Summary

Add a new method for counting leading non-negative bytes in a byte array, effectively exposing publicly an efficient, intrinsified method we use internally in String-related code to perform quick ASCII tests.

Problem

The internal solution serves the current String implementation well, including various built-in CharsetEncoder/-Decoder implementations. There are however a number of optional decoders and encoders out there that are left out, as well as third-party code operating on ByteBuffers et.c.

Solution

Expose the internal java.lang.StringCoding::countPositives by means of a new method java.util.Arrays::numberOfLeadingPositives.

Specification

The new method takes a naming cue from Integer::numberOfLeadingZeros, while aligning parameter naming and error handling with other methods in java.util.Arrays:

    /**
     * Counts the number of consecutive bytes in the range, starting from
     * {@code fromIndex}, with a value greater than or equal to zero.
     *
     * @implNote this implementation will attempt to optimize this count using
     * platform-specific SIMD instructions, and was implemented to speed up
     * various ASCII tests.
     *
     * @param a the byte array to count leading positive bytes in
     * @param fromIndex the index of the first element, inclusive, to be counted
     * @param toIndex the index of the last element, exclusive, to be counted
     * @return the number of consecutive bytes in the range, starting from
     *         {@code fromIndex}, with a value greater than or equal to zero.
     *
     * @throws IllegalArgumentException if {@code fromIndex > toIndex}
     * @throws ArrayIndexOutOfBoundsException
     *     if {@code fromIndex < 0} or {@code toIndex > a.length}
     * @since 21
     */
    public static int countLeadingNonNegatives(byte[] a, int fromIndex, int toIndex)