/*
 * Copyright 2016 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.palantir.javaformat.java.javadoc;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Verify.verify;
import static com.google.common.collect.Iterators.peekingIterator;
import static com.palantir.javaformat.java.javadoc.Token.Type.BEGIN_JAVADOC;
import static com.palantir.javaformat.java.javadoc.Token.Type.BLOCKQUOTE_CLOSE_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.BLOCKQUOTE_OPEN_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.BR_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.CODE_CLOSE_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.CODE_OPEN_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.END_JAVADOC;
import static com.palantir.javaformat.java.javadoc.Token.Type.FOOTER_JAVADOC_TAG_START;
import static com.palantir.javaformat.java.javadoc.Token.Type.FORCED_NEWLINE;
import static com.palantir.javaformat.java.javadoc.Token.Type.HEADER_CLOSE_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.HEADER_OPEN_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.HTML_COMMENT;
import static com.palantir.javaformat.java.javadoc.Token.Type.INLINE_TAG_CLOSE;
import static com.palantir.javaformat.java.javadoc.Token.Type.INLINE_TAG_OPEN;
import static com.palantir.javaformat.java.javadoc.Token.Type.LIST_CLOSE_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.LIST_ITEM_CLOSE_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.LIST_ITEM_OPEN_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.LIST_OPEN_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.LITERAL;
import static com.palantir.javaformat.java.javadoc.Token.Type.MOE_BEGIN_STRIP_COMMENT;
import static com.palantir.javaformat.java.javadoc.Token.Type.MOE_END_STRIP_COMMENT;
import static com.palantir.javaformat.java.javadoc.Token.Type.OPTIONAL_LINE_BREAK;
import static com.palantir.javaformat.java.javadoc.Token.Type.PARAGRAPH_CLOSE_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.PARAGRAPH_OPEN_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.PRE_CLOSE_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.PRE_OPEN_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.TABLE_CLOSE_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.TABLE_OPEN_TAG;
import static com.palantir.javaformat.java.javadoc.Token.Type.WHITESPACE;
import static java.lang.String.format;
import static java.util.regex.Pattern.CASE_INSENSITIVE;
import static java.util.regex.Pattern.DOTALL;
import static java.util.regex.Pattern.compile;

import com.google.common.base.CharMatcher;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.PeekingIterator;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Deque;
import java.util.List;
import java.util.regex.Pattern;

/** Lexer for the Javadoc formatter. */
final class JavadocLexer {
    /** Takes a Javadoc comment, including ∕✱✱ and ✱∕, and returns tokens, including ∕✱✱ and ✱∕. */
    static ImmutableList<Token> lex(String input) throws LexException {
        /*
         * TODO(cpovirk): In theory, we should interpret Unicode escapes (yet output them in their
         * original form). This would mean mean everything from an encoded ∕✱✱ to an encoded <pre> tag,
         * so we'll probably never bother.
         */
        input = stripJavadocBeginAndEnd(input);
        input = normalizeLineEndings(input);
        return new JavadocLexer(new CharStream(input)).generateTokens();
    }

    /** The lexer crashes on windows line endings, so for now just normalize to `\n`. */
    // TODO(cushon): use the platform line separator for output
    private static String normalizeLineEndings(String input) {
        return NON_UNIX_LINE_ENDING.matcher(input).replaceAll("\n");
    }

    private static final Pattern NON_UNIX_LINE_ENDING = Pattern.compile("\r\n?");

    private static String stripJavadocBeginAndEnd(String input) {
        /*
         * We do this ahead of time so that the main part of the lexer need not say things like
         * "(?![*]/)" to avoid accidentally swallowing ✱∕ when consuming a newline.
         */
        checkArgument(input.startsWith("/**"), "Missing /**: %s", input);
        checkArgument(input.endsWith("*/") && input.length() > 4, "Missing */: %s", input);
        return input.substring("/**".length(), input.length() - "*/".length());
    }

    private final CharStream input;
    private final NestingCounter braceDepth = new NestingCounter();
    private final NestingCounter preDepth = new NestingCounter();
    private final NestingCounter codeDepth = new NestingCounter();
    private final NestingCounter tableDepth = new NestingCounter();
    private boolean somethingSinceNewline;

    private JavadocLexer(CharStream input) {
        this.input = checkNotNull(input);
    }

    private ImmutableList<Token> generateTokens() throws LexException {
        ImmutableList.Builder<Token> tokens = ImmutableList.builder();

