cbm_float.ts

import { hex } from "./utils";

/*
 *  IEEE double precision to CBM 5 byte float conversion
 *
 *  written 2008-06-19 by Michael Kircher
 */

export function CBMFloat(number: number/*S: string*/): string[]
{
  //let number = Number(S); // atof

  let cbm_mantissa: number;
  let cbm_exponent: number;

  let [mantissa, exponent] = frexp(number);

  cbm_mantissa = (4294967296.0*Math.abs(mantissa)) & 0x7FFFFFFF + 2147483648*(mantissa<0?1:0);
  cbm_exponent = 128+exponent;

  if(number == 0.0)
  {
     cbm_exponent = 0;
     cbm_mantissa = 0;
  }  

  let R = [ 
     "$" + hex((cbm_exponent      ) & 0xFF) ,
     "$" + hex((cbm_mantissa >> 24) & 0xFF) ,
     "$" + hex((cbm_mantissa >> 16) & 0xFF) ,
     "$" + hex((cbm_mantissa >>  8) & 0xFF) ,
     "$" + hex((cbm_mantissa >>  0) & 0xFF)
   ];

  return R;
}

// taken from http://locutus.io/c/math/frexp/
function frexp(arg: number) {
   //  discuss at: http://locutus.io/c/frexp/
   // original by: Oskar Larsson Högfeldt (http://oskar-lh.name/)
   //      note 1: Instead of
   //      note 1: double frexp( double arg, int* exp );
   //      note 1: this is built as
   //      note 1: [double, int] frexp( double arg );
   //      note 1: due to the lack of pointers in JavaScript.
   //      note 1: See code comments for further information.
   //   example 1: frexp(1)
   //   returns 1: [0.5, 1]
   //   example 2: frexp(1.5)
   //   returns 2: [0.75, 1]
   //   example 3: frexp(3 * Math.pow(2, 500))
   //   returns 3: [0.75, 502]
   //   example 4: frexp(-4)
   //   returns 4: [-0.5, 3]
   //   example 5: frexp(Number.MAX_VALUE)
   //   returns 5: [0.9999999999999999, 1024]
   //   example 6: frexp(Number.MIN_VALUE)
   //   returns 6: [0.5, -1073]
   //   example 7: frexp(-Infinity)
   //   returns 7: [-Infinity, 0]
   //   example 8: frexp(-0)
   //   returns 8: [-0, 0]
   //   example 9: frexp(NaN)
   //   returns 9: [NaN, 0]
 
   // Potential issue with this implementation:
   // the precisions of Math.pow and the ** operator are undefined in the ECMAScript standard,
   // however, sane implementations should give the same results for Math.pow(2, <integer>) operations
 
   // Like frexp of C and std::frexp of C++,
   // but returns an array instead of using a pointer argument for passing the exponent result.
   // Object.is(n, frexp(n)[0] * 2 ** frexp(n)[1]) for all number values of n except when Math.isFinite(n) && Math.abs(n) > 2**1023
   // Object.is(n, (2 * frexp(n)[0]) * 2 ** (frexp(n)[1] - 1)) for all number values of n
   // Object.is(n, frexp(n)[0]) for these values of n: 0, -0, NaN, Infinity, -Infinity
   // Math.abs(frexp(n)[0]) is >= 0.5 and < 1.0 for any other number-type value of n
   // See http://en.cppreference.com/w/c/numeric/math/frexp for a more detailed description
 
   arg = Number(arg)
 
   const result = [arg, 0]
 
   if (arg !== 0 && Number.isFinite(arg)) {
     const absArg = Math.abs(arg)
     // Math.log2 was introduced in ES2015, use it when available
     const log2 = Math.log2 || function log2 (n) { return Math.log(n) * Math.LOG2E }
     let exp = Math.max(-1023, Math.floor(log2(absArg)) + 1)
     let x = absArg * Math.pow(2, -exp)
 
     // These while loops compensate for rounding errors that sometimes occur because of ECMAScript's Math.log2's undefined precision
     // and also works around the issue of Math.pow(2, -exp) === Infinity when exp <= -1024
     while (x < 0.5) {
       x *= 2
       exp--
     }
     while (x >= 1) {
       x *= 0.5
       exp++
     }
 
     if (arg < 0) {
       x = -x
     }
     result[0] = x
     result[1] = exp
   }
   return result
}