<!-- livebook:{"app_settings":{"access_type":"public","output_type":"rich","show_source":true,"slug":"github-stars"}} -->

# Github Repo Star History

```elixir
Mix.install([
  {:req, "~> 0.3.9"},
  {:kino, github: "livebook-dev/kino", override: true},
  {:kino_vega_lite, "~> 0.1.7"},
  {:kino_explorer, "~> 0.1.7"}
])
```

## About this notebook

```elixir
Kino.Markdown.new("""
## What is this?

This is a simple example of a [Livebook](https://livebook.dev/?utm_source=github-stars-app&utm_medium=livebook-apps) app that connects to an API, gets some data, and displays that data in a chart.

It uses Github's API to get data about the number of stars of repo had over time and plots that data.

### What are Livebook apps?

A Livebook app is a notebook-based app written in Elixir and using [Livebook](https://livebook.dev/?utm_source=github-stars-app&utm_medium=livebook-apps).
If you want to know more about Livebook apps, read or watch that [blog post](https://news.livebook.dev/deploy-notebooks-as-apps-quality-of-life-upgrades---launch-week-1---day-1-2OTEWI) or [video](https://www.youtube.com/watch?v=q7T6ue7cw1Q).

No HTML or Javascript was written for that app, only Elixir. 😉

You can view the source code by clicking the icon at the top corner.
""")
```

## Github API client

This notebook uses GitHub API to retrieve repository metadata. It needs a personal access token acess the API, otherwise it would easily rit the rate limit for unauthenticated requests.

Here's the URL to generate a Github API token: https://github.com/settings/tokens/new

```elixir
defmodule Github do
  def get_star_dates(repo_name) do
    case Req.get!(base_req(), url: "/repos/#{repo_name}/stargazers?per_page=100") do
      %Req.Response{status: 200, headers: headers} ->
        last_page = get_last_page_number(headers)
        star_dates = concurret_paginate(repo_name, last_page)
        {:ok, star_dates}

      %Req.Response{status: 404, body: body} ->
        {:error, body["message"]}
    end
  end

  defp base_req() do
    Req.new(
      base_url: "https://api.github.com",
      auth: {:bearer, github_token()},
      headers: [
        accept: "application/vnd.github.star+json",
        "X-GitHub-Api-Version": "2022-11-28"
      ]
    )
  end

  defp github_token do
    System.fetch_env!("LB_GITHUB_TOKEN")
  end

  defp get_last_page_number(headers) do
    link_header =
      headers
      |> Enum.find(fn {key, _value} -> key == "link" end)
      |> elem(1)

    last_link =
      link_header
      |> String.split(",")
      |> Enum.map(fn link ->
        [url, rel] = String.split(link, ";")
        [url] = Regex.run(~r/<(.*)>/, url, capture: :all_but_first)
        [_, rel] = String.split(rel, "=")
        rel = String.trim(rel)
        [url, rel]
      end)
      |> Enum.find(fn [_url, rel] -> rel == "\"last\"" end)

    if last_link == nil do
      ""
    else
      [page, _] = last_link

      %{"page_number" => page_number} =
        Regex.named_captures(~r/.*&page=(?<page_number>\d+)/, page)

      String.to_integer(page_number)
    end
  end

  defp concurret_paginate(repo_name, last_page) do
    1..last_page
    |> Task.async_stream(
      fn page ->
        response =
          Req.get!(base_req(), url: "/repos/#{repo_name}/stargazers?per_page=100&page=#{page}")

        if response.status != 200, do: IO.inspect("BAM!")
        parse_star_dates(response.body)
      end,
      max_concurrency: 60
    )
    |> Enum.reduce([], fn {:ok, star_dates}, star_dates_acc ->
      [star_dates | star_dates_acc]
    end)
    |> List.flatten()
  end

  defp parse_star_dates(body) do
    body
    |> Enum.map(fn %{"starred_at" => date} ->
      {:ok, datetime, _} = DateTime.from_iso8601(date)
      DateTime.to_date(datetime)
    end)
  end
end
```

## Data processing

```elixir
process_data = fn star_dates ->
  star_dates
  |> List.flatten()
  |> Enum.group_by(& &1)
  |> Enum.map(fn {date, dates} -> {date, Enum.count(dates)} end)
  |> List.keysort(0, {:asc, Date})
  |> Enum.reduce(%{date: [], stars: []}, fn {date, stars}, data ->
    %{date: dates_acc, stars: stars_acc} = data

    cumulative_stars =
      if List.first(stars_acc) == nil do
        0 + stars
      else
        List.first(stars_acc) + stars
      end

    %{date: [date | dates_acc], stars: [cumulative_stars | stars_acc]}
  end)
end
```

## Chart generation

```elixir
generate_chart = fn data, repo_name ->
  VegaLite.new(
    width: 850,
    height: 550,
    title: [text: "⭐️ Github Stars History for #{repo_name}", font_size: 20]
  )
  |> VegaLite.data_from_values(data, only: ["date", "stars"])
  |> VegaLite.mark(:line, tooltip: true)
  |> VegaLite.encode_field(:x, "date",
    type: :temporal,
    axis: [
      label_expr:
        "[timeFormat(datum.value, '%b'), timeFormat(datum.value, '%m') == '01' ? timeFormat(datum.value, '%Y') : ['']]",
      grid_dash: [
        condition: [test: [field: "value", time_unit: "month", equal: 1], value: []],
        value: [2, 2]
      ],
      tick_dash: [
        condition: [test: [field: "value", time_unit: "month", equal: 1], value: []],
        value: [2, 2]
      ]
    ]
  )
  |> VegaLite.encode_field(:y, "stars", type: :quantitative)
end
```

## UI

```elixir
display_data = fn frame, %{data: data, repo_name: repo_name}, to: origin ->
  chart = generate_chart.(data, repo_name)

  Kino.Frame.render(
    frame,
    Kino.Layout.tabs(
      Chart: chart,
      Table: Explorer.DataFrame.new(data)
    ),
    to: origin
  )
end

display_error = fn frame, message, to: origin ->
  Kino.Frame.render(
    frame,
    Kino.Markdown.new("<p style='font-style:italic; color:red'>#{message}</p"),
    to: origin
  )
end
```

```elixir
form =
  Kino.Control.form(
    [
      repo_name: Kino.Input.text("Github full repo name", default: "livebook-dev/livebook")
    ],
    submit: "Generate Chart"
  )

frame = Kino.Frame.new()

Kino.listen(form, fn %{data: %{repo_name: repo_name}, origin: origin} ->
  Kino.Frame.clear(frame, to: origin)

  if repo_name == "" or repo_name == nil do
    display_error.(frame, "repo can't be blank", to: origin)
  else
    Kino.Frame.append(frame, "Collecting stars from Github API...", to: origin)

    case Github.get_star_dates(repo_name) do
      {:ok, star_dates} ->
        data = process_data.(star_dates)
        display_data.(frame, %{data: data, repo_name: repo_name}, to: origin)

      {:error, "Not Found"} ->
        display_error.(frame, "repo not found", to: origin)
    end
  end
end)

Kino.Layout.grid([form, frame], boxed: true)
```