path: root/tools/build/v2/build/feature.jam

# Copyright 2001, 2002, 2003 Dave Abrahams
# Copyright 2002, 2006 Rene Rivera
# Copyright 2002, 2003, 2004, 2005, 2006 Vladimir Prus
# Distributed under the Boost Software License, Version 1.0.
# (See accompanying file LICENSE_1_0.txt or copy at
# http://www.boost.org/LICENSE_1_0.txt)

import assert : * ;
import "class" : * ;
import indirect ;
import modules ;
import regex ;
import sequence ;
import set ;
import utility ;


local rule setup ( )
{
    .all-attributes =
        implicit
        composite
        optional
        symmetric
        free
        incidental
        path
        dependency
        propagated
        link-incompatible
        subfeature
        order-sensitive
    ;

    .all-features = ;
    .all-subfeatures = ;
    .all-top-features = ;  # non-subfeatures
    .all-implicit-values = ;
}
setup ;


# Prepare a fresh space to test in by moving all global variable settings into
# the given temporary module and erasing them here.
#
rule prepare-test ( temp-module )
{
    DELETE_MODULE $(temp-module) ;

    # Transfer globals to temp-module.
    for local v in [ VARNAMES feature ]
    {
        if [ MATCH (\\.) : $(v) ]
        {
            modules.poke $(temp-module) : $(v) : $($(v)) ;
            $(v) = ;
        }
    }
    setup ;
}


# Clear out all global variables and recover all variables from the given
# temporary module.
#
rule finish-test ( temp-module )
{
    # Clear globals.
    for local v in [ VARNAMES feature ]
    {
        if [ MATCH (\\.) : $(v) ]
        {
            $(v) = ;
        }
    }

    for local v in [ VARNAMES $(temp-module) ]
    {
        $(v) = [ modules.peek $(temp-module) : $(v) ] ;
    }
    DELETE_MODULE $(temp-module) ;
}


# Transform features by bracketing any elements which are not already bracketed
# by "<>".
#
local rule grist ( features * )
{
    local empty = "" ;
    return $(empty:G=$(features)) ;
}


# Declare a new feature with the given name, values, and attributes.
#
rule feature (
      name          # Feature name.
    : values *      # Allowable values - may be extended later using feature.extend.
    : attributes *  # Feature attributes (e.g. implicit, free, propagated...).
)
{
    name = [ grist $(name) ] ;

    local error ;

    # Check for any unknown attributes.
    if ! ( $(attributes) in $(.all-attributes) )
    {
        error = unknown attributes:
            [ set.difference $(attributes) : $(.all-attributes) ] ;
    }
    else if $(name) in $(.all-features)
    {
        error = feature already defined: ;
    }
    else if implicit in $(attributes) && free in $(attributes)
    {
        error = free features cannot also be implicit ;
    }
    else if free in $(attributes) && propagated in $(attributes)
    {
        error = free features cannot be propagated ;
    }
    else
    {
        local m = [ MATCH (.*=.*) : $(values) ] ;
        if $(m[1])
        {
            error = "feature value may not contain '='" ;
        }
    }

    if $(error)
    {
        import errors ;
        errors.error $(error)
              : "in" feature declaration:
              : feature [ errors.lol->list $(1) : $(2) : $(3) ] ;
    }

    $(name).values ?= ;
    $(name).attributes = $(attributes) ;
    $(name).subfeatures ?= ;
    $(attributes).features += $(name) ;

    .all-features += $(name) ;
    if subfeature in $(attributes)
    {
        .all-subfeatures += $(name) ;
    }
    else
    {
        .all-top-features += $(name) ;
    }
    extend $(name) : $(values) ;
}


# Sets the default value of the given feature, overriding any previous default.
#
rule set-default ( feature : value )
{
    local f = [ grist $(feature) ] ;
    local a = $($(f).attributes) ;
    local bad-attribute = ;
    if free in $(a)
    {
        bad-attribute = free ;
    }
    else if optional in $(a)
    {
        bad-attribute = optional ;
    }
    if $(bad-attribute)
    {
        import errors ;
        errors.error $(bad-attribute) property $(f) cannot have a default. ;
    }
    if ! $(value) in $($(f).values)
    {
        import errors ;
        errors.error The specified default value, '$(value)' is invalid :
            allowed values are: $($(f).values) ;
    }
    $(f).default = $(value) ;
}


# Returns the default property values for the given features.
#
rule defaults ( features * )
{
    local result ;
    for local f in $(features)
    {
        local gf = $(:E=:G=$(f)) ;
        local a = $($(gf).attributes) ;
        if ( free in $(a) ) || ( optional in $(a) )
        {
        }
        else
        {
            result += $(gf)$($(gf).default) ;
        }
    }
    return $(result) ;
}