        Token token = new Token(BEGIN_JAVADOC, "/**");
        tokens.add(token);

        while (!input.isExhausted()) {
            token = readToken();
            tokens.add(token);
        }

        checkMatchingTags();

        token = new Token(END_JAVADOC, "*/");
        tokens.add(token);

        ImmutableList<Token> result = tokens.build();
        result = joinAdjacentLiteralsAndAdjacentWhitespace(result);
        result = inferParagraphTags(result);
        result = optionalizeSpacesAfterLinks(result);
        result = deindentPreCodeBlocks(result);
        return result;
    }

    private Token readToken() throws LexException {
        Token.Type type = consumeToken();
        String value = input.readAndResetRecorded();
        return new Token(type, value);
    }

    private Token.Type consumeToken() throws LexException {
        boolean preserveExistingFormatting = preserveExistingFormatting();

        if (input.tryConsumeRegex(NEWLINE_PATTERN)) {
            somethingSinceNewline = false;
            return preserveExistingFormatting ? FORCED_NEWLINE : WHITESPACE;
        } else if (input.tryConsume(" ") || input.tryConsume("\t")) {
            // TODO(cpovirk): How about weird whitespace chars? Ideally we'd distinguish breaking vs. not.
            // Returning LITERAL here prevent us from breaking a <pre> line. For more info, see LITERAL.
            return preserveExistingFormatting ? LITERAL : WHITESPACE;
        }

        /*
         * TODO(cpovirk): Maybe try to detect things like "{@code\n@GwtCompatible}" that aren't intended
         * as tags. But in the most likely case, in which that happens inside <pre>{@code, we have no
         * great options for fixing it.
         * https://github.com/google/google-java-format/issues/7#issuecomment-197383926
         */
        if (!somethingSinceNewline && input.tryConsumeRegex(FOOTER_TAG_PATTERN)) {
            checkMatchingTags();
            somethingSinceNewline = true;
            return FOOTER_JAVADOC_TAG_START;
        }
        somethingSinceNewline = true;

        if (input.tryConsumeRegex(INLINE_TAG_OPEN_PATTERN)) {
            braceDepth.increment();
            return INLINE_TAG_OPEN;
        } else if (input.tryConsume("{")) {
            braceDepth.incrementIfPositive();
            return LITERAL;
        } else if (input.tryConsume("}")) {
            braceDepth.decrementIfPositive();
            return braceDepth.isPositive() ? LITERAL : INLINE_TAG_CLOSE;
        }

        // Inside an inline tag, don't do any HTML interpretation.
        if (braceDepth.isPositive()) {
            verify(input.tryConsumeRegex(LITERAL_PATTERN));
            return LITERAL;
        }

        if (input.tryConsumeRegex(PRE_OPEN_PATTERN)) {
            preDepth.increment();
            return preserveExistingFormatting ? LITERAL : PRE_OPEN_TAG;
        } else if (input.tryConsumeRegex(PRE_CLOSE_PATTERN)) {
            preDepth.decrementIfPositive();
            return preserveExistingFormatting() ? LITERAL : PRE_CLOSE_TAG;
        }

        if (input.tryConsumeRegex(CODE_OPEN_PATTERN)) {
            codeDepth.increment();
            return preserveExistingFormatting ? LITERAL : CODE_OPEN_TAG;
        } else if (input.tryConsumeRegex(CODE_CLOSE_PATTERN)) {
            codeDepth.decrementIfPositive();
            return preserveExistingFormatting() ? LITERAL : CODE_CLOSE_TAG;
        }

        if (input.tryConsumeRegex(TABLE_OPEN_PATTERN)) {
            tableDepth.increment();
            return preserveExistingFormatting ? LITERAL : TABLE_OPEN_TAG;
        } else if (input.tryConsumeRegex(TABLE_CLOSE_PATTERN)) {
            tableDepth.decrementIfPositive();
            return preserveExistingFormatting() ? LITERAL : TABLE_CLOSE_TAG;
        }

