/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You 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 org.apache.commons.jexl3.internal;
  
  import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.Reader;
  import java.io.StringReader;
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Set;
  
  import org.apache.commons.jexl3.JexlCache;
  import org.apache.commons.jexl3.JexlContext;
  import org.apache.commons.jexl3.JexlException;
  import org.apache.commons.jexl3.JexlInfo;
  import org.apache.commons.jexl3.JexlOptions;
  import org.apache.commons.jexl3.JxltEngine;
  import org.apache.commons.jexl3.parser.ASTJexlScript;
  import org.apache.commons.jexl3.parser.JexlNode;
  import org.apache.commons.jexl3.parser.StringParser;
  import org.apache.commons.logging.Log;
  
  /**
   * A JxltEngine implementation.
   * @since 3.0
   */
  public final class TemplateEngine extends JxltEngine {
      /**
       * Abstract the source fragments, verbatim or immediate typed text blocks.
       */
      static final class Block {
          /** The type of block, verbatim or directive. */
          private final BlockType type;
          /** The block start line info. */
          private final int line;
          /** The actual content. */
          private final String body;
  
          /**
           * Creates a new block.
           * @param theType  the block type
           * @param theLine  the line number
           * @param theBlock the content
           */
          Block(final BlockType theType, final int theLine, final String theBlock) {
              type = theType;
              line = theLine;
              body = theBlock;
          }
  
          /**
           * @return body
           */
          String getBody() {
              return body;
          }
  
          /**
           * @return line
           */
          int getLine() {
              return line;
          }
  
          /**
           * @return type
           */
          BlockType getType() {
              return type;
          }
  
          /**
           * Appends this block string representation to a builder.
           * @param strb   the string builder to append to
           * @param prefix the line prefix (immediate or deferred)
           */
          protected void toString(final StringBuilder strb, final String prefix) {
              if (BlockType.VERBATIM.equals(type)) {
                  strb.append(body);
              } else {
                  final Iterator<CharSequence> lines = readLines(new StringReader(body));
                  while (lines.hasNext()) {
                     strb.append(prefix).append(lines.next());
                 }
             }
         }
     }
     /**
      * The enum capturing the difference between verbatim and code source fragments.
      */
     enum BlockType {
         /** Block is to be output "as is" but may be a unified expression. */
         VERBATIM,
         /** Block is a directive, ie a fragment of JEXL code. */
         DIRECTIVE
     }
     /** A composite unified expression: "... ${...} ... #{...} ...". */
     final class CompositeExpression extends TemplateExpression {
         /** Bit encoded (deferred count > 0) bit 1, (immediate count > 0) bit 0. */
         private final int meta;
         /** The list of sub-expression resulting from parsing. */
         protected final TemplateExpression[] exprs;
 
         /**
          * Creates a composite expression.
          * @param counters counters of expressions per type
          * @param list     the sub-expressions
          * @param src      the source for this expression if any
          */
         CompositeExpression(final int[] counters, final List<TemplateExpression> list, final TemplateExpression src) {
             super(src);
             this.exprs = list.toArray(new TemplateExpression[0]);
             this.meta = (counters[ExpressionType.DEFERRED.getIndex()] > 0 ? 2 : 0)
                     | (counters[ExpressionType.IMMEDIATE.getIndex()] > 0 ? 1 : 0);
         }
 
         @Override
         public StringBuilder asString(final StringBuilder strb) {
             for (final TemplateExpression e : exprs) {
                 e.asString(strb);
             }
             return strb;
         }
 
         @Override
         protected Object evaluate(final Interpreter interpreter) {
             Object value;
             // common case: evaluate all expressions & concatenate them as a string
             final StringBuilder strb = new StringBuilder();
             for (final TemplateExpression expr : exprs) {
                 value = expr.evaluate(interpreter);
                 if (value != null) {
                     strb.append(value.toString());
                 }
             }
             return strb.toString();
         }
 
         @Override
         ExpressionType getType() {
             return ExpressionType.COMPOSITE;
         }
 
         @Override
         public Set<List<String>> getVariables() {
             final Engine.VarCollector collector = jexl.varCollector();
             for (final TemplateExpression expr : exprs) {
                 expr.getVariables(collector);
             }
             return collector.collected();
         }
 