# Returns true iff all 'names' elements are valid features.
#
rule valid ( names + )
{
    if $(names) in $(.all-features)
    {
        return true ;
    }
}


# Returns the attibutes of the given feature.
#
rule attributes ( feature )
{
    return $($(feature).attributes) ;
}


# Returns the values of the given feature.
#
rule values ( feature )
{
    return $($(:E=:G=$(feature)).values) ;
}


# Returns true iff 'value-string' is a value-string of an implicit feature.
#
rule is-implicit-value ( value-string )
{
    local v = [ regex.split $(value-string) - ] ;
    local failed ;
    if ! $(v[1]) in $(.all-implicit-values)
    {
        failed = true ;
    }
    else
    {
        local feature = $($(v[1]).implicit-feature) ;
        for local subvalue in $(v[2-])
        {
            if ! [ find-implied-subfeature $(feature) $(subvalue) : $(v[1]) ]
            {
                failed = true ;
            }
        }
    }

    if ! $(failed)
    {
        return true ;
    }
}


# Returns the implicit feature associated with the given implicit value.
#
rule implied-feature ( implicit-value )
{
    local components = [ regex.split $(implicit-value) "-" ] ;
    local feature = $($(components[1]).implicit-feature) ;
    if ! $(feature)
    {
        import errors ;
        errors.error \"$(implicit-value)\" is not an implicit feature value ;
        feature = "" ;  # Keep testing happy; it expects a result.
    }
    return $(feature) ;
}


local rule find-implied-subfeature ( feature subvalue : value-string ? )
{
    # Feature should be of the form <feature-name>.
    if $(feature) != $(feature:G)
    {
        import errors ;
        errors.error invalid feature $(feature) ;
    }
    return $($(feature)$(value-string:E="")<>$(subvalue).subfeature) ;
}


# Given a feature and a value of one of its subfeatures, find the name of the
# subfeature. If value-string is supplied, looks for implied subfeatures that
# are specific to that value of feature
#
rule implied-subfeature (
      feature         # The main feature name.
      subvalue        # The value of one of its subfeatures.
    : value-string ?  # The value of the main feature.
)
{
    local subfeature = [ find-implied-subfeature $(feature) $(subvalue)
        : $(value-string) ] ;
    if ! $(subfeature)
    {
        value-string ?= "" ;
        import errors ;
        errors.error \"$(subvalue)\" is not a known subfeature value of
            $(feature)$(value-string) ;
    }
    return $(subfeature) ;
}


# Generate an error if the feature is unknown.
#
local rule validate-feature ( feature )
{
    if ! $(feature) in $(.all-features)
    {
        import errors ;
        errors.error unknown feature \"$(feature)\" ;
    }
}


# Given a feature and its value or just a value corresponding to an implicit
# feature, returns a property set consisting of all component subfeatures and
# their values. For example all the following calls:
#
#   expand-subfeatures-aux <toolset>gcc-2.95.2-linux-x86
#   expand-subfeatures-aux gcc-2.95.2-linux-x86
#
# return:
#
#   <toolset>gcc <toolset-version>2.95.2 <toolset-os>linux <toolset-cpu>x86
#
local rule expand-subfeatures-aux (
      feature ?        # Feature name or empty if value corresponds to an
                       # implicit property.
    : value            # Feature value.
    : dont-validate ?  # If set, no value string validation will be done.
)
{
    if $(feature)
    {
        feature = $(feature) ;
    }

    if ! $(feature)
    {
        feature = [ implied-feature $(value) ] ;
    }
    else
    {
        validate-feature $(feature) ;
    }
    if ! $(dont-validate)
    {
        validate-value-string $(feature) $(value) ;
    }

    local components = [ regex.split $(value) "-" ] ;

    # Get the top-level feature's value.
    local value = $(components[1]:G=) ;

    local result = $(components[1]:G=$(feature)) ;

    local subvalues = $(components[2-]) ;
    while $(subvalues)
    {
        local subvalue = $(subvalues[1]) ;  # Pop the head off of subvalues.
        subvalues = $(subvalues[2-]) ;

        local subfeature = [ find-implied-subfeature $(feature) $(subvalue) :
            $(value) ] ;

