Support UXXXXXX escape sequences

Author: stasm

fluent.syntax/src/main/kotlin/org/projectfluent/syntax/processor/Processor.kt

@@ -2,6 +2,7 @@ package org.projectfluent.syntax.processor
 
 import org.projectfluent.syntax.ast.* // ktlint-disable no-wildcard-imports
 import java.lang.Exception
+import java.lang.StringBuilder
 
 /**
  * Process patterns by returning new patterns with elements transformed.
@@ -147,16 +148,34 @@ class Processor {
     }
 
     private val special =
-        """\\(([\\"])|(u[0-9a-fA-F]{4}))""".toRegex()
+        """\\(([\\"])|(u[0-9a-fA-F]{4})|(U[0-90a-fA-F]{6}))""".toRegex()
 
     private fun unescape(matchResult: MatchResult): CharSequence {
         val matches = matchResult.groupValues.drop(2).listIterator()
         val simple = matches.next()
-        if (simple != "") { return simple }
+        if (simple != "") {
+            return simple
+        }
+
         val uni4 = matches.next()
         if (uni4 != "") {
-            return uni4.substring(1).toInt(16).toChar().toString()
+            val codepoint = uni4.substring(1).toInt(16)
+            if (Character.isBmpCodePoint(codepoint)) {
+                return codepoint.toChar().toString()
+            }
         }
+
+        val uni6 = matches.next()
+        if (uni6 != "") {
+            val codepoint = uni6.substring(1).toInt(16)
+            if (Character.isValidCodePoint(codepoint)) {
+                val builder = StringBuilder()
+                builder.append(Character.highSurrogate(codepoint))
+                builder.append(Character.lowSurrogate(codepoint))
+                return builder
+            }
+        }
+
         throw Exception("Unexpected")
     }
 }

fluent.syntax/src/test/kotlin/org/projectfluent/syntax/processor/ProcessorTest.kt

@@ -65,6 +65,22 @@ internal class ProcessorTest {
             processor.unescapeLiteralsToText(pattern)
         )
 
+        pattern.elements.clear()
+        pattern.elements.addAll(
+            arrayOf(
+                TextElement("Emoji: "),
+                Placeable(expression = StringLiteral("""\U01f602"""))
+            )
+        )
+        assertEquals(
+            Pattern(TextElement("Emoji: \uD83D\uDE02")),
+            processor.unescapeLiteralsToText(pattern)
+        )
+        assertEquals(
+            Pattern(TextElement("Emoji: 😂")),
+            processor.unescapeLiteralsToText(pattern)
+        )
+
         pattern.elements.clear()
         pattern.elements.addAll(
             arrayOf(