engine-proto/gram-test/Engine.jl

@@ -8,13 +8,28 @@ export rand_on_shell, Q, DescentHistory, realize_gram
 
 # === guessing ===
 
-##[TO DO] write a test to confirm that the outputs are on the correct shells
-##[TO DO] this can fail at generating spacelike vectors
-function rand_on_shell(rng::AbstractRNG, shells::Array{T}) where T <: Number
-  space_parts = randn(rng, T, 4, size(shells, 1))
-  space_self_prods = [dot(x, x) for x in eachcol(space_parts)]
-  return [space_parts; sqrt.(space_self_prods .- shells)']
+sconh(t, u) = 0.5*(exp(t) + u*exp(-t))
+
+function rand_on_sphere(rng::AbstractRNG, ::Type{T}, n) where T
+  out = randn(rng, T, n)
+  tries_left = 2
+  while dot(out, out) < 1e-6 && tries_left > 0
+    out = randn(rng, T, n)
+    tries_left -= 1
   end
+  normalize(out)
+end
+
+##[TO DO] write a test to confirm that the outputs are on the correct shells
+function rand_on_shell(rng::AbstractRNG, shell::T) where T <: Number
+  space_part = rand_on_sphere(rng, T, 4)
+  rapidity = randn(rng, T)
+  sig = sign(shell)
+  [sconh(rapidity, sig)*space_part; sconh(rapidity, -sig)]
+end
+
+rand_on_shell(rng::AbstractRNG, shells::Array{T}) where T <: Number =
+  hcat([rand_on_shell(rng, sh) for sh in shells]...)
 
 rand_on_shell(shells::Array{<:Number}) = rand_on_shell(Random.default_rng(), shells)