source: addons/HeuristicLab.PushGP/HeuristicLab.Problems.ProgramSynthesis/Push/Expressions/StringExpressions.cs @ 16958

namespace HeuristicLab.Problems.ProgramSynthesis.Push.Expressions {
  using System;
  using System.Globalization;
  using System.Linq;

  using Attributes;

  using HeuristicLab.Persistence.Default.CompositeSerializers.Storable;
  using HeuristicLab.Problems.ProgramSynthesis.Push.Extensions;

  using Interpreter;
  using Stack;

  /// <summary>
  /// Stringifies and pushes the top INTEGER.,
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.FROMINTEGER",
    "Stringifies and pushes the top INTEGER.",
    StackTypes.Integer)]
  public class StringFromIntegerExpression : StatelessExpression {
    public StringFromIntegerExpression() { }
    [StorableConstructor]
    protected StringFromIntegerExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.IntegerStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var value = interpreter.IntegerStack.Pop();
      interpreter.StringStack.Push(value.ToString());
    }
  }

  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.FROMFLOAT",
    "Stringifies and pushes the top FLOAT.",
    StackTypes.Float)]
  public class StringFromFloatExpression : StatelessExpression {
    public StringFromFloatExpression() { }
    [StorableConstructor]
    protected StringFromFloatExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.FloatStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var value = interpreter.FloatStack.Pop();
      var str = value.ToString(interpreter.Configuration.FloatStringFormat, CultureInfo.InvariantCulture);

      interpreter.StringStack.Push(str);
    }
  }

  /// <summary>
  /// Stringifies and pushes the top BOOLEAN.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.FROMBOOLEAN",
    "Stringifies and pushes the top BOOLEAN.",
    StackTypes.Boolean)]
  public class StringFromBooleanExpression : StatelessExpression {
    public StringFromBooleanExpression() { }
    [StorableConstructor]
    protected StringFromBooleanExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.BooleanStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var value = interpreter.BooleanStack.Pop();
      interpreter.StringStack.Push(value.ToString());
    }
  }

  /// <summary>
  /// Stringifies and pushes the top CHAR.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.FROMCHAR",
    "Stringifies and pushes the top CHAR.",
    StackTypes.Char)]
  public class StringFromCharExpression : StatelessExpression {
    public StringFromCharExpression() { }
    [StorableConstructor]
    protected StringFromCharExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.CharStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var value = interpreter.CharStack.Pop();
      interpreter.StringStack.Push(value.ToString());
    }
  }

  /// <summary>
  /// Concats the top 2 strings.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.CONCAT",
    "Concats the top 2 strings.")]
  public class StringConcatExpression : StatelessExpression {
    public StringConcatExpression() { }
    [StorableConstructor]
    protected StringConcatExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.Count < 2 ||
             interpreter.StringStack.Top.Length + interpreter.StringStack[1].Length > interpreter.Configuration.MaxStringLength;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Pop();
      interpreter.StringStack.Top += str;
    }
  }

  /// <summary>
  /// Conj char onto string
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.CONJCHAR",
    "Conj top CHAR onto top STRING",
    StackTypes.Char)]
  public class StringConjCharExpression : StatelessExpression {
    public StringConjCharExpression() { }
    [StorableConstructor]
    protected StringConjCharExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.CharStack.IsEmpty ||
             interpreter.StringStack.Top.Length + 1 > interpreter.Configuration.MaxStringLength;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var c = interpreter.CharStack.Pop();
      interpreter.StringStack.Top += c;
    }
  }

  /// <summary>
  /// Same as substring 0-n, whereby n is taken from the INTEGER stack.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.TAKE",
    "Same as substring 0-n, whereby n is taken from the INTEGER stack.",
    StackTypes.Integer)]
  public class StringTakeExpression : StatelessExpression {
    public StringTakeExpression() { }
    [StorableConstructor]
    protected StringTakeExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.IntegerStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Top;

      if (str.Length == 0)
        return;

      var endIndex = interpreter.IntegerStack.Pop().AsInt(str.Length);
      interpreter.StringStack.Top = str.Substring(0, endIndex);
    }
  }

  /// <summary>
  /// Replaces top STRING with a substring from index A to B, whereby A and B are taken from the INTEGER STACK.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.SUBSTRING",
    "Replaces top STRING with a substring from index A to B, whereby A and B are taken from the INTEGER STACK.",
    StackTypes.Integer)]
  public class StringSubstringExpression : StatelessExpression {
    public StringSubstringExpression() { }
    [StorableConstructor]
    protected StringSubstringExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.IntegerStack.Count < 2;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Top;
      var firstValue = interpreter.IntegerStack[1];
      var secondValue = interpreter.IntegerStack.Top;
      interpreter.IntegerStack.Remove(2);

      var first = Math.Min(str.Length - 1, Math.Max(firstValue, 0));
      var second = Math.Min(str.Length - 1, Math.Max(secondValue, first));
      var length = second - first;

      if (length > 0)
        interpreter.StringStack.Top = str.Substring((int)first, (int)length);
    }
  }

