Kotlin File extension for easily editing the contents of a file

FileEditor.kt

/**
 * Edit the contents of a File with a [FileEditor]. The new contents will override the old.
 *
 * @param lineTransform Called with each line of the file in order.
 * Return the new value you would like written in the file, or null to delete the line.
 * You can also call [FileEditor.insertNext] to insert a line immediately above the given line, or
 * [FileEditor.insertPrevious] to insert a line immediately after the given line.
 */
fun File.edit(lineTransform: FileEditor.(line: String) -> String) {
    FileEditor(this).edit(lineTransform)
}

/** Used from [File.edit]. */
class FileEditor(val file: File) {
    private var lines: MutableList<String> = file.readLines().toMutableList()
    private val lineCount: Int get() = lines.size
    private var currentLineIndex = 0

    fun insertPrevious(line: String) {
        lines.add(currentLineIndex, line)
        currentLineIndex++
    }

    fun insertNext(line: String) {
        lines.add(currentLineIndex + 1, line)
    }

    fun edit(lineTransform: FileEditor.(line: String) -> String?): File {
        currentLineIndex = 0

        while (currentLineIndex < lineCount) {
            val line = lines.getOrNull(currentLineIndex) ?: throw IllegalStateException("No line at $currentLineIndex")
            val newLine = lineTransform(line)
            if (newLine == null) {
                lines.removeAt(currentLineIndex)
            } else {
                lines[currentLineIndex] = newLine
                currentLineIndex++
            }
        }

        return file.replaceContents(lines)
    }
}

/**
 * Replaces the contents of this file with the given lines.
 * Overwrites the old file and returns a new File instance with the new content.
 */
fun File.replaceContents(lines: List<String>): File {
    val tempFile = createTempFile()
    tempFile.printWriter().use { writer ->
        lines.forEach { writer.println(it) }
    }
    check(this.delete() && tempFile.renameTo(this)) { "failed to replace file" }
    return tempFile
}