[KT-80748] Strings add ensurePrefix/ensureSuffix extension functions

libraries/stdlib/src/kotlin/text/Strings.kt

@@ -630,6 +630,28 @@ public fun String.removePrefix(prefix: CharSequence): String {
     return this
 }
 
+/**
+ * If this char sequence does not start with the given [prefix], returns a new char sequence
+ * with the prefix added. Otherwise, returns a new char sequence with the same characters.
+ */
+public fun CharSequence.ensurePrefix(prefix: CharSequence): CharSequence {
+    if (!this.startsWith(prefix)) {
+        return prefix.toString() + this.toString()
+    }
+    return subSequence(0, length)
+}
+
+/**
+ * If this string does not start with the given [prefix], returns a copy of this string
+ * with the prefix added. Otherwise, returns this string.
+ */
+public fun String.ensurePrefix(prefix: CharSequence): String {
+    if (!this.startsWith(prefix)) {
+        return prefix.toString() + this
+    }
+    return this
+}
+
 /**
  * If this char sequence ends with the given [suffix], returns a new char sequence
  * with the suffix removed. Otherwise, returns a new char sequence with the same characters.
@@ -652,6 +674,28 @@ public fun String.removeSuffix(suffix: CharSequence): String {
     return this
 }
 
+/**
+ * If the char sequence not ends with the given [suffix], returns a copy of this string
+ * with the suffix appended. Otherwise, returns a new char sequence with the same characters.
+ */
+public fun CharSequence.ensureSuffix(suffix: CharSequence): CharSequence {
+    if (!endsWith(suffix)) {
+        return this.toString() + suffix.toString()
+    }
+    return subSequence(0, length)
+}
+
+/**
+ * If the string not ends with the given [suffix], returns a copy of this string
+ * with the suffix appended. Otherwise, returns this string.
+ */
+public fun String.ensureSuffix(suffix: CharSequence): String {
+    if (!endsWith(suffix)) {
+        return this + suffix.toString()
+    }
+    return this
+}
+
 /**
  * When this char sequence starts with the given [prefix] and ends with the given [suffix],
  * returns a new char sequence having both the given [prefix] and [suffix] removed.

libraries/stdlib/test/text/StringTest.kt

@@ -596,6 +596,18 @@ class StringTest {
         assertEquals("sample", "sample".removeSuffix(""))
     }
 
+    @Test fun ensureSuffix() = withOneCharSequenceArg("fix") { suffix ->
+        assertEquals("suffix", "suf".ensureSuffix(suffix), "Appends suffix")
+        assertEquals("suffix", "suffix".ensureSuffix(suffix))
+        assertEquals("sample", "sample".ensureSuffix(""))
+    }
+
+    @Test fun ensurePrefix() = withOneCharSequenceArg("suf") { prefix ->
+        assertEquals("suffix", "fix".ensurePrefix(prefix), "Appends prefix")
+        assertEquals("suffix", "suffix".ensurePrefix(prefix))
+        assertEquals("sample", "sample".ensurePrefix(""))
+    }
+
     @Test fun removeSurrounding() = withOneCharSequenceArg { arg1 ->
         val pre = arg1("<")
         val post = arg1(">")
@@ -628,6 +640,24 @@ class StringTest {
         assertContentEquals("sample", "sample".removeSuffix(""))
     }
 
+    @Test fun ensurePrefixCharSequence() = withTwoCharSequenceArgs { arg1, arg2 ->
+        fun String.ensurePrefix(prefix: String) = arg1(this).ensurePrefix(arg2(prefix))
+        val prefix = "suf"
+
+        assertContentEquals("suffix", "fix".ensurePrefix(prefix), "Appends prefix")
+        assertContentEquals("suffix", "suffix".ensurePrefix(prefix))
+        assertContentEquals("sample", "sample".ensurePrefix(""))
+    }
+
+    @Test fun ensureSuffixCharSequence() = withTwoCharSequenceArgs { arg1, arg2 ->
+        fun String.ensureSuffix(suffix: String) = arg1(this).ensureSuffix(arg2(suffix))
+        val suffix = "fix"
+
+        assertContentEquals("suffix", "suf".ensureSuffix(suffix), "Appends suffix")
+        assertContentEquals("suffix", "suffix".ensureSuffix(suffix))
+        assertContentEquals("sample", "sample".ensureSuffix(""))
+    }
+
     @Test fun removeSurroundingCharSequence() = withTwoCharSequenceArgs { arg1, arg2 ->
         fun String.removeSurrounding(prefix: String, postfix: String) = arg1(this).removeSurrounding(arg2(prefix), arg2(postfix))