Switch to copied implementation of tensor_pow_by_squaring

Author: akirakyle

src/tensor.jl

@@ -14,4 +14,30 @@ tensor(states::Vector{T}) where T<:StateVector = reduce(tensor, states)
 
 Gives the tensor product of `a` `N` times.
 """
-tensor_pow(a, N) = Base.power_by_squaring(a, N; mul=⊗)
+tensor_pow(a, N) = tensor_pow_by_squaring(a, N)
+
+# Copied from Base.power_by_squaring as `mul` keyword dosn't work without implementing `*`
+function tensor_pow_by_squaring(x, p::Integer)
+    if p == 1
+        return x
+    elseif p == 2
+        return tensor(x, x)
+    elseif p < 1
+        throw(DomainError("Cannot take tensor_pow to power less than one"))
+    end
+    t = trailing_zeros(p) + 1
+    p >>= t
+    while (t -= 1) > 0
+        x = tensor(x, x)
+    end
+    y = x
+    while p > 0
+        t = trailing_zeros(p) + 1
+        p >>= t
+        while (t -= 1) >= 0
+            x = tensor(x, x)
+        end
+        y = tensor(y, x)
+    end
+    return y
+end