use provided gradient for dphi_0 in HZ

Author: mateuszbaran

src/plans/stepsize/stepsize.jl

@@ -2594,7 +2594,12 @@ function (hzls::HagerZhangLinesearchStepsize)(
     M = get_manifold(mp)
     p = get_iterate(s)
 
-    dphi_0 = get_differential(mp, p, η; Y = hzls.temporary_tangent)
+    local dphi_0 # COV_EXCL_LINE
+    if :gradient in keys(kwargs)
+        dphi_0 = real(inner(M, p, η, kwargs[:gradient]))
+    else
+        dphi_0 = get_differential(mp, p, η; Y = hzls.temporary_tangent)
+    end
     hzls.triples[1] = UnivariateTriple(0.0, fp, dphi_0)
     hzls.last_evaluation_index = 1
 

test/plans/test_stepsize.jl

@@ -293,6 +293,8 @@ end
         α = hzls(dmp, gs, 1, η)
         @test isfinite(α)
         @test α > 0
+        α2 = hzls(dmp, gs, 1, η; gradient = grad_f_sum_sq(M, p))
+        @test α2 ≈ α
         @test hzls.last_stepsize == α
         @test hzls.last_cost <= f_sum_sq(M, p) + 1.0e-12