Can we kotlinify Java’s `Objects.hash(foo, bar, ba...

# stdlib

[Marc Knaup]

Can we kotlinify Java’s Objects.hash(foo, bar, baz, …) to something like this?
hash { foo x bar x baz x … }

Just the boxing is a little tricky as well as getting it to work with just a single non-Int value without introducing boxing.

Implementation:

inline fun hash(block: HashScope.() -> Int) =
	HashScope.block()

object HashScope {

	inline infix fun Any?.x(hashable: Any?) =
		(31 * hashCode()) + hashable.hashCode()

	inline infix fun Any?.x(hashable: Int) =
		(31 * hashCode()) + hashable

	inline infix fun Int.x(hashable: Any?) =
		(31 * this) + hashable.hashCode()

	inline infix fun Int.x(hashable: Int) =
		(31 * this) + hashable
}

class Example(val foo: String, val bar: List<Any>?, val baz: Int) {

	override fun hashCode() =
		hash { foo x bar x baz }
}

[jw]

What's the advantage over just hashOf(vararg args: Any?) which seems more idiomatic than a somewhat-cryptic DSL? Then you can add an intrinsic in the compiler, similar to what's being explored in javac.

[Marc Knaup]

Java’s isn’t multiplatform.
Also, doesn’t Objects.hash() cause a lot of wrapping (primitives/inline classes)? + additional array and ArrayList allocation.
Or are calls replaced by something intrinsic at runtime?

[Marc Knaup]

Also intrinsic would need compiler support. My suggestion would be pure Kotlin within stdlib.

[jw]

Have you observed the boxing to be a problem in practice?

[jw]

Mine also is a pure Kotlin suggestion that could be aliases to the JDK version for the JVM. The intrinsic is only for increasing performance, probably through invoke dynamic.

[Marc Knaup]

I can only judge from the Kotlin decompiler in IntelliJ that a lot of wrapping and one array allocation would occur.
I don’t know whether that will actually disappear at runtime.

[marcinmoskala]

@Marc Knaup here is a simple and multiplatform implementation. Still thinking about the name though 🤔 @jw What do you think?

override fun hashCode(): Int =
    hashCodeFrom(timeZone, millis)

inline fun hashCodeFrom(vararg values: Any?) =
    values.fold(0) { acc, value ->
        (acc * 31) + value.hashCode()
    }