From a4fd6d8428775ca83b77dcad4142bd4e8fbdf55c Mon Sep 17 00:00:00 2001
From: Aayush Sabharwal <aayush.sabharwal@gmail.com>
Date: Thu, 28 Nov 2024 16:05:01 +0530
Subject: [PATCH] feat: do not require `nlsolve_alg` for trivial `OverrideInit`

---
 src/initialization.jl  | 46 +++++++++++++++++++++++++-----------------
 test/initialization.jl | 26 ++++++++++++++++++++++++
 2 files changed, 53 insertions(+), 19 deletions(-)

diff --git a/src/initialization.jl b/src/initialization.jl
index 4e976e168..1179e606c 100644
--- a/src/initialization.jl
+++ b/src/initialization.jl
@@ -172,6 +172,9 @@ Keyword arguments:
   provided to the `OverrideInit` constructor takes priority over this keyword argument.
   If the former is `nothing`, this keyword argument will be used. If it is also not provided,
   an error will be thrown.
+
+In case the initialization problem is trivial, `nlsolve_alg`, `abstol` and `reltol` are
+not required.
 """
 function get_initial_values(prob, valp, f, alg::OverrideInit,
         iip::Union{Val{true}, Val{false}}; nlsolve_alg = nothing, abstol = nothing, reltol = nothing, kwargs...)
@@ -185,35 +188,40 @@ function get_initial_values(prob, valp, f, alg::OverrideInit,
     initdata::OverrideInitData = f.initialization_data
     initprob = initdata.initializeprob
 
-    nlsolve_alg = something(nlsolve_alg, alg.nlsolve, Some(nothing))
-    if nlsolve_alg === nothing && state_values(initprob) !== nothing
-        throw(OverrideInitMissingAlgorithm())
-    end
-
     if initdata.update_initializeprob! !== nothing
         initdata.update_initializeprob!(initprob, valp)
     end
 
-    if alg.abstol !== nothing
-        _abstol = alg.abstol
-    elseif abstol !== nothing
-        _abstol = abstol
-    else
-        throw(OverrideInitNoTolerance(:abstol))
-    end
-    if alg.reltol !== nothing
-        _reltol = alg.reltol
-    elseif reltol !== nothing
-        _reltol = reltol
+    if state_values(initprob) === nothing
+        nlsol = initprob
+        success = true
     else
-        throw(OverrideInitNoTolerance(:reltol))
+        nlsolve_alg = something(nlsolve_alg, alg.nlsolve, Some(nothing))
+        if nlsolve_alg === nothing && state_values(initprob) !== nothing
+            throw(OverrideInitMissingAlgorithm())
+        end
+        if alg.abstol !== nothing
+            _abstol = alg.abstol
+        elseif abstol !== nothing
+            _abstol = abstol
+        else
+            throw(OverrideInitNoTolerance(:abstol))
+        end
+        if alg.reltol !== nothing
+            _reltol = alg.reltol
+        elseif reltol !== nothing
+            _reltol = reltol
+        else
+            throw(OverrideInitNoTolerance(:reltol))
+        end
+        nlsol = solve(initprob, nlsolve_alg; abstol = _abstol, reltol = _reltol)
+        success = SciMLBase.successful_retcode(nlsol)
     end
-    nlsol = solve(initprob, nlsolve_alg; abstol = _abstol, reltol = _reltol)
 
     u0 = initdata.initializeprobmap(nlsol)
     if initdata.initializeprobpmap !== nothing
         p = initdata.initializeprobpmap(valp, nlsol)
     end
 
-    return u0, p, SciMLBase.successful_retcode(nlsol)
+    return u0, p, success
 end
diff --git a/test/initialization.jl b/test/initialization.jl
index 1ea1da694..7d5dfb01d 100644
--- a/test/initialization.jl
+++ b/test/initialization.jl
@@ -244,4 +244,30 @@ end
         @test p ≈ 0.0
         @test success
     end
+
+    @testset "Trivial initialization" begin
+        initprob = NonlinearProblem(Returns(nothing), nothing, [1.0])
+        update_initializeprob! = function (iprob, integ)
+            iprob.p[1] = integ.u[1]
+        end
+        initprobmap = function (nlsol)
+            u1 = parameter_values(nlsol)[1]
+            return [u1, u1]
+        end
+        initprobpmap = function (_, nlsol)
+            return 0.0
+        end
+        initialization_data = SciMLBase.OverrideInitData(
+            initprob, update_initializeprob!, initprobmap, initprobpmap)
+        fn = ODEFunction(rhs2; initialization_data)
+        prob = ODEProblem(fn, [2.0, 0.0], (0.0, 1.0), 0.0)
+        integ = init(prob; initializealg = NoInit())
+
+        u0, p, success = SciMLBase.get_initial_values(
+            prob, integ, fn, SciMLBase.OverrideInit(), Val(false)
+        )
+        @test u0 ≈ [2.0, 2.0]
+        @test p ≈ 0.0
+        @test success
+    end
 end