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package org.apache.juneau.svl;

import java.io.*;
import java.util.*;

import org.apache.juneau.svl.vars.*;

/**
 * Utility class for resolving variables of the form <js>"$X{key}"</js> in strings.
 * 
 * <p>
 * Variables are of the form <code>$X{key}</code>, where <code>X</code> can consist of zero or more ASCII characters.
 * <br>The variable key can contain anything, even nested variables that get recursively resolved.
 * 
 * <p>
 * Variables are defined through the {@link VarResolverBuilder#vars(Class[])} method.
 * 
 * <p>
 * The {@link Var} interface defines how variables are converted to values.
 * 
 * <h5 class='section'>Example:</h5>
 * <p class='bcode'>
 *    <jk>public class</jk> SystemPropertiesVar <jk>extends</jk> SimpleVar {
 * 
 *       <jc>// Must have a no-arg constructor!</jc>
 *       <jk>public</jk> SystemPropertiesVar() {
 *          <jk>super</jk>(<js>"S"</js>);
 *       }
 * 
 *       <ja>@Override</ja>
 *       <jk>public</jk> String resolve(VarResolverSession session, String varVal) {
 *          <jk>return</jk> System.<jsm>getProperty</jsm>(varVal);
 *       }
 *    }
 * 
 *    <jc>// Create a variable resolver that resolves system properties (e.g. "$S{java.home}")</jc>
 *    VarResolver r = VarResolver.<jsm>create</jsm>().vars(SystemPropertiesVar.<jk>class</jk>).build();
 * 
 *    <jc>// Use it!</jc>
 *    System.<jsf>out</jsf>.println(r.resolve(<js>"java.home is set to $S{java.home}"</js>));
 * </p>
 * 
 * <h5 class='section'>See Also:</h5>
 * <ul>
 *    <li class='link'><a class="doclink" href="../../../../overview-summary.html#juneau-svl.VarResolvers">Overview &gt; juneau-svl &gt; VarResolvers and VarResolverSessions</a>
 * </ul>
 */
public class VarResolver {

   /**
    * Default string variable resolver with support for system properties and environment variables:
    * 
    * <ul>
    *    <li><code>$S{key[,default]}</code> - {@link SystemPropertiesVar}
    *    <li><code>$E{key[,default]}</code> - {@link EnvVariablesVar}
    *    <li><code>$A{key[,default]}</code> - {@link ArgsVar}
    *    <li><code>$MF{key[,default]}</code> - {@link ManifestFileVar}
    *    <li><code>$IF{arg,then[,else]}</code> - {@link IfVar}
    *    <li><code>$SW{arg,pattern1:then1[,pattern2:then2...]}</code> - {@link SwitchVar}
    *    <li><code>$CO{arg[,arg2...]}</code> - {@link CoalesceVar}
    *    <li><code>$PM{arg,pattern}</code> - {@link PatternMatchVar}
    *    <li><code>$UC{arg}</code> - {@link UpperCaseVar}
    *    <li><code>$LC{arg}</code> - {@link LowerCaseVar}
    *    <li><code>$NE{arg}</code> - {@link NotEmptyVar}
    * </ul>
    * 
    * @see SystemPropertiesVar
    * @see EnvVariablesVar
    */
   public static final VarResolver DEFAULT = new VarResolverBuilder().defaultVars().build();

   final VarResolverContext ctx;

   /**
    * Instantiates a new clean-slate {@link VarResolverBuilder} object.
    * 
    * <p>
    * This is equivalent to simply calling <code><jk>new</jk> VarResolverBuilder()</code>.
    * 
    * @return A new {@link VarResolverBuilder} object.
    */
   public static VarResolverBuilder create() {
      return new VarResolverBuilder();
   }

   /**
    * Constructor.
    * 
    * @param vars The var classes
    * @param contextObjects
    */
   VarResolver(Class<? extends Var>[] vars, Map<String,Object> contextObjects) {
      this.ctx = new VarResolverContext(vars, contextObjects);
   }

   /**
    * Returns a new builder object using the settings in this resolver as a base.
    * 
    * @return A new var resolver builder.
    */
   public VarResolverBuilder builder() {
      return new VarResolverBuilder()
         .vars(ctx.getVars())
         .contextObjects(ctx.getContextObjects());
   }

   /**
    * Returns the read-only properties on this variable resolver.
    * 
    * @return The read-only properties on this variable resolver.
    */
   public VarResolverContext getContext() {
      return ctx;
   }

   /**
    * Creates a new resolver session with no session objects.
    * 
    * <p>
    * Session objects can be associated with the specified session using the {@link VarResolverSession#sessionObject(String, Object)}
    * method.
    * 
    * @return A new resolver session.
    */
   public VarResolverSession createSession() {
      return new VarResolverSession(ctx, null);
   }

   /**
    * Same as {@link #createSession()} except allows you to specify session objects as a map.
    * 
    * @param sessionObjects The session objects to associate with the session.
    * @return A new resolver session.
    */
   public VarResolverSession createSession(Map<String,Object> sessionObjects) {
      return new VarResolverSession(ctx, sessionObjects);
   }

   /**
    * Resolve variables in the specified string.
    * 
    * <p>
    * This is a shortcut for calling <code>createSession(<jk>null</jk>).resolve(s);</code>.
    * <br>This method can only be used if the string doesn't contain variables that rely on the existence of session
    * variables.
    * 
    * @param s The input string.
    * @return The string with variables resolved, or the same string if it doesn't contain any variables to resolve.
    */
   public String resolve(String s) {
      return createSession(null).resolve(s);
   }

   /**
    * Resolve variables in the specified string and sends the results to the specified writer.
    * 
    * <p>
    * This is a shortcut for calling <code>createSession(<jk>null</jk>).resolveTo(s, w);</code>.
    * <br>This method can only be used if the string doesn't contain variables that rely on the existence of session
    * variables.
    * 
    * @param s The input string.
    * @param w The writer to send the result to.
    * @throws IOException
    */
   public void resolveTo(String s, Writer w) throws IOException {
      createSession(null).resolveTo(s, w);
   }
}