From 5fb3588aee77cdc8f20c3672ad6e0e5127a686d5 Mon Sep 17 00:00:00 2001
From: Lukas Devos <ldevos98@gmail.com>
Date: Thu, 8 Jan 2026 15:37:26 +0100
Subject: [PATCH 1/3] try to make truncation GPU-friendly

---
 src/factorizations/truncation.jl | 49 +++++++++++++++++++++-----------
 1 file changed, 33 insertions(+), 16 deletions(-)

diff --git a/src/factorizations/truncation.jl b/src/factorizations/truncation.jl
index b9d060fec..3bc1ebf48 100644
--- a/src/factorizations/truncation.jl
+++ b/src/factorizations/truncation.jl
@@ -191,6 +191,23 @@ function _sort_and_perm(values::SectorVector; by = identity, rev::Bool = false)
     return values_sorted, perms
 end
 
+function _findtruncvalue_order(values::SectorVector, n::Int; by = identity, rev::Bool = false)
+    I = sectortype(values)
+    p = sortperm(parent(values); by, rev)
+
+    if FusionStyle(I) isa UniqueFusion # dimensions are all 1
+        return n <= 0 ? nothing : p[min(n, length(p))]
+    else
+        dims = similar(values, Base.promote_op(dim, I))
+        for (c, v) in pairs(dims)
+            fill!(v, dim(c))
+        end
+        cumulative_dim = cumsum(Base.permute!(parent(dims), p))
+        k = findlast(<=(n), cumulative_dim)
+        return isnothing(k) ? k : p[k]
+    end
+end
+
 # findtruncated
 # -------------
 # Generic fallback
@@ -202,25 +219,25 @@ function MAK.findtruncated(values::SectorVector, ::NoTruncation)
     return SectorDict(c => Colon() for c in keys(values))
 end
 
+# TruncationByOrder strategy:
+# - find the howmany'th value of the input sorted according to the strategy
+# - discard everything that is ordered after that value
+
 function MAK.findtruncated(values::SectorVector, strategy::TruncationByOrder)
-    values_sorted, perms = _sort_and_perm(values; strategy.by, strategy.rev)
-    inds = MAK.findtruncated_svd(values_sorted, truncrank(strategy.howmany))
-    return SectorDict(c => perms[c][I] for (c, I) in inds)
-end
-function MAK.findtruncated_svd(values::SectorVector, strategy::TruncationByOrder)
-    I = keytype(values)
-    truncdim = SectorDict{I, Int}(c => length(d) for (c, d) in pairs(values))
-    totaldim = sum(dim(c) * d for (c, d) in truncdim; init = 0)
-    while totaldim > strategy.howmany
-        next = _findnexttruncvalue(values, truncdim; strategy.by, strategy.rev)
-        isnothing(next) && break
-        _, cmin = next
-        truncdim[cmin] -= 1
-        totaldim -= dim(cmin)
-        truncdim[cmin] == 0 && delete!(truncdim, cmin)
+    k = _findtruncvalue_order(values, strategy.howmany; strategy.by, strategy.rev)
+
+    if isnothing(k)
+        # discard everything
+        return SectorDict{sectortype(values), UnitRange{Int}}()
+    else
+        val = strategy.by(values[k])
+        strategy = trunctol(; atol = val, strategy.by, keep_below = !strategy.rev)
+        return MAK.findtruncated_svd(values, strategy)
     end
-    return SectorDict(c => Base.OneTo(d) for (c, d) in truncdim)
 end
+# disambiguate
+MAK.findtruncated_svd(values::SectorVector, strategy::TruncationByOrder) =
+    MAK.findtruncated(values, strategy)
 
 function MAK.findtruncated(values::SectorVector, strategy::TruncationByFilter)
     return SectorDict(c => findall(strategy.filter, d) for (c, d) in pairs(values))

From 77f0ffa0879bd84e3cd304ec6c9e0b29a20b12de Mon Sep 17 00:00:00 2001
From: Katharine Hyatt <kslimes@gmail.com>
Date: Tue, 20 Jan 2026 11:21:17 +0100
Subject: [PATCH 2/3] Temporarily fix StridedViews version

---
 Project.toml | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Project.toml b/Project.toml
index 934bdb6ed..3e0c7a02d 100644
--- a/Project.toml
+++ b/Project.toml
@@ -52,6 +52,7 @@ Random = "1"
 SafeTestsets = "0.1"
 ScopedValues = "1.3.0"
 Strided = "2"
+StridedViews = "=0.4.1"
 TensorKitSectors = "0.3.3"
 TensorOperations = "5.1"
 Test = "1"
@@ -75,6 +76,7 @@ GPUArrays = "0c68f7d7-f131-5f86-a1c3-88cf8149b2d7"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 Mooncake = "da2b9cff-9c12-43a0-ae48-6db2b0edb7d6"
 SafeTestsets = "1bc83da4-3b8d-516f-aca4-4fe02f6d838f"
+StridedViews = "4db3bf67-4bd7-4b4e-b153-31dc3fb37143"
 TensorOperations = "6aa20fa7-93e2-5fca-9bc0-fbd0db3c71a2"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 TestExtras = "5ed8adda-3752-4e41-b88a-e8b09835ee3a"

From 5e5d87b4f113bc9a25dc55f61079ac8571091cde Mon Sep 17 00:00:00 2001
From: Katharine Hyatt <kslimes@gmail.com>
Date: Tue, 20 Jan 2026 16:16:49 +0100
Subject: [PATCH 3/3] Revert "Temporarily fix StridedViews version"

This reverts commit 77f0ffa0879bd84e3cd304ec6c9e0b29a20b12de.
---
 Project.toml | 2 --
 1 file changed, 2 deletions(-)

diff --git a/Project.toml b/Project.toml
index 3e0c7a02d..934bdb6ed 100644
--- a/Project.toml
+++ b/Project.toml
@@ -52,7 +52,6 @@ Random = "1"
 SafeTestsets = "0.1"
 ScopedValues = "1.3.0"
 Strided = "2"
-StridedViews = "=0.4.1"
 TensorKitSectors = "0.3.3"
 TensorOperations = "5.1"
 Test = "1"
@@ -76,7 +75,6 @@ GPUArrays = "0c68f7d7-f131-5f86-a1c3-88cf8149b2d7"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 Mooncake = "da2b9cff-9c12-43a0-ae48-6db2b0edb7d6"
 SafeTestsets = "1bc83da4-3b8d-516f-aca4-4fe02f6d838f"
-StridedViews = "4db3bf67-4bd7-4b4e-b153-31dc3fb37143"
 TensorOperations = "6aa20fa7-93e2-5fca-9bc0-fbd0db3c71a2"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 TestExtras = "5ed8adda-3752-4e41-b88a-e8b09835ee3a"