# Mage Build Tool by Matt McCutchen
# http://www.kepreon.com/~matt/mage/

# Remember the original default goal so we can restore it at the end of mage.mk.
mg-orig-default-goal := $(.DEFAULT_GOAL)

# TEXT UTILITIES
empty :=
bs := \$(empty)
define nl


endef
# Shell-quote: a'b => 'a'\''b'
sq = '$(subst ','\'',$1)'
# Make-quote: a$\nb => a$$$(nl)b
# This is enough to assign the value, *but not to use it as an argument!*
mqas = $(subst $(nl),$$(nl),$(subst $$,$$$$,$1))
# Return nonempty if the strings are equal, empty otherwise.
# If the strings have only nice