feat(model): add sheet and cell model

lib/src/sheet.dart

@@ -0,0 +1,90 @@
+import 'address.dart';
+import 'ast.dart';
+import 'format.dart';
+import 'values.dart';
+import 'workbook.dart';
+
+typedef GlobalCell = (Sheet sheet, CellAddress address);
+
+class Cell {
+  String raw;
+  Expr? formula;
+  CellValue value;
+  NumberFormat? format;
+  Set<GlobalCell> precedents;
+  Set<GlobalCell> dependents;
+
+  Cell()
+      : raw = '',
+        formula = null,
+        value = const BlankValue(),
+        format = null,
+        precedents = <GlobalCell>{},
+        dependents = <GlobalCell>{};
+
+  bool get isFormula => formula != null;
+
+  bool get isOccupied => raw.isNotEmpty || formula != null;
+}
+
+class Sheet {
+  final String name;
+  final Workbook workbook;
+  final Map<CellAddress, Cell> cells = {};
+
+  Sheet.internal(this.name, this.workbook);
+
+  factory Sheet([String name = 'Sheet1']) {
+    final book = Workbook.empty();
+    return book.addSheet(name);
+  }
+
+  Iterable<MapEntry<CellAddress, Cell>> get occupiedCells =>
+      cells.entries.where((entry) => entry.value.isOccupied);
+
+  Cell? cellAt(CellAddress address) => cells[address];
+
+  CellValue valueAt(CellAddress address) => cells[address]?.value ?? const BlankValue();
+
+  CellValue valueOf(String reference) => valueAt(CellAddress.parse(reference));
+
+  String rawAt(CellAddress address) => cells[address]?.raw ?? '';
+
+  String displayAt(CellAddress address) {
+    final cell = cells[address];
+    if (cell == null) return '';
+    final format = cell.format;
+    if (format != null) return format.apply(cell.value);
+    return cell.value.display();
+  }
+
+  NumberFormat? formatAt(CellAddress address) => cells[address]?.format;
+
+  void setFormat(String reference, NumberFormat? format) =>
+      setFormatAt(CellAddress.parse(reference), format);
+
+  void setFormatAt(CellAddress address, NumberFormat? format) {
+    final cell = cells.putIfAbsent(address, Cell.new);
+    cell.format = format;
+  }
+
+  void setValue(String reference, String input) => setCell(CellAddress.parse(reference), input);
+
+  void setCell(CellAddress address, String input, {bool recalculate = true}) {
+    workbook.setCell(this, address, input, recalculate: recalculate);
+  }
+
+  void clear(CellAddress address) => workbook.clearCell(this, address);
+
+  void recalculate() => workbook.recalculate();
+
+  ({int cols, int rows}) dimensions() {
+    var maxCol = -1;
+    var maxRow = -1;
+    for (final entry in occupiedCells) {
+      if (entry.key.col > maxCol) maxCol = entry.key.col;
+      if (entry.key.row > maxRow) maxRow = entry.key.row;
+    }
+    return (cols: maxCol + 1, rows: maxRow + 1);
+  }
+}