        if (preserveExistingFormatting) {
            verify(input.tryConsumeRegex(LITERAL_PATTERN));
            return LITERAL;
        }

        if (input.tryConsumeRegex(PARAGRAPH_OPEN_PATTERN)) {
            return PARAGRAPH_OPEN_TAG;
        } else if (input.tryConsumeRegex(PARAGRAPH_CLOSE_PATTERN)) {
            return PARAGRAPH_CLOSE_TAG;
        } else if (input.tryConsumeRegex(LIST_OPEN_PATTERN)) {
            return LIST_OPEN_TAG;
        } else if (input.tryConsumeRegex(LIST_CLOSE_PATTERN)) {
            return LIST_CLOSE_TAG;
        } else if (input.tryConsumeRegex(LIST_ITEM_OPEN_PATTERN)) {
            return LIST_ITEM_OPEN_TAG;
        } else if (input.tryConsumeRegex(LIST_ITEM_CLOSE_PATTERN)) {
            return LIST_ITEM_CLOSE_TAG;
        } else if (input.tryConsumeRegex(BLOCKQUOTE_OPEN_PATTERN)) {
            return BLOCKQUOTE_OPEN_TAG;
        } else if (input.tryConsumeRegex(BLOCKQUOTE_CLOSE_PATTERN)) {
            return BLOCKQUOTE_CLOSE_TAG;
        } else if (input.tryConsumeRegex(HEADER_OPEN_PATTERN)) {
            return HEADER_OPEN_TAG;
        } else if (input.tryConsumeRegex(HEADER_CLOSE_PATTERN)) {
            return HEADER_CLOSE_TAG;
        } else if (input.tryConsumeRegex(BR_PATTERN)) {
            return BR_TAG;
        } else if (input.tryConsumeRegex(MOE_BEGIN_STRIP_COMMENT_PATTERN)) {
            return MOE_BEGIN_STRIP_COMMENT;
        } else if (input.tryConsumeRegex(MOE_END_STRIP_COMMENT_PATTERN)) {
            return MOE_END_STRIP_COMMENT;
        } else if (input.tryConsumeRegex(HTML_COMMENT_PATTERN)) {
            return HTML_COMMENT;
        } else if (input.tryConsumeRegex(LITERAL_PATTERN)) {
            return LITERAL;
        }
        throw new IllegalStateException();
    }

    private boolean preserveExistingFormatting() {
        return preDepth.isPositive() || tableDepth.isPositive() || codeDepth.isPositive();
    }

    private void checkMatchingTags() throws LexException {
        if (braceDepth.isPositive() || preDepth.isPositive() || tableDepth.isPositive() || codeDepth.isPositive()) {
            throw new LexException();
        }
    }

    /**
     * Join together adjacent literal tokens, and join together adjacent whitespace tokens.
     *
     * <p>For literal tokens, this means something like {@code ["<b>", "foo", "</b>"] => ["<b>foo</b>"]}. See
     * {@link #LITERAL_PATTERN} for discussion of why those tokens are separate to begin with.
     *
     * <p>Whitespace tokens are treated analogously. We don't really "want" to join whitespace tokens, but in the course
     * of joining literals, we incidentally join whitespace, too. We do take advantage of the joining later on: It
     * simplifies {@link #inferParagraphTags}.
     *
     * <p>Note that we do <i>not</i> merge a literal token and a whitespace token together.
     */
    private static ImmutableList<Token> joinAdjacentLiteralsAndAdjacentWhitespace(List<Token> input) {
        /*
         * Note: Our final token is always END_JAVADOC. This saves us some trouble:
         *
         * - Our inner while() doesn't need a hasNext() check.
         *
         * - We don't need to check for leftover accumulated literals after we exit the loop.
         */
        ImmutableList.Builder<Token> output = ImmutableList.builder();
        StringBuilder accumulated = new StringBuilder();
        NestingCounter inlineTagDepth = new NestingCounter();
        boolean lastTagWasInlineTagStart = false;

