Short circuiting mod1 to speed offsetc (#184)

* Short circuiting `mod1` to speed `offsetc` for simple models

src/System/System.jl

@@ -165,6 +165,12 @@ const Site = Union{NTuple{4, Int}, CartesianIndex{4}}
 @inline to_cartesian(i::CartesianIndex{N}) where N = i
 @inline to_cartesian(i::NTuple{N, Int})    where N = CartesianIndex(i)
 
+# Like mod1(x, L), but short-circuits early in the common case. See
+# https://github.com/SunnySuite/Sunny.jl/pull/184 for discussion.
+@inline function altmod1(x::Int, L::Int)
+    1 <= x <= L ? x : mod1(x, L)
+end
+
 # Offset a `cell` by `ncells`
 @inline offsetc(cell::CartesianIndex{3}, ncells, latsize) = CartesianIndex(mod1.(Tuple(cell) .+ Tuple(ncells), latsize))