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package jdk.nashorn.internal.runtime.linker;

/**

 * <p>

 * Implements the name mangling and demangling as specified by John Rose's

 * <a href="https://blogs.oracle.com/jrose/entry/symbolic_freedom_in_the_vm"

 * target="_blank">"Symbolic Freedom in the VM"</a> article. Normally, you would

 * mangle the names in the call sites as you're generating bytecode, and then

 * demangle them when you receive them in bootstrap methods.

 * </p>

 * <p>

 * This code is derived from sun.invoke.util.BytecodeName. Apart from subsetting that

 * class, we don't want to create dependency between non-exported package from java.base

 * to nashorn module.

 * </p>

 *

 * <h3>Comment from BytecodeName class reproduced here:</h3>

 *

 * Includes universal mangling rules for the JVM.

 *

 * <h3>Avoiding Dangerous Characters </h3>

 *

 * <p>

 * The JVM defines a very small set of characters which are illegal

 * in name spellings.  We will slightly extend and regularize this set

 * into a group of <cite>dangerous characters</cite>.

 * These characters will then be replaced, in mangled names, by escape sequences.

 * In addition, accidental escape sequences must be further escaped.

 * Finally, a special prefix will be applied if and only if

 * the mangling would otherwise fail to begin with the escape character.

 * This happens to cover the corner case of the null string,

 * and also clearly marks symbols which need demangling.

 * </p>

 * <p>

 * Dangerous characters are the union of all characters forbidden

 * or otherwise restricted by the JVM specification,

 * plus their mates, if they are brackets

 * There is no distinction between type, method, and field names.

 * This makes it easier to convert between mangled names of different

 * types, since they do not need to be decoded (demangled).

 * </p>

 * <p>

 * (also known as reverse solidus).

 * This character is, until now, unheard of in bytecode names,

 * but traditional in the proposed role.

 *

 * </p>

 * <h3> Replacement Characters </h3>

 *

 *

 * <p>

 * Every escape sequence is two characters

 * (in fact, two UTF8 bytes) beginning with

 * the escape character and followed by a

 * <cite>replacement character</cite>.

 * (Since the replacement character is never a backslash,

 * iterated manglings do not double in size.)

 * </p>

 * <p>

 * Each dangerous character has some rough visual similarity

 * to its corresponding replacement character.

 * This makes mangled symbols easier to recognize by sight.

 * </p>

 * <p>

 * The dangerous characters are

 * Their replacements are, respectively,

 * In addition, the replacement character for the escape character itself is

 * and the replacement character for the null prefix is

 * </p>

 * <p>

 * followed by any of these replacement characters

 * is an escape sequence, and there are no other escape sequences.

 * An equal sign is only part of an escape sequence

 * if it is the second character in the whole string, following a backslash.

 * Two consecutive backslashes do <em>not</em> form an escape sequence.

 * </p>

 * <p>

 * Each escape sequence replaces a so-called <cite>original character</cite>

 * which is either one of the dangerous characters or the escape character.

 * A null prefix replaces an initial null string, not a character.

 * </p>

 * <p>

 * All this implies that escape sequences cannot overlap and may be

 * determined all at once for a whole string.  Note that a spelling

 * string can contain <cite>accidental escapes</cite>, apparent escape

 * sequences which must not be interpreted as manglings.

 * These are disabled by replacing their leading backslash with an

 * are required, though they may be carried out in one pass:

 * </p>

 * <ol>

 *   <li>In each accidental escape, replace the backslash with an escape sequence

 *   <li>Replace each dangerous character with an escape sequence

 *   <li>If the first two steps introduced any change, <em>and</em>

 * </ol>

 *

 * To demangle a mangled string that begins with an escape,

 * remove any null prefix, and then replace (in parallel)

 * each escape sequence by its original character.

 * <p>Spelling strings which contain accidental

 * escapes <em>must</em> have them replaced, even if those

 * strings do not contain dangerous characters.

 * This restriction means that mangling a string always

 * requires a scan of the string for escapes.

 * But then, a scan would be required anyway,

 * to check for dangerous characters.

 *

 * </p>

 * <h3> Nice Properties </h3>

 *

 * <p>

 * If a bytecode name does not contain any escape sequence,

 * demangling is a no-op:  The string demangles to itself.

 * Such a string is called <cite>self-mangling</cite>.

 * Almost all strings are self-mangling.