        # If no subfeature was found reconstitute the value string and use that.
        if ! $(subfeature)
        {
            result = $(components:J=-) ;
            result = $(result:G=$(feature)) ;
            subvalues = ;  # Stop looping.
        }
        else
        {
            local f = [ MATCH ^<(.*)>$ : $(feature) ] ;
            result += $(subvalue:G=$(f)-$(subfeature)) ;
        }
    }

    return $(result) ;
}


# Make all elements of properties corresponding to implicit features explicit,
# and express all subfeature values as separate properties in their own right.
# For example, all of the following properties
#
#    gcc-2.95.2-linux-x86
#    <toolset>gcc-2.95.2-linux-x86
#
# might expand to
#
#   <toolset>gcc <toolset-version>2.95.2 <toolset-os>linux <toolset-cpu>x86
#
rule expand-subfeatures (
    properties *       # Property set with elements of the form
                       # <feature>value-string or just value-string in the case
                       # of implicit features.
    : dont-validate ?
)
{
    local result ;
    for local p in $(properties)
    {
        # Don't expand subfeatures in subfeatures
        if ! [ MATCH "(:)" : $(p:G) ]
        {
            result += [ expand-subfeatures-aux $(p:G) : $(p:G=) : $(dont-validate) ] ;
        }
        else
        {
            result += $(p) ;
        }
    }
    return $(result) ;
}


# Helper for extend, below. Handles the feature case.
#
local rule extend-feature ( feature : values * )
{
    feature = [ grist $(feature) ] ;
    validate-feature $(feature) ;
    if implicit in $($(feature).attributes)
    {
        for local v in $(values)
        {
            if $($(v).implicit-feature)
            {
                import errors ;
                errors.error $(v) is already associated with the
                    \"$($(v).implicit-feature)\" feature ;
            }
            $(v).implicit-feature = $(feature) ;
        }

        .all-implicit-values += $(values) ;
    }
    if ! $($(feature).values)
    {
        # This is the first value specified for this feature so make it be the
        # default.
        $(feature).default = $(values[1]) ;
    }
    $(feature).values += $(values) ;
}


# Checks that value-string is a valid value-string for the given feature.
#
rule validate-value-string ( feature value-string )
{
    if ! (
        free in $($(feature).attributes)
        || ( $(value-string) in $(feature).values )
    )
    {
        local values = $(value-string) ;

        if $($(feature).subfeatures)
        {
            if ! $(value-string) in $($(feature).values)
                $($(feature).subfeatures)
            {
                values = [ regex.split $(value-string) - ] ;
            }
        }

        if ! ( $(values[1]) in $($(feature).values) ) &&

            # An empty value is allowed for optional features.
            ( $(values[1]) || ! ( optional in $($(feature).attributes) ) )
        {
            import errors ;
            errors.error \"$(values[1])\" is not a known value of feature
                $(feature) : legal values: \"$($(feature).values)\" ;
        }

        for local v in $(values[2-])
        {
            # This will validate any subfeature values in value-string.
            implied-subfeature $(feature) $(v) : $(values[1]) ;
        }
    }
}


# A helper that computes:
#  * name(s) of module-local variable(s) used to record the correspondence
#    between subvalue(s) and a subfeature
#  * value of that variable when such a subfeature/subvalue has been defined and
# returns a list consisting of the latter followed by the former.
#
local rule subvalue-var (
    feature         # Main feature name.
    value-string ?  # If supplied, specifies a specific value of the main
                    # feature for which the subfeature values are valid.
    : subfeature    # Subfeature name.
    : subvalues *   # Subfeature values.
)
{
    feature = [ grist $(feature) ] ;
    validate-feature $(feature) ;
    if $(value-string)
    {
        validate-value-string $(feature) $(value-string) ;
    }

    local subfeature-name = [ get-subfeature-name $(subfeature) $(value-string) ] ;

    return $(subfeature-name)
    $(feature)$(value-string:E="")<>$(subvalues).subfeature ;
}


# Extends the given subfeature with the subvalues. If the optional value-string
# is provided, the subvalues are only valid for the given value of the feature.
# Thus, you could say that <target-platform>mingw is specific to
# <toolset>gcc-2.95.2 as follows:
#
#       extend-subfeature toolset gcc-2.95.2 : target-platform : mingw ;
#
rule extend-subfeature (
    feature         # The feature whose subfeature is being extended.

    value-string ?  # If supplied, specifies a specific value of the main
                    # feature for which the new subfeature values are valid.

