Support for boolean lists in FlatBuffer.

Support for boolean lists in FlatBuffer.

R=paulberry@google.com
BUG=

Committed: https://github.com/dart-lang/sdk/commit/4ca8d2d4a871fc1e3ac04a2f2f828e7875c7c57e

[Paul Berry, 2016-02-29 21:12:32]

The encoding you've chosen (32 bit length followed by bits packed into 32 bits
each) would work, but it's not compatible with the way booleans are encoded
using standard flatbuffers.  Standard flatbuffers use a 32 bit length followed
by unpacked booleans (one boolean per byte, with 0x00 representing false and
0x01 representing true).  So this approach would break compatibility with
standard flatbuffer tools.

Also, I have an efficiency concern: Dart uses native integers for values up to
+/- 2^30, and a slower implementation for anything larger.  Which means that 3/4
of the time when we try to access a bool, we're going to wind up taking the slow
path.

Consider the following alternative implementation:

- Bytes 0-3: the number of bytes that follow.
- Bytes 4 through n-1: packed bools.
- Last byte: the number of padding bits at the end of the array of packed bools.

So, for instance, the array [true, true, true, false, false, false, true, true,
true] (9 bools) would be encoded as:

0x00000003 (3 bytes follow)
0xc7 (first 8 bools)
0x01 (last bool, and 7 padding bits)
0x07 (there were 7 padding bits at the end)

This would have the advantage that:

- The arithmetic necessary to access the bits would operate on very small
values, so it would use native integers.

- When generating the `format.fbs` file, we would encode a boolean vector as
[ubyte], and so standard flatbuffer tools would still consider our data to be
valid.

https://codereview.chromium.org/1747663002/diff/1/pkg/analyzer/lib/src/summar...
File pkg/analyzer/lib/src/summary/flat_buffers.dart (right):

https://codereview.chromium.org/1747663002/diff/1/pkg/analyzer/lib/src/summar...
pkg/analyzer/lib/src/summary/flat_buffers.dart:378: if (hasPartial) {
Change this to `if (values.isNotEmpty)`.  Otherwise when `values.length` is a
nonzero multiple of 32, we won't store the last int.

https://codereview.chromium.org/1747663002/diff/1/pkg/analyzer/test/src/summa...
File pkg/analyzer/test/src/summary/flat_buffers_test.dart (right):

https://codereview.chromium.org/1747663002/diff/1/pkg/analyzer/test/src/summa...
pkg/analyzer/test/src/summary/flat_buffers_test.dart:195: void
test_writeList_ofBool() {
This test only exercises a List<bool> of length 100.  IMHO there are enough
tricky corner cases that we should also test the following lengths:

- 0
- 1
- 31
- 32
- 33
- 63
- 64

[scheglov, 2016-02-29 21:59:54]

PTAL

[Paul Berry, 2016-02-29 22:36:12]

lgtm

https://codereview.chromium.org/1747663002/diff/20001/pkg/analyzer/lib/src/su...
File pkg/analyzer/lib/src/summary/flat_buffers.dart (right):

https://codereview.chromium.org/1747663002/diff/20001/pkg/analyzer/lib/src/su...
pkg/analyzer/lib/src/summary/flat_buffers.dart:359: if (bitLength % 8 == 0) {
Consider replacing this if/else with:

int padding = (-bitLength) % 8;
int byteLength = (bitLength + padding) ~/ 8;

[scheglov, 2016-03-01 02:06:36]

https://codereview.chromium.org/1747663002/diff/20001/pkg/analyzer/lib/src/su...
File pkg/analyzer/lib/src/summary/flat_buffers.dart (right):

https://codereview.chromium.org/1747663002/diff/20001/pkg/analyzer/lib/src/su...
pkg/analyzer/lib/src/summary/flat_buffers.dart:359: if (bitLength % 8 == 0) {
On 2016/02/29 22:36:12, Paul Berry wrote:
> Consider replacing this if/else with:
> 
> int padding = (-bitLength) % 8;
> int byteLength = (bitLength + padding) ~/ 8;

Done.
Thank you!

[scheglov, 2016-03-01 02:08:46]

Description was changed from

==========
Support for boolean lists in FlatBuffer.

R=paulberry@google.com
BUG=
==========

to

==========
Support for boolean lists in FlatBuffer.

R=paulberry@google.com
BUG=

Committed:
https://github.com/dart-lang/sdk/commit/4ca8d2d4a871fc1e3ac04a2f2f828e7875c7c57e
==========

[scheglov, 2016-03-01 02:08:48]

Committed patchset #2 (id:20001) manually as
4ca8d2d4a871fc1e3ac04a2f2f828e7875c7c57e (presubmit successful).