         /**
          * Fills up the list of variables accessed by this unified expression.
          * @param collector the variable collector
          */
         @Override
         protected void getVariables(final Engine.VarCollector collector) {
             for (final TemplateExpression expr : exprs) {
                 expr.getVariables(collector);
             }
         }
 
         @Override
         public boolean isImmediate() {
             // immediate if no deferred
             return (meta & 2) == 0;
         }
 
         @Override
         protected TemplateExpression prepare(final Interpreter interpreter) {
             // if this composite is not its own source, it is already prepared
             if (source != this) {
                 return this;
             }
             // we need to prepare all sub-expressions
             final int size = exprs.length;
             final ExpressionBuilder builder = new ExpressionBuilder(size);
             // tracking whether prepare will return a different expression
             boolean eq = true;
             for (final TemplateExpression expr : exprs) {
                 final TemplateExpression prepared = expr.prepare(interpreter);
                 // add it if not null
                 if (prepared != null) {
                     builder.add(prepared);
                 }
                 // keep track of TemplateExpression equivalence
                 eq &= expr == prepared;
             }
             return eq ? this : builder.build(TemplateEngine.this, this);
         }
     }
     /** A constant unified expression. */
     final class ConstantExpression extends TemplateExpression {
         /** The constant held by this unified expression. */
         private final Object value;
 
         /**
          * Creates a constant unified expression.
          * <p>
          * If the wrapped constant is a string, it is treated
          * as a JEXL strings with respect to escaping.
          * </p>
          * @param val    the constant value
          * @param source the source TemplateExpression if any
          */
         ConstantExpression(final Object val, final TemplateExpression source) {
             super(source);
             if (val == null) {
                 throw new NullPointerException("constant can not be null");
             }
             this.value = val instanceof String
                     ? StringParser.buildTemplate((String) val, false)
                     : val;
         }
 
         @Override
         public StringBuilder asString(final StringBuilder strb) {
             if (value != null) {
                 strb.append(value.toString());
             }
             return strb;
         }
 
         @Override
         protected Object evaluate(final Interpreter interpreter) {
             return value;
         }
 
         @Override
         ExpressionType getType() {
             return ExpressionType.CONSTANT;
         }
     }
     /** A deferred unified expression: #{jexl}. */
     final class DeferredExpression extends JexlBasedExpression {
         /**
          * Creates a deferred unified expression.
          * @param expr   the unified expression as a string
          * @param node   the unified expression as an AST
          * @param source the source unified expression if any
          */
         DeferredExpression(final CharSequence expr, final JexlNode node, final TemplateExpression source) {
             super(expr, node, source);
         }
 
         @Override
         ExpressionType getType() {
             return ExpressionType.DEFERRED;
         }
 
         @Override
         protected void getVariables(final Engine.VarCollector collector) {
             // noop
         }
 
         @Override
         public boolean isImmediate() {
             return false;
         }
 
         @Override
         protected TemplateExpression prepare(final Interpreter interpreter) {
             return new ImmediateExpression(expr, node, source);
         }
     }
     /**
      * A helper class to build expressions.
      * Keeps count of sub-expressions by type.
      */
     static final class ExpressionBuilder {
         /** Per TemplateExpression type counters. */
         private final int[] counts;
         /** The list of expressions. */
         private final List<TemplateExpression> expressions;
 
         /**
          * Creates a builder.
          * @param size the initial TemplateExpression array size
          */
         ExpressionBuilder(final int size) {
             counts = new int[]{0, 0, 0};
             expressions = new ArrayList<>(size <= 0 ? 3 : size);
         }
 
         /**
          * Adds an TemplateExpression to the list of expressions, maintain per-type counts.
          * @param expr the TemplateExpression to add
          */
         void add(final TemplateExpression expr) {
             counts[expr.getType().getIndex()] += 1;
             expressions.add(expr);
         }
 
         /**
          * Builds an TemplateExpression from a source, performs checks.
          * @param el     the unified el instance
          * @param source the source TemplateExpression
          * @return an TemplateExpression
          */
         TemplateExpression build(final TemplateEngine el, final TemplateExpression source) {
             int sum = 0;
             for (final int count : counts) {
                 sum += count;
             }
             if (expressions.size() != sum) {
                 final StringBuilder error = new StringBuilder("parsing algorithm error: ");
                 throw new IllegalStateException(toString(error).toString());
             }
             // if only one sub-expr, no need to create a composite
             if (expressions.size() == 1) {
                 return expressions.get(0);
             }
             return el.new CompositeExpression(counts, expressions, source);
         }
 
