#  Copyright (C) 2003 Vladimir Prus
#  Use, modification, and distribution is subject to the Boost Software
#  License, Version 1.0. (See accompanying file LICENSE.txt or copy
#  at https://www.bfgroup.xyz/b2/LICENSE.txt)

#  This module defines a class which allows to order arbitrary object with
# regard to arbitrary binary relation.
#
#  The primary use case is the gcc toolset, which is sensitive to library order:
#  if library 'a' uses symbols from library 'b', then 'a' must be present before
#  'b' on the linker's command line.
#
#  This requirement can be lifted for gcc with GNU ld, but for gcc with Solaris
#  LD (and for Solaris toolset as well), the order always matters.
#
#  So, we need to store order requirements and then order libraries according to
#  them. It is not possible to use the dependency graph as order requirements.
#  What we need is a "use symbols" relationship while dependency graph provides
#  the "needs to be updated" relationship.
#
#  For example::
#    lib a : a.cpp b;
#    lib b ;
#
#  For static linking, library 'a' need not depend on 'b'. However, it should
#  still come before 'b' on the command line.

class order
{
    rule __init__ ( )
    {
    }

    # Adds the constraint that 'first' should preceede 'second'.
    rule add-pair ( first second )
    {
        .constraits += $(first)--$(second) ;
    }
    NATIVE_RULE class@order : add-pair ;

    # Given a list of objects, reorder them so that the constraints specified by
    # 'add-pair' are satisfied.
    #
    # The algorithm was adopted from an awk script by Nikita Youshchenko
    # (yoush at cs dot msu dot su)
    rule order ( objects * )
    {
        # The algorithm used is the same is standard transitive closure, except
        # that we're not keeping in-degree for all vertices, but rather removing
        # edges.
        local result ;
        if $(objects)
        {
            local constraints = [ eliminate-unused-constraits $(objects) ] ;

            # Find some library that nobody depends upon and add it to the
            # 'result' array.
            local obj ;
            while $(objects)
            {
                local new_objects ;
                while $(objects)
                {
                    obj = $(objects[1]) ;
                    if [ has-no-dependents $(obj) : $(constraints) ]
                    {
                        # Emulate break ;
                        new_objects += $(objects[2-]) ;
                        objects = ;
                    }
                    else
                    {
                        new_objects += $(obj) ;
                        obj = ;
                        objects = $(objects[2-]) ;
                    }
                }

                if ! $(obj)
                {
                    errors.error "Circular order dependencies" ;
                }
                # No problem with placing first.
                result += $(obj) ;
                # Remove all constraints where 'obj' comes first, since they are
                # already satisfied.
                constraints = [ remove-satisfied $(constraints) : $(obj) ] ;

                # Add the remaining objects for further processing on the next
                # iteration
                objects = $(new_objects) ;
            }

        }
        return $(result) ;
    }
    NATIVE_RULE class@order : order ;

    # Eliminate constraints which mention objects not in 'objects'. In
    # graph-theory terms, this is finding a subgraph induced by ordered
    # vertices.
    rule eliminate-unused-constraits ( objects * )
    {
        local result ;
        for local c in $(.constraints)
        {
            local m = [ MATCH (.*)--(.*) : $(c) ] ;
            if $(m[1]) in $(objects) && $(m[2]) in $(objects)
            {
                result += $(c) ;
            }
        }
        return $(result) ;
    }

    # Returns true if there's no constraint in 'constaraints' where 'obj' comes
    # second.
    rule has-no-dependents ( obj : constraints * )
    {
        local failed ;
        while $(constraints) && ! $(failed)
        {
            local c = $(constraints[1]) ;
            local m = [ MATCH (.*)--(.*) : $(c) ] ;
            if $(m[2]) = $(obj)
            {
                failed = true ;
            }
            constraints = $(constraints[2-]) ;
        }
        if ! $(failed)
        {
            return true ;
        }
    }

    rule remove-satisfied ( constraints * : obj )
    {
        local result ;
        for local c in $(constraints)
        {
            local m = [ MATCH (.*)--(.*) : $(c) ] ;
            if $(m[1]) != $(obj)
            {
                result += $(c) ;
            }
        }
        return $(result) ;
    }
}


rule __test__ ( )
{
    import "class" : new ;
    import assert ;

    c1 = [ new order ] ;
    $(c1).add-pair l1 l2 ;

    assert.result l1 l2 : $(c1).order l1 l2 ;
    assert.result l1 l2 : $(c1).order l2 l1 ;

    $(c1).add-pair l2 l3 ;
    assert.result l1 l2 : $(c1).order l2 l1 ;
    $(c1).add-pair x l2 ;
    assert.result l1 l2 : $(c1).order l2 l1 ;
    assert.result l1 l2 l3 : $(c1).order l2 l3 l1 ;

    # The output should be stable for unconstrained
    # elements.
    assert.result l4 l5 : $(c1).order l4 l5 ;
}
