Unpacks the flags of an Int by name

flagUnwrapper.kt

package com.cyphercove.flagunwrapper

import java.lang.reflect.Modifier
import kotlin.reflect.KClass

/** Return a set of String field names for flags used in the input value.
 *
 * Usage:
 * ```
 * Log.d("Flags", unwrapFlags(WindowManager.LayoutParams::class, 2097152).joinToString())
 * ```
 *
 * @param cls The class containing the constant flag values.
 * @param flags The Int containing the flag values
 * @param fieldNamePrefix The start of every field name that is a flag value.
 */
fun unwrapFlags(cls: KClass<*>, flags: Int, fieldNamePrefix: String = "FLAG_"): Set<String> {
    val flagValues = cls.java.declaredFields
        .filter {
            Modifier.isStatic(it.modifiers) &&
                    Modifier.isPublic(it.modifiers) &&
                    Modifier.isFinal(it.modifiers) &&
                    it.type == Int::class.javaPrimitiveType &&
                    it.getInt(null).toBits().count { bit -> bit } == 1 &&
                    it.name.startsWith(fieldNamePrefix)
        }.map {
            val bits = it.getInt(null).toBits()
            val bitIndex = (0..31).first { i -> bits[i] }
            it.name to bitIndex
        }
    val bits = flags.toBits()
    val usedFlags = mutableSetOf<String>()
    for (i in 0..31) {
        if (bits[i]) {
            flagValues.firstOrNull { it.second == i }?.first?.run {
                usedFlags += this
            }
        }
    }
    return usedFlags
}

fun Int.toBits(): BooleanArray {
    val bits = BooleanArray(32)
    for (i in 31 downTo 0) {
        bits[i] = this and (1 shl i) != 0
    }
    return bits
}

fun BooleanArray.toBitString() = joinToString("") { if (it) "1" else "0" }