         @Override
         public String toString() {
             return toString(new StringBuilder()).toString();
         }
 
         /**
          * Base for to-string.
          * @param error the builder to fill
          * @return the builder
          */
         StringBuilder toString(final StringBuilder error) {
             error.append("exprs{");
             error.append(expressions.size());
             error.append(", constant:");
             error.append(counts[ExpressionType.CONSTANT.getIndex()]);
             error.append(", immediate:");
             error.append(counts[ExpressionType.IMMEDIATE.getIndex()]);
             error.append(", deferred:");
             error.append(counts[ExpressionType.DEFERRED.getIndex()]);
             error.append("}");
             return error;
         }
     }
 
     /**
      * Types of expressions.
      * Each instance carries a counter index per (composite sub-) template expression type.
      * @see ExpressionBuilder
      */
     enum ExpressionType {
         /** Constant TemplateExpression, count index 0. */
         CONSTANT(0),
         /** Immediate TemplateExpression, count index 1. */
         IMMEDIATE(1),
         /** Deferred TemplateExpression, count index 2. */
         DEFERRED(2),
         /** Nested (which are deferred) expressions, count
          * index 2. */
         NESTED(2),
         /** Composite expressions are not counted, index -1. */
         COMPOSITE(-1);
         /** The index in arrays of TemplateExpression counters for composite expressions. */
         private final int index;
 
         /**
          * Creates an ExpressionType.
          * @param idx the index for this type in counters arrays.
          */
         ExpressionType(final int idx) {
             this.index = idx;
         }
 
         /**
          * @return the index member
          */
         int getIndex() {
             return index;
         }
     }
 
     /** An immediate unified expression: ${jexl}. */
     final class ImmediateExpression extends JexlBasedExpression {
         /**
          * Creates an immediate unified expression.
          * @param expr   the unified expression as a string
          * @param node   the unified expression as an AST
          * @param source the source unified expression if any
          */
         ImmediateExpression(final CharSequence expr, final JexlNode node, final TemplateExpression source) {
             super(expr, node, source);
         }
 
         @Override
         ExpressionType getType() {
             return ExpressionType.IMMEDIATE;
         }
 
         @Override
         protected TemplateExpression prepare(final Interpreter interpreter) {
             // evaluate immediate as constant
             final Object value = evaluate(interpreter);
             return value != null ? new ConstantExpression(value, source) : null;
         }
     }
 
     /** The base for JEXL based unified expressions. */
     abstract class JexlBasedExpression extends TemplateExpression {
         /** The JEXL string for this unified expression. */
         protected final CharSequence expr;
         /** The JEXL node for this unified expression. */
         protected final JexlNode node;
 
         /**
          * Creates a JEXL interpretable unified expression.
          * @param theExpr   the unified expression as a string
          * @param theNode   the unified expression as an AST
          * @param theSource the source unified expression if any
          */
         protected JexlBasedExpression(final CharSequence theExpr, final JexlNode theNode, final TemplateExpression theSource) {
             super(theSource);
             this.expr = theExpr;
             this.node = theNode;
         }
 
         @Override
         public StringBuilder asString(final StringBuilder strb) {
             strb.append(isImmediate() ? immediateChar : deferredChar);
             strb.append("{");
             strb.append(expr);
             strb.append("}");
             return strb;
         }
 
         @Override
         protected Object evaluate(final Interpreter interpreter) {
             return interpreter.interpret(node);
         }
 
         @Override
         JexlInfo getInfo() {
             return node.jexlInfo();
         }
 
         @Override
         public Set<List<String>> getVariables() {
             final Engine.VarCollector collector = jexl.varCollector();
             getVariables(collector);
             return collector.collected();
         }
 
         @Override
         protected void getVariables(final Engine.VarCollector collector) {
             jexl.getVariables(node instanceof ASTJexlScript? (ASTJexlScript) node : null, node, collector);
         }
 
         @Override
         protected JexlOptions options(final JexlContext context) {
             return jexl.evalOptions(node instanceof ASTJexlScript? (ASTJexlScript) node : null, context);
         }
     }
 
