feat(io): add csv parsing and serialization

lib/src/csv.dart

@@ -0,0 +1,102 @@
+import 'address.dart';
+import 'sheet.dart';
+
+List<List<String>> parseCsv(String input) {
+  final rows = <List<String>>[];
+  var field = StringBuffer();
+  var record = <String>[];
+  var inQuotes = false;
+  var fieldStarted = false;
+  var index = 0;
+
+  while (index < input.length) {
+    final ch = input[index];
+    if (inQuotes) {
+      if (ch == '"') {
+        if (index + 1 < input.length && input[index + 1] == '"') {
+          field.write('"');
+          index += 2;
+          continue;
+        }
+        inQuotes = false;
+        index++;
+        continue;
+      }
+      field.write(ch);
+      index++;
+      continue;
+    }
+
+    switch (ch) {
+      case '"':
+        inQuotes = true;
+        fieldStarted = true;
+        index++;
+      case ',':
+        record.add(field.toString());
+        field = StringBuffer();
+        fieldStarted = false;
+        index++;
+      case '\r':
+        index++;
+      case '\n':
+        record.add(field.toString());
+        rows.add(record);
+        field = StringBuffer();
+        record = <String>[];
+        fieldStarted = false;
+        index++;
+      default:
+        field.write(ch);
+        fieldStarted = true;
+        index++;
+    }
+  }
+
+  if (field.isNotEmpty || record.isNotEmpty || fieldStarted) {
+    record.add(field.toString());
+    rows.add(record);
+  }
+  return rows;
+}
+
+String encodeCsv(List<List<String>> rows) =>
+    rows.map((record) => record.map(_encodeField).join(',')).join('\n');
+
+String _encodeField(String value) {
+  if (value.contains(',') || value.contains('"') || value.contains('\n') || value.contains('\r')) {
+    return '"${value.replaceAll('"', '""')}"';
+  }
+  return value;
+}
+
+void importCsv(Sheet sheet, String input) {
+  final rows = parseCsv(input);
+  for (var rowIndex = 0; rowIndex < rows.length; rowIndex++) {
+    final record = rows[rowIndex];
+    for (var colIndex = 0; colIndex < record.length; colIndex++) {
+      final field = record[colIndex];
+      if (field.isEmpty) continue;
+      sheet.setCell(CellAddress(colIndex, rowIndex), field, recalculate: false);
+    }
+  }
+  sheet.recalculate();
+}
+
+String exportValues(Sheet sheet) => _export(sheet, formulas: false);
+
+String exportFormulas(Sheet sheet) => _export(sheet, formulas: true);
+
+String _export(Sheet sheet, {required bool formulas}) {
+  final dimensions = sheet.dimensions();
+  final rows = <List<String>>[];
+  for (var row = 0; row < dimensions.rows; row++) {
+    final record = <String>[];
+    for (var col = 0; col < dimensions.cols; col++) {
+      final address = CellAddress(col, row);
+      record.add(formulas ? sheet.rawAt(address) : sheet.valueAt(address).display());
+    }
+    rows.add(record);
+  }
+  return encodeCsv(rows);
+}