Event Handler Implementation

src/MOI_wrapper.jl

@@ -47,6 +47,7 @@ mutable struct Optimizer <: MOI.AbstractOptimizer
             Dict(),
             Dict(),
             Dict(),
+            Dict(),
             [],
         )
 
@@ -285,6 +286,7 @@ function MOI.empty!(o::Optimizer)
         Dict(),
         Dict(),
         Dict(),
+        Dict(),
         [],
     )
     # reapply parameters

src/SCIP.jl

@@ -43,4 +43,6 @@ include("convenience.jl")
 # warn about rewrite
 include("compat.jl")
 
+# Event handler
+include("event_handler.jl")
 end

src/event_handler.jl

@@ -0,0 +1,104 @@
+# Wrapper for implementing event handlers in SCIP.
+# Before using please familiaze yourself with https://scipopt.org/doc/html/EVENT.php
+# 
+# The basic idea here is the same as with the separator wrappers. First, you need
+# to define a structure that implements the abstract type `AbstractEventhdlr`.
+# Second you should implement the function `eventexec` where the argument is an
+# instance of your event handler structure. Third, you should at runtime instantiate
+# the structure and call `include_event_handler` to register the event handler with SCIP.
+# 
+# See eventhdlr.jl in the test folder for an example.
+# 
+abstract type AbstractEventhdlr end
+
+""" 
+This is a virtual function that must be implemented by the user. Its Only
+argument is the event handler object.
+"""
+function eventexec(event::T) where {T<:AbstractEventhdlr}
+    error("eventexec not implemented for type $(T)")
+end
+
+"""
+This is the function that will be converted to a C function. It signature
+matches the one given in the SCIP documentation for SCIP_DECL_EVENTEXEC.
+
+Only the eventhdlr object is passed to the function which is user defined.
+"""
+function _eventexec(
+    scip::Ptr{SCIP_},
+    eventhdlr::Ptr{SCIP_Eventhdlr},
+    event::Ptr{SCIP_Event},
+    eventdata::Ptr{SCIP_EventData},
+)
+    # Get Julia object out of eventhandler data
+    data::Ptr{SCIP_EventData} = SCIPeventhdlrGetData(eventhdlr)
+    event = unsafe_pointer_to_objref(data)
+
+    #call user method
+    eventexec(event)
+
+    return SCIP_OKAY
+end
+
+"""
+    include_event_handler(scipd::SCIP.SCIPData, event_handler::EVENTHDLR; name="", desc="")
+
+Include the event handler in SCIP. WARNING! In contrast to the separator wrapper you only need to 
+pass the SCIPData rather than the SCIP pointer and dictionary. 
+
+# Arguments
+- scipd::SCIP.SCIPData: The SCIPData object
+- event_handler::EVENTHDLR: The event handler object
+- name::String: The name of the event handler
+- desc::String: The description of the event handler
+"""
+function include_event_handler(
+    scipd::SCIP.SCIPData,
+    event_handler::EVENTHDLR;
+    name="",
+    desc="",
+) where {EVENTHDLR<:AbstractEventhdlr}
+    _eventexec = @cfunction(
+        _eventexec,
+        SCIP_RETCODE,
+        (Ptr{SCIP_}, Ptr{SCIP_Eventhdlr}, Ptr{SCIP_Event}, Ptr{SCIP_EventData})
+    )
+
+    eventhdlrptr = Ref{Ptr{SCIP_Eventhdlr}}(C_NULL)
+    eventhdlr = pointer_from_objref(event_handler)
+
+    if name == ""
+        name = "__eventhdlr__$(length(eventhdlrs))"
+    end
+
+    @SCIP_CALL SCIPincludeEventhdlrBasic(
+        scipd.scip[],
+        eventhdlrptr,
+        name,
+        desc,
+        _eventexec,
+        eventhdlr,
+    )
+
+    @assert eventhdlrptr[] != C_NULL
+    #Persist in scip store against GC
+    scipd.eventhdlrs[event_handler] = eventhdlrptr[]
+end
+
+"""
+    catch_event(scipd::SCIP.SCIPData, eventtype::SCIP_EVENTTYPE, eventhdlr::EVENTHDLR)
+
+Catch an event in SCIP. This function is a wrapper around the SCIPcatchEvent function.
+Warning! This function should only be called after the SCIP has been transformed.
+"""
+function catch_event(
+    scipd::SCIP.SCIPData,
+    eventtype::SCIP_EVENTTYPE,
+    eventhdlr::EVENTHDLR,
+) where {EVENTHDLR<:AbstractEventhdlr}
+    @assert SCIPgetStage(scipd) != SCIP_STAGE_INIT
+    @assert SCIPgetStage(scipd) != SCIP_STAGE_PROBLEM
+    eventhdlrptr = scipd.eventhdlrs[eventhdlr]
+    @SCIP_CALL SCIPcatchEvent(scipd, eventtype, eventhdlrptr, C_NULL, C_NULL)
+end

src/scip_data.jl

@@ -34,6 +34,7 @@ mutable struct SCIPData
     scip::Ref{Ptr{SCIP_}}
     vars::Dict{VarRef,Ref{Ptr{SCIP_VAR}}}
     conss::Dict{ConsRef,Ref{Ptr{SCIP_CONS}}}
+
     var_count::Int64
     cons_count::Int64
 