     /**
      * An immediate unified expression nested into a deferred unified expression.
      * #{...${jexl}...}
      * Note that the deferred syntax is JEXL's.
      */
     final class NestedExpression extends JexlBasedExpression {
         /**
          * Creates a nested unified expression.
          * @param expr   the unified expression as a string
          * @param node   the unified expression as an AST
          * @param source the source unified expression if any
          */
         NestedExpression(final CharSequence expr, final JexlNode node, final TemplateExpression source) {
             super(expr, node, source);
             if (this.source != this) {
                 throw new IllegalArgumentException("Nested TemplateExpression can not have a source");
             }
         }
 
         @Override
         public StringBuilder asString(final StringBuilder strb) {
             strb.append(expr);
             return strb;
         }
 
         @Override
         protected Object evaluate(final Interpreter interpreter) {
             return prepare(interpreter).evaluate(interpreter);
         }
 
         @Override
         ExpressionType getType() {
             return ExpressionType.NESTED;
         }
 
         @Override
         public boolean isImmediate() {
             return false;
         }
 
         @Override
         protected TemplateExpression prepare(final Interpreter interpreter) {
             final String value = interpreter.interpret(node).toString();
             final JexlNode dnode = jexl.parse(node.jexlInfo(), noscript, value, null);
             return new ImmediateExpression(value, dnode, this);
         }
     }
 
     /** The different parsing states. */
     private enum ParseState {
         /** Parsing a constant. */
         CONST,
         /** Parsing after $ . */
         IMMEDIATE0,
         /** Parsing after # . */
         DEFERRED0,
         /** Parsing after ${ . */
         IMMEDIATE1,
         /** Parsing after #{ . */
         DEFERRED1,
         /** Parsing after \ . */
         ESCAPE
     }
 
     /**
      * The abstract base class for all unified expressions, immediate '${...}' and deferred '#{...}'.
      */
     abstract class TemplateExpression implements Expression {
         /** The source of this template expression(see {@link TemplateEngine.TemplateExpression#prepare}). */
         protected final TemplateExpression source;
 
         /**
          * Creates an TemplateExpression.
          * @param src the source TemplateExpression if any
          */
         TemplateExpression(final TemplateExpression src) {
             this.source = src != null ? src : this;
         }
 
         @Override
         public String asString() {
             final StringBuilder strb = new StringBuilder();
             asString(strb);
             return strb.toString();
         }
 
         /**
          * Interprets a sub-expression.
          * @param interpreter a JEXL interpreter
          * @return the result of interpretation
          * @throws JexlException (only for nested and composite)
          */
         protected abstract Object evaluate(Interpreter interpreter);
 
         @Override
         public final Object evaluate(final JexlContext context) {
             return evaluate(context, null, null);
         }
 
         /**
          * Evaluates this expression.
          *
          * @param frame the frame storing parameters and local variables
          * @param context the context storing global variables
          * @param options flags and properties that can alter the evaluation behavior.
          * @return the expression value
          * @throws JexlException
          */
         protected final Object evaluate(final JexlContext context, final Frame frame, final JexlOptions options) {
             try {
                 final TemplateInterpreter.Arguments args = new TemplateInterpreter.Arguments(jexl).context(context)
                         .options(options != null ? options : options(context)).frame(frame);
                 final Interpreter interpreter = jexl.createTemplateInterpreter(args);
                 return evaluate(interpreter);
             } catch (final JexlException xjexl) {
                 final JexlException xuel = createException(xjexl.getInfo(), "evaluate", this, xjexl);
                 if (jexl.isSilent()) {
                     if (logger.isWarnEnabled()) {
                         logger.warn(xuel.getMessage(), xuel.getCause());
                     }
                     return null;
                 }
                 throw xuel;
             }
         }
 
         /** @return the info */
         JexlInfo getInfo() {
             return null;
         }
 
         @Override
         public final TemplateExpression getSource() {
             return source;
         }
 
         /**
          * Gets this TemplateExpression type.
          * @return its type
          */
         abstract ExpressionType getType();
 
         @Override
         public Set<List<String>> getVariables() {
             return Collections.emptySet();
         }
 
         /**
          * Fills up the list of variables accessed by this unified expression.
          * @param collector the variable collector
          */
         protected void getVariables(final Engine.VarCollector collector) {
             // nothing to do
         }
 
