floating point number from crypto.randomBytes() in javascript

Question

How do I convert the array of bytes returned from crypto.randomBytes into a to a floating point number? I'm writing a small replacement for Math.random()

Answer 1

Suppose you have a series of bytes randomly selected with uniform distribution over [0, 256). Take seven of those bytes, say a ₀ , a ₁ ,… a ₆ . Calculate (((((((a ₆ % 32)/32 + a ₅ )/256 + a ₄ )/256 + a ₃ )/256 + a ₂ )/256 + a ₁ )/256 + a ₀ )/256.

Explanation: a ₆ % 32 denotes the residue of a ₆ modulo 32. This takes five bits of a ₆ . Then division by 32 “shifts” these bits to the right of the radix point, eight new bits are added, a division by 256 “shifts” right eight bits, and so on until we have 53 bits.

This provides 2 ⁵³ possible results in [0, 1). It is possible to provide more since the floating-point resolution is finer as values get closer to zero, but then there are problems with uniformity and other issues.