File I/O — Read, Write, and Process

import java.io.File
import java.nio.file.*
import java.io.BufferedReader

fun main() {
    val dir = File("temp-demo").also { it.mkdirs() }

    // Write
    val file = File(dir, "example.txt")
    file.writeText("Hello, Kotlin!\nLine 2\nLine 3")
    println("Written: ${file.absolutePath}")

    // Append
    file.appendText("\nLine 4 (appended)")

    // Read entire file
    val content = file.readText()
    println("Content:\n$content")

    // Read lines (into list)
    val lines = file.readLines()
    println("\nLines: ${lines.size}")
    lines.forEachIndexed { i, line -> println("  ${i + 1}: $line") }

    // useLines — lazy line-by-line (memory efficient)
    println("\nFiltered lines:")
    file.useLines { sequence ->
        sequence
            .filter { it.contains("Line") }
            .map { it.uppercase() }
            .forEach { println("  $it") }
    }

    // Buffered reader
    file.bufferedReader().use { reader: BufferedReader ->
        val first = reader.readLine()
        println("\nFirst line: $first")
    }

    // Write structured data
    data class Record(val name: String, val score: Int)
    val records = listOf(
        Record("Alice", 95),
        Record("Bob", 82),
        Record("Charlie", 91)
    )
    val csvFile = File(dir, "scores.csv")
    csvFile.printWriter().use { out ->
        out.println("name,score")
        records.forEach { out.println("${it.name},${it.score}") }
    }
    println("\nCSV written: ${csvFile.readText()}")

    // File properties
    println("\nFile info:")
    println("  Name: ${file.name}")
    println("  Size: ${file.length()} bytes")
    println("  Exists: ${file.exists()}")
    println("  Is file: ${file.isFile}")
    println("  Extension: ${file.extension}")
    println("  Name without ext: ${file.nameWithoutExtension}")

    // Walk directory tree
    println("\nDirectory tree:")
    dir.walkTopDown().forEach { f ->
        val indent = "  ".repeat(f.relativeTo(dir).path.count { it == File.separatorChar })
        println("$indent${f.name}${if (f.isDirectory) "/" else ""}")
    }

    // Copy and move
    val copy = File(dir, "example-copy.txt")
    file.copyTo(copy, overwrite = true)
    println("\nCopied to: ${copy.name}")

    // NIO Path operations
    val nioPath = Path.of(dir.path, "nio-test.txt")
    Files.writeString(nioPath, "NIO content")
    println("NIO read: ${Files.readString(nioPath)}")

    // Cleanup
    dir.deleteRecursively()
    println("\nCleaned up demo directory")
}