         @Override
         public final boolean isDeferred() {
             return !isImmediate();
         }
 
         @Override
         public boolean isImmediate() {
             return true;
         }
 
         /**
          * The options to use during evaluation.
          * @param context the context
          * @return the options
          */
         protected JexlOptions options(final JexlContext context) {
             return jexl.evalOptions(null, context);
         }
 
         /**
          * Prepares a sub-expression for interpretation.
          * @param interpreter a JEXL interpreter
          * @return a prepared unified expression
          * @throws JexlException (only for nested and composite)
          */
         protected TemplateExpression prepare(final Interpreter interpreter) {
             return this;
         }
 
         @Override
         public final TemplateExpression prepare(final JexlContext context) {
                 return prepare(context, null, null);
         }
 
         /**
          * Prepares this expression.
          *
          * @param frame the frame storing parameters and local variables
          * @param context the context storing global variables
          * @param opts flags and properties that can alter the evaluation behavior.
          * @return the expression value
          * @throws JexlException
          */
         protected final TemplateExpression prepare(final JexlContext context, final Frame frame, final JexlOptions opts) {
             try {
                 final JexlOptions interOptions = opts != null ? opts : jexl.evalOptions(context);
                 final Interpreter interpreter = jexl.createInterpreter(context, frame, interOptions);
                 return prepare(interpreter);
             } catch (final JexlException xjexl) {
                 final JexlException xuel = createException(xjexl.getInfo(), "prepare", this, xjexl);
                 if (jexl.isSilent()) {
                     if (logger.isWarnEnabled()) {
                         logger.warn(xuel.getMessage(), xuel.getCause());
                     }
                     return null;
                 }
                 throw xuel;
             }
         }
 
         @Override
         public final String toString() {
             final StringBuilder strb = new StringBuilder();
             asString(strb);
             if (source != this) {
                 strb.append(" /*= ");
                 strb.append(source.toString());
                 strb.append(" */");
             }
             return strb.toString();
         }
 
     }
 
     /**
      * Helper for expression dealing with embedded strings.
      * @param strb the expression buffer to copy characters into
      * @param expr the source
      * @param position the offset into the source
      * @param c the separator character
      * @return the new position to read the source from
      */
     private static int append(final StringBuilder strb, final CharSequence expr, final int position, final char c) {
         strb.append(c);
         if (c != '"' && c != '\'') {
             return position;
         }
         // read thru strings
         final int end = expr.length();
         boolean escape= false;
         int index = position + 1;
         for (; index < end; ++index) {
             final char ec = expr.charAt(index);
             strb.append(ec);
             if (ec == '\\') {
                 escape = !escape;
             } else if (escape) {
                 escape = false;
             } else if (ec == c) {
                 break;
             }
         }
         return index;
     }
 
     /**
      * Creates a JxltEngine.Exception from a JexlException.
      * @param info   the source info
      * @param action createExpression, prepare, evaluate
      * @param expr   the template expression
      * @param xany   the exception
      * @return an exception containing an explicit error message
      */
     static Exception createException(final JexlInfo info,
                                      final String action,
                                      final TemplateExpression expr,
                                      final java.lang.Exception xany) {
         final StringBuilder strb = new StringBuilder("failed to ");
         strb.append(action);
         if (expr != null) {
             strb.append(" '");
             strb.append(expr.toString());
             strb.append("'");
         }
         final Throwable cause = xany.getCause();
         if (cause != null) {
             final String causeMsg = cause.getMessage();
             if (causeMsg != null) {
                 strb.append(", ");
                 strb.append(causeMsg);
             }
         }
         return new Exception(info, strb.toString(), xany);
     }
 
     /**
      * Read lines from a (buffered / mark-able) reader keeping all new-lines and line-feeds.
      * @param reader the reader
      * @return the line iterator
      */
     protected static Iterator<CharSequence> readLines(final Reader reader) {
         if (!reader.markSupported()) {
             throw new IllegalArgumentException("mark support in reader required");
         }
         return new Iterator<CharSequence>() {
             private CharSequence next = doNext();
 
             private CharSequence doNext() {
                 final StringBuilder strb = new StringBuilder(64); // CSOFF: MagicNumber
                 int c;
                 boolean eol = false;
                 try {
                     while ((c = reader.read()) >= 0) {
                         if (eol) {// && (c != '\n' && c != '\r')) {
                             reader.reset();
                             break;
                         }
                         if (c == '\n') {
                             eol = true;
                         }
                         strb.append((char) c);
                         reader.mark(1);
                     }
                 } catch (final IOException xio) {
                     return null;
                 }
                 return strb.length() > 0 ? strb : null;
             }
 
