# This file is a part of Julia. License is MIT: https://julialang.org/license

const LineNum = Int

mutable struct InferenceState
    params::InferenceParams
    result::InferenceResult # remember where to put the result
    linfo::MethodInstance
    sptypes::Vector{Any}    # types of static parameter
    slottypes::Vector{Any}
    mod::Module
    currpc::LineNum
    pclimitations::IdSet{InferenceState} # causes of precision restrictions (LimitedAccuracy) on currpc ssavalue
    limitations::IdSet{InferenceState} # causes of precision restrictions (LimitedAccuracy) on return

    # info on the state of inference and the linfo
    src::CodeInfo
    world::UInt
    valid_worlds::WorldRange
    nargs::Int
    stmt_types::Vector{Union{Nothing, Vector{Any}}} # ::Vector{Union{Nothing, VarTable}}
    stmt_edges::Vector{Union{Nothing, Vector{Any}}}
    stmt_info::Vector{Any}
    # return type
    bestguess #::Type
    # current active instruction pointers
    ip::BitSet
    pc´´::LineNum
    nstmts::Int
    # current exception handler info
    cur_hand #::Union{Nothing, Pair{LineNum, prev_handler}}
    handler_at::Vector{Any}
    n_handlers::Int
    # ssavalue sparsity and restart info
    ssavalue_uses::Vector{BitSet}
    throw_blocks::BitSet

    cycle_backedges::Vector{Tuple{InferenceState, LineNum}} # call-graph backedges connecting from callee to caller
    callers_in_cycle::Vector{InferenceState}
    parent::Union{Nothing, InferenceState}

    # TODO: move these to InferenceResult / Params?
    cached::Bool
    inferred::Bool
    dont_work_on_me::Bool

    # The place to look up methods while working on this function.
    # In particular, we cache method lookup results for the same function to
    # fast path repeated queries.
    method_table::CachedMethodTable{InternalMethodTable}

    # The interpreter that created this inference state. Not looked at by
    # NativeInterpreter. But other interpreters may use this to detect cycles
    interp::AbstractInterpreter

    # src is assumed to be a newly-allocated CodeInfo, that can be modified in-place to contain intermediate results
    function InferenceState(result::InferenceResult, src::CodeInfo,
                            cached::Bool, interp::AbstractInterpreter)
        linfo = result.linfo
        code = src.code::Array{Any,1}
        toplevel = !isa(linfo.def, Method)

        sp = sptypes_from_meth_instance(linfo::MethodInstance)

        nssavalues = src.ssavaluetypes::Int
        src.ssavaluetypes = Any[ NOT_FOUND for i = 1:nssavalues ]
        stmt_info = Any[ nothing for i = 1:length(code) ]

        n = length(code)
        s_edges = Union{Nothing, Vector{Any}}[ nothing for i = 1:n ]
        s_types = Union{Nothing, Vector{Any}}[ nothing for i = 1:n ]

        # initial types
        nslots = length(src.slotflags)
        argtypes = result.argtypes
        nargs = length(argtypes)
        s_argtypes = VarTable(undef, nslots)
        slottypes = Vector{Any}(undef, nslots)
        for i in 1:nslots
            at = (i > nargs) ? Bottom : argtypes[i]
            s_argtypes[i] = VarState(at, i > nargs)
            slottypes[i] = at
        end
        s_types[1] = s_argtypes

        ssavalue_uses = find_ssavalue_uses(code, nssavalues)
        throw_blocks = find_throw_blocks(code)

        # exception handlers
        cur_hand = nothing
        handler_at = Any[ nothing for i=1:n ]
        n_handlers = 0

        W = BitSet()
        push!(W, 1) #initial pc to visit

        if !toplevel
            meth = linfo.def
            inmodule = meth.module
        else
            inmodule = linfo.def::Module
        end

        valid_worlds = WorldRange(src.min_world,
            src.max_world == typemax(UInt) ? get_world_counter() : src.max_world)
        frame = new(
            InferenceParams(interp), result, linfo,
            sp, slottypes, inmodule, 0,
            IdSet{InferenceState}(), IdSet{InferenceState}(),
            src, get_world_counter(interp), valid_worlds,
            nargs, s_types, s_edges, stmt_info,
            Union{}, W, 1, n,
            cur_hand, handler_at, n_handlers,
            ssavalue_uses, throw_blocks,
            Vector{Tuple{InferenceState,LineNum}}(), # cycle_backedges
            Vector{InferenceState}(), # callers_in_cycle
            #=parent=#nothing,
            cached, false, false,
            CachedMethodTable(method_table(interp)),
            interp)
        result.result = frame
        cached && push!(get_inference_cache(interp), result)
        return frame
    end
end

