/*
 * Copyright (C) 2009 The Guava Authors
 *
 * 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.google.common.escape;

import static com.google.common.base.Preconditions.checkNotNull;

import java.util.HashMap;
import java.util.Map;

import javax.annotation.Nullable;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;

/**
 * Static utility methods pertaining to {@link Escaper} instances.
 *
 * @author Sven Mawson
 * @author David Beaumont
 * @since 15.0
 */
@Beta
@GwtCompatible
public final class Escapers {
	private Escapers() {
	}

	/**
	 * Returns an {@link Escaper} that does no escaping, passing all character data
	 * through unchanged.
	 */
	public static Escaper nullEscaper() {
		return NULL_ESCAPER;
	}

	// An Escaper that efficiently performs no escaping.
	// Extending CharEscaper (instead of Escaper) makes Escapers.compose() easier.
	private static final Escaper NULL_ESCAPER = new CharEscaper() {
		@Override
		public String escape(String string) {
			return checkNotNull(string);
		}

		@Override
		protected char[] escape(char c) {
			// TODO: Fix tests not to call this directly and make it throw an error.
			return null;
		}
	};

	/**
	 * Returns a builder for creating simple, fast escapers. A builder instance can
	 * be reused and each escaper that is created will be a snapshot of the current
	 * builder state. Builders are not thread safe.
	 *
	 * <p>
	 * The initial state of the builder is such that:
	 * <ul>
	 * <li>There are no replacement mappings
	 * <li>
	 * <li>{@code safeMin == Character.MIN_VALUE}</li>
	 * <li>{@code safeMax == Character.MAX_VALUE}</li>
	 * <li>{@code unsafeReplacement == null}</li>
	 * </ul>
	 * <p>
	 * For performance reasons escapers created by this builder are not Unicode
	 * aware and will not validate the well-formedness of their input.
	 */
	public static Builder builder() {
		return new Builder();
	}

	/**
	 * A builder for simple, fast escapers.
	 *
	 * <p>
	 * Typically an escaper needs to deal with the escaping of high valued
	 * characters or code points. In these cases it is necessary to extend either
	 * {@link ArrayBasedCharEscaper} or {@link ArrayBasedUnicodeEscaper} to provide
	 * the desired behavior. However this builder is suitable for creating escapers
	 * that replace a relative small set of characters.
	 *
	 * @author David Beaumont
	 * @since 15.0
	 */
	@Beta
	public static final class Builder {
		private final Map<Character, String> replacementMap = new HashMap<Character, String>();
		private char safeMin = Character.MIN_VALUE;
		private char safeMax = Character.MAX_VALUE;
		private String unsafeReplacement = null;

		// The constructor is exposed via the builder() method above.
		private Builder() {
		}

		/**
		 * Sets the safe range of characters for the escaper. Characters in this range
		 * that have no explicit replacement are considered 'safe' and remain unescaped
		 * in the output. If {@code safeMax < safeMin} then the safe range is empty.
		 *
		 * @param safeMin the lowest 'safe' character
		 * @param safeMax the highest 'safe' character
		 * @return the builder instance
		 */
		public Builder setSafeRange(char safeMin, char safeMax) {
			this.safeMin = safeMin;
			this.safeMax = safeMax;
			return this;
		}

		/**
		 * Sets the replacement string for any characters outside the 'safe' range that
		 * have no explicit replacement. If {@code unsafeReplacement} is {@code null}
		 * then no replacement will occur, if it is {@code ""} then the unsafe
		 * characters are removed from the output.
		 *
		 * @param unsafeReplacement the string to replace unsafe chracters
		 * @return the builder instance
		 */
		public Builder setUnsafeReplacement(@Nullable String unsafeReplacement) {
			this.unsafeReplacement = unsafeReplacement;
			return this;
		}

		/**
		 * Adds a replacement string for the given input character. The specified
		 * character will be replaced by the given string whenever it occurs in the
		 * input, irrespective of whether it lies inside or outside the 'safe' range.
		 *
		 * @param c           the character to be replaced
		 * @param replacement the string to replace the given character
		 * @return the builder instance
		 * @throws NullPointerException if {@code replacement} is null
		 */
		public Builder addEscape(char c, String replacement) {
			checkNotNull(replacement);
			// This can replace an existing character (the builder is re-usable).
			replacementMap.put(c, replacement);
			return this;
		}

		/**
		 * Returns a new escaper based on the current state of the builder.
		 */
		public Escaper build() {
			return new ArrayBasedCharEscaper(replacementMap, safeMin, safeMax) {
				private final char[] replacementChars = unsafeReplacement != null ? unsafeReplacement.toCharArray()
						: null;

