

• Generating professional-looking PDF invoices from invoice data
• Storing these PDFs in the cloud
• Updating invoice records with PDF URLs - to provide a button on the frontend for download

Future Scope

• Customisable templates
• Emails?

Questions

• Should we trigger invoice generation when request received from the frontend
• Or, should we trigger invoice generation as a background job when invoices are finalised

Key considerations:

• If we go with option 1, it could take some time to generate
• If we go with option 2, we can re-use temporal for this, we need to think about failure handling
• Option 2 is better for automated emails in the future as well
• Multiple generations should not re-create files
• Background generation should not affect API

Storage

• s3?

[BRO3886]

https://github.com/unidoc/unipdf?tab=readme-ov-file

• seems maintained
• lots of features
• invoice generation is explained here - https://unidoc.io/post/simple-invoices/
• pure go implementation

[nkmishra1997]

Wow this looks interesting

[BRO3886]

repo code is obfuscated, really really difficult to debug, documentation also not extensive for doing things more than simple generations

[BRO3886]

requires license also

Unlicensed copy of UniPDF
- Get a free trial license on https://unidoc.io
2025/03/07 18:34:05 Error generating template 1: unipdf license code required
exit status 1

[BRO3886]

Skipping this for now, would see if any open source solution is available

[BRO3886]

https://zerodha.tech/blog/1-5-million-pdfs-in-25-minutes/

Initially, PDFs were generated from HTML using Puppeteer, which involved spawning headless instances of Chrome. This of course grew to be slow and quite resource-intensive as our volume grew.

gotenberg also spins up a chrome browser

https://github.com/typst/typst

In our benchmarks, Typst performed be 2 to 3 times faster than LaTeX when compiling small files to PDF. For larger documents, such as those with tables extending over thousands of pages, Typst dramatically outperforms lualatex. A 2000-page contract note takes approximately 1 minute to compile with Typst, in stark contrast to lualatex’s 18 minutes.

invoices might not reach this size but the comparison is interesting!

Thus, to fully benefit from S3’s partitioning capabilities, it is crucial to consistently utilize prefixes that have already been partitioned to achieve higher transactions per second (TPS).

I had no idea about this at all

[BRO3886]

https://github.com/angelodlfrtr/go-invoice-generator

to take inspiration from, uses github.com/go-pdf/fpdf

[BRO3886]

fpdf does not seem very scalable for generating custom templates, as required for future use-cases. Everything works on the concept of cells, Cells are added at specific points (default mm).

func (t *BaseTemplate) Generate(pdf *fpdf.Fpdf, inv *models.Invoice) error {
	pdf.AddPage()
	pdf.SetFont(t.options.HeaderFont, "", 24)
	pdf.SetTextColor(0, 0, 0)

	pdf.ImageOptions(t.options.LogoPath, 24, 24, 100, 0, false, fpdf.ImageOptions{}, 0, "")

	pdf.Ln(48)
	pdf.SetFont(t.options.BodyFont, "", 12)
	pdf.MultiCell(0, 18, "Flexprice Inc\n123 Main St,\nAnytown, USA\ninfo@flexprice.io\n+123 456 7890", "", "", false)
	pdf.MoveTo(80-24, 48+28)
	pdf.MultiCell(0, 18, "Nikhil Mishra\n123 Main St,\nAnytown, USA\ndemo-user-v2@flexprice.io\n+123 456 7890", "", "R", false)
	....
}

[BRO3886]

https://github.com/signintech/gopdf

[BRO3886]

ye bhi not too easy for highly customisable templates

[BRO3886]

https://github.com/johnfercher/maroto

uses gofpdf under the hood

[BRO3886]

go (inbuilt) without extra infra → same codebase

mjml:
generators / templates → gotenberg → html template → pdf generate → headless chromium
https://mjml.io/
mjml template file -> golang -> html -> xyz tool to convert to pdf

typst:

• ai generated?
• pre built
• production launch
• how to convert to pdf

gotenberg: html -> pdf

basic customisability
logo
performance oriented

[BRO3886]

https://github.com/ad-si/invoice-maker/blob/main/invoice-maker.typ

[BRO3886]

typst is pretty good, I'm impressed. I particularly like the focus on markdown / code like templating. It feels natural while writing.

We can call functions with params like so - there are also fields like dictionaries (recipient) and arrays (items) in the language

#show: invoice.default-invoice.with(
  invoice-number: "INV-202503-00084",
  issuing-date: "2025-03-05",
  due-date: "2025-04-06",
  amount-due: 120065,
  amount-paid: 0,
  amount-remaining: 120065,
  vat: 0.18,
  biller: (
    website: "flexprice.io",
    name: "Flexprice",
    email: "payments@flexprice.io",
  ),
  recipient: (
    name: "Nikhil Mishra",
    email: "nikhil [at] flexprice.io",
  ),
 items: (
    (
      plan-display-name: "Basic AI API Access",
      display-name: "Monthly Subscription",
      period-start: "2025-03-05",
      period-end: "2025-04-05",
      quantity: 1,
      amount: 49,
    ),
    (
      plan-display-name: "GPT-4 Usage",
      display-name: "5k tokens consumed",
      period-start: "2025-03-05",
      period-end: "2025-04-05",
      quantity: 5000,
      amount: 24,
    ),
    (
      plan-display-name: "Custom Model Training",
      display-name: "One-time service",
      period-start: "2025-03-05",
      period-end: "2025-03-05",
      quantity: 1,
      amount: 16,
    ),
  ),
)

We can also create custom functions. Here are a few helper methods I created:

To get the date object from date string

#let parse-date = (date-str) => {
  let parts = date-str.split("-")
  if parts.len() != 3 {
    panic(
      "Invalid date string: " + date-str + "\n" +
      "Expected format: YYYY-MM-DD"
    )
  }
  datetime(
    year: int(parts.at(0)),
    month: int(parts.at(1)),
    day: int(parts.at(2)),
  )
}

To format the date object into a human readable date

#let format-date = (date) => {
  let month-names = (
    "Jan", "Feb", "Mar", "Apr", "May", "Jun",
    "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
  )
  
  let day = if date.day() < 10 {
    "0" + str(date.day())
  } else {
    str(date.day())
  }
  let month = month-names.at(date.month() - 1)
  let year = str(date.year()).slice(2) // Get last 2 digits
  
  day + " " + month + " " + year
}

To convert bigger numbers: 100000 -> 100,000

#let format-number = (num) => {
  let str-num = str(num)
  let parts = str-num.split(".")
  let integer-part = str(parts.at(0))
  let decimal-part = if parts.len() > 1 { "." + parts.at(1) } else { "" }
  
  // Add commas every 3 digits from the right
  let chars = integer-part.rev().clusters()
  let result = ""
  for (i, c) in chars.enumerate() {
    if calc.rem-euclid(i, 3) == 0 and i != 0 {
      result += ","
    }
    result += c
  }
  
  result.rev() + decimal-part
}

Generative AI support is not that great, even when setting the documentation link on cursor, I had to check the documentation to refer methods for arrays, strings, calculations etc. Claude does an okayish job.

Typst docs are robust - https://typst.app/docs/

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Full invoice.typ template - gist