             @Override
             public boolean hasNext() {
                 return next != null;
             }
 
             @Override
             public CharSequence next() {
                 final CharSequence current = next;
                 if (current != null) {
                     next = doNext();
                 }
                 return current;
             }
         };
     }
 
     /** The TemplateExpression cache. */
     final JexlCache<String, TemplateExpression> cache;
 
     /** The JEXL engine instance. */
     final Engine jexl;
 
     /** The logger. */
     final Log logger;
 
     /** The first character for immediate expressions. */
     final char immediateChar;
 
     /** The first character for deferred expressions. */
     final char deferredChar;
 
     /** Whether expressions can use JEXL script or only expressions (ie, no for, var, etc). */
     final boolean noscript;
 
     /**
      * Creates a new instance of {@link JxltEngine} creating a local cache.
      * @param jexl     the JexlEngine to use.
      * @param noScript  whether this engine only allows JEXL expressions or scripts
      * @param cacheSize the number of expressions in this cache, default is 256
      * @param immediate the immediate template expression character, default is '$'
      * @param deferred  the deferred template expression character, default is '#'
      */
     public TemplateEngine(final Engine jexl,
                           final boolean noScript,
                           final int cacheSize,
                           final char immediate,
                           final char deferred) {
         this.jexl = jexl;
         this.logger = jexl.logger;
         this.cache = (JexlCache<String, TemplateExpression>) jexl.cacheFactory.apply(cacheSize);
         immediateChar = immediate;
         deferredChar = deferred;
         noscript = noScript;
     }
 
     /**
      * Clears the cache.
      */
     @Override
     public void clearCache() {
         synchronized (cache) {
             cache.clear();
         }
     }
 
     @Override
     public JxltEngine.Expression createExpression(final JexlInfo jexlInfo, final String expression) {
         final JexlInfo info = jexlInfo == null ?  jexl.createInfo() : jexlInfo;
         Exception xuel = null;
         TemplateExpression stmt = null;
         try {
             stmt = cache.get(expression);
             if (stmt == null) {
                 stmt = parseExpression(info, expression, null);
                 cache.put(expression, stmt);
             }
         } catch (final JexlException xjexl) {
             xuel = new Exception(xjexl.getInfo(), "failed to parse '" + expression + "'", xjexl);
         }
         if (xuel != null) {
             if (!jexl.isSilent()) {
                 throw xuel;
             }
             if (logger.isWarnEnabled()) {
                 logger.warn(xuel.getMessage(), xuel.getCause());
             }
             stmt = null;
         }
         return stmt;
     }
 
     @Override
     public TemplateScript createTemplate(final JexlInfo info, final String prefix, final Reader source, final String... parms) {
         return new TemplateScript(this, info, prefix, source,  parms);
     }
 
     /**
      * @return the deferred character
      */
     char getDeferredChar() {
         return deferredChar;
     }
 
     /**
      * Gets the JexlEngine underlying this JxltEngine.
      * @return the JexlEngine
      */
     @Override
     public Engine getEngine() {
         return jexl;
     }
 
     /**
      * @return the immediate character
      */
     char getImmediateChar() {
         return immediateChar;
     }
 
