59: add support for \uXXXX escapes within string literals

This is in response to edn-format/edn#65 .

This is an extension as string literals as currently documented
do not specify support for \uXXXX escapes.

  https://github.com/edn-format/edn/tree/a51127aecd318096667ae0dafa25353ecb07c9c3

Notes:

- Unicode escape must begin with "\u". This is case sensitive "\U" will
  be rejected.
- "\u" must be followed by exactly four hex digits taken from this set:
  0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F
- The digits are not case sensitive.
- Each such Unicode escape encodes a single 16-bit Java char. Since Java
  uses UTF-16 internally (for historical reasons) code points beyond
  the basic multilingual plane as a pair of unicode escapes.
  (see also "surrogate pairs")

src/main/java/us/bpsm/edn/parser/ScannerImpl.java

@@ -377,6 +377,23 @@ private String readStringLiteral(Parseable pbr) throws IOException {
                 case '\\':
                     b.append('\\');
                     break;
+                case 'u':
+                    // Support for reading unicode escapes within string
+                    // literals is an extension to EDN. It is not currently
+                    // part of the spec described here:
+                    // https://github.com/edn-format/edn
+                    int v = 0;
+                    for (int i = 0; i < 4; i++) {
+                        curr = pbr.read();
+                        int d = Character.digit(curr, 16);
+                        if (d == -1) {
+                            throw new EdnSyntaxException(
+                              "Invalid \\u Unicode escape in string.");
+                        }
+                        v = v * 16 + d;
+                    }
+                    b.append((char)v);
+                    break;
                 default:
                     throw new EdnSyntaxException("Unsupported '"+ ((char)curr)
                             +"' escape in string");

src/test/java/us/bpsm/edn/parser/ScannerTest.java

@@ -13,6 +13,7 @@
 import org.junit.Test;
 
 import us.bpsm.edn.EdnException;
+import us.bpsm.edn.EdnSyntaxException;
 import us.bpsm.edn.Keyword;
 import us.bpsm.edn.Symbol;
 import us.bpsm.edn.parser.Parseable;
@@ -366,6 +367,35 @@ public void unicodeEscapeCharacterLiterals() {
             assertEquals(c, s.nextToken(pbr));
     }
 
+    @Test
+    public void unicodeEscapesInStringLiterals() {
+        String txt = "\"" +
+          "\\" + "u0000" +
+          "\\" + "u1234" +
+          "\\" + "u0Ff0" +
+          "\"";
+        String expected = "\u0000\u1234\u0Ff0";
+        assertEquals(3, expected.length());
+        Parseable pbr = Parsers.newParseable(txt);
+        Scanner s = scanner();
+        assertEquals(expected, s.nextToken(pbr));
+    }
+
+    @Test(expected = EdnSyntaxException.class)
+    public void truncatedUnicodeEscapeInStringLiteral() {
+        scanner().nextToken(Parsers.newParseable("\"\\" + "u123\""));
+    }
+
+    @Test(expected = EdnSyntaxException.class)
+    public void truncatedInputInUnicodeEscapeInStringLiteral() {
+        scanner().nextToken(Parsers.newParseable("\"\\" + "u123"));
+    }
+
+    @Test(expected = EdnSyntaxException.class)
+    public void nonDigitInUnicodeEscapeInStringLiteral() {
+        scanner().nextToken(Parsers.newParseable("\"\\" + "u123?\""));
+    }
+
     @Test
     public void simpleStringWithLinebreak() {
         assertEquals("\n", scan("\"\n\""));