    : subfeature    # Subfeature name.
    : subvalues *   # Additional subfeature values.
)
{
    local subfeature-vars = [ subvalue-var $(feature) $(value-string)
        : $(subfeature) : $(subvalues) ] ;

    local f = [ utility.ungrist [ grist $(feature) ] ] ;
    extend $(f)-$(subfeature-vars[1]) : $(subvalues) ;

    # Provide a way to get from the given feature or property and subfeature
    # value to the subfeature name.
    $(subfeature-vars[2-]) = $(subfeature-vars[1]) ;
}


# Returns true iff the subvalues are valid for the feature. When the optional
# value-string is provided, returns true iff the subvalues are valid for the
# given value of the feature.
#
rule is-subvalue ( feature : value-string ? : subfeature : subvalue )
{
    local subfeature-vars = [ subvalue-var $(feature) $(value-string)
        : $(subfeature) : $(subvalue) ] ;

    if $($(subfeature-vars[2])) = $(subfeature-vars[1])
    {
        return true ;
    }
}


# Can be called three ways:
#
#    1. extend feature : values *
#    2. extend <feature> subfeature : values *
#    3. extend <feature>value-string subfeature : values *
#
# * Form 1 adds the given values to the given feature.
# * Forms 2 and 3 add subfeature values to the given feature.
# * Form 3 adds the subfeature values as specific to the given property
#   value-string.
#
rule extend ( feature-or-property subfeature ? : values * )
{
    local feature ;       # If a property was specified this is its feature.
    local value-string ;  # E.g., the gcc-2.95-2 part of <toolset>gcc-2.95.2.

    # If a property was specified.
    if $(feature-or-property:G) && $(feature-or-property:G=)
    {
        # Extract the feature and value-string, if any.
        feature = $(feature-or-property:G) ;
        value-string = $(feature-or-property:G=) ;
    }
    else
    {
        feature = [ grist $(feature-or-property) ] ;
    }

    # Dispatch to the appropriate handler.
    if $(subfeature)
    {
        extend-subfeature $(feature) $(value-string) : $(subfeature)
            : $(values) ;
    }
    else
    {
        # If no subfeature was specified, we do not expect to see a
        # value-string.
        if $(value-string)
        {
            import errors ;
            errors.error can only specify a property as the first argument when
                extending a subfeature
                : usage:
                : "    extend" feature ":" values...
                : "  | extend" <feature>value-string subfeature ":" values... ;
        }

        extend-feature $(feature) : $(values) ;
    }
}


local rule get-subfeature-name ( subfeature value-string ? )
{
    local prefix = $(value-string): ;
    return $(prefix:E="")$(subfeature) ;
}


# Declares a subfeature.
#
rule subfeature (
    feature         # Root feature that is not a subfeature.
    value-string ?  # A value-string specifying which feature or subfeature
                    # values this subfeature is specific to, if any.
    : subfeature    # The name of the subfeature being declared.
    : subvalues *   # The allowed values of this subfeature.
    : attributes *  # The attributes of the subfeature.
)
{
    feature = [ grist $(feature) ] ;
    validate-feature $(feature) ;

    # Add grist to the subfeature name if a value-string was supplied.
    local subfeature-name = [ get-subfeature-name $(subfeature) $(value-string) ] ;

    if $(subfeature-name) in $($(feature).subfeatures)
    {
        import errors ;
        errors.error \"$(subfeature)\" already declared as a subfeature of
            \"$(feature)\" "specific to "$(value-string) ;
    }
    $(feature).subfeatures += $(subfeature-name) ;

    # First declare the subfeature as a feature in its own right.
    local f = [ utility.ungrist $(feature) ] ;
    feature $(f)-$(subfeature-name) : $(subvalues) : $(attributes) subfeature ;

    # Now make sure the subfeature values are known.
    extend-subfeature $(feature) $(value-string) : $(subfeature) : $(subvalues) ;
}


# Set components of the given composite property.
#
rule compose ( composite-property : component-properties * )
{
    local feature = $(composite-property:G) ;
    if ! ( composite in [ attributes $(feature) ] )
    {
        import errors ;
        errors.error "$(feature)" is not a composite feature ;
    }

    $(composite-property).components ?= ;
    if $($(composite-property).components)
    {
        import errors ;
        errors.error components of "$(composite-property)" already set:
            $($(composite-property).components) ;
    }

    if $(composite-property) in $(component-properties)
    {
        import errors ;
        errors.error composite property "$(composite-property)" cannot have itself as a component ;
    }
    $(composite-property).components = $(component-properties) ;
}


local rule expand-composite ( property )
{
    return $(property)
        [ sequence.transform expand-composite : $($(property).components) ] ;
}


# Return all values of the given feature specified by the given property set.
#
rule get-values ( feature : properties * )
{
    local result ;

    feature = $(:E=:G=$(feature)) ;  # Add <> if necessary.
    for local p in $(properties)
    {
        if $(p:G) = $(feature)
        {
            # Use MATCH instead of :G= to get the value, in order to preserve
            # the value intact instead of having bjam treat it as a decomposable
            # path.
            result += [ MATCH ">(.*)" : $(p) ] ;
        }
    }
    return $(result) ;
}


rule free-features ( )
{
    return $(free.features) ;
}


# Expand all composite properties in the set so that all components are
# explicitly expressed.
#
rule expand-composites ( properties * )
{
    local explicit-features = $(properties:G) ;
    local result ;

