TRAP formatting: adopt Java's standards

* Encode dates with D"" strings
* Truncate exceedingly long string values
* Note that floats don't require any special handling

java/kotlin-extractor/src/main/java/com/semmle/util/unicode/UTF8Util.java

@@ -0,0 +1,52 @@
+package com.semmle.util.unicode;
+
+public class UTF8Util {
+	/**
+	 * Get the length (in Unicode code units, not code points) of the longest prefix of
+	 * a string that can be UTF-8 encoded in no more than the given number of bytes.
+	 *
+	 * <p>
+	 * Unencodable characters (such as lone surrogate halves or low surrogates
+	 * that do not follow a high surrogate) are treated as being encoded in
+	 * three bytes. This is safe since on encoding they will be replaced by
+	 * a replacement character, which in turn will take at most three bytes to
+	 * encode.
+	 * </p>
+	 *
+	 * @param str string to encode
+	 * @param maxEncodedLength maximum number of bytes for the encoded prefix
+	 * @return length of the prefix
+	 */
+	public static int encodablePrefixLength(String str, int maxEncodedLength) {
+		// no character takes more than three bytes to encode
+		if (str.length() > maxEncodedLength / 3) {
+			int encodedLength = 0;
+			for (int i = 0; i < str.length(); ++i) {
+				int oldI = i;
+				char c = str.charAt(i);
+				if (c <= 0x7f) {
+					encodedLength += 1;
+				} else if (c <= 0x7ff) {
+					encodedLength += 2;
+				} else if (Character.isHighSurrogate(c)) {
+					// surrogate pairs take four bytes to encode
+					if (i+1 < str.length() && Character.isLowSurrogate(str.charAt(i+1))) {
+						encodedLength += 4;
+						++i;
+					} else {
+						// lone high surrogate, assume length three
+						encodedLength += 3;
+					}
+				} else {
+					encodedLength += 3;
+				}
+
+				if (encodedLength > maxEncodedLength) {
+					return oldI;
+				}
+			}
+		}
+
+		return str.length();
+	}
+}

java/kotlin-extractor/src/main/kotlin/KotlinExtractorExtension.kt

@@ -8,6 +8,7 @@ import java.io.FileOutputStream
 import java.nio.file.Files
 import java.nio.file.Paths
 import com.semmle.util.files.FileUtil
+import com.semmle.util.unicode.UTF8Util
 import kotlin.system.exitProcess
 
 class KotlinExtractorExtension(
@@ -117,6 +118,10 @@ class FileExtractionProblems(val invocationExtractionProblems: ExtractionProblem
 
 fun escapeTrapString(str: String) = str.replace("\"", "\"\"")
 
+const val MAX_STRLEN = 1.shl(20) // 1 megabyte
+
+fun truncateString(str: String) = str.substring(0, UTF8Util.encodablePrefixLength(str, MAX_STRLEN))
+
 private fun equivalentTrap(f1: File, f2: File): Boolean {
     f1.bufferedReader().use { bw1 ->
         f2.bufferedReader().use { bw2 ->