        for (PeekingIterator<Token> tokens = peekingIterator(input.iterator()); tokens.hasNext(); ) {
            Token.Type nextType = tokens.peek().getType();
            if (nextType == INLINE_TAG_OPEN) {
                inlineTagDepth.increment();
            } else if (nextType == INLINE_TAG_CLOSE) {
                inlineTagDepth.decrementIfPositive();
            }

            if (nextType == INLINE_TAG_OPEN) {
                // This remains true until we hit some whitespace.
                lastTagWasInlineTagStart = true;
            }

            if (nextType == LITERAL || nextType == INLINE_TAG_OPEN || nextType == INLINE_TAG_CLOSE) {
                accumulated.append(tokens.peek().getValue());
                tokens.next();
                continue;
            }

            /*
             * IF we have accumulated some literals to join together (say, "foo<b>bar</b>"), and IF we'll
             * next see whitespace followed by a "@" literal, we need to join that together with the
             * previous literals. That ensures that we won't insert a line break before the "@," turning
             * it into a tag.
             */

            if (accumulated.length() == 0) {
                output.add(tokens.peek());
                tokens.next();
                continue;
            }

            StringBuilder seenWhitespace = new StringBuilder();
            while (tokens.peek().getType() == WHITESPACE) {
                seenWhitespace.append(tokens.next().getValue());
            }

            if (tokens.peek().getType() == LITERAL
                    && (tokens.peek().getValue().startsWith("@") || lastTagWasInlineTagStart)) {
                lastTagWasInlineTagStart = false;
                // OK, we're in the case described above.
                accumulated.append(" ");
                accumulated.append(tokens.peek().getValue());
                tokens.next();
                continue;
            }

            output.add(new Token(LITERAL, accumulated.toString()));
            accumulated.setLength(0);

            if (seenWhitespace.length() > 0) {
                output.add(new Token(WHITESPACE, seenWhitespace.toString()));
            }

            lastTagWasInlineTagStart = false;
            // We have another token coming, possibly of type OTHER. Leave it for the next iteration.
        }

        /*
         * TODO(cpovirk): Another case where we could try to join tokens is if a line ends with
         * /[^ -]-/, as in "non-\nblocking."
         */
        return output.build();
    }

    /**
     * Where the input has two consecutive line breaks between literals, insert a {@code <p>} tag between the literals.
     *
     * <p>This method must be called after {@link #joinAdjacentLiteralsAndAdjacentWhitespace}, as it assumes that
     * adjacent whitespace tokens have already been joined.
     */
    private static ImmutableList<Token> inferParagraphTags(List<Token> input) {
        ImmutableList.Builder<Token> output = ImmutableList.builder();

        for (PeekingIterator<Token> tokens = peekingIterator(input.iterator()); tokens.hasNext(); ) {
            if (tokens.peek().getType() == LITERAL) {
                output.add(tokens.next());

                if (tokens.peek().getType() == WHITESPACE
                        && hasMultipleNewlines(tokens.peek().getValue())) {
                    output.add(tokens.next());

                    if (tokens.peek().getType() == LITERAL) {
                        output.add(new Token(PARAGRAPH_OPEN_TAG, "<p>"));
                    }
                }
            } else {
                // TODO(cpovirk): Or just `continue` from the <p> case and move this out of the `else`?
                output.add(tokens.next());
            }
        }

        return output.build();

        /*
         * Note: We do not want to insert <p> tags inside <pre>. Fortunately, the formatter gets that
         * right without special effort on our part. The reason: Line breaks inside a <pre> section are
         * of type FORCED_NEWLINE rather than WHITESPACE.
         */
    }

    /**
     * Replaces whitespace after a {@code href=...>} token with an "optional link break." This allows us to output
     * either {@code <a href=foo>foo</a>} or {@code <a href=foo>\nfoo</a>}, depending on how much space we have left on
     * the line.
     *
     * <p>This method must be called after {@link #joinAdjacentLiteralsAndAdjacentWhitespace}, as it assumes that
     * adjacent whitespace tokens have already been joined.
     */
    private static ImmutableList<Token> optionalizeSpacesAfterLinks(List<Token> input) {
        ImmutableList.Builder<Token> output = ImmutableList.builder();

        for (PeekingIterator<Token> tokens = peekingIterator(input.iterator()); tokens.hasNext(); ) {
            if (tokens.peek().getType() == LITERAL && tokens.peek().getValue().matches("^href=[^>]*>")) {
                output.add(tokens.next());

                if (tokens.peek().getType() == WHITESPACE) {
                    output.add(new Token(OPTIONAL_LINE_BREAK, tokens.next().getValue()));
                }
            } else {
                output.add(tokens.next());
            }
        }

        return output.build();

