Add String utils

Sources/AOC2018/tools.swift

@@ -27,6 +27,14 @@ struct Tools {
         }
         return retVal
     }
+    
+    // Assumes THE string is 'polymer'
+    static func stringIndexRange(withString str: String, fromRange range: Range<Int>) -> Range<String.Index> {
+        let myrange = Range(uncheckedBounds: (lower: max(0, min(str.length, range.lowerBound)), upper: min(str.length, max(0, range.upperBound))))
+        let start = str.index(str.startIndex, offsetBy: myrange.lowerBound)
+        let end = str.index(start, offsetBy: myrange.upperBound - myrange.lowerBound)
+        return start ..< end
+    }
 }
 
 extension Character {
@@ -39,7 +47,41 @@ extension Character {
 }
 
 extension StringProtocol {
+    // var string: String { return String(self) }
+
     var ascii: [UInt32] {
         return compactMap { $0.ascii }
     }
+
+    // subscript(offset: Int) -> Element {
+    //     return self[index(startIndex, offsetBy: offset)]
+    // }
+}
+
+extension String {
+
+  var length: Int {
+    return count
+  }
+
+  subscript (i: Int) -> String {
+    return self[i ..< i + 1]
+  }
+
+  func substring(fromIndex: Int) -> String {
+    return self[min(fromIndex, length) ..< length]
+  }
+
+  func substring(toIndex: Int) -> String {
+    return self[0 ..< max(0, toIndex)]
+  }
+
+  subscript (r: Range<Int>) -> String {
+    let range = Range(uncheckedBounds: (lower: max(0, min(length, r.lowerBound)),
+                                        upper: min(length, max(0, r.upperBound))))
+    let start = index(startIndex, offsetBy: range.lowerBound)
+    let end = index(start, offsetBy: range.upperBound - range.lowerBound)
+    return String(self[start ..< end])
+  }
+
 }