Price fraction: Difference between revisions

=={{header|Mercury}}==

<lang Mercury>:- module price.

:- interface.

:- import_module int.

:- type price == int.

:- func standard(price) = price.

:- implementation.

:- import_module require, list.

standard(P) = SP :-

require(P >= 0, "P must be positive"),

Cents = P `mod` 100,

P + adjust(Cents) = SP.

:- func adjust(int) = int.

adjust(Cents) = adjust(Cents, rules).

:- func adjust(int, list(price_rule)) = int.

adjust(_, []) = unexpected("price", "adjust/2", "exhausted rules").

adjust(N, [rule(Low, High, To)|T]) = R :-

( N >= Low, N < High -> To - N = R ; adjust(N, T) = R ).

:- type price_rule ---> rule(int, int, int).

:- func rules = list(price_rule).

rules = [rule(00, 06, 10),

rule(06, 11, 18),

rule(11, 16, 26),

rule(16, 21, 32),

rule(21, 26, 38),

rule(26, 31, 44),

rule(31, 36, 50),

rule(36, 41, 54),

rule(41, 46, 58),

rule(46, 51, 62),

rule(51, 56, 66),

rule(56, 61, 70),

rule(61, 66, 74),

rule(66, 71, 78),

rule(71, 76, 82),

rule(76, 81, 86),

rule(81, 86, 90),

rule(86, 91, 94),

rule(91, 96, 98),

rule(96, 101, 100)].</lang>

A build system might turn the text of the table into the definition of a hundred-element array of adjustments. In that case,

<lang Mercury>adjust(Cents) = array.lookup(price_table, Cents).</lang>