From 790b559508c96a852ddf63e085e1644d7e7ee192 Mon Sep 17 00:00:00 2001
From: Avik Pal <avikpal@mit.edu>
Date: Wed, 20 Sep 2023 20:50:33 -0400
Subject: [PATCH 1/2] Add NonlinearLeastSquaresProblem

---
 src/SciMLBase.jl               |  6 +--
 src/problems/basic_problems.jl | 79 ++++++++++++++++++++++++++++++++++
 src/remake.jl                  | 32 ++++++++++++++
 3 files changed, 114 insertions(+), 3 deletions(-)

diff --git a/src/SciMLBase.jl b/src/SciMLBase.jl
index d206ca07d..8da9ff267 100644
--- a/src/SciMLBase.jl
+++ b/src/SciMLBase.jl
@@ -769,9 +769,9 @@ export isinplace
 
 export solve, solve!, init, discretize, symbolic_discretize
 
-export LinearProblem,
-    NonlinearProblem, IntervalNonlinearProblem,
-    IntegralProblem, SampledIntegralProblem, OptimizationProblem
+export LinearProblem, NonlinearProblem, IntervalNonlinearProblem,
+    IntegralProblem, SampledIntegralProblem, OptimizationProblem,
+    NonlinearLeastSquaresProblem
 
 export DiscreteProblem, ImplicitDiscreteProblem
 export SteadyStateProblem, SteadyStateSolution
diff --git a/src/problems/basic_problems.jl b/src/problems/basic_problems.jl
index 1207fe79b..3c2430c58 100644
--- a/src/problems/basic_problems.jl
+++ b/src/problems/basic_problems.jl
@@ -312,6 +312,85 @@ function NonlinearProblem(prob::AbstractODEProblem)
     NonlinearProblem{isinplace(prob)}(prob.f, prob.u0, prob.p)
 end
 
+@doc doc"""
+Defines a nonlinear least squares problem.
+
+## Mathematical Specification of a Nonlinear Least Squares Problem
+
+To define a Nonlinear Problem, you simply need to give the function ``f`` which defines the
+nonlinear system:
+
+```math
+\underset{x}{\min} \| f(x, p) \|
+```
+
+and an initial guess ``u_0`` for the minimization problem. ``f`` should be specified as
+``f(u, p)`` (or in-place as ``f(du, u, p)``), and ``u_0``` should be an AbstractArray (or
+number) whose geometry matches the desired geometry of ``u``. Note that we are not limited
+to numbers or vectors for ``u_0``; one is allowed to provide ``u_0`` as arbitrary
+matrices / higher-dimension tensors as well.
+
+## Problem Type
+
+### Constructors
+
+```julia
+NonlinearLeastSquaresProblem(f::NonlinearFunction, u0, p=NullParameters(); kwargs...)
+NonlinearLeastSquaresProblem{isinplace}(f, u0, p=NullParameters(); kwargs...)
+```
+
+`isinplace` optionally sets whether the function is in-place or not. This is
+determined automatically, but not inferred.
+
+Parameters are optional, and if not given, then a `NullParameters()` singleton
+will be used, which will throw nice errors if you try to index non-existent
+parameters.
+
+For specifying Jacobians and mass matrices, see the
+[NonlinearFunctions](@ref nonlinearfunctions) page.
+
+### Fields
+
+* `f`: The function in the problem.
+* `u0`: The initial guess for the solution.
+* `p`: The parameters for the problem. Defaults to `NullParameters`.
+* `kwargs`: The keyword arguments passed on to the solvers.
+"""
+struct NonlinearLeastSquaresProblem{uType, isinplace, P, F, K} <:
+       AbstractNonlinearProblem{uType, isinplace}
+    f::F
+    u0::uType
+    p::P
+    kwargs::K
+
+    @add_kwonly function NonlinearLeastSquaresProblem{iip}(f::AbstractNonlinearFunction{
+            iip}, u0, p = NullParameters(); kwargs...) where {iip}
+        warn_paramtype(p)
+        return new{typeof(u0), iip, typeof(p), typeof(f), typeof(kwargs)}(f, u0, p, kwargs)
+    end
+
+    function NonlinearLeastSquaresProblem{iip}(f, u0, p = NullParameters()) where {iip}
+        return NonlinearLeastSquaresProblem{iip}(NonlinearFunction{iip}(f), u0, p)
+    end
+end
+
+TruncatedStacktraces.@truncate_stacktrace NonlinearLeastSquaresProblem 2 1
+
+"""
+$(SIGNATURES)
+
+Define a nonlinear least squares problem using an instance of
+[`AbstractNonlinearFunction`](@ref AbstractNonlinearFunction).
+"""
+function NonlinearLeastSquaresProblem(f::AbstractNonlinearFunction, u0,
+    p = NullParameters(); kwargs...)
+    return NonlinearLeastSquaresProblem{isinplace(f)}(f, u0, p; kwargs...)
+end
+
+function NonlinearLeastSquaresProblem(f, u0, p = NullParameters(); kwargs...)
+    return NonlinearLeastSquaresProblem(NonlinearFunction(f), u0, p; kwargs...)
+end
+
 @doc doc"""
 
 Defines an integral problem.
diff --git a/src/remake.jl b/src/remake.jl
index 8fe0feb1f..083f0fda6 100644
--- a/src/remake.jl
+++ b/src/remake.jl
@@ -369,6 +369,38 @@ function remake(prob::NonlinearProblem;
     end
 end
 
+
+"""
+    remake(prob::NonlinearLeastSquaresProblem; f = missing, u0 = missing, p = missing,
+        kwargs = missing, _kwargs...)
+
+Remake the given `NonlinearLeastSquaresProblem`.
+"""
+function remake(prob::NonlinearLeastSquaresProblem; f = missing, u0 = missing, p = missing,
+    kwargs = missing, _kwargs...)
+    if p === missing && u0 === missing
+        p, u0 = prob.p, prob.u0
+    else # at least one of them has a value
+        if p === missing
+            p = prob.p
+        end
+        if u0 === missing
+            u0 = prob.u0
+        end
+    end
+
+    if f === missing
+        f = prob.f
+    end
+
+    if kwargs === missing
+        return NonlinearLeastSquaresProblem{isinplace(prob)}(; f, u0, p, prob.kwargs...,
+            _kwargs...)
+    else
+        return NonlinearLeastSquaresProblem{isinplace(prob)}(; f, u0, p, kwargs...)
+    end
+end
+
 # overloaded in MTK to intercept symbolic remake
 function process_p_u0_symbolic(prob, p, u0)
     if typeof(prob) <: Union{AbstractDEProblem, OptimizationProblem, NonlinearProblem}

From 5ddf69ab2cc37ca1c469bd87753b2de646339f76 Mon Sep 17 00:00:00 2001
From: Avik Pal <avikpal@mit.edu>
Date: Sun, 8 Oct 2023 15:09:57 -0400
Subject: [PATCH 2/2] Update Project

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

diff --git a/Project.toml b/Project.toml
index 7c190f21b..f2cf68910 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "SciMLBase"
 uuid = "0bca4576-84f4-4d90-8ffe-ffa030f20462"
 authors = ["Chris Rackauckas <accounts@chrisrackauckas.com> and contributors"]
-version = "2.3.1"
+version = "2.4.0"
 
 [deps]
 ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"