 * In practice, to demangle almost any name “found in nature”,

 * simply verify that it does not begin with a backslash.

 * </p>

 * <p>

 * Mangling is a one-to-one function, while demangling

 * is a many-to-one function.

 * A mangled string is defined as <cite>validly mangled</cite> if

 * it is in fact the unique mangling of its spelling string.

 * If a language back-end or runtime is using mangled names,

 * it should never present an invalidly mangled bytecode

 * name to the JVM.  If the runtime encounters one,

 * it should also report an error, since such an occurrence

 * probably indicates a bug in name encoding which

 * will lead to errors in linkage.

 * However, this note does not propose that the JVM verifier

 * detect invalidly mangled names.

 * </p>

 * <p>

 * As a result of these rules, it is a simple matter to

 * compute validly mangled substrings and concatenations

 * of validly mangled strings, and (with a little care)

 * these correspond to corresponding operations on their

 * spelling strings.

 * </p>

 * <ul>

 *   <li>Any prefix of a validly mangled string is also validly mangled,

 * although a null prefix may need to be removed.</li>

 *   <li>Any suffix of a validly mangled string is also validly mangled,

 * although a null prefix may need to be added.</li>

 *   <li>Two validly mangled strings, when concatenated,

 * are also validly mangled, although any null prefix

 * must be removed from the second string,

 * and a trailing backslash on the first string may need escaping,

 * if it would participate in an accidental escape when followed

 * by the first character of the second string.</li>

 * </ul>

 * <p>If languages that include non-Java symbol spellings use this

 * mangling convention, they will enjoy the following advantages:

 * </p>

 * <ul>

 *   <li>They can interoperate via symbols they share in common.</li>

 *   <li>Low-level tools, such as backtrace printers, will have readable displays.</li>

 *   <li>Future JVM and language extensions can safely use the dangerous characters

 * for structuring symbols, but will never interfere with valid spellings.</li>

 *   <li>Runtimes and compilers can use standard libraries for mangling and demangling.</li>

 *   <li>Occasional transliterations and name composition will be simple and regular,

 * for classes, methods, and fields.</li>

 *   <li>Bytecode names will continue to be compact.

 * When mangled, spellings will at most double in length, either in

 * UTF8 or UTF16 format, and most will not change at all.</li>

 * </ul>

 *

 *

 * <h3> Suggestions for Human Readable Presentations </h3>

 *

 *

 * <p>

 * For human readable displays of symbols,

 * it will be better to present a string-like quoted

 * representation of the spelling, because JVM users

 * are generally familiar with such tokens.

 * We suggest using single or double quotes before and after

 * mangled symbols which are not valid Java identifiers,

 * with quotes, backslashes, and non-printing characters

 * escaped as if for literals in the Java language.

 * </p>

 * <p>

 * For example, an HTML-like spelling

 * display more cleanly as

 * with the quotes included.

 * Such string-like conventions are <em>not</em> suitable

 * for mangled bytecode names, in part because

 * dangerous characters must be eliminated, rather

 * than just quoted.  Otherwise internally structured

 * strings like package prefixes and method signatures

 * could not be reliably parsed.

 * </p>

 * <p>

 * In such human-readable displays, invalidly mangled

 * names should <em>not</em> be demangled and quoted,

 * for this would be misleading.  Likewise, JVM symbols

 * which contain dangerous characters (like dots in field

 * names or brackets in method names) should not be

 * simply quoted.  The bytecode names

 * and in demangled displays they should be presented as

 * </p>

 */

public final class NameCodec {

    private NameCodec() {

    }

    private static final char ESCAPE_C = '\\';

    // empty escape sequence to avoid a null name or illegal prefix

    private static final char NULL_ESCAPE_C = '=';

    private static final String NULL_ESCAPE = ESCAPE_C+""+NULL_ESCAPE_C;

    /**

     * Canonical encoding for the empty name.

     */

    public static final String EMPTY_NAME =  new String(new char[] { ESCAPE_C, NULL_ESCAPE_C });

    /**

     * Encodes ("mangles") an unencoded symbolic name.

     * @param name the symbolic name to mangle

     * @return the mangled form of the symbolic name.

     */

    public static String encode(final String name) {

        final String bn = mangle(name);

        assert((Object)bn == name || looksMangled(bn)) : bn;

        assert(name.equals(decode(bn))) : name;

        return bn;

    }

    /**

     * Decodes ("demangles") an encoded symbolic name.