    # Now expand composite features.
    for local p in $(properties)
    {
        local expanded = [ expand-composite $(p) ] ;

        for local x in $(expanded)
        {
            if ! $(x) in $(result)
            {
                local f = $(x:G) ;

                if $(f) in $(free.features)
                {
                    result += $(x) ;
                }
                else if ! $(x) in $(properties)  # x is the result of expansion
                {
                    if ! $(f) in $(explicit-features)  # not explicitly-specified
                    {
                        if $(f) in $(result:G)
                        {
                            import errors ;
                            errors.error expansions of composite features result
                                in conflicting values for $(f)
                                : values: [ get-values $(f) : $(result) ] $(x:G=)
                                : one contributing composite property was $(p) ;
                        }
                        else
                        {
                            result += $(x) ;
                        }
                    }
                }
                else if $(f) in $(result:G)
                {
                    import errors ;
                    errors.error explicitly-specified values of non-free feature
                        $(f) conflict :
                        "existing values:" [ get-values $(f) : $(properties) ] :
                        "value from expanding " $(p) ":" $(x:G=) ;
                }
                else
                {
                    result += $(x) ;
                }
            }
        }
    }
    return $(result) ;
}


# Return true iff f is an ordinary subfeature of the parent-property's feature,
# or if f is a subfeature of the parent-property's feature specific to the
# parent-property's value.
#
local rule is-subfeature-of ( parent-property f )
{
    if subfeature in $($(f).attributes)
    {
        local specific-subfeature = [ MATCH <(.*):(.*)> : $(f) ] ;
        if $(specific-subfeature)
        {
            # The feature has the form <topfeature-topvalue:subfeature>, e.g.
            # <toolset-msvc:version>.
            local feature-value = [ split-top-feature $(specific-subfeature[1])
                ] ;
            if <$(feature-value[1])>$(feature-value[2]) = $(parent-property)
            {
                return true ;
            }
        }
        else
        {
            # The feature has the form <topfeature-subfeature>, e.g.
            # <toolset-version>
            local top-sub = [ split-top-feature [ utility.ungrist $(f) ] ] ;
            if $(top-sub[2]) && <$(top-sub[1])> = $(parent-property:G)
            {
                return true ;
            }
        }
    }
}


# As for is-subfeature-of but for subproperties.
#
local rule is-subproperty-of ( parent-property p )
{
    return [ is-subfeature-of $(parent-property) $(p:G) ] ;
}


# Given a property, return the subset of features consisting of all ordinary
# subfeatures of the property's feature, and all specific subfeatures of the
# property's feature which are conditional on the property's value.
#
local rule select-subfeatures ( parent-property : features * )
{
    return [ sequence.filter is-subfeature-of $(parent-property) : $(features) ] ;
}


# As for select-subfeatures but for subproperties.
#
local rule select-subproperties ( parent-property : properties * )
{
    return [ sequence.filter is-subproperty-of $(parent-property) : $(properties) ] ;
}


# Given a property set which may consist of composite and implicit properties
# and combined subfeature values, returns an expanded, normalized property set
# with all implicit features expressed explicitly, all subfeature values
# individually expressed, and all components of composite properties expanded.
# Non-free features directly expressed in the input properties cause any values
# of those features due to composite feature expansion to be dropped. If two
# values of a given non-free feature are directly expressed in the input, an
# error is issued.
#
rule expand ( properties * )
{
    local expanded = [ expand-subfeatures $(properties) ] ;
    return [ expand-composites $(expanded) ] ;
}


# Helper rule for minimize. Returns true iff property's feature is present in
# the contents of the variable named by feature-set-var.
#
local rule in-features ( feature-set-var property )
{
    if $(property:G) in $($(feature-set-var))
    {
        return true ;
    }
}


# Helper rule for minimize. Returns the list with the same properties, but with
# all subfeatures moved to the end of the list.
#
local rule move-subfeatures-to-the-end ( properties * )
{
    local x1 ;
    local x2 ;
    for local p in $(properties)
    {
        if subfeature in $($(p:G).attributes)
        {
            x2 += $(p) ;
        }
        else
        {
            x1 += $(p) ;
        }
    }
    return $(x1) $(x2) ;
}


