RFR: 8197594 - String and character repeat

[Kevin Bourrillion]

Just to add another dimension to this data:  most of the usages of our
repeat method (~75%) are in test code. These tests usually just want any
old test string of a certain length. Repeating a single character is the
obvious way to get that.

Among production code usages (~25%), there are a few roughly equal use
cases: ascii indentation/alignment, redaction, and Martin's expected case
of "drawing" with ASCII symbols, and "other".



On Thu, Feb 15, 2018 at 12:52 PM, Louis Wasserman <lowasser at google.com>
wrote:

> I don't think there's a case for demand to merit having a
> repeat(CharSequence, int) at all.
>
> I did an analysis of usages of Guava's Strings.repeat on Google's
> codebase.  Users might be rolling their own implementations, too, but this
> should be a very good proxy for demand.
>
> StringRepeat_SingleConstantChar = 4.475 K // strings with .length() ==  1
> StringRepeat_SingleConstantCodePoint = 28 // strings with
> .codePointCount(...) == 1
> StringRepeat_MultiCodePointConstant = 1.156 K // constant strings neither
> of the above
> StringRepeat_CharSequenceToString = 2 //
> Strings.repeat(CharSequence.toString(), n)
> StringRepeat_NoneOfTheAbove = 248
>
> Notably, it seems like basically nobody needs to repeat a CharSequence --
> definitely not enough demand to merit the awkwardness of e.g.
> Rope.repeat(n) inheriting a repeat returning a String.
>
> Based on this data, I'd recommend providing one and only one method of this
> type: String.repeat(int).  There's no real advantage to a static
> repeat(char, int) method when the overwhelming majority of these are
> constants: e.g. compare SomeUtilClass.repeat('*', n) versus "*".repeat(n).
> Character.toString(c).repeat(n) isn't a bad workaround if you don't have a
> constant char.  There also isn't much demand for dealing with the code
> point case specially; the String.repeat(int) method seems like it'd handle
> that just fine.
>
> On Thu, Feb 15, 2018 at 11:44 AM Jim Laskey <james.laskey at oracle.com>
> wrote:
>
> >
> >
> > > On Feb 15, 2018, at 3:36 PM, Ivan Gerasimov <ivan.gerasimov at oracle.com
> >
> > wrote:
> > >
> > > Hello!
> > >
> > > The link with the webrev returned 404, but I could find it at this
> > location: http://cr.openjdk.java.net/~jlaskey/8197594/webrev-00/
> > >
> > > A few minor comments:
> > >
> > > 1)
> > >
> > > This check:
> > >
> > > 2992             final long limit = (long)count * 2L;
> > > 2993             if ((long)Integer.MAX_VALUE < limit) {
> > >
> > > can be possibly simplified as
> > > if (count > Integer.MAX_VALUE - count) {
> >
> > Good.
> >
> > >
> > > 2)
> > > Should String repeat(final int codepoint, final int count) be optimized
> > for codepoints that can be represented with a single char?
> > >
> > > E.g. like this:
> > >
> > > public static String repeat(final int codepoint, final int count) {
> > >    return Character.isBmpCodePoint(codepoint))
> > >        ? repeat((char) codepoint, count)
> > >        : (new String(Character.toChars(codepoint))).repeat(count);
> > > }
> >
> > Yes, avoid array allocation.
> >
> > >
> > > 3)
> > > Using long arithmetic can possibly be avoided in the common path of
> > repeat(final int count):
> > >
> > > E.g. like this:
> > >
> > >         if (count < 0) {
> > >             throw new IllegalArgumentException("count is negative, " +
> > count);
> > >         } else if (count == 1) {
> > >             return this;
> > >         } else if (count == 0) {
> > >             return "";
> > > }
> > >         final int len = value.length;
> > >         if (Integer.MAX_VALUE / count < len) {
> > >             throw new IllegalArgumentException(
> > >                     "Resulting string exceeds maximum string length: "
> +
> > ((long)len * (long)count));
> > >         }
> > >         final int limit = count * len;
> >
> > Good.
> >
> > Thank you.
> >
> > >
> > > With kind regards,
> > > Ivan
> > >
> > > On 2/15/18 9:20 AM, Jim Laskey wrote:
> > >> This is a pre-CSR code review [1] for String repeat methods
> > (Enhancement).
> > >>
> > >> The proposal is to introduce four new methods;
> > >>
> > >> 1. public String repeat(final int count)
> > >> 2. public static String repeat(final char ch, final int count)
> > >> 3. public static String repeat(final int codepoint, final int count)
> > >> 4. public static String repeat(final CharSequence seq, final int
> count)
> > >>
> > >> For the sake of transparency, only 1 is necessary, 2-4 are convenience
> > methods.
> > >> In the case of 2, “*”.repeat(10) performs as well as
> String.repeat(‘*’,
> > 10).
> > >> 3 and 4 convert to String before calling 1.
> > >>
> > >> Performance runs with jmh (results as comment in [2]) show that these
> > >> methods are significantly faster that StringBuilder equivalents.
> > >>  - fewer memory allocations
> > >>  - fewer char to byte array conversions
> > >>  - faster pyramid replication vs O(N) copying
> > >>
> > >> I left StringBuilder out of scope. It falls under the category of
> > >> Appendables#append with repeat. A much bigger project.
> > >>
> > >> All comments welcome. Especially around the need for convenience
> > >> methods, the JavaDoc content and expanding the tests.
> > >>
> > >> — Jim
> > >>
> > >> [1] webrev:
> > http://cr.openjdk.java.net//oj/home/jlaskey/8197594/webrev-00
> > >> [2] jbs: https://bugs.openjdk.java.net/browse/JDK-8197594
> > >>
> > >>
> > >
> > > --
> > > With kind regards,
> > > Ivan Gerasimov
> > >
> >
> >
>



-- 
Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at google.com