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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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package jdk.internal.util;

import jdk.internal.vm.annotation.ForceInline;

/// Utilities for string encoding and decoding with the
/// [Modified UTF-8][java.io.DataInput##modified-utf-8] format.
public final class ModifiedUtf {
    /// Maximum number of bytes allowed for a Modified UTF-8 encoded string
    /// in a [java.lang.classfile.constantpool.Utf8Entry] or a hotspot `Symbol`.
    public static final int CONSTANT_POOL_UTF8_MAX_BYTES = 65535;

    private ModifiedUtf() {
    }

    /// Writes a char to the pre-sized modified UTF buffer.
    @ForceInline
    public static int putChar(byte[] buf, int offset, char c) {
        if (c != 0 && c < 0x80) {
            buf[offset++] = (byte) c;
        } else if (c >= 0x800) {
            buf[offset    ] = (byte) (0xE0 | c >> 12 & 0x0F);
            buf[offset + 1] = (byte) (0x80 | c >> 6  & 0x3F);
            buf[offset + 2] = (byte) (0x80 | c       & 0x3F);
            offset += 3;
        } else {
            buf[offset    ] = (byte) (0xC0 | c >> 6 & 0x1F);
            buf[offset + 1] = (byte) (0x80 | c      & 0x3F);
            offset += 2;
        }
        return offset;
    }

    /// Calculate the encoded length of an input String.
    /// For many workloads that have fast paths for ASCII-only prefixes,
    /// [#utfLen(String, int)] skips scanning that prefix.
    ///
    /// @param str input string
    public static long utfLen(String str) {
        return utfLen(str, 0);
    }

    /// Calculate the encoded length of trailing parts of an input String,
    /// after [jdk.internal.access.JavaLangAccess#countNonZeroAscii(String)]
    /// calculates the number of contiguous single-byte characters in the
    /// beginning of the string.
    ///
    /// @param str input string
    /// @param countNonZeroAscii the number of non-zero ascii characters in the
    ///        prefix calculated by JLA.countNonZeroAscii(str)
    @ForceInline
    public static long utfLen(String str, int countNonZeroAscii) {
        long utflen = str.length();
        for (int i = (int)utflen - 1; i >= countNonZeroAscii; i--) {
            int c = str.charAt(i);
            if (c >= 0x80 || c == 0)
                utflen += (c >= 0x800) ? 2L : 1L;
        }
        return utflen;
    }

    /// Checks whether an input String can be encoded in a
    /// [java.lang.classfile.constantpool.Utf8Entry], or represented as a
    /// hotspot `Symbol` (which has the same length limit).
    ///
    /// @param str input string
    @ForceInline
    public static boolean isValidLengthInConstantPool(String str) {
        // Quick approximation: each char can be at most 3 bytes in Modified UTF-8.
        // If the string is short enough, it definitely fits.
        int strLen = str.length();
        if (strLen <= CONSTANT_POOL_UTF8_MAX_BYTES / 3) {
            return true;
        }
        if (strLen > CONSTANT_POOL_UTF8_MAX_BYTES) {
            return false;
        }
        // Check exact Modified UTF-8 length.
        long utfLen = utfLen(str);
        return utfLen <= CONSTANT_POOL_UTF8_MAX_BYTES;
    }
}