How a Developer Used Differential Testing to Validate Ancient I Ching Logic

A software developer building a deterministic engine for I Ching hexagram casting encountered a core validation problem: tests written from the same flawed understanding as the code will pass even when both are wrong. With no authoritative modern spec — the classical Chinese source texts are 3,000 years old and interpreted inconsistently by competing scholars — traditional expert review proved unscalable and unreliable. To break this self-validation loop, the developer borrowed a technique from compiler engineering called differential testing, comparing outputs against two independent MIT-licensed implementations: najia and yigram-najia-rules. A field in the output was only considered correct when all three separate codebases agreed, dramatically reducing the chance that two implementations shared the same underlying bug. The approach reframes correctness as a reproducible, automated engineering fact rather than an appeal to human authority.