Short-circuit truncrank to notrunc when kept rank is larger than input

Author: Yue-Zhengyuan

src/factorizations/truncation.jl

@@ -171,6 +171,9 @@ function MAK.findtruncated(values::SectorVector, strategy::TruncationByOrder)
 
     # dimensions are all 1 so no need to account for weight
     if FusionStyle(I) isa UniqueFusion
+        if length(parent(values)) <= strategy.howmany
+            return MAK.findtruncated(values, NoTruncation())
+        end
         perm = partialsortperm(parent(values), 1:strategy.howmany; strategy.by, strategy.rev)
         result = similar(values, Bool)
         fill!(parent(result), false)
@@ -183,6 +186,9 @@ function MAK.findtruncated(values::SectorVector, strategy::TruncationByOrder)
     for (c, v) in pairs(dims)
         fill!(v, dim(c))
     end
+    if sum(dims) <= strategy.howmany
+        return MAK.findtruncated(values, NoTruncation())
+    end
 
     # allocate logical array for the output
     result = similar(values, Bool)

test/tensors/factorizations.jl

@@ -254,6 +254,14 @@ for V in spacelist
                 @test isisometric(U)
                 @test isisometric(Vᴴ; side = :right)
 
+                # when rank of t is already smaller than truncrank
+                t_rank = ceil(Int, min(dim(codomain(t)), dim(domain(t))))
+                U, S, Vᴴ, ϵ = @constinferred svd_trunc(t; trunc = truncrank(t_rank + 1))
+                @test U * S * Vᴴ ≈ t
+                @test ϵ ≈ 0
+                @test isisometric(U)
+                @test isisometric(Vᴴ; side = :right)
+
                 # dimension of S is a float for IsingBimodule
                 nvals = round(Int, dim(domain(S)) / 2)
                 trunc = truncrank(nvals)