js-hexfloat

js-hexfloat

Rudimentary C99 Hexadecimal Float Support in JS

SYNOPSIS

var pi = parseHexFloat('0x1.921fb54442d18p+1'); // 3.14159265358982
var piHex = Math.PI.toHexString();              // '0x1.921fb54442d18p+1'
parseHexFloat(piHex) == Math.PI;                // true

DESCRIPTION

This script adds the following:

parseHexFloat(theString)

Parses C99 hexadecimal floating point in theString. Returns NaN if fails.

Unlike parseInt and parseFloat, the number must be prepended with '0x' and ended with 'p' and exponent in decimal number.

also available as Number.parseHexFloat().

RE_HEXFLOAT

RegExp object used in parseHexFloat:

/([\+\-]?)0x([0-9A-F]+).?([0-9A-F]*)p([\+\-]?[0-9]*)/i

Number.prototype.toHexString()

Stringifies the number as a C99 hexadecimal notation like "%a" in C99 sprintf().

Why Canonical Form?

From version 0.3.0, .toHexString() always returns the canonical notation.

Math.PI.toString(16);   // 3.243f6a8885a3 -> 0x3.243f6a8885a3p0 is valid yet uncanonical
Math.PI.toHexString();  // 0x1.921fb54442d18p+1 is valid and canonical

It seems ok to just prepend '0x' and append 'p0' to .toString(16) to make a hex float. Turns out it isn't. It sometimes drops the last bit in some platforms.

Math.log(2).toString(16)  // 0.b17217f7d1cf78
Math.log(2).toHexString() // 0x1.62e42fefa39efp-1

% perl -E 'say 0x0.b17217f7d1cf78p0 - 0x1.62e42fefa39efp-1'
-1.11022302462516e-16
% perl -E 'say sprintf "%a", 0x0.b17217f7d1cf78p0 - 0x1.62e42fefa39efp-1'
-0x1p-53

And abs(-0x1p-53) is DBL_EPSILON.

SEE ALSO

• http://en.cppreference.com/w/c/language/floating_constant
• https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html
• http://search.cpan.org/dist/perl-5.22.0/pod/perldelta.pod#Floating_point_parsing_has_been_improved