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Kotlin Standard Library for JVM

There is a newer version: 2.1.0-Beta1

package kotlin

import java.util.Comparator

/**
* Helper method for implementing [[Comparable]] methods using a list of functions
* to calculate the values to compare
*/
fun  compareBy(a: T?, b: T?, vararg functions: T.() -> Comparable<*>?): Int {
    require(functions.size > 0)
    if (a === b) return 0
    if (a == null) return - 1
    if (b == null) return 1
    for (fn in functions) {
        val v1 = a.fn()
        val v2 = b.fn()
        val diff = compareValues(v1, v2)
        if (diff != 0) return diff
    }
    return 0
}

/**
 * Compares the two values which may be [[Comparable]] otherwise
 * they are compared via [[#equals()]] and if they are not the same then
 * the [[#hashCode()]] method is used as the difference
 */
public fun > compareValues(a: T?, b: T?): Int {
    if (a === b) return 0
    if (a == null) return - 1
    if (b == null) return 1

    return (a as Comparable).compareTo(b)
}

/**
 * Creates a comparator using the sequence of functions used to calculate a value to compare on
 */
public fun  comparator(vararg functions: T.() -> Comparable<*>?): Comparator {
    return FunctionComparator(*functions)
}


private class FunctionComparator(vararg val functions: T.() -> Comparable<*>?):  Comparator {

    public override fun toString(): String {
        return "FunctionComparator${functions.toList()}"
    }

    public override fun compare(o1: T, o2: T): Int {
        return compareBy(o1, o2, *functions)
    }

    public override fun equals(obj: Any?): Boolean {
        return this == obj
    }
}

/**
 * Creates a comparator using the sequence of functions used to calculate a value to compare on
 */
public fun  comparator(fn: (T,T) -> Int): Comparator {
    return Function2Comparator(fn)
}
private class Function2Comparator(val compareFn: (T,T) -> Int):  Comparator {

    public override fun toString(): String {
        return "Function2Comparator${compareFn}"
    }

    public override fun compare(a: T, b: T): Int {
        if (a === b) return 0
        if (a == null) return - 1
        if (b == null) return 1
        return (compareFn)(a, b)
    }

    public override fun equals(obj: Any?): Boolean {
        return this == obj
    }
}