From 15d6163c9749f1090376fdb6ac18cfeb9ca46f18 Mon Sep 17 00:00:00 2001
From: Ryan Levy <rlevy@flatironinstitute.org>
Date: Thu, 25 Apr 2024 15:29:45 -0400
Subject: [PATCH] Initial zeroing out operator

---
 src/ITensorNumericalAnalysis.jl |  3 ++-
 src/elementary_operators.jl     | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/ITensorNumericalAnalysis.jl b/src/ITensorNumericalAnalysis.jl
index 850bc81..e6dd1eb 100644
--- a/src/ITensorNumericalAnalysis.jl
+++ b/src/ITensorNumericalAnalysis.jl
@@ -44,7 +44,8 @@ export const_itensornetwork,
   fourth_derivative_operator,
   identity_operator,
   delta_x,
-  delta_xyz
+  delta_xyz,
+  zero_point_op
 export const_itn,
   poly_itn, cosh_itn, sinh_itn, tanh_itn, exp_itn, sin_itn, cos_itn, rand_itn
 export calculate_fx, calculate_fxyz
diff --git a/src/elementary_operators.jl b/src/elementary_operators.jl
index aac4d37..2aa4f18 100644
--- a/src/elementary_operators.jl
+++ b/src/elementary_operators.jl
@@ -191,6 +191,27 @@ function identity_operator(s::IndsNetworkMap; kwargs...)
   return ITensorNetwork(Op("I"), operator_inds)
 end
 
+" Create an operator which is 0 for points in xs "
+function zero_point_op(s::IndsNetworkMap, xs::Vector, dimension::Int; truncate_kwargs...)
+  ttn_op = OpSum()
+  # build I- ∑_p ∏(bit string p)
+  for p in xs
+    ind_to_ind_value_map = calculate_ind_values(s, p, dimension)
+    string_site = []
+    for v in dimension_vertices(s, dimension)
+      op = ind_to_ind_value_map[only(s[v])] == 1 ? "Dup" : "Ddn"
+      push!(string_site, (op, v))
+    end
+    add!(ttn_op, -1.0, (string_site...)...)
+  end
+  add!(ttn_op, 1.0, "I", first(dimension_vertices(s, dimension)))
+  return ttn(ttn_op, indsnetwork(s); algorithm="svd", truncate_kwargs...)
+end
+
+function zero_point_op(s::IndsNetworkMap, x::Number, dimension::Int; truncate_kwargs...)
+  return zero_point_op(s, [x], dimension; truncate_kwargs...)
+end
+
 function operator(fx::ITensorNetworkFunction)
   fx = copy(fx)
   operator = itensornetwork(fx)