Well, hdb is pretty close, except there's not really a need to guess or search through the array. The smallest step in the compared values is 
$ perl -we 'print log(78894000000/63115200000)'
0.22314355131421

Using a suitably small log base, one is assured that no more than one value step can occur between log()..log()+1. Here, int(4.5*log(...)) will do nicely. 

#! /usr/bin/perl -l

use List::Util qw(first min max);
use Data::Dumper;

my @G = (
    [ 25000,        2500        ],
    [ 50000,        5000        ],
    [ 150000,       12500       ],
    [ 225000,       25000       ],
    [ 300000,       37500       ],
    [ 600000,       60000       ],
    [ 1200000,      120000      ],
    [ 3600000,      300000      ],
    [ 5400000,      600000      ],
    [ 10800000,     900000      ],
    [ 21600000,     1800000     ],
    [ 43200000,     3600000     ],
    [ 64800000,     7200000     ],
    [ 129600000,    10800000    ],
    [ 216000000,    21600000    ],
    [ 432000000,    43200000    ],
    [ 864000000,    86400000    ],
    [ 1728000000,   172800000   ],
    [ 3024000000,   345600000   ],
    [ 6048000000,   604800000   ],
    [ 12096000000,  1209600000  ],
    [ 31557600000,  2629800000  ],
    [ 63115200000,  5259600000  ],
    [ 78894000000,  7889400000  ],
    [ 157788000000, 15778800000 ],
);

# construct the lookup table.
my @TT = map { my $i=$_; first { $i <= tti($_->[0]) } @G } 0..tti(~0);
push @TT, [~0, 31557600000];
print Dumper \@TT;

sub tti { -45 + int (4.5 * log(min(max(25000,shift),157788000000))) }

sub v2r {
    my ($v, $i) = ($_[0], tti($_[0]));

    $TT[$i + ($v >= $TT[$i][0])][1];
}

for (map { $_-1, $_, int 1.2*$_ } map $_->[0], @G) {
    print "v2r($_) == @{[v2r($_)]}";
}

[download]

Split array and combined array variants ought be benched as well. In practice, computing the log might be more cumbersome than a small search.

In reply to Re: A better way of lookup? by Anonymous Monk
in thread A better way of lookup? by BrowserUk

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