[samples/ffi] Add bitfield sample

Issue: https://github.com/dart-lang/sdk/issues/38954
Change-Id: I04b2b81d83a19c5ff88d4364256b23594fe2139b
Cq-Include-Trybots: luci.dart.try:vm-ffi-android-debug-arm-try,vm-ffi-android-debug-arm64-try,analyzer-analysis-server-linux-try,analyzer-linux-release-try,front-end-linux-release-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/122762
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>

samples/ffi/sample_ffi_bitfield.dart

@@ -0,0 +1,123 @@
+// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:ffi';
+
+import 'package:ffi/ffi.dart';
+import 'package:expect/expect.dart';
+
+/// typedef struct {
+///     unsigned int bold      : 1;
+///     unsigned int underline : 2;
+///     unsigned int italic    : 1;
+///     unsigned int blink     : 1;
+///     unsigned int reverse   : 1;
+///     unsigned int strike    : 1;
+///     unsigned int font      : 4;
+/// } ScreenCellAttrs;
+class ScreenCellAttrs extends Struct {
+  @Int16()
+  int bits;
+
+  int get bold => getBits(kBoldFieldOffset, kBoldFieldLength);
+  void set bold(int value) =>
+      setBits(kBoldFieldOffset, kBoldFieldLength, value);
+
+  int get underline => getBits(kUnderlineFieldOffset, kUnderlineFieldLength);
+  void set underline(int value) =>
+      setBits(kUnderlineFieldOffset, kUnderlineFieldLength, value);
+
+  int get italic => getBits(kItalicFieldOffset, kItalicFieldLength);
+  void set italic(int value) =>
+      setBits(kItalicFieldOffset, kItalicFieldLength, value);
+
+  int get blink => getBits(kBlinkFieldOffset, kBlinkFieldLength);
+  void set blink(int value) =>
+      setBits(kBlinkFieldOffset, kBlinkFieldLength, value);
+
+  int get reverse => getBits(kReverseFieldOffset, kReverseFieldLength);
+  void set reverse(int value) =>
+      setBits(kReverseFieldOffset, kReverseFieldLength, value);
+
+  int get strike => getBits(kStrikeFieldOffset, kStrikeFieldLength);
+  void set strike(int value) =>
+      setBits(kStrikeFieldOffset, kStrikeFieldLength, value);
+
+  int get font => getBits(kFontFieldOffset, kFontFieldLength);
+  void set font(int value) =>
+      setBits(kFontFieldOffset, kFontFieldLength, value);
+
+  int getBits(int offset, int length) => bits.getBits(offset, length);
+
+  void setBits(int offset, int length, int value) {
+    bits = bits.setBits(offset, length, value);
+  }
+}
+
+const int kBoldFieldOffset = 0;
+const int kBoldFieldLength = 1;
+const int kUnderlineFieldOffset = kBoldFieldOffset + kBoldFieldLength;
+const int kUnderlineFieldLength = 2;
+const int kItalicFieldOffset = kUnderlineFieldOffset + kUnderlineFieldLength;
+const int kItalicFieldLength = 1;
+const int kBlinkFieldOffset = kItalicFieldOffset + kItalicFieldLength;
+const int kBlinkFieldLength = 1;
+const int kReverseFieldOffset = kBlinkFieldOffset + kBlinkFieldLength;
+const int kReverseFieldLength = 1;
+const int kStrikeFieldOffset = kReverseFieldOffset + kReverseFieldLength;
+const int kStrikeFieldLength = 1;
+const int kFontFieldOffset = kStrikeFieldOffset + kStrikeFieldLength;
+const int kFontFieldLength = 4;
+
+/// Extension to use a 64-bit integer as bit field.
+extension IntBitField on int {
+  static int _bitMask(int offset, int lenght) => ((1 << lenght) - 1) << offset;
+
+  /// Read `length` bits at `offset`.
+  ///
+  /// Truncates everything.
+  int getBits(int offset, int length) {
+    final mask = _bitMask(offset, length);
+    return (this & mask) >> offset;
+  }
+
+  /// Returns a new integer value in which `length` bits are overwritten at
+  /// `offset`.
+  ///
+  /// Truncates everything.
+  int setBits(int offset, int length, int value) {
+    final mask = _bitMask(offset, length);
+    return (this & ~mask) | ((value << offset) & mask);
+  }
+}
+
+main() {
+  final p = allocate<ScreenCellAttrs>(count: 3);
+
+  // Zeroes out all fields.
+  p.ref.bits = 0;
+
+  // Set individual fields.
+  p.ref.blink = 0;
+  p.ref.bold = 1;
+  p.ref.font = 15;
+  p.ref.italic = 1;
+  p.ref.reverse = 0;
+  p.ref.strike = 0;
+  p.ref.underline = 2;
+
+  // Read individual fields.
+  print(p.ref.blink);
+  print(p.ref.bold);
+  print(p.ref.font);
+  print(p.ref.italic);
+  print(p.ref.reverse);
+  print(p.ref.strike);
+  print(p.ref.underline);
+
+  // A check for automated testing.
+  Expect.equals(1933, p.ref.bits);
+
+  free(p);
+}

samples/ffi/samples_test.dart

@@ -6,6 +6,7 @@
 //
 // SharedObjects=ffi_test_dynamic_library ffi_test_functions
 
+import 'sample_ffi_bitfield.dart' as sample0;
 import 'sample_ffi_data.dart' as sample1;
 import 'sample_ffi_dynamic_library.dart' as sample2;
 import 'sample_ffi_functions_callbacks.dart' as sample3;
@@ -14,6 +15,7 @@ import 'sample_ffi_functions.dart' as sample5;
 import 'sample_ffi_structs.dart' as sample6;
 
 main() {
+  sample0.main();
   sample1.main();
   sample2.main();
   sample3.main();