feat: issue #32 markdown editor for TUI

lib/bds/ui/code_editor/widget.ex

@@ -0,0 +1,205 @@
+defmodule BDS.UI.CodeEditor do
+  @moduledoc """
+  Syntax-highlighted, word-wrapping code editor widget (issue #32).
+
+  The editing engine stays Rust-owned (`ExRatatui.textarea_*` — buffer,
+  cursor, undo/redo, clipboard); this widget only re-renders the buffer
+  every frame through a styling + wrap pipeline:
+
+    1. read the buffer and cursor off the textarea reference
+    2. highlight with `BDS.UI.CodeEditor.Highlight`
+    3. patch the current-line background into the cursor's source line
+    4. cut the span at the cursor column and splice in a cursor-styled
+       cell (the cell-caret rides the paragraph's wrapping)
+    5. compute the vertical scroll offset so the cursor's visual row
+       stays inside the window
+    6. emit a wrapping `ExRatatui.Widgets.Paragraph`
+
+  Fields:
+
+    * `:textarea` — the Rust textarea reference (required)
+    * `:language` — `:markdown_macros | :liquid | :lua | :text`
+    * `:theme` — syntect theme, default `:base16_ocean_dark`
+    * `:wrap` — soft-wrap long lines, default `true`
+    * `:cursor_style` — patch style for the cell under the cursor
+    * `:line_highlight_style` — patch style for the cursor's source line
+    * `:block` — optional `%ExRatatui.Widgets.Block{}` container
+    * `:scroll` — previous `{vertical, horizontal}` offset; the render
+      adjusts it minimally so the cursor stays visible
+
+  There is deliberately no line-number gutter. The wrap math assumes
+  Latin cell widths (1 char = 1 cell) — wide graphemes (CJK, emoji)
+  can scroll slightly off; noted as a TODO, out of scope for now.
+  """
+
+  alias ExRatatui.Style
+  alias ExRatatui.Widgets.Block
+  alias ExRatatui.Widgets.CodeBlock
+
+  @type language :: BDS.UI.CodeEditor.Highlight.language()
+
+  @type t :: %__MODULE__{
+          textarea: reference() | nil,
+          language: language(),
+          theme: CodeBlock.theme(),
+          wrap: boolean(),
+          cursor_style: Style.t(),
+          line_highlight_style: Style.t(),
+          block: Block.t() | nil,
+          scroll: {non_neg_integer(), non_neg_integer()}
+        }
+
+  defstruct textarea: nil,
+            language: :text,
+            theme: :base16_ocean_dark,
+            wrap: true,
+            cursor_style: %Style{bg: :cyan},
+            line_highlight_style: %Style{},
+            block: nil,
+            scroll: {0, 0}
+end
+
+defimpl ExRatatui.Widget, for: BDS.UI.CodeEditor do
+  alias BDS.UI.CodeEditor
+  alias BDS.UI.CodeEditor.Highlight
+  alias BDS.UI.CodeEditor.Overlay
+  alias ExRatatui.Layout.Rect
+  alias ExRatatui.Style
+  alias ExRatatui.Text.Line
+  alias ExRatatui.Text.Span
+  alias ExRatatui.Widgets.Block
+  alias ExRatatui.Widgets.Paragraph
+
+  def render(%CodeEditor{textarea: nil} = editor, %Rect{} = rect) do
+    [{%Paragraph{text: "", wrap: editor.wrap, block: editor.block}, rect}]
+  end
+
+  def render(%CodeEditor{} = editor, %Rect{} = rect) do
+    content = ExRatatui.textarea_get_value(editor.textarea)
+    {row, col} = ExRatatui.textarea_cursor(editor.textarea)
+    source_lines = String.split(content, "\n")
+
+    inner = inner_rect(rect, editor.block)
+    width = max(inner.width, 1)
+    height = max(inner.height, 1)
+
+    lines =
+      content
+      |> Highlight.highlight(editor.language, editor.theme)
+      |> highlight_current_line(row, editor.line_highlight_style)
+      |> splice_cursor(source_lines, row, col, editor)
+