  /// <summary>
  /// Pushes first char of top STRING.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.FIRST",
    "Pushes first char of top STRING.")]
  public class StringFirstExpression : StatelessExpression {
    public StringFirstExpression() { }
    [StorableConstructor]
    protected StringFirstExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.StringStack.Top.Length == 0;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      interpreter.StringStack.Top = interpreter.StringStack.Top[0].ToString();
    }
  }

  /// <summary>
  /// Pushes last char of top STRING.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.LAST",
    "Pushes last char of top STRING.")]
  public class StringLastExpression : StatelessExpression {
    public StringLastExpression() { }
    [StorableConstructor]
    protected StringLastExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.StringStack.Top.Length == 0;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Top;
      var c = str[str.Length - 1].ToString();
      interpreter.StringStack.Top = c;
    }
  }

  /// <summary>
  /// Pushes nth char of top STRING, whereby n is take from the INTEGER stack.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.NTH",
    "Pushes nth char of top STRING, whereby n is take from the INTEGER stack.",
    StackTypes.Integer)]
  public class StringNthExpression : StatelessExpression {
    public StringNthExpression() { }
    [StorableConstructor]
    protected StringNthExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.IntegerStack.IsEmpty ||
             interpreter.StringStack.Top.Length == 0;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Top;
      var index = str.Length == 1 ? 0 : interpreter.IntegerStack.Pop().AsInt(str.Length);
      var c = str[index].ToString();
      interpreter.StringStack.Top = c;
    }
  }

  /// <summary>
  /// Removes first char of top STRING.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.REST",
    "Removes first char of top STRING.")]
  public class StringRestExpression : StatelessExpression {
    public StringRestExpression() { }
    [StorableConstructor]
    protected StringRestExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.StringStack.Top.Length == 0;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Top;
      interpreter.StringStack.Top = str.Length == 1 ? string.Empty : str.Substring(1, str.Length - 1);
    }
  }

  /// <summary>
  /// Removes last char of top STRING.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.BUTLAST",
    "Removes last char of top STRING.")]
  public class StringButLastExpression : StatelessExpression {
    public StringButLastExpression() { }
    [StorableConstructor]
    protected StringButLastExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.StringStack.Top.Length == 0;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Top;
      interpreter.StringStack.Top = str.Length == 1 ? string.Empty : str.Substring(0, str.Length - 1);
    }
  }

  /// <summary>
  /// Pushes length of top STRING onto the INTEGER stack.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.LENGTH",
    "Pushes length of top STRING onto the INTEGER stack.",
    StackTypes.Integer)]
  public class StringLengthExpression : StatelessExpression {
    public StringLengthExpression() { }
    [StorableConstructor]
    protected StringLengthExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Pop();
      interpreter.IntegerStack.Push(str.Length);
    }
  }

  /// <summary>
  /// Reverses the top STRING.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.REVERSE",
    "Reverses the top STRING.")]
  public class StringReverseExpression : StatelessExpression {
    public StringReverseExpression() { }
    [StorableConstructor]
    protected StringReverseExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty || interpreter.StringStack.Top.Length <= 1;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var chars = interpreter.StringStack.Top.ToArray();
      Array.Reverse(chars);
      interpreter.StringStack.Top = new string(chars);
    }
  }

  /// <summary>
  /// Maps every char of top STRING as separate string.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.PARSETOCHARS",
    "Maps every char of top STRING as separate string.")]
  public class StringParseToCharsExpression : StatelessExpression {
    public StringParseToCharsExpression() { }
    [StorableConstructor]
    protected StringParseToCharsExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      if (interpreter.StringStack.Top.Length == 0) {
        interpreter.StringStack.Pop();
        return;
      }

      var str = interpreter.StringStack.Top;
      interpreter.StringStack.Top = str[0].ToString();

      if (str.Length > 1) {
        var chars = new string[str.Length - 1];
        for (var i = 0; i < str.Length - 1; i++) {
          chars[i] = str[i + 1].ToString();
        }

        interpreter.StringStack.Push(chars);
      }
    }
  }

  /// <summary>
  /// Splits top STRING by whitespace chars.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.SPLIT",
    "Splits top STRING by whitespace chars.")]
  public class StringSplitExpression : StatelessExpression {
    public StringSplitExpression() { }
    [StorableConstructor]
    protected StringSplitExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var words = interpreter.StringStack.Top
        .Trim()
        .Split()
        .Where(x => !string.IsNullOrWhiteSpace(x))
        .Reverse()
        .ToList();

      if (words.Count == 0) {
        interpreter.StringStack.Pop();
      } else {
        interpreter.StringStack.Top = words[0];
        interpreter.StringStack.Push(words, 1);
      }
    }
  }

