Remove codePointOffsetsToStringIndices and inline its logic:

Due to checks performed at its callers, one of the branches in codePointOffsetsToStringIndices
was unreachable and untested. It's clearer (and likely more performant)
to eliminate this method and inline specialized versions of its logic at
its former callsites.

Author: JoshRosen

sjsonnet/src/sjsonnet/Evaluator.scala

@@ -379,7 +379,8 @@ class Evaluator(
         val unicodeLength = str.codePointCount(0, str.length)
         if (int >= unicodeLength)
           Error.fail(s"string bounds error: $int not within [0, $unicodeLength)", pos)
-        val (startUtf16, endUtf16) = Util.codePointOffsetsToStringIndices(str, int, int + 1)
+        val startUtf16 = if (int == 0) 0 else str.offsetByCodePoints(0, int)
+        val endUtf16 = str.offsetByCodePoints(startUtf16, 1)
         Val.Str(pos, str.substring(startUtf16, endUtf16))
       case (v: Val.Obj, i: Val.Str) =>
         v.value(i.value, pos)

sjsonnet/src/sjsonnet/Std.scala

@@ -543,8 +543,8 @@ class Std(
       if (safeLength <= 0) {
         Val.Str(pos, "")
       } else {
-        val (startUtf16, endUtf16) =
-          Util.codePointOffsetsToStringIndices(str, safeOffset, safeOffset + safeLength)
+        val startUtf16 = if (safeOffset == 0) 0 else str.offsetByCodePoints(0, safeOffset)
+        val endUtf16 = str.offsetByCodePoints(startUtf16, safeLength)
         Val.Str(pos, str.substring(startUtf16, endUtf16))
       }
     }

sjsonnet/src/sjsonnet/Util.scala

@@ -72,28 +72,6 @@ object Util {
     res: Val
   }
 
-  /**
-   * Converts Unicode codepoint positions to Java String indices. For example, the string "🌍!" has
-   * a length of 3 UTF-16 code units, but only 2 Unicode codepoints, so this function would map the
-   * range (0, 2) to (0, 3).
-   */
-  def codePointOffsetsToStringIndices(
-      s: String,
-      startCodePointOffset: Int,
-      endCodePointOffset: Int): (Int, Int) = {
-    val unicodeLength = s.codePointCount(0, s.length)
-    val safeStart = math.max(0, math.min(startCodePointOffset, unicodeLength))
-    val safeEnd = math.max(safeStart, math.min(endCodePointOffset, unicodeLength))
-
-    if (safeStart == safeEnd) {
-      val utf16Pos = if (safeStart == 0) 0 else s.offsetByCodePoints(0, safeStart)
-      (utf16Pos, utf16Pos)
-    } else {
-      val startUtf16 = if (safeStart == 0) 0 else s.offsetByCodePoints(0, safeStart)
-      val endUtf16 = s.offsetByCodePoints(startUtf16, safeEnd - safeStart)
-      (startUtf16, endUtf16)
-    }
-  }
 
   private def sliceStr(s: String, start: Int, end: Int, step: Int): String = {
     val unicodeLength = s.codePointCount(0, s.length)
@@ -102,7 +80,11 @@ object Util {
     } else {
       step match {
         case 1 =>
-          val (startUtf16, endUtf16) = codePointOffsetsToStringIndices(s, start, end)
+          // Preconditions: start >= 0, start < end, start < unicodeLength
+          val safeEnd = math.min(end, unicodeLength)
+          val sliceLength = safeEnd - start
+          val startUtf16 = if (start == 0) 0 else s.offsetByCodePoints(0, start)
+          val endUtf16 = s.offsetByCodePoints(startUtf16, sliceLength)
           s.substring(startUtf16, endUtf16)
         case _ =>
           val result =