The test module is a small integer simulation: up to 64 bodies bouncing in a 10 m box (lengths in millimetres), with a stage axis (apply input, gravity, move, bookkeeping). All integers are little endian.
magic 4 bytes "OSTM" version u16 1 flags u16 bit 0: hash rows present; bit 1: a start snapshot present; bit 2: a link block follows the setup; other bits must be 0 setup length u32 6, or 10 when flag bit 2 is set setup 6 bytes u8 dimensions (2 or 3), u8 body count (1..64), u32 seed link block 4 bytes (only with flag bit 2) u8 role: 1 sends a body away, 2 receives it, 3 neither (declares the density and occupancy fields only); u8 body index (roles 1 and 2); u16 transfer step step count u32 at most 16 777 216 start snapshot u32 length, then the world snapshot, only if flag bit 1: the state a live attach began from; its first 8 bytes are the host's step at that moment, which is the recording's start step hash rows u64 x (step count + 1), only if flag bit 0; row i is the world hash at the start step + i per step u16 input length, then that many bytes
A step input is a sequence of operations: 1 body dx dy dz (impulse), 2 body x y z (teleport), 3 body (remove), 4 gravity, 5 x y z (spawn a body at rest; it takes the next free index, and does nothing when the world already holds 64 bodies). body is a u8 index, the rest are i32. Files written before spawn existed stay valid.
Every length is checked against the bytes that remain before anything is allocated; trailing bytes are an error. Errors are ScopeFormatException, naming the input and what was expected at which offset.
A world with a link block also declares the two scalar fields (density, per grid vertex, and occupancy, per triangle, both over an 8 by 8 floor grid) and their overlays. Role 1 removes the body at the transfer step and role 2 holds it from that step on (it is absent before, and appears at the middle of the box with a fixed velocity); the sender declares the link as ContinuedBy and the receiver as Continues, with link keys tm-left and tm-right. TestGhost writes a pose cache from a run, with an offset, as a stand-in for a reference engine's dump.
testdata/test-10000.ostm (3D, 24 bodies, 10 000 steps) and testdata/test-2d-600.ostm are produced by tools/GenTestData, as are test-field-2d.ostm, test-field-3d.ostm, the linked pair test-link-left.ostm and test-link-right.ostm (body 3 moves at step 150) and the pose cache test-2d-ghost.ospc (the 2D replay's bodies for 40 frames from step 200, drawn 400 mm to the right).
Live stream
The test module is also a live module. Its probe snapshot is the 6-byte setup followed by the world snapshot (a 64-bit host step, gravity, then per body (as many as the world holds, up to 64, spawned ones included) alive, still count, position and velocity as 32-bit integers). Each record is one step's input. A viewer's world starts at the snapshot at the host's step, so the viewer's step numbers are the host's and hashes compare by step number. A live source written as a replay sets flag bit 1 and holds the snapshot, so reopening it reproduces the same hashes.