     * @param name the symbolic name to demangle

     * @return the demangled form of the symbolic name.

     */

    public static String decode(final String name) {

        String sn = name;

        if (!sn.isEmpty() && looksMangled(name)) {

            sn = demangle(name);

            assert(name.equals(mangle(sn))) : name+" => "+sn+" => "+mangle(sn);

        }

        return sn;

    }

    private static boolean looksMangled(final String s) {

        return s.charAt(0) == ESCAPE_C;

    }

    private static String mangle(final String s) {

        if (s.length() == 0)

            return NULL_ESCAPE;

        // build this lazily, when we first need an escape:

        StringBuilder sb = null;

        for (int i = 0, slen = s.length(); i < slen; i++) {

            final char c = s.charAt(i);

            boolean needEscape = false;

            if (c == ESCAPE_C) {

                if (i+1 < slen) {

                    final char c1 = s.charAt(i+1);

                    if ((i == 0 && c1 == NULL_ESCAPE_C)

                        || c1 != originalOfReplacement(c1)) {

                        // an accidental escape

                        needEscape = true;

                    }

                }

            } else {

                needEscape = isDangerous(c);

            }

            if (!needEscape) {

                if (sb != null)  sb.append(c);

                continue;

            }

            // build sb if this is the first escape

            if (sb == null) {

                sb = new StringBuilder(s.length()+10);

                // mangled names must begin with a backslash:

                if (s.charAt(0) != ESCAPE_C && i > 0)

                    sb.append(NULL_ESCAPE);

                // append the string so far, which is unremarkable:

                sb.append(s, 0, i);

            }

            // rewrite \ to \-, / to \|, etc.

            sb.append(ESCAPE_C);

            sb.append(replacementOf(c));

        }

        if (sb != null)   return sb.toString();

        return s;

    }

    private static String demangle(final String s) {

        // build this lazily, when we first meet an escape:

        StringBuilder sb = null;

        int stringStart = 0;

        if (s.startsWith(NULL_ESCAPE))

            stringStart = 2;

        for (int i = stringStart, slen = s.length(); i < slen; i++) {

            char c = s.charAt(i);

            if (c == ESCAPE_C && i+1 < slen) {

                // might be an escape sequence

                final char rc = s.charAt(i+1);

                final char oc = originalOfReplacement(rc);

                if (oc != rc) {

                    // build sb if this is the first escape

                    if (sb == null) {

                        sb = new StringBuilder(s.length());

                        // append the string so far, which is unremarkable:

                        sb.append(s, stringStart, i);

                    }

                    ++i;  // skip both characters

                    c = oc;

                }

            }

            if (sb != null)

                sb.append(c);

        }

        if (sb != null)   return sb.toString();

        return s.substring(stringStart);

    }

    private static final String DANGEROUS_CHARS   = "\\/.;:$[]<>"; // \\ must be first

    private static final String REPLACEMENT_CHARS =  "-|,?!%{}^_";

    private static final int DANGEROUS_CHAR_FIRST_INDEX = 1; // index after \\

    private static final long[] SPECIAL_BITMAP = new long[2];  // 128 bits

    static {

        final String SPECIAL = DANGEROUS_CHARS + REPLACEMENT_CHARS;

        for (final char c : SPECIAL.toCharArray()) {

            SPECIAL_BITMAP[c >>> 6] |= 1L << c;

        }

    }

    private static boolean isSpecial(final char c) {

        if ((c >>> 6) < SPECIAL_BITMAP.length)

            return ((SPECIAL_BITMAP[c >>> 6] >> c) & 1) != 0;

        else

            return false;

    }

    private static char replacementOf(final char c) {

        if (!isSpecial(c))  return c;

        final int i = DANGEROUS_CHARS.indexOf(c);

        if (i < 0)  return c;

        return REPLACEMENT_CHARS.charAt(i);

    }

    private static char originalOfReplacement(final char c) {

        if (!isSpecial(c))  return c;

        final int i = REPLACEMENT_CHARS.indexOf(c);

        if (i < 0)  return c;

        return DANGEROUS_CHARS.charAt(i);

    }

    private static boolean isDangerous(final char c) {

        if (!isSpecial(c))  return false;

        return (DANGEROUS_CHARS.indexOf(c) >= DANGEROUS_CHAR_FIRST_INDEX);

    }

}