two highlighting fixes (counts and surrogates)

src/main/java/io/github/homchom/recode/feature/visual/EditBoxExpressionFormatter.kt

@@ -119,7 +119,7 @@ class EditBoxExpressionFormatter(
                 min(startIndex, partialEnd) - partialStart
             ))
 
-            if (partialRange.first in startIndex..<endIndex || partialRange.last in startIndex..<endIndex) {
+            if (startIndex in partialRange || endIndex - 1 in partialRange) {
                 append(highlighted.text.subSequence(
                     max(startIndex, partialStart) - startIndex,
                     min(endIndex, partialEnd) - startIndex

src/main/java/io/github/homchom/recode/ui/text/FormattedCharSequenceTransformations.kt

@@ -8,11 +8,9 @@ import net.minecraft.util.FormattedCharSequence
  * @see CharSequence.subSequence
  */
 fun FormattedCharSequence.subSequence(startIndex: Int, endIndex: Int) = FormattedCharSequence { sink ->
-    var index = 0
-    var adjustedIndex = 0
-    accept { _, style, codePoint ->
-        if (index++ in startIndex..<endIndex) {
-            sink.accept(adjustedIndex++, style, codePoint)
+    acceptWithAbsoluteIndex { index, style, codePoint ->
+        if (index in startIndex..<endIndex) {
+            sink.accept(index - startIndex, style, codePoint)
         } else true
     }
 }

src/main/java/io/github/homchom/recode/ui/text/TextBuilders.kt

@@ -32,9 +32,9 @@ fun emptyText(style: StyleWrapper = style()) =
 /**
  * Builds a [Component] by adding [style] to [root] and applying [builder].
  *
- * Use [translatedText] and [literalText] when applicable, as their output is more optimized.
- * Use [formattedCharSequence] if all text is literal and a [Component] representation is not necessary, as it
- * is the most optimized.
+ * Use [translatedText] and [literalText] when applicable, as their output is more optimized. Also note
+ * that some functions take a [net.minecraft.util.FormattedCharSequence]; in those cases you should build
+ * a [formattedCharSequence] directly.
  *
  * @see TextBuilder
  */

src/main/java/io/github/homchom/recode/ui/text/TextFunctions.kt

@@ -2,10 +2,12 @@
 
 package io.github.homchom.recode.ui.text
 
+import io.github.homchom.recode.util.fromCodePoint
 import net.kyori.adventure.text.Component
 import net.kyori.adventure.text.format.Style
 import net.kyori.adventure.text.serializer.plain.PlainTextComponentSerializer
 import net.minecraft.util.FormattedCharSequence
+import net.minecraft.util.FormattedCharSink
 
 /**
  * @return A new [Component] created by merging this Component's style with [style], using [strategy] and [merges].
@@ -78,4 +80,19 @@ fun Regex.matchEntirePlain(text: Component) = matchEntire(text.plainText)
 /**
  * @return Whether [text]'s entire [plainText] matches this [Regex] pattern.
  */
-fun Regex.matchesPlain(text: Component) = matches(text.plainText)
+fun Regex.matchesPlain(text: Component) = matches(text.plainText)
+
+/**
+ * [FormattedCharSequence.accept]s this [FormattedCharSequence], adjusting the `index` parameter
+ * passed to [sink] to be absolute instead of relative. Surrogate pairs are handled but not validated.
+ */
+fun FormattedCharSequence.acceptWithAbsoluteIndex(sink: FormattedCharSink): Boolean {
+    var absoluteIndex = 0
+    return accept { _, style, codePoint ->
+        val shouldContinue = sink.accept(absoluteIndex++, style, codePoint)
+        if (String.fromCodePoint(codePoint)[0].isHighSurrogate()) {
+            absoluteIndex++
+        }
+        shouldContinue
+    }
+}

src/main/java/io/github/homchom/recode/util/BasicTypeExtensions.kt

@@ -11,6 +11,11 @@ fun Boolean.unitOrNull() = if (this) Unit else null
 
 // strings
 
+/**
+ * @see Character.toString
+ */
+fun String.Companion.fromCodePoint(codePoint: Int): String = Character.toString(codePoint)
+
 /**
  * Appends multiple [substrings] to this [StringBuilder].
  */