More efficient double generation in JS

libraries/stdlib/js/src/kotlin/random/PlatformRandom.kt

@@ -5,6 +5,8 @@
 
 package kotlin.random
 
+import kotlin.math.pow
+
 internal actual fun defaultPlatformRandom(): Random =
     Random(js("(Math.random() * Math.pow(2, 32)) | 0").unsafeCast<Int>())
 
@@ -19,3 +21,8 @@ internal actual fun fastLog2(value: Int): Int {
     }
     return log
 }
+
+private val INV_2_26: Double = 2.0.pow(-26)
+private val INV_2_53: Double = 2.0.pow(-53)
+internal actual fun doubleFromParts(hi26: Int, low27: Int): Double =
+    hi26 * INV_2_26 + low27 * INV_2_53

libraries/stdlib/jvm/src/kotlin/random/PlatformRandom.kt

@@ -31,6 +31,9 @@ internal actual inline fun defaultPlatformRandom(): Random =
 internal actual fun fastLog2(value: Int): Int =
     31 - Integer.numberOfLeadingZeros(value)
 
+internal actual fun doubleFromParts(hi26: Int, low27: Int): Double =
+    (hi26.toLong().shl(27) + low27) / (1L shl 53).toDouble()
+
 
 internal abstract class AbstractPlatformRandom : Random() {
     abstract val impl: java.util.Random

libraries/stdlib/src/kotlin/random/Random.kt

@@ -179,7 +179,7 @@ public abstract class Random {
     /**
      * Gets the next random [Double] value uniformly distributed between 0 (inclusive) and 1 (exclusive).
      */
-    public open fun nextDouble(): Double = (nextBits(26).toLong().shl(27) + nextBits(27)) / (1L shl 53).toDouble()
+    public open fun nextDouble(): Double = doubleFromParts(nextBits(26), nextBits(27))
 
     /**
      * Gets the next random non-negative `Double` from the random number generator not greater than the specified [bound].
@@ -327,6 +327,7 @@ public fun Random(seed: Long): Random = XorWowRandom(seed.toInt(), seed.shr(32).
 
 internal expect fun defaultPlatformRandom(): Random
 internal expect fun fastLog2(value: Int): Int //  31 - Integer.numberOfLeadingZeros(value)
+internal expect fun doubleFromParts(hi26: Int, low27: Int): Double
 
 /** Takes upper [bitCount] bits (0..32) from this number. */
 internal fun Int.takeUpperBits(bitCount: Int): Int =