Testing

The library's tests

tests/Unit holds TaxRateTest, TaxBreakdownTest and MultiRateTaxCalculatorTest: rounding on both sides of the half, on negated bases and in currencies of 0, 2 and 3 decimals; the largest base taxed and the next refused; codes, shares and percentages accepted and refused, percentages read and printed back; breakdowns on one base, refused on another base, in another currency or with a code twice; reported amounts; reversal line by line; taxes taken out of a price, the base rounded on both sides of the half, a negated price, the largest price and the next refused.

From the root of the monorepo:

vendor/bin/phpunit packages/tax/tests

Installed on its own:

composer install
vendor/bin/phpunit --no-configuration tests

Mutation testing holds the library to the bar of a domain layer: every mutant of src/ is killed.

Testing code that taxes

• Assert on minor units, line by line: self::assertSame(199, $breakdown->line('QST')?->amount->minorUnits).
• Keep a case whose lines round in opposite directions, such as 19.99 at 5 % and 9.975 %: a computation that rounds the sum of the rates once, or compounds them, fails it.
• Assert that a reversal is the exact negation: $credit->totalWithTax()->equals($invoice->totalWithTax()->negate()).
• The library has no clock, container or I/O: no double is needed.

Benchmarks

The phpbench benchmarks in tests/Benchmark/ measure calculate() and extract() over carts of one to two hundred lines, with one to three rates. From the monorepo root:

composer bench -- packages/tax/tests/Benchmark