# Given an expanded property set, eliminate all redundancy: properties that are
# elements of other (composite) properties in the set will be eliminated.
# Non-symmetric properties equal to default values will be eliminated unless
# they override a value from some composite property. Implicit properties will
# be expressed without feature grist, and sub-property values will be expressed
# as elements joined to the corresponding main property.
#
rule minimize ( properties * )
{
    # Precondition checking
    local implicits = [ set.intersection $(p:G=) : $(p:G) ] ;
    if $(implicits)
    {
        import errors ;
        errors.error minimize requires an expanded property set, but
            \"$(implicits[1])\" appears to be the value of an un-expanded
            implicit feature ;
    }

    # Remove properties implied by composite features.
    local components = $($(properties).components) ;
    local x = [ set.difference $(properties) : $(components) ] ;

    # Handle subfeatures and implicit features.
    x = [ move-subfeatures-to-the-end $(x) ] ;
    local result ;
    while $(x)
    {
        local p fullp = $(x[1]) ;
        local f = $(p:G) ;
        local v = $(p:G=) ;

        # Eliminate features in implicit properties.
        if implicit in [ attributes $(f) ]
        {
            p = $(v) ;
        }

        # Locate all subproperties of $(x[1]) in the property set.
        local subproperties = [ select-subproperties $(fullp) : $(x) ] ;
        if $(subproperties)
        {
            # Reconstitute the joined property name.
            local sorted = [ sequence.insertion-sort $(subproperties) ] ;
            result += $(p)-$(sorted:G="":J=-) ;

            x = [ set.difference $(x[2-]) : $(subproperties) ] ;
        }
        else
        {
            # Eliminate properties whose value is equal to feature's default,
            # which are not symmetric and which do not contradict values implied
            # by composite properties.

            # Since all component properties of composites in the set have been
            # eliminated, any remaining property whose feature is the same as a
            # component of a composite in the set must have a non-redundant
            # value.
            if $(fullp) != [ defaults $(f) ]
                || symmetric in [ attributes $(f) ]
                || $(fullp:G) in $(components:G)
            {
                result += $(p) ;
            }

            x = $(x[2-]) ;
        }
    }
    return $(result) ;
}


# Combine all subproperties into their parent properties
#
# Requires: for every subproperty, there is a parent property. All features are
# explicitly expressed.
#
# This rule probably should not be needed, but build-request.expand-no-defaults
# is being abused for unintended purposes and it needs help.
#
rule compress-subproperties ( properties * )
{
    local all-subs ;
    local matched-subs ;
    local result ;

    for local p in $(properties)
    {
        if ! $(p:G)
        {
            # Expecting fully-gristed properties.
            assert.variable-not-empty p:G ;
        }

        if ! subfeature in $($(p:G).attributes)
        {
            local subs = [ sequence.insertion-sort
                [ sequence.filter is-subproperty-of $(p) : $(properties) ] ] ;

            matched-subs += $(subs) ;

            local subvalues = -$(subs:G=:J=-) ;
            subvalues ?= "" ;
            result += $(p)$(subvalues) ;
        }
        else
        {
            all-subs += $(p) ;
        }
    }
    assert.result true : set.equal $(all-subs) : $(matched-subs) ;
    return $(result) ;
}


# Given an ungristed string, finds the longest prefix which is a top-level
# feature name followed by a dash, and return a pair consisting of the parts
# before and after that dash. More interesting than a simple split because
# feature names may contain dashes.
#
local rule split-top-feature ( feature-plus )
{
    local e = [ regex.split $(feature-plus) - ] ;
    local f = $(e[1]) ;
    local v ;
    while $(e)
    {
        if <$(f)> in $(.all-top-features)
        {
            v = $(f) $(e[2-]:J=-) ;
        }
        e = $(e[2-]) ;
        f = $(f)-$(e[1]) ;
    }
    return $(v) ;
}


# Given a set of properties, add default values for features not represented in
# the set.
#
# Note: if there's an ordinary feature F1 and a composite feature F2 which
# includes some value for F1 and both feature have default values then the
# default value of F1 will be added (as opposed to the value in F2). This might
# not be the right idea, e.g. consider:
#
#   feature variant : debug ... ;
#        <variant>debug : .... <runtime-debugging>on
#   feature <runtime-debugging> : off on ;
#
#   Here, when adding default for an empty property set, we'll get
#
#     <variant>debug <runtime_debugging>off
#
#   and that's kind of strange.
#
rule add-defaults ( properties * )
{
    for local v in $(properties:G=)
    {
        if $(v) in $(properties)
        {
            import errors ;
            errors.error add-defaults requires explicitly specified features,
                but \"$(v)\" appears to be the value of an un-expanded implicit
                feature ;
        }
    }
    # We don't add default for elements with ":" inside. This catches:
    # 1. Conditional properties --- we don't want <variant>debug:<define>DEBUG
    #    to be takes as specified value for <variant>
    # 2. Free properties with ":" in values. We don't care, since free
    #    properties don't have defaults.
    local xproperties = [ MATCH "^([^:]+)$" : $(properties) ] ;
    local missing-top = [ set.difference $(.all-top-features) : $(xproperties:G) ] ;
    local more =  [ defaults $(missing-top) ] ;
    properties += $(more) ;
    xproperties += $(more) ;