				@Override
				protected char[] escapeUnsafe(char c) {
					return replacementChars;
				}
			};
		}
	}

	/**
	 * Returns a {@link UnicodeEscaper} equivalent to the given escaper instance. If
	 * the escaper is already a UnicodeEscaper then it is simply returned, otherwise
	 * it is wrapped in a UnicodeEscaper.
	 *
	 * <p>
	 * When a {@link CharEscaper} escaper is wrapped by this method it acquires
	 * extra behavior with respect to the well-formedness of Unicode character
	 * sequences and will throw {@link IllegalArgumentException} when given bad
	 * input.
	 *
	 * @param escaper the instance to be wrapped
	 * @return a UnicodeEscaper with the same behavior as the given instance
	 * @throws NullPointerException     if escaper is null
	 * @throws IllegalArgumentException if escaper is not a UnicodeEscaper or a
	 *                                  CharEscaper
	 */
	static UnicodeEscaper asUnicodeEscaper(Escaper escaper) {
		checkNotNull(escaper);
		if (escaper instanceof UnicodeEscaper) {
			return (UnicodeEscaper) escaper;
		} else if (escaper instanceof CharEscaper) {
			return wrap((CharEscaper) escaper);
		}
		// In practice this shouldn't happen because it would be very odd not to
		// extend either CharEscaper or UnicodeEscaper for non trivial cases.
		throw new IllegalArgumentException("Cannot create a UnicodeEscaper from: " + escaper.getClass().getName());
	}

	/**
	 * Returns a string that would replace the given character in the specified
	 * escaper, or {@code null} if no replacement should be made. This method is
	 * intended for use in tests through the {@code EscaperAsserts} class;
	 * production users of {@link CharEscaper} should limit themselves to its public
	 * interface.
	 *
	 * @param c the character to escape if necessary
	 * @return the replacement string, or {@code null} if no escaping was needed
	 */
	public static String computeReplacement(CharEscaper escaper, char c) {
		return stringOrNull(escaper.escape(c));
	}

	/**
	 * Returns a string that would replace the given character in the specified
	 * escaper, or {@code null} if no replacement should be made. This method is
	 * intended for use in tests through the {@code EscaperAsserts} class;
	 * production users of {@link UnicodeEscaper} should limit themselves to its
	 * public interface.
	 *
	 * @param cp the Unicode code point to escape if necessary
	 * @return the replacement string, or {@code null} if no escaping was needed
	 */
	public static String computeReplacement(UnicodeEscaper escaper, int cp) {
		return stringOrNull(escaper.escape(cp));
	}

	private static String stringOrNull(char[] in) {
		return (in == null) ? null : new String(in);
	}

	/** Private helper to wrap a CharEscaper as a UnicodeEscaper. */
	private static UnicodeEscaper wrap(final CharEscaper escaper) {
		return new UnicodeEscaper() {
			@Override
			protected char[] escape(int cp) {
				// If a code point maps to a single character, just escape that.
				if (cp < Character.MIN_SUPPLEMENTARY_CODE_POINT) {
					return escaper.escape((char) cp);
				}
				// Convert the code point to a surrogate pair and escape them both.
				// Note: This code path is horribly slow and typically allocates 4 new
				// char[] each time it is invoked. However this avoids any
				// synchronization issues and makes the escaper thread safe.
				char[] surrogateChars = new char[2];
				Character.toChars(cp, surrogateChars, 0);
				char[] hiChars = escaper.escape(surrogateChars[0]);
				char[] loChars = escaper.escape(surrogateChars[1]);

				// If either hiChars or lowChars are non-null, the CharEscaper is trying
				// to escape the characters of a surrogate pair separately. This is
				// uncommon and applies only to escapers that assume UCS-2 rather than
				// UTF-16. See: http://en.wikipedia.org/wiki/UTF-16/UCS-2
				if (hiChars == null && loChars == null) {
					// We expect this to be the common code path for most escapers.
					return null;
				}
				// Combine the characters and/or escaped sequences into a single array.
				int hiCount = hiChars != null ? hiChars.length : 1;
				int loCount = loChars != null ? loChars.length : 1;
				char[] output = new char[hiCount + loCount];
				if (hiChars != null) {
					// TODO: Is this faster than System.arraycopy() for small arrays?
					for (int n = 0; n < hiChars.length; ++n) {
						output[n] = hiChars[n];
					}
				} else {
					output[0] = surrogateChars[0];
				}
				if (loChars != null) {
					for (int n = 0; n < loChars.length; ++n) {
						output[hiCount + n] = loChars[n];
					}
				} else {
					output[hiCount] = surrogateChars[1];
				}
				return output;
			}
		};
	}
}