  /// <summary>
  /// True if top string is empty.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.EMPTYSTRING",
    "True if top STRING is empty.",
    StackTypes.Boolean)]
  public class StringEmptyStringExpression : StatelessExpression {
    public StringEmptyStringExpression() { }
    [StorableConstructor]
    protected StringEmptyStringExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Pop();
      interpreter.BooleanStack.Push(str.Length == 0);
    }
  }

  /// <summary>
  /// True if top string is a substring of second string; false otherwise
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.CONTAINS",
    "True if top STRING is a substring of second STRING, false otherwise.",
    StackTypes.Boolean)]
  public class StringContainsExpression : StatelessExpression {
    public StringContainsExpression() { }
    [StorableConstructor]
    protected StringContainsExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.Count < 2;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var first = interpreter.StringStack[1];
      var second = interpreter.StringStack.Top;
      interpreter.StringStack.Remove(2);

      interpreter.BooleanStack.Push(first.IndexOf(second, StringComparison.Ordinal) >= 0);
    }
  }

  /// <summary>
  /// True if the top char is in the top string
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.CONTAINSCHAR",
    "True if the top CHAR is in the top STRING.",
    StackTypes.Boolean | StackTypes.Char)]
  public class StringContainsCharExpression : StatelessExpression {
    public StringContainsCharExpression() { }
    [StorableConstructor]
    protected StringContainsCharExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.CharStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Pop();
      var c = interpreter.CharStack.Pop();

      interpreter.BooleanStack.Push(str.IndexOf(c) >= 0);
    }
  }

  /// <summary>
  /// Puts on the integer stack the index of the top char in the top string
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.INDEXOFCHAR",
    "Puts on the INTEGER stack the index of the top CHAR in the top STRING.",
    StackTypes.Integer | StackTypes.Char)]
  public class StringIndexOfCharExpression : StatelessExpression {
    public StringIndexOfCharExpression() { }
    [StorableConstructor]
    protected StringIndexOfCharExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.CharStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Pop();
      var c = interpreter.CharStack.Pop();

      interpreter.IntegerStack.Push(str.IndexOf(c));
    }
  }

  /// <summary>
  /// The number of times the top char is in the top string
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.OCCURRENCESOFCHAR",
    "The number of times the top CHAR is in the top STRING.",
    StackTypes.Integer | StackTypes.Char)]
  public class StringOccurrencesOfCharExpression : StatelessExpression {
    public StringOccurrencesOfCharExpression() { }
    [StorableConstructor]
    protected StringOccurrencesOfCharExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.CharStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Pop();
      var c = interpreter.CharStack.Pop();

      var count = 0;
      for (var i = 0; i < str.Length; i++)
        if (str[i] == c) count++;

      interpreter.IntegerStack.Push(count);
    }
  }

  /// <summary>
  /// In third STRING, replaces second string with first string
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.REPLACE",
    "In third STRING, replaces second STRING with first STRING.")]
  public class StringReplaceExpression : StatelessExpression {
    public StringReplaceExpression() { }
    [StorableConstructor]
    protected StringReplaceExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.Count < 3;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var result = GetResultString(interpreter);

      // Should be checked in isNoop but would require to calculate result twice which is an expensive operation
      if (result.Length > interpreter.Configuration.MaxStringLength)
        return;

      interpreter.StringStack.Remove(2);
      interpreter.StringStack.Top = result;
    }

    private static string GetResultString(IInternalPushInterpreter interpreter) {
      var third = interpreter.StringStack[2];
      var second = interpreter.StringStack[1];
      var first = interpreter.StringStack[0];

      var result = third.Length == 0 || second.Length == 0
        ? third
        : third.Replace(second, first);

      return result;
    }
  }

  /// <summary>
  /// In third STRING, replaces first occurrence of second string with first string
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.REPLACEFIRST",
    "In third STRING, replaces first occurrence of second STRING with first STRING")]
  public class StringReplaceFirstExpression : StatelessExpression {
    public StringReplaceFirstExpression() { }
    [StorableConstructor]
    protected StringReplaceFirstExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.Count < 3;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var result = GetResultString(interpreter);

      // Should be checked in isNoop but would require to calculate result twice which is an expensive operation
      if (result.Length > interpreter.Configuration.MaxStringLength)
        return;

      interpreter.StringStack.Remove(2);
      interpreter.StringStack.Top = result;
    }

    private static string GetResultString(IInternalPushInterpreter interpreter) {
      var third = interpreter.StringStack[2];
      var second = interpreter.StringStack[1];
      var first = interpreter.StringStack[0];

      var result = third.Length == 0 || second.Length == 0
        ? third
        : ReplaceFirst(third, second, first);
      return result;
    }

    private static string ReplaceFirst(string text, string search, string replace) {
      var pos = text.IndexOf(search, StringComparison.Ordinal);
      return pos < 0 ? text : text.Substring(0, pos) + replace + text.Substring(pos + search.Length);
    }
  }