     /**
      * Parses a unified expression.
      * @param info  the source info
      * @param expr  the string expression
      * @param scope the template scope
      * @return the unified expression instance
      * @throws JexlException if an error occur during parsing
      */
     TemplateExpression parseExpression(final JexlInfo info, final String expr, final Scope scope) {  // CSOFF: MethodLength
         final int size = expr.length();
         final ExpressionBuilder builder = new ExpressionBuilder(0);
         final StringBuilder strb = new StringBuilder(size);
         ParseState state = ParseState.CONST;
         int immediate1 = 0;
         int deferred1 = 0;
         int inner1 = 0;
         boolean nested = false;
         int inested = -1;
         int lineno = info.getLine();
         for (int column = 0; column < size; ++column) {
             final char c = expr.charAt(column);
             switch (state) {
                 case CONST:
                     if (c == immediateChar) {
                         state = ParseState.IMMEDIATE0;
                     } else if (c == deferredChar) {
                         inested = column;
                         state = ParseState.DEFERRED0;
                     } else if (c == '\\') {
                         state = ParseState.ESCAPE;
                     } else {
                         // do buildup expr
                         strb.append(c);
                     }
                     break;
                 case IMMEDIATE0: // $
                     if (c == '{') {
                         state = ParseState.IMMEDIATE1;
                         // if chars in buffer, create constant
                         if (strb.length() > 0) {
                             final TemplateExpression cexpr = new ConstantExpression(strb.toString(), null);
                             builder.add(cexpr);
                             strb.delete(0, Integer.MAX_VALUE);
                         }
                     } else {
                         // revert to CONST
                         strb.append(immediateChar);
                         state = ParseState.CONST;
                         // 'unread' the current character
                         column -= 1;
                         continue;
                     }
                     break;
                 case DEFERRED0: // #
                     if (c == '{') {
                         state = ParseState.DEFERRED1;
                         // if chars in buffer, create constant
                         if (strb.length() > 0) {
                             final TemplateExpression cexpr = new ConstantExpression(strb.toString(), null);
                             builder.add(cexpr);
                             strb.delete(0, Integer.MAX_VALUE);
                         }
                     } else {
                         // revert to CONST
                         strb.append(deferredChar);
                         state = ParseState.CONST;
                         // 'unread' the current character
                         column -= 1;
                         continue;
                     }
                     break;
                 case IMMEDIATE1: // ${...
                     if (c == '}') {
                         if (immediate1 > 0) {
                             immediate1 -= 1;
                             strb.append(c);
                         } else {
                             // materialize the immediate expr
                             final String src = strb.toString();
                             final TemplateExpression iexpr = new ImmediateExpression(
                                     src,
                                     jexl.parse(info.at(lineno, column), noscript, src, scope),
                                     null);
                             builder.add(iexpr);
                             strb.delete(0, Integer.MAX_VALUE);
                             state = ParseState.CONST;
                         }
                     } else {
                         if (c == '{') {
                             immediate1 += 1;
                         }
                         // do buildup expr
                         column = append(strb, expr, column, c);
                     }
                     break;
                 case DEFERRED1: // #{...
                     // skip inner strings (for '}')
                     // nested immediate in deferred; need to balance count of '{' & '}'
                     // closing '}'
                     switch (c) {
                         case '"':
                         case '\'':
                             strb.append(c);
                             column = StringParser.readString(strb, expr, column + 1, c);
                             continue;
                         case '{':
                             if (expr.charAt(column - 1) == immediateChar) {
                                 inner1 += 1;
                                 strb.deleteCharAt(strb.length() - 1);
                                 nested = true;
                             } else {
                                 deferred1 += 1;
                                 strb.append(c);
                             }
                             continue;
                         case '}':
                             // balance nested immediate
                             if (deferred1 > 0) {
                                 deferred1 -= 1;
                                 strb.append(c);
                             } else if (inner1 > 0) {
                                 inner1 -= 1;
                             } else  {
                                 // materialize the nested/deferred expr
                                 final String src = strb.toString();
                                 TemplateExpression dexpr;
                                 if (nested) {
                                     dexpr = new NestedExpression(
                                             expr.substring(inested, column + 1),
                                             jexl.parse(info.at(lineno, column), noscript, src, scope),
                                             null);
                                 } else {
                                     dexpr = new DeferredExpression(
                                             strb.toString(),
                                             jexl.parse(info.at(lineno, column), noscript, src, scope),
                                             null);
                                 }
                                 builder.add(dexpr);
                                 strb.delete(0, Integer.MAX_VALUE);
                                 nested = false;
                                 state = ParseState.CONST;
                             }
                             break;
                         default:
                             // do buildup expr
                             column = append(strb, expr, column, c);
                             break;
                         }
                     break;
                 case ESCAPE:
                     if (c == deferredChar) {
                         strb.append(deferredChar);
                     } else if (c == immediateChar) {
                         strb.append(immediateChar);
                     } else {
                         strb.append('\\');
                         strb.append(c);
                     }
                     state = ParseState.CONST;
                     break;
                 default: // in case we ever add new unified expression type
                     throw new UnsupportedOperationException("unexpected unified expression type");
             }
             if (c == '\n') {
                 lineno += 1;
             }
         }
         // we should be in that state
         if (state != ParseState.CONST) {
             // otherwise, we ended a line with a \, $ or #
             switch (state) {
                 case ESCAPE:
                     strb.append('\\');
                     strb.append('\\');
                     break;
                 case DEFERRED0:
                     strb.append(deferredChar);
                     break;
                 case IMMEDIATE0:
                     strb.append(immediateChar);
                     break;
                 default:
                     throw new Exception(info.at(lineno, 0), "malformed expression: " + expr, null);
             }
         }
         // if any chars were buffered, add them as a constant
         if (strb.length() > 0) {
             final TemplateExpression cexpr = new ConstantExpression(strb.toString(), null);
             builder.add(cexpr);
         }
         return builder.build(this, null);
     }
 
