1Open Location Code

An Open Location Code library for Common Lisp.

1.1Description

Open Location Code is a location encoding system for addresses, independent of street names and building numbers. The encoded location identifier is called Plus Code since it always contains a plus sign character. For example, 8FX8QJ2G+QH is the Plus Code of the Roman bridge in Trier, Germany.

See <https://plus.codes>.

The methods requested by the Open Location Code specification are implemented as follows:

• encode (latitude /longitude/) ⇒ full-code \\Convert a latitude and longitude into a 10 digit Plus Code.
• encode (latitude /longitude/ precision) ⇒ full-code \\Convert a latitude and longitude into an arbitrary length Plus Code.
• decode (code) ⇒ code-area \\Decode a Plus Code into, at a minimum, the latitude and longitude ofthe south-west corner and the area's height and width.
• validp (code) ⇒ boolean \\Determine if a string is a valid sequence of Plus Code characters.
• fullp (code) ⇒ boolean \\Determine if a string is a valid full Plus Code.
• shortp (code) ⇒ boolean \\Determine if a string is a valid short Plus Code.
• shorten (full-code /latitude/ longitude) ⇒ short-code \\Remove four or six digits from the front of a Plus Code given areference location.
• recover (short-code /latitude/ longitude) ⇒ full-code \\Recover a full Plus Code from a short code and a reference location.

See <https://github.com/google/open-location-code/> for more documentation and sample implementations.

1.2Installation

The Open Location Code library is available in Quicklisp.

(ql:quickload "open-location-code")

The test suite can be run by evaluating the following form.

(asdf:test-system "open-location-code")

1.3Notes

The Open Location Code library uses integer arithmetic for converting locations to Plus Codes and vice versa. The Common Lisp rationalize function is used to convert locations given in floating-point numbers into rational numbers.

The Google developers finally realized that floating-point arithmetic is inaccurate by definition. They are now in the transition phase to integer arithmetic. However, there is IMHO still some inconsistency between the integer algorithms and the numbers in the test data. Due to that, one encoding test fails with an off-by-one error for the time being:

 | Failed Form: (OPEN-LOCATION-CODE:ENCODE OPEN-LOCATION-CODE-TESTS::LAT
                                           OPEN-LOCATION-CODE-TESTS::LON
                                           OPEN-LOCATION-CODE-TESTS::PREC)
 | Expected "CHGW2H5C+X2RRRRR" but saw "CHGW2H5C+X2RRRRJ"
 | OPEN-LOCATION-CODE-TESTS::LAT => 80.00999996d0
 | OPEN-LOCATION-CODE-TESTS::LON => 58.57d0
 | OPEN-LOCATION-CODE-TESTS::PREC => 10

The test succeeds if you write 80.00999996 as 8000999996/100000000.Changing 80.00999996 to 80.009999960000009 fixes the floating-pointinaccuracy issue since the rational number approximation for thisnumber is always closer to the next larger floating-point numberand thus greater than 80.00999996.