#!/usr/bin/env perl
use strict;
use warnings;

use Math::BigFloat;
use Math::BigInt;
use Math::Int128 qw(int128 int128_to_hex);
use Math::Int128::die_on_overflow;
$\ = $/;

# https://www.youtube.com/watch?v=S12qx1DwjVk
my $SCALE_BITS = 124;
my $SCALE = Math::BigInt->new(2)->bpow($SCALE_BITS);
print "  Scale: 2**$SCALE_BITS = $SCALE";

sub number2fixed { # Math::Big* --> fixed-pt Math::Int128
    return int128( shift->copy->bmul($SCALE)->as_int->bstr );
}
sub fixed2number { # fixed-pt Math::Int128 --> Math::BigFloat
    return scalar Math::BigFloat->new(''.shift)->bdiv($SCALE);
}

my $EPSILON = fixed2number(int128(1));
print "Epsilon: ", $EPSILON->bstr;

print "//// example: Pi";
my $pi1 = Math::BigFloat->bpi(45);
print " orig: $pi1";
my $fpi = number2fixed($pi1);
print "fixed: ", int128_to_hex($fpi);
my $pi2 = fixed2number($fpi);
print " conv: $pi2";

# https://www.youtube.com/watch?v=npQF28g6s_k
sub mul_fixed { # 2x Math::Int128 --> Math::Int128
    return (shift>>($SCALE_BITS/2)) * (shift>>($SCALE_BITS/2));
}

print "//// properties of mul_fixed";
print "smallest possible multiplicand: ",
    fixed2number(int128(1)<<($SCALE_BITS/2));
# demo effect of the smallest possible multiplicand
my $mul = number2fixed(Math::BigInt->new(3));
my $toosmall = number2fixed(
    Math::BigFloat->new("0.000000000000000000216"));
print "  too small: ", int128_to_hex($toosmall);
print "\t* 3 = ", fixed2number(mul_fixed($mul,$toosmall));
my $justenough = number2fixed(
    Math::BigFloat->new("0.000000000000000000217") );
print "just enough: ", int128_to_hex($justenough);
print "\t* 3 = ", fixed2number(mul_fixed($mul,$justenough));

print "//// test case: (45e-10)**2";
my $val = Math::BigFloat->new("0.0000000045");
print " input value: ", $val->bstr;
my $fval = number2fixed($val);
print " as fixed-pt: ", int128_to_hex($fval);
my $p = mul_fixed($fval,$fval);
print "fixed-pt mul: ", int128_to_hex($p);
print "   converted: ", fixed2number($p);
print "    expected: ", $val->copy->bpow(2);