        /*
         * Note: We do not want to insert <p> tags inside <pre>. Fortunately, the formatter gets that
         * right without special effort on our part. The reason: Line breaks inside a <pre> section are
         * of type FORCED_NEWLINE rather than WHITESPACE.
         */
    }

    /**
     * Adjust indentation inside `<pre>{@code` blocks.
     *
     * <p>Also trim leading and trailing blank lines, and move the trailing `}` to its own line.
     */
    private static ImmutableList<Token> deindentPreCodeBlocks(List<Token> input) {
        ImmutableList.Builder<Token> output = ImmutableList.builder();
        for (PeekingIterator<Token> tokens = peekingIterator(input.iterator()); tokens.hasNext(); ) {
            if (tokens.peek().getType() != PRE_OPEN_TAG) {
                output.add(tokens.next());
                continue;
            }

            output.add(tokens.next());
            List<Token> initialNewlines = new ArrayList<>();
            while (tokens.hasNext() && tokens.peek().getType() == FORCED_NEWLINE) {
                initialNewlines.add(tokens.next());
            }
            if (tokens.peek().getType() != LITERAL || !tokens.peek().getValue().matches("[ \t]*[{]@code")) {
                output.addAll(initialNewlines);
                output.add(tokens.next());
                continue;
            }

            deindentPreCodeBlock(output, tokens);
        }
        return output.build();
    }

    private static void deindentPreCodeBlock(ImmutableList.Builder<Token> output, PeekingIterator<Token> tokens) {
        Deque<Token> saved = new ArrayDeque<>();
        output.add(new Token(LITERAL, tokens.next().getValue().trim()));
        while (tokens.hasNext() && tokens.peek().getType() != PRE_CLOSE_TAG) {
            Token token = tokens.next();
            saved.addLast(token);
        }
        while (!saved.isEmpty() && saved.peekFirst().getType() == FORCED_NEWLINE) {
            saved.removeFirst();
        }
        while (!saved.isEmpty() && saved.peekLast().getType() == FORCED_NEWLINE) {
            saved.removeLast();
        }
        if (saved.isEmpty()) {
            return;
        }

        // move the trailing `}` to its own line
        Token last = saved.peekLast();
        boolean trailingBrace = false;
        if (last.getType() == LITERAL && last.getValue().endsWith("}")) {
            saved.removeLast();
            if (last.length() > 1) {
                saved.addLast(new Token(
                        LITERAL, last.getValue().substring(0, last.getValue().length() - 1)));
                saved.addLast(new Token(FORCED_NEWLINE, null));
            }
            trailingBrace = true;
        }

        int trim = -1;
        for (Token token : saved) {
            if (token.getType() == LITERAL) {
                int idx = CharMatcher.isNot(' ').indexIn(token.getValue());
                if (idx != -1 && (trim == -1 || idx < trim)) {
                    trim = idx;
                }
            }
        }

        output.add(new Token(FORCED_NEWLINE, "\n"));
        for (Token token : saved) {
            if (token.getType() == LITERAL) {
                output.add(new Token(
                        LITERAL,
                        trim > 0 && token.length() > trim ? token.getValue().substring(trim) : token.getValue()));
            } else {
                output.add(token);
            }
        }

        if (trailingBrace) {
            output.add(new Token(LITERAL, "}"));
        } else {
            output.add(new Token(FORCED_NEWLINE, "\n"));
        }
    }

    private static final CharMatcher NEWLINE = CharMatcher.is('\n');

    private static boolean hasMultipleNewlines(String s) {
        return NEWLINE.countIn(s) > 1;
    }