  /// <summary>
  /// In top STRING, replaces all occurrences of second char with first char
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.REPLACECHAR",
    "In top STRING, replaces all occurrences of second CHAR with first CHAR.",
    StackTypes.Char)]
  public class StringReplaceCharExpression : StatelessExpression {
    public StringReplaceCharExpression() { }
    [StorableConstructor]
    protected StringReplaceCharExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.CharStack.Count < 2;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var first = interpreter.CharStack[1];
      var second = interpreter.CharStack.Top;
      interpreter.CharStack.Remove(2);

      var result = interpreter.StringStack.Top.Replace(first, second);
      interpreter.StringStack.Top = result;
    }
  }

  /// <summary>
  /// In top STRING, replaces first occurrence of second char with first char
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.REPLACEFIRSTCHAR",
    "In top STRING, replaces first occurrence of second CHAR with first CHAR.",
    StackTypes.Char)]
  public class StringReplaceFirstCharExpression : StatelessExpression {
    public StringReplaceFirstCharExpression() { }
    [StorableConstructor]
    protected StringReplaceFirstCharExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.CharStack.Count < 2;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Top;
      var first = interpreter.CharStack[1];
      var second = interpreter.CharStack.Top;
      interpreter.CharStack.Remove(2);

      var pos = str.IndexOf(first);

      if (pos < 0)
        return;

      var result = str.Substring(0, pos) +
                   second +
                   str.Substring(Math.Min(pos + 1, str.Length - 1));

      interpreter.StringStack.Top = result;
    }
  }

  /// <summary>
  /// In top STRING, remove all occurrences of char
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.REMOVECHAR",
    "In top STRING, remove all occurrences of top CHAR.",
    StackTypes.Char)]
  public class StringRemoveCharExpression : StatelessExpression {
    public StringRemoveCharExpression() { }
    [StorableConstructor]
    protected StringRemoveCharExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.CharStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var c = interpreter.CharStack.Pop();
      var result = interpreter.StringStack.Top.Trim(c);
      interpreter.StringStack.Top = result;
    }
  }

  /// <summary>
  /// Sets char at index in string
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.SETCHAR",
    "Sets top CHAR at index in top STRING, whereby the index is taken from the INTEGER stack.",
    StackTypes.Char | StackTypes.Integer)]
  public class StringSetCharExpression : StatelessExpression {
    public StringSetCharExpression() { }
    [StorableConstructor]
    protected StringSetCharExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.CharStack.IsEmpty ||
             interpreter.IntegerStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Top;
      var c = interpreter.CharStack.Pop();
      var x = interpreter.IntegerStack.Pop();

      if (str.Length == 0) return;
      if (str.Length == 1) {
        interpreter.StringStack.Top = c.ToString();
        return;
      }

      var pos = x.AsInt(str.Length);
      var result = str.Substring(0, pos) + c + str.Substring(Math.Min(pos + 1, str.Length - 1));

      interpreter.StringStack.Top = result;
    }
  }

  /// <summary>
  /// Iterates over a string using the code on the exec stack.
  /// </summary>
  [StorableClass]
  [PushExpression(
    StackTypes.String,
    "STRING.ITERATE",
    "Iterates over the top STRING using the CODE on the EXEC stack.",
    StackTypes.Exec | StackTypes.Char, requiredBlockCount: 1)]
  public class StringIterateExpression : StatelessExpression {
    public StringIterateExpression() { }
    [StorableConstructor]
    protected StringIterateExpression(bool deserializing) : base(deserializing) { }

    public override bool IsNoop(IInternalPushInterpreter interpreter) {
      return interpreter.StringStack.IsEmpty ||
             interpreter.ExecStack.IsEmpty;
    }

    public override void Eval(IInternalPushInterpreter interpreter) {
      var str = interpreter.StringStack.Pop();

      if (str.Length == 0) {
        interpreter.ExecStack.Pop();
      }
      // If the rest of the string is empty, we're done iterating.
      else if (str.Length == 1) {
        interpreter.CharStack.Push(str[0]);
      } else {
        var pool = interpreter.PoolContainer.GetStatefulExpressionPool<StringPushExpression>();

        interpreter.CharStack.Push(str[0]);
        interpreter.ExecStack.Push(
          this,
          StringPushExpression.Create(pool, str.Substring(1)),
          interpreter.ExecStack.Top);
      }
    }
  }
}