@@ -48,6 +49,10 @@ mutable struct SCIPData
     # corresponding SCIP objects.
     sepas::Dict{Any,Ptr{SCIP_SEPA}}
 
+    # Map from user-defined types (keys are <: AbstractEventHandler)
+    # to the corresponding SCIP objects.
+    eventhdlrs::Dict{Any,Ptr{SCIP_Eventhdlr}}
+
     # User-defined cut selectors and branching rules
     cutsel_storage::Dict{Any,Ptr{SCIP_CUTSEL}}
     branchrule_storage::Dict{Any,Ptr{SCIP_BRANCHRULE}}

test/eventhdlr.jl

@@ -0,0 +1,80 @@
+# A simple testcase to test the event handler functionality.
+# It is assumed that test/sepa_support.jl is already included
+using SCIP
+import MathOptInterface as MOI
+
+module FirstLPEventTest
+# A simple event handler that stores the objective value of the first LP solve at the root node
+using SCIP
+
+mutable struct FirstLPEvent <: SCIP.AbstractEventhdlr
+    scip::SCIP.SCIPData
+    firstlpobj::Float64
+end
+
+function SCIP.eventexec(event::FirstLPEvent)
+    # Only consider the root node
+    current_node = SCIP.SCIPgetFocusNode(event.scip)
+    depth = SCIP.SCIPnodeGetDepth(current_node)
+    if depth == 0
+        scip = event.scip
+        event.firstlpobj = SCIP.SCIPgetLPObjval(scip)
+    end
+end
+end
+@testset "Try To Listen To First LP Solve" begin
+    # create an empty problem
+    optimizer = SCIP.Optimizer()
+    inner = optimizer.inner
+    sepa_set_scip_parameters((par, val) -> SCIP.set_parameter(inner, par, val))
+
+    # add variables
+    x, y = MOI.add_variables(optimizer, 2)
+    MOI.add_constraint(optimizer, x, MOI.ZeroOne())
+    MOI.add_constraint(optimizer, y, MOI.ZeroOne())
+
+    # add constraint: x + y ≤ 1.5
+    MOI.add_constraint(
+        optimizer,
+        MOI.ScalarAffineFunction(
+            MOI.ScalarAffineTerm.([1.0, 1.0], [x, y]),
+            0.0,
+        ),
+        MOI.LessThan(1.5),
+    )
+
+    # minimize -x - y
+    MOI.set(
+        optimizer,
+        MOI.ObjectiveFunction{MOI.ScalarAffineFunction{Float64}}(),
+        MOI.ScalarAffineFunction(
+            MOI.ScalarAffineTerm.([-1.0, -1.0], [x, y]),
+            0.0,
+        ),
+    )
+    MOI.set(optimizer, MOI.ObjectiveSense(), MOI.MIN_SENSE)
+
+    # add eventhandler 
+    eventhdlr = FirstLPEventTest.FirstLPEvent(inner, 10)
+    SCIP.include_event_handler(
+        inner,
+        eventhdlr;
+        name="firstlp",
+        desc="Store the objective value of the first LP solve at the root node",
+    )
+
+    # transform the problem into SCIP
+    SCIP.@SCIP_CALL SCIP.SCIPtransformProb(inner)
+
+    # catch the event. Again this can only be done after the problem is transformed
+    SCIP.catch_event(inner, SCIP.SCIP_EVENTTYPE_FIRSTLPSOLVED, eventhdlr)
+
+    # solve the problem
+    SCIP.@SCIP_CALL SCIP.SCIPsolve(inner.scip[])
+
+    # test if the event handler worked 
+    @test eventhdlr.firstlpobj != 10
+
+    # free the problem
+    finalize(inner)
+end

test/runtests.jl

@@ -43,6 +43,11 @@ include("sepa_support.jl")
 @testset "separators" begin
     include("sepa.jl")
 end
+
+@testset "event handlers" begin
+    include("eventhdlr.jl")
+end
+
 @testset "cut callbacks" begin
     include("cutcallback.jl")
 end