neiam /gitgud

defmodule GitGudWeb.AvatarComponent do
  @moduledoc """
  Deterministic identicon-style avatars that pull their colors from the
  active daisyUI theme.

  No image storage. The avatar is an inline SVG whose `fill`s reference
  CSS custom properties (`var(--color-primary)`, etc.) — so switching
  themes recolors every avatar on the page without re-rendering.

  Pattern: 5×5 grid of cells, left half + center driven by 15 bits from
  a SHA-256 of the seed name; right half mirrors the left. Identicon
  shape used by GitHub, picked because it stays legible at small sizes.

  Usage:

      <.avatar name="alice" size="md" />
      <.avatar name={@user.handle} size="lg" alt={@user.handle} />
  """

  use Phoenix.Component

  @sizes %{
    "xs" => "size-4",
    "sm" => "size-6",
    "md" => "size-8",
    "lg" => "size-12",
    "xl" => "size-16",
    "2xl" => "size-24"
  }

  # daisyUI color slots we'll pick from for the foreground. Background
  # is fixed to `base-200` so the avatar reads against any panel.
  @palette ~w(primary secondary accent info success warning)

  attr :name, :string, required: true, doc: "Seed string — handle, repo name, anything stable"
  attr :size, :string, default: "md", values: Map.keys(@sizes)
  attr :alt, :string, default: ""
  attr :class, :string, default: ""

  def avatar(assigns) do
    {fg_color, cells} = render_spec(assigns.name || "?")

    assigns =
      assigns
      |> assign(:size_class, Map.get(@sizes, assigns.size, "size-8"))
      |> assign(:fg_color, fg_color)
      |> assign(:cells, cells)

    ~H"""
    <span
      class={[
        "inline-block overflow-hidden rounded-full bg-base-200 align-middle shrink-0",
        @size_class,
        @class
      ]}
      role="img"
      aria-label={@alt}
    >
      <svg viewBox="0 0 5 5" xmlns="http://www.w3.org/2000/svg" class="w-full h-full block">
        <rect
          :for={{x, y} <- @cells}
          x={x}
          y={y}
          width="1"
          height="1"
          style={"fill: var(--color-" <> @fg_color <> ")"}
        />
      </svg>
    </span>
    """
  end

  @doc """
  For a given seed name, return `{fg_color, cells}` where `fg_color` is
  the chosen daisyUI palette slot (without the `--color-` prefix) and
  `cells` is the list of `{x, y}` grid positions to fill.
  """
  @spec render_spec(String.t()) :: {String.t(), [{0..4, 0..4}]}
  def render_spec(name) when is_binary(name) do
    <<color_byte, bits1, bits2, _rest::binary>> = :crypto.hash(:sha256, name)

    fg = Enum.at(@palette, rem(color_byte, length(@palette)))
    # 15 bits — 5 rows × 3 left/center columns. Take the low 8 of bits1
    # and the low 7 of bits2 as a 15-bit field.
    bit_field = bits1 * 128 + Bitwise.band(bits2, 0x7F)

    cells =
      for row <- 0..4, col <- 0..2, bit?(bit_field, row * 3 + col), reduce: [] do
        acc ->
          left = {col, row}
          mirror = {4 - col, row}
          if left == mirror, do: [left | acc], else: [left, mirror | acc]
      end

    {fg, cells}
  end

  defp bit?(field, position),
    do: Bitwise.band(Bitwise.bsr(field, position), 1) == 1

  @doc false
  def palette, do: @palette

  @doc false
  def sizes, do: @sizes
end