On Sun, 22 Jan 2023 09:50:21 GMT, Sergey Tsypanov <stsypa...@openjdk.org> wrote:
>> Currently it's O(n) - we do `n` shifts of bytes within `StringBuilder`. This
>> can be reduced to O(1) improving the code like:
>>
>> DateTimeFormatter dtf = new DateTimeFormatterBuilder()
>> .appendLiteral("Date:")
>> .padNext(20, ' ')
>> .append(DateTimeFormatter.ISO_DATE)
>> .toFormatter();
>> String text = dtf.format(LocalDateTime.now());
>
> Sergey Tsypanov has updated the pull request with a new target base due to a
> merge or a rebase. The incremental webrev excludes the unrelated changes
> brought in by the merge/rebase. The pull request contains two additional
> commits since the last revision:
>
> - Merge branch 'master' into dtfb
> - Improve padding of DateTimeFormatter
Changes requested by redestad (Reviewer).
src/java.base/share/classes/java/time/format/DateTimeFormatterBuilder.java line
2603:
> 2601: public boolean format(DateTimePrintContext context,
> StringBuilder buf) {
> 2602: int preLen = buf.length();
> 2603: if (!printerParser.format(context, buf)) {
Non-standard as it may be, this style of using `expr == false` instead of
`!expr` is a style choice by the original author. I would advice against
changing these piecemeal without discussion and agreement that we should
consistently enforce the `!expr` style.
src/java.base/share/classes/java/time/format/DateTimeFormatterBuilder.java line
2611:
> 2609: "Cannot print as output of " + len + " characters
> exceeds pad width of " + padWidth);
> 2610: }
> 2611: buf.insert(preLen, String.valueOf(padChar).repeat(padWidth
> - len));
Have you checked with a microbenchmark that this added allocation can be elided
by JITs and that there's a significant speed-up with your changes? I don't have
the necessary domain expertise to assert anything here but I suspect padding
widths are typically short. Such as 2 or 4 (for day/month and year fields) so a
micro should examine there's no regression for little to no padding. Unlike the
original code you call `insert` even if `padWidth - len == 0`. This might be
optimized away by JITs, but it'd be good to verify which is best.
-------------
PR: https://git.openjdk.org/jdk/pull/12131
