Evolution's choice in build 4.2.6.401

Evolution chooses a plant in three places: the evolution of a source plant, the rank-4 spawn in an empty cell, and the rank-3 periodic spawn. Up to build 4.2.4.399, each of them shuffled its whole candidate list with the library's std::random_shuffle, on the engine shared by every such shuffle in the process, and took the first element, while the periodic spawn took its column and row from the game's own generator.

In 4.2.6.401, none of the three shuffles its list. Each passes the length of its candidate list to a new helper, receives one index, and takes that entry of the unshuffled list. The periodic spawn chooses its column and row through the same helper, with counts 9 and 5. The executable no longer contains the artifact's std::random_shuffle instance.¹1The index helper of build 4.2.6.401All addresses are in the 4.2.6.401 executable. The first five are the calls from the evolution selection, the rank-4 spawn, and the rank-3 periodic type, column and row; the others are the helper, its uniform_int_distribution routine, the mt19937 step routine and the engine constructor. The helper compares its count with 2 and returns 0 below it. The column and row calls pass 9 and 5. The constructor seeds the engine from eight random_device words through seed_seq, and only the helper references the engine directly. In the 4.2.4.399 executable the three selections call random_shuffle and the column and row come from the game generator; the 4.2.6.401 executable has no random_shuffle instance for the artifact. PLANTTYPES.json and ARTIFACT.json are identical in both builds.0x100a22dc4 · 0x100a20f84 · 0x100a22008 · 0x100a21a6c · 0x100a21a78 · 0x100a23148 · 0x100a58c28 · 0x1002278f8 · 0x100a5873c

For a list of one entry or none, the helper returns 0 without drawing. For a list of $n \ge 2$ entries, it draws a uniform index from 0 to $n-1$ with libc++'s uniform_int_distribution. Each attempt takes one 32-bit output and masks it to the bits the range needs; a value that is too large is drawn again.

The engine is a std::mt19937 that only this helper uses. It is built the first time the helper runs after a launch with at least two candidates: eight 32-bit words from std::random_device, passed through std::seed_seq, seed its state. Nothing else in the executable reads the engine by a direct reference, and nothing reseeds it.¹

The candidate lists and their order, the order in which sources are processed, and the placement rules are the same as in 4.2.4.399. The declared data is the same as well: PLANTTYPES.json and ARTIFACT.json are identical to those of 4.2.4.399.

Other list shuffles in the game still use the shared engine. They include the Devolution artifact's previews and the gravestone bags a level shuffles as it loads.