     /**
      * Reads lines of a template grouping them by typed blocks.
      * @param prefix the directive prefix
      * @param source the source reader
      * @return the list of blocks
      */
     protected List<Block> readTemplate(final String prefix, final Reader source) {
         final ArrayList<Block> blocks = new ArrayList<>();
         final BufferedReader reader;
         if (source instanceof BufferedReader) {
             reader = (BufferedReader) source;
         } else {
             reader = new BufferedReader(source);
         }
         final StringBuilder strb = new StringBuilder();
         BlockType type = null;
         int prefixLen;
         final Iterator<CharSequence> lines = readLines(reader);
         int lineno = 1;
         int start = 0;
         while (lines.hasNext()) {
             final CharSequence line = lines.next();
             if (line == null) {
                 break;
             }
             if (type == null) {
                 // determine starting type if not known yet
                 prefixLen = startsWith(line, prefix);
                 if (prefixLen >= 0) {
                     type = BlockType.DIRECTIVE;
                     strb.append(line.subSequence(prefixLen, line.length()));
                 } else {
                     type = BlockType.VERBATIM;
                     strb.append(line.subSequence(0, line.length()));
                 }
                 start = lineno;
             } else if (type == BlockType.DIRECTIVE) {
                 // switch to verbatim if necessary
                 prefixLen = startsWith(line, prefix);
                 if (prefixLen < 0) {
                     final Block directive = new Block(BlockType.DIRECTIVE, start, strb.toString());
                     strb.delete(0, Integer.MAX_VALUE);
                     blocks.add(directive);
                     type = BlockType.VERBATIM;
                     strb.append(line.subSequence(0, line.length()));
                     start = lineno;
                 } else {
                     // still a directive
                     strb.append(line.subSequence(prefixLen, line.length()));
                 }
             } else if (type == BlockType.VERBATIM) {
                 // switch to directive if necessary
                 prefixLen = startsWith(line, prefix);
                 if (prefixLen >= 0) {
                     final Block verbatim = new Block(BlockType.VERBATIM, start, strb.toString());
                     strb.delete(0, Integer.MAX_VALUE);
                     blocks.add(verbatim);
                     type = BlockType.DIRECTIVE;
                     strb.append(line.subSequence(prefixLen, line.length()));
                     start = lineno;
                 } else {
                     strb.append(line.subSequence(0, line.length()));
                 }
             }
             lineno += 1;
         }
         // input may be null
         if (type != null && strb.length() > 0) {
             final Block block = new Block(type, start, strb.toString());
             blocks.add(block);
         }
         blocks.trimToSize();
         return blocks;
     }
 
     /**
      * Whether a sequence starts with a given set of characters (following spaces).
      * <p>Space characters at beginning of line before the pattern are discarded.</p>
      * @param sequence the sequence
      * @param pattern  the pattern to match at start of sequence
      * @return the first position after end of pattern if it matches, -1 otherwise
      */
     protected int startsWith(final CharSequence sequence, final CharSequence pattern) {
         final int length = sequence.length();
         int s = 0;
         while (s < length && Character.isSpaceChar(sequence.charAt(s))) {
             s += 1;
         }
         if (s < length && pattern.length() <= length - s) {
             final CharSequence subSequence = sequence.subSequence(s, length);
             if (subSequence.subSequence(0, pattern.length()).equals(pattern)) {
                 return s + pattern.length();
             }
         }
         return -1;
     }
 }