    # Add defaults for subfeatures of features which are present.
    for local p in $(xproperties)
    {
        local s = $($(p:G).subfeatures) ;
        local f = [ utility.ungrist $(p:G) ] ;
        local missing-subs = [ set.difference <$(f)-$(s)> : $(properties:G) ] ;
        properties += [ defaults [ select-subfeatures $(p) : $(missing-subs) ] ] ;
    }

    return $(properties)  ;
}


# Given a property-set of the form
#       v1/v2/...vN-1/<fN>vN/<fN+1>vN+1/...<fM>vM
#
# Returns
#       v1 v2 ... vN-1 <fN>vN <fN+1>vN+1 ... <fM>vM
#
# Note that vN...vM may contain slashes. This needs to be resilient to the
# substitution of backslashes for slashes, since Jam, unbidden, sometimes swaps
# slash direction on NT.
#
rule split ( property-set )
{
    local pieces = [ regex.split $(property-set) [\\/] ] ;
    local result ;

    for local x in $(pieces)
    {
        if ( ! $(x:G) ) && $(result[-1]:G)
        {
            result = $(result[1--2]) $(result[-1])/$(x) ;
        }
        else
        {
            result += $(x) ;
        }
    }

    return $(result) ;
}


# Tests of module feature.
#
rule __test__ ( )
{
    # Use a fresh copy of the feature module.
    prepare-test feature-test-temp ;

    import assert ;
    import errors : try catch ;

    # These are local rules and so must be explicitly reimported into the
    # testing module.
    import feature : extend-feature validate-feature select-subfeatures ;

    feature toolset : gcc : implicit ;
    feature define : : free ;
    feature runtime-link : dynamic static : symmetric ;
    feature optimization : on off ;
    feature variant : debug release profile : implicit composite symmetric ;
    feature stdlib : native stlport ;
    feature magic : : free ;

    compose <variant>debug : <define>_DEBUG <optimization>off ;
    compose <variant>release : <define>NDEBUG <optimization>on ;

    assert.result dynamic static : values <runtime-link> ;
    assert.result dynamic static : values runtime-link ;

    try ;
    {
        compose <variant>profile : <variant>profile ;
    }
    catch composite property <variant>profile cannot have itself as a component ;

    extend-feature toolset : msvc metrowerks ;
    subfeature toolset gcc : version : 2.95.2 2.95.3 2.95.4 3.0 3.0.1 3.0.2 ;

    assert.true is-subvalue toolset : gcc : version : 2.95.3 ;
    assert.false is-subvalue toolset : gcc : version : 1.1 ;

    assert.false is-subvalue toolset : msvc : version : 2.95.3 ;
    assert.false is-subvalue toolset : : version : yabba ;

    feature yabba ;
    subfeature yabba : version : dabba ;
    assert.true is-subvalue yabba : : version : dabba ;

    subfeature toolset gcc : platform : linux cygwin : optional ;

    assert.result <toolset-gcc:version>
        : select-subfeatures <toolset>gcc
        : <toolset-gcc:version>
          <toolset-msvc:version>
          <toolset-version>
          <stdlib> ;

    subfeature stdlib : version : 3 4 : optional ;

    assert.result <stdlib-version>
        : select-subfeatures <stdlib>native
        : <toolset-gcc:version>
          <toolset-msvc:version>
          <toolset-version>
          <stdlib-version> ;

    assert.result <toolset>gcc <toolset-gcc:version>3.0.1
        : expand-subfeatures <toolset>gcc-3.0.1 ;

    assert.result <toolset>gcc <toolset-gcc:version>3.0.1 <toolset-gcc:platform>linux
        : expand-subfeatures <toolset>gcc-3.0.1-linux ;

    assert.result <toolset>gcc <toolset-gcc:version>3.0.1
        : expand <toolset>gcc <toolset-gcc:version>3.0.1  ;

    assert.result <define>foo=x-y
        : expand-subfeatures <define>foo=x-y ;

    assert.result <toolset>gcc <toolset-gcc:version>3.0.1
        : expand-subfeatures gcc-3.0.1 ;

    assert.result a c e
        : get-values <x> : <x>a <y>b <x>c <y>d <x>e ;

    assert.result <toolset>gcc <toolset-gcc:version>3.0.1
        <variant>debug <define>_DEBUG <optimization>on
        : expand gcc-3.0.1 debug <optimization>on ;

    assert.result <variant>debug <define>_DEBUG <optimization>on
        : expand debug <optimization>on ;

    assert.result <optimization>on <variant>debug <define>_DEBUG
        : expand <optimization>on debug ;

    assert.result <runtime-link>dynamic <optimization>on
        : defaults <runtime-link> <define> <optimization> ;

