stdlib: Add indentation extensions for strings

backend.native/tests/external/stdlib/text/StringTest/trimIndent.kt

@@ -60,7 +60,12 @@ ${"    "}
     """.trimIndent()
 
     assertEquals(23, deindented.lines().size)
-    val indents = deindented.lines().map { "^\\s*".toRegex().find(it)!!.value.length }
+    val indents = deindented.lines().map {
+        var i = 0
+        while (i < it.length && it[i].isWhitespace())
+            i++
+        return@map i
+    }
     assertEquals(0, indents.min())
     assertEquals(42, indents.max())
     assertEquals(1, deindented.lines().count { it.isEmpty() })

runtime/src/main/kotlin/kotlin/text/Indent.kt

@@ -0,0 +1,108 @@
+package kotlin.text
+
+/**
+ * Trims leading whitespace characters followed by [marginPrefix] from every line of a source string and removes
+ * the first and the last lines if they are blank (notice difference blank vs empty).
+ *
+ * Doesn't affect a line if it doesn't contain [marginPrefix] except the first and the last blank lines.
+ *
+ * Doesn't preserve the original line endings.
+ *
+ * @param marginPrefix non-blank string, which is used as a margin delimiter. Default is `|` (pipe character).
+ *
+ * @sample samples.text.Strings.trimMargin
+ * @see trimIndent
+ * @see kotlin.text.isWhitespace
+ */
+public fun String.trimMargin(marginPrefix: String = "|"): String =
+    replaceIndentByMargin("", marginPrefix)
+
+/**
+ * Detects indent by [marginPrefix] as it does [trimMargin] and replace it with [newIndent].
+ *
+ * @param marginPrefix non-blank string, which is used as a margin delimiter. Default is `|` (pipe character).
+ */
+public fun String.replaceIndentByMargin(newIndent: String = "", marginPrefix: String = "|"): String {
+    require(marginPrefix.isNotBlank()) { "marginPrefix must be non-blank string." }
+    val lines = lines()
+
+    return lines.reindent(length + newIndent.length * lines.size, getIndentFunction(newIndent), { line ->
+        val firstNonWhitespaceIndex = line.indexOfFirst { !it.isWhitespace() }
+
+        when {
+            firstNonWhitespaceIndex == -1 -> null
+            line.startsWith(marginPrefix, firstNonWhitespaceIndex) -> line.substring(firstNonWhitespaceIndex + marginPrefix.length)
+            else -> null
+        }
+    })
+}
+
+/**
+ * Detects a common minimal indent of all the input lines, removes it from every line and also removes the first and the last
+ * lines if they are blank (notice difference blank vs empty).
+ *
+ * Note that blank lines do not affect the detected indent level.
+ *
+ * In case if there are non-blank lines with no leading whitespace characters (no indent at all) then the
+ * common indent is 0, and therefore this function doesn't change the indentation.
+ *
+ * Doesn't preserve the original line endings.
+ *
+ * @sample samples.text.Strings.trimIndent
+ * @see trimMargin
+ * @see kotlin.text.isBlank
+ */
+public fun String.trimIndent(): String = replaceIndent("")
+
+/**
+ * Detects a common minimal indent like it does [trimIndent] and replaces it with the specified [newIndent].
+ */
+public fun String.replaceIndent(newIndent: String = ""): String {
+    val lines = lines()
+
+    val minCommonIndent = lines
+            .filter(String::isNotBlank)
+            .map(String::indentWidth)
+            .min() ?: 0
+
+    return lines.reindent(length + newIndent.length * lines.size, getIndentFunction(newIndent), { line -> line.drop(minCommonIndent) })
+}
+
+/**
+ * Prepends [indent] to every line of the original string.
+ *
+ * Doesn't preserve the original line endings.
+ */
+public fun String.prependIndent(indent: String = "    "): String =
+    lineSequence()
+    .map {
+        when {
+            it.isBlank() -> {
+                when {
+                    it.length < indent.length -> indent
+                    else -> it
+                }
+            }
+            else -> indent + it
+        }
+    }
+    .joinToString("\n")
+
+private fun String.indentWidth(): Int = indexOfFirst { !it.isWhitespace() }.let { if (it == -1) length else it }
+
+private fun getIndentFunction(indent: String) = when {
+    indent.isEmpty() -> { line: String -> line }
+    else -> { line: String -> indent + line }
+}
+
+private inline fun List<String>.reindent(resultSizeEstimate: Int, indentAddFunction: (String) -> String, indentCutFunction: (String) -> String?): String {
+    val lastIndex = lastIndex
+    return mapIndexedNotNull { index, value ->
+            if ((index == 0 || index == lastIndex) && value.isBlank())
+                null
+            else
+                indentCutFunction(value)?.let(indentAddFunction) ?: value
+        }
+        .joinTo(StringBuilder(resultSizeEstimate), "\n")
+        .toString()
+}