"""
    Iterate through all callers of the given InferenceState in the abstract
    interpretation stack (including the given InferenceState itself), vising
    children before their parents (i.e. ascending the tree from the given
    InferenceState). Note that cycles may be visited in any order.
"""
struct InfStackUnwind
    inf::InferenceState
end
iterate(unw::InfStackUnwind) = (unw.inf, (unw.inf, 0))
function iterate(unw::InfStackUnwind, (infstate, cyclei)::Tuple{InferenceState, Int})
    # iterate through the cycle before walking to the parent
    if cyclei < length(infstate.callers_in_cycle)
        cyclei += 1
        infstate = infstate.callers_in_cycle[cyclei]
    else
        cyclei = 0
        infstate = infstate.parent
    end
    infstate === nothing && return nothing
    (infstate::InferenceState, (infstate, cyclei))
end

method_table(interp::AbstractInterpreter, sv::InferenceState) = sv.method_table

function InferenceState(result::InferenceResult, cached::Bool, interp::AbstractInterpreter)
    # prepare an InferenceState object for inferring lambda
    src = retrieve_code_info(result.linfo)
    src === nothing && return nothing
    validate_code_in_debug_mode(result.linfo, src, "lowered")
    return InferenceState(result, src, cached, interp)
end

function sptypes_from_meth_instance(linfo::MethodInstance)
    toplevel = !isa(linfo.def, Method)
    if !toplevel && isempty(linfo.sparam_vals) && isa(linfo.def.sig, UnionAll)
        # linfo is unspecialized
        sp = Any[]
        sig = linfo.def.sig
        while isa(sig, UnionAll)
            push!(sp, sig.var)
            sig = sig.body
        end
    else
        sp = collect(Any, linfo.sparam_vals)
    end
    for i = 1:length(sp)
        v = sp[i]
        if v isa TypeVar
            fromArg = 0
            # if this parameter came from arg::Type{T}, then `arg` is more precise than
            # Type{T} where lb<:T<:ub
            sig = linfo.def.sig
            temp = sig
            for j = 1:i-1
                temp = temp.body
            end
            Pi = temp.var
            while temp isa UnionAll
                temp = temp.body
            end
            sigtypes = temp.parameters
            for j = 1:length(sigtypes)
                tj = sigtypes[j]
                if isType(tj) && tj.parameters[1] === Pi
                    fromArg = j
                    break
                end
            end
            if fromArg > 0
                ty = fieldtype(linfo.specTypes, fromArg)
            else
                ub = v.ub
                while ub isa TypeVar
                    ub = ub.ub
                end
                if has_free_typevars(ub)
                    ub = Any
                end
                lb = v.lb
                while lb isa TypeVar
                    lb = lb.lb
                end
                if has_free_typevars(lb)
                    lb = Bottom
                end
                if Any <: ub && lb <: Bottom
                    ty = Any
                else
                    tv = TypeVar(v.name, lb, ub)
                    ty = UnionAll(tv, Type{tv})
                end
            end
        elseif isa(v, Core.TypeofVararg)
            ty = Int
        else
            ty = Const(v)
        end
        sp[i] = ty
    end
    return sp
end

_topmod(sv::InferenceState) = _topmod(sv.mod)

# work towards converging the valid age range for sv
function update_valid_age!(sv::InferenceState, worlds::WorldRange)
    sv.valid_worlds = intersect(worlds, sv.valid_worlds)
    @assert(sv.world in sv.valid_worlds, "invalid age range update")
    nothing
end

update_valid_age!(edge::InferenceState, sv::InferenceState) = update_valid_age!(sv, edge.valid_worlds)

function record_ssa_assign(ssa_id::Int, @nospecialize(new), frame::InferenceState)
    old = frame.src.ssavaluetypes[ssa_id]
    if old === NOT_FOUND || !(new ⊑ old)
        # typically, we expect that old ⊑ new (that output information only
        # gets less precise with worse input information), but to actually
        # guarantee convergence we need to use tmerge here to ensure that is true
        frame.src.ssavaluetypes[ssa_id] = old === NOT_FOUND ? new : tmerge(old, new)
        W = frame.ip
        s = frame.stmt_types
        for r in frame.ssavalue_uses[ssa_id]
            if s[r] !== nothing # s[r] === nothing => unreached statement
                if r < frame.pc´´
                    frame.pc´´ = r
                end
                push!(W, r)
            end
        end
    end
    nothing
end

function add_cycle_backedge!(frame::InferenceState, caller::InferenceState, currpc::Int)
    update_valid_age!(frame, caller)
    backedge = (caller, currpc)
    contains_is(frame.cycle_backedges, backedge) || push!(frame.cycle_backedges, backedge)
    add_backedge!(frame.linfo, caller)
    return frame
end

# temporarily accumulate our edges to later add as backedges in the callee
function add_backedge!(li::MethodInstance, caller::InferenceState)
    isa(caller.linfo.def, Method) || return # don't add backedges to toplevel exprs
    edges = caller.stmt_edges[caller.currpc]
    if edges === nothing
        edges = caller.stmt_edges[caller.currpc] = []
    end
    push!(edges, li)
    nothing
end

# used to temporarily accumulate our no method errors to later add as backedges in the callee method table
function add_mt_backedge!(mt::Core.MethodTable, @nospecialize(typ), caller::InferenceState)
    isa(caller.linfo.def, Method) || return # don't add backedges to toplevel exprs
    edges = caller.stmt_edges[caller.currpc]
    if edges === nothing
        edges = caller.stmt_edges[caller.currpc] = []
    end
    push!(edges, mt)
    push!(edges, typ)
    nothing
end

function print_callstack(sv::InferenceState)
    while sv !== nothing
        print(sv.linfo)
        !sv.cached && print("  [uncached]")
        println()
        for cycle in sv.callers_in_cycle
            print(' ', cycle.linfo)
            println()
        end
        sv = sv.parent
    end
end