    /*
     * This also eats any trailing whitespace. We would be smart enough to ignore that, anyway --
     * except in the case of <pre>/<table>, inside which we otherwise leave whitespace intact.
     *
     * We'd remove the trailing whitespace later on (in JavaCommentsHelper.rewrite), but I feel safer
     * stripping it now: It otherwise might confuse our line-length count, which we use for wrapping.
     */
    private static final Pattern NEWLINE_PATTERN = compile("^[ \t]*\n[ \t]*[*]?[ \t]?");

    // We ensure elsewhere that we match this only at the beginning of a line.
    // Only match tags that start with a lowercase letter, to avoid false matches on unescaped
    // annotations inside code blocks.
    // Match "@param <T>" specially in case the <T> is a <P> or other HTML tag we treat specially.
    private static final Pattern FOOTER_TAG_PATTERN = compile("^@(param\\s+<\\w+>|[a-z]\\w*)");
    private static final Pattern MOE_BEGIN_STRIP_COMMENT_PATTERN =
            compile("^<!--\\s*MOE:begin_intracomment_strip\\s*-->");
    private static final Pattern MOE_END_STRIP_COMMENT_PATTERN = compile("^<!--\\s*MOE:end_intracomment_strip\\s*-->");
    private static final Pattern HTML_COMMENT_PATTERN = fullCommentPattern();
    private static final Pattern PRE_OPEN_PATTERN = openTagPattern("pre");
    private static final Pattern PRE_CLOSE_PATTERN = closeTagPattern("pre");
    private static final Pattern CODE_OPEN_PATTERN = openTagPattern("code");
    private static final Pattern CODE_CLOSE_PATTERN = closeTagPattern("code");
    private static final Pattern TABLE_OPEN_PATTERN = openTagPattern("table");
    private static final Pattern TABLE_CLOSE_PATTERN = closeTagPattern("table");
    private static final Pattern LIST_OPEN_PATTERN = openTagPattern("ul|ol|dl");
    private static final Pattern LIST_CLOSE_PATTERN = closeTagPattern("ul|ol|dl");
    private static final Pattern LIST_ITEM_OPEN_PATTERN = openTagPattern("li|dt|dd");
    private static final Pattern LIST_ITEM_CLOSE_PATTERN = closeTagPattern("li|dt|dd");
    private static final Pattern HEADER_OPEN_PATTERN = openTagPattern("h[1-6]");
    private static final Pattern HEADER_CLOSE_PATTERN = closeTagPattern("h[1-6]");
    private static final Pattern PARAGRAPH_OPEN_PATTERN = openTagPattern("p");
    private static final Pattern PARAGRAPH_CLOSE_PATTERN = closeTagPattern("p");
    private static final Pattern BLOCKQUOTE_OPEN_PATTERN = openTagPattern("blockquote");
    private static final Pattern BLOCKQUOTE_CLOSE_PATTERN = closeTagPattern("blockquote");
    private static final Pattern BR_PATTERN = openTagPattern("br");
    private static final Pattern INLINE_TAG_OPEN_PATTERN = compile("^[{]@\\w*");
    /*
     * We exclude < so that we don't swallow following HTML tags. This lets us fix up "foo<p>" (~400
     * hits in Google-internal code). We will join unnecessarily split "words" (like "foo<b>bar</b>")
     * in a later step. There's a similar story for braces. I'm not sure I actually need to exclude @
     * or *. TODO(cpovirk): Try removing them.
     *
     * Thanks to the "rejoin" step in joinAdjacentLiteralsAndAdjacentWhitespace(), we could get away
     * with matching only one character here. That would eliminate the need for the regex entirely.
     * That might be faster or slower than what we do now.
     */
    private static final Pattern LITERAL_PATTERN = compile("^.[^ \t\n@<{}*]*", DOTALL);

    private static Pattern fullCommentPattern() {
        return compile("^<!--.*?-->", DOTALL);
    }

    private static Pattern openTagPattern(String namePattern) {
        return compile(format("^<(?:%s)\\b[^>]*>", namePattern), CASE_INSENSITIVE);
    }

    private static Pattern closeTagPattern(String namePattern) {
        return compile(format("^</(?:%s)\\b[^>]*>", namePattern), CASE_INSENSITIVE);
    }

    static class LexException extends Exception {}
}