    # Make sure defaults is resilient to missing grist.
    assert.result <runtime-link>dynamic <optimization>on
        : defaults runtime-link define optimization ;

    feature dummy : dummy1 dummy2 ;
    subfeature dummy : subdummy : x y z : optional ;

    feature fu : fu1 fu2 : optional ;
    subfeature fu : subfu : x y z : optional ;
    subfeature fu : subfu2 : q r s ;

    assert.result optional : attributes <fu> ;

    assert.result <runtime-link>static <define>foobar <optimization>on
        <toolset>gcc:<define>FOO <toolset>gcc <variant>debug <stdlib>native
        <dummy>dummy1 <toolset-gcc:version>2.95.2
        : add-defaults <runtime-link>static <define>foobar <optimization>on
          <toolset>gcc:<define>FOO ;

    assert.result <runtime-link>static <define>foobar <optimization>on
        <toolset>gcc:<define>FOO <fu>fu1 <toolset>gcc <variant>debug
        <stdlib>native <dummy>dummy1 <fu-subfu2>q <toolset-gcc:version>2.95.2
        : add-defaults <runtime-link>static <define>foobar <optimization>on
          <toolset>gcc:<define>FOO <fu>fu1 ;

    set-default <runtime-link> : static ;
    assert.result <runtime-link>static : defaults <runtime-link> ;

    assert.result gcc-3.0.1 debug <optimization>on
        : minimize [ expand gcc-3.0.1 debug <optimization>on <stdlib>native ] ;

    assert.result gcc-3.0.1 debug <runtime-link>dynamic
        : minimize
          [ expand gcc-3.0.1 debug <optimization>off <runtime-link>dynamic ] ;

    assert.result gcc-3.0.1 debug
        : minimize [ expand gcc-3.0.1 debug <optimization>off ] ;

    assert.result debug <optimization>on
        : minimize [ expand debug <optimization>on ] ;

    assert.result gcc-3.0
        : minimize <toolset>gcc <toolset-gcc:version>3.0 ;

    assert.result gcc-3.0
        : minimize <toolset-gcc:version>3.0 <toolset>gcc ;

    assert.result <x>y/z <a>b/c <d>e/f
        : split <x>y/z/<a>b/c/<d>e/f ;

    assert.result <x>y/z <a>b/c <d>e/f
        : split <x>y\\z\\<a>b\\c\\<d>e\\f ;

    assert.result a b c <d>e/f/g <h>i/j/k
        : split a/b/c/<d>e/f/g/<h>i/j/k ;

    assert.result a b c <d>e/f/g <h>i/j/k
        : split a\\b\\c\\<d>e\\f\\g\\<h>i\\j\\k ;

    # Test error checking.

    try ;
    {
        expand release <optimization>off <optimization>on ;
    }
    catch explicitly-specified values of non-free feature <optimization> conflict ;

    try ;
    {
        validate-feature <foobar> ;
    }
    catch unknown feature ;

    validate-value-string <toolset> gcc ;
    validate-value-string <toolset> gcc-3.0.1 ;

    try ;
    {
        validate-value-string <toolset> digital_mars ;
    }
    catch \"digital_mars\" is not a known value of <toolset> ;

    try ;
    {
        feature foobar : : baz ;
    }
    catch unknown attributes: baz ;

    feature feature1 ;
    try ;
    {
        feature feature1 ;
    }
    catch feature already defined: ;

    try ;
    {
        feature feature2 : : free implicit ;
    }
    catch free features cannot also be implicit ;

    try ;
    {
        feature feature3 : : free propagated ;
    }
    catch free features cannot be propagated ;

    try ;
    {
        implied-feature lackluster ;
    }
    catch \"lackluster\" is not an implicit feature value ;

    try ;
    {
        implied-subfeature <toolset> 3.0.1 ;
    }
    catch \"3.0.1\" is not a known subfeature value of <toolset> ;

    try ;
    {
        implied-subfeature <toolset> not-a-version : gcc ;
    }
    catch \"not-a-version\" is not a known subfeature value of <toolset>gcc ;

    # Leave a clean copy of the features module behind.
    finish-test feature-test-temp ;
}