CompoundString.swift

/*
 * The MIT License (MIT)
 *
 * Copyright (C) 2019, CosmicMind, Inc. <http://cosmicmind.com>.
 * All rights reserved.
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 * The above copyright notice and this permission notice shall be included in
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 *
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import Foundation

/**
 Compound strings into a single Predicate.
 ### Example
 ```swift
 let compoundString: CompoundString = ("T1" && "T2") || !"T3"
 let predicate = compoundString.predicate(with "X ==")
 print(predicate)
 // prints: (X == "T1" AND X == "T2") OR NOT (X == "T3")
 ```
 */
public struct CompoundString: ExpressibleByStringLiteral {
  /// A reference to NSPredicate.
  private let predicate: NSPredicate
  
  /**
   Initialize a CompoundString with given array of CompoundString by compounding
   them into a single predicate using provided logical type.
   - Parameter _ compounds: An array of CompoundString.
   - Parameter type: An NSCompoundPredicate.LogicalType.
   */
  fileprivate init(_ compounds: [CompoundString], type: NSCompoundPredicate.LogicalType) {
    self.predicate = NSCompoundPredicate(type: type, subpredicates: compounds.map { $0.predicate })
  }
  
  /// Initialze CompoundString with FALSEPREDICATE.
  init() {
    self.predicate = NSPredicate(value: false)
  }
  
  /**
   Initialize CompoundString with string literal value.
   - Parameter stringLiteral value: An StringLiteralType.
   */
  public init(stringLiteral value: StringLiteralType) {
    self.predicate = NSPredicate(format: "__REPLACE__ == %@", value)
  }
  
  /**
   Create a Predicate by replacing placeholder with the given format.
   - Parameter with format: A String.
   - Returns: A Predicate having the provided format.
   */
  internal func predicate(with format: String) -> Predicate {
    let format = predicate.predicateFormat.replacingOccurrences(of: "__REPLACE__ ==", with: format)
    let predicates = [NSPredicate(format: format)]
    return Predicate(predicates)
  }
}

/**
 An operator to compound given two CompoundString into
 a single CompoundString using AND logic.
 - Parameter left: A CompoundString.
 - Parameter right: A CompoundString.
 - Returns: A single CompoundString.
 */
public func &&(left: CompoundString, right: CompoundString) -> CompoundString {
  return CompoundString([left, right], type: .and)
}

/**
 An operator to compound given two CompoundString into
 a single CompoundString using OR logic.
 - Parameter left: A CompoundString.
 - Parameter right: A CompoundString.
 - Returns: A single CompoundString.
 */
public func ||(left: CompoundString, right: CompoundString) -> CompoundString {
  return CompoundString([left, right], type: .or)
}

/**
 A prefix operator to negate given CompoundString.
 - Parameter compound: A CompoundString.
 - Returns: A negated CompoundString.
 */
public prefix func !(compound: CompoundString) -> CompoundString {
  return CompoundString([compound], type: .not)
}