feat: issue #32 markdown editor for TUI

lib/bds/ui/code_editor/highlight.ex

@@ -0,0 +1,72 @@
+defmodule BDS.UI.CodeEditor.Highlight do
+  @moduledoc """
+  Syntax highlighting for the TUI code editor (issue #32).
+
+  Wraps `ExRatatui.CodeBlock.highlight/3` with the editor's language
+  set: base lexing via syntect, plus pure-Elixir overlays for the
+  syntaxes syntect does not know — post macros (`[[gallery names="x"]]`)
+  over the markdown base and Liquid tags (`{{ … }}` / `{% … %}`) over
+  the HTML base. Mirrors the GUI Monaco language registrations in
+  `assets/js/monaco/languages.js`.
+  """
+
+  alias BDS.UI.CodeEditor.Overlay
+  alias ExRatatui.Text.Line
+  alias ExRatatui.Widgets.CodeBlock
+
+  @type language :: :markdown_macros | :liquid | :lua | :text
+
+  @doc """
+  Highlights `content` for `language` using `theme`, returning one
+  `%ExRatatui.Text.Line{}` per source line.
+
+  The returned line count always matches `String.split(content, "\\n")`
+  (syntect drops trailing empty lines, but the textarea's line count is
+  authoritative for the cursor row, so they are padded back). Span
+  contents never contain newline characters.
+  """
+  @spec highlight(String.t(), language(), CodeBlock.theme()) :: [Line.t()]
+  def highlight(content, language, theme) when is_binary(content) do
+    content
+    |> base_highlight(language, theme)
+    |> sanitize_lines()
+    |> pad_lines(source_line_count(content))
+    |> apply_overlay(language)
+  end
+
+  defp base_highlight(content, :markdown_macros, theme),
+    do: ExRatatui.CodeBlock.highlight(content, "markdown", theme)
+
+  defp base_highlight(content, :liquid, theme),
+    do: ExRatatui.CodeBlock.highlight(content, "html", theme)
+
+  defp base_highlight(content, :lua, theme),
+    do: ExRatatui.CodeBlock.highlight(content, "lua", theme)
+
+  defp base_highlight(content, :text, theme),
+    do: ExRatatui.CodeBlock.highlight(content, nil, theme)
+
+  # syntect appends the line terminator to the last span of a line;
+  # spans fed to `ExRatatui.Widgets.Paragraph` must not contain "\n".
+  defp sanitize_lines(lines) do
+    Enum.map(lines, fn %Line{spans: spans} = line ->
+      spans =
+        spans
+        |> Enum.map(&%{&1 | content: String.replace(&1.content, ~r/[\r\n]/, "")})
+        |> Enum.reject(&(&1.content == ""))
+
+      %{line | spans: spans}
+    end)
+  end
+
+  defp source_line_count(content), do: content |> String.split("\n") |> length()
+
+  defp pad_lines(lines, count) when length(lines) >= count, do: lines
+
+  defp pad_lines(lines, count),
+    do: lines ++ List.duplicate(%Line{}, count - length(lines))
+
+  defp apply_overlay(lines, :markdown_macros), do: Enum.map(lines, &Overlay.markdown_macros/1)
+  defp apply_overlay(lines, :liquid), do: Enum.map(lines, &Overlay.liquid/1)
+  defp apply_overlay(lines, _language), do: lines
+end