cljs.math/copy-sign

function

since v1.11.50

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Source docstring:

Returns a double with the magnitude of the first argument and the sign of
the second.
See: https://docs.oracle.com/javase/8/docs/api/java/lang/Math.html#copySign-double-double-

(defn ^number copy-sign
  {:added "1.11.10"}
  [magnitude sign]
  ;; create a buffer large enough for 2 doubles
  (let [a (js/ArrayBuffer. 16)
        ;; represent the buffer as a double array
        d (js/Float64Array. a)
        ;; represent the buffer as bytes
        b (js/Uint8Array. a)
        ;; find the offset of the byte that holds the sign bit
        sbyte (if little-endian? 7 0)]
    ;; the first double holds the magnitude, the second holds the sign value
    (aset d 0 magnitude)
    (aset d 1 sign)
    ;; read the sign bit from the sign value
    (let [sign-sbyte (bit-and 0x80 (aget b (+ 8 sbyte)))
          ;; read all the bits that aren't the sign bit in the same byte of the magnitude
          mag-sbyte (bit-and 0x7F (aget b sbyte))]
      ;; combine the sign bit from the sign value and the non-sign-bits from the magnitude value
      ;; write it back into the byte in the magnitude
      (aset b sbyte (bit-or sign-sbyte mag-sbyte))
      ;; retrieve the full magnitude value with the updated byte
      (aget d 0))))

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