Blog · page 21
Notes on engineering, design, and everything in between.
URL encoding: spaces, plus signs and double encoding
A plus is a space in the query string and a literal plus in the path; the two positions encode the same character differently; encoding twice yields a plausible value that never matches.
Four places Chinese chess rules go wrong
Horse-leg blocking, elephant-eye blocking, cannon screens and the flying-general rule. Splitting move generation into a geometry layer and a legality layer removes most of the mistakes.
Writing a cron parser: five fields, the union rule, and February 30th
The syntax takes half an hour. The semantics are the work: day-of-month and day-of-week unite rather than intersect, impossible dates need a stopping condition, and 7 and 0 are both Sunday.
CSV is not split on commas: quoting and newlines from RFC 4180
The moment a field is quoted, split stops working: quotes hold delimiters and newlines, and a quote inside a field is doubled. A thirty-line state machine beats any regex.
Legal moves in Reversi: eight directions, one scan
A square is legal if at least one of eight directions can sandwich the opponent. Testing and flipping share a single scan, so there is no simulated move to roll back.
A sudoku generator: digging holes, unique solutions, and the bug that froze the page
Generate a full solution, then dig cells out while checking uniqueness. I let zero mean unlimited in the solver, so an empty grid enumerated every solution and the new-game button hung.