+    top = scroll_top(source_lines, row, col, width, height, elem(editor.scroll, 0))
+
+    [
+      {%Paragraph{
+         text: lines,
+         wrap: editor.wrap,
+         scroll: {top, 0},
+         block: editor.block
+       }, rect}
+    ]
+  end
+
+  defp inner_rect(rect, nil), do: rect
+
+  defp inner_rect(rect, %Block{borders: borders}) do
+    left = if border?(borders, :left), do: 1, else: 0
+    right = if border?(borders, :right), do: 1, else: 0
+    top = if border?(borders, :top), do: 1, else: 0
+    bottom = if border?(borders, :bottom), do: 1, else: 0
+
+    %Rect{
+      x: rect.x + left,
+      y: rect.y + top,
+      width: max(rect.width - left - right, 1),
+      height: max(rect.height - top - bottom, 1)
+    }
+  end
+
+  defp border?(borders, side), do: :all in borders or side in borders
+
+  defp highlight_current_line(lines, row, %Style{} = style) do
+    if style == %Style{} do
+      lines
+    else
+      List.update_at(lines, row, fn %Line{} = line ->
+        %{line | spans: Enum.map(line.spans, &%{&1 | style: Overlay.merge_style(&1.style, style)})}
+      end)
+    end
+  end
+
+  defp splice_cursor(lines, source_lines, row, col, editor) do
+    List.update_at(lines, row, fn %Line{} = line ->
+      if col >= cursor_line_length(source_lines, row) do
+        append_cursor_cell(line, editor)
+      else
+        splice_cursor_cell(line, col, editor.cursor_style)
+      end
+    end)
+  end
+
+  defp cursor_line_length(source_lines, row) do
+    source_lines |> Enum.at(row, "") |> String.length()
+  end
+
+  # Past the last character there is no span to cut into — append a
+  # cursor cell instead. The line highlight bleeds through when the
+  # cursor style leaves the background alone.
+  defp append_cursor_cell(%Line{} = line, editor) do
+    style = Overlay.merge_style(editor.line_highlight_style, editor.cursor_style)
+    %{line | spans: line.spans ++ [%Span{content: " ", style: style}]}
+  end
+
+  defp splice_cursor_cell(%Line{} = line, col, cursor_style) do
+    {new_spans, _offset} =
+      Enum.map_reduce(line.spans, 0, fn span, offset ->
+        len = String.length(span.content)
+
+        if col >= offset and col < offset + len do
+          {cut_span(span, col - offset, cursor_style), offset + len}
+        else
+          {[span], offset + len}
+        end
+      end)
+
+    %{line | spans: List.flatten(new_spans)}
+  end
+
+  defp cut_span(span, at, cursor_style) do
+    [
+      %Span{content: String.slice(span.content, 0, at), style: span.style},
+      %Span{
+        content: String.slice(span.content, at, 1),
+        style: Overlay.merge_style(span.style, cursor_style)
+      },
+      %Span{content: String.slice(span.content, at + 1, String.length(span.content)), style: span.style}
+    ]
+    |> Enum.reject(&(&1.content == ""))
+  end
+
+  # Latin-width assumption (1 char = 1 cell): wide graphemes wrap
+  # differently than counted here. TODO: CJK/emoji cell widths.
+  defp scroll_top(source_lines, row, col, width, height, prev_top) do
+    visual_row =
+      source_lines
+      |> Enum.take(row)
+      |> Enum.map(&visual_rows(&1, width))
+      |> Enum.sum()
+      |> Kernel.+(div(col, width))
+
+    cond do
+      visual_row < prev_top -> visual_row
+      visual_row >= prev_top + height -> visual_row - height + 1
+      true -> prev_top
+    end
+    |> max(0)
+  end
+
+  defp visual_rows(line, width) do
+    line
+    |> String.length()
+    |> max(1)
+    |> then(&div(&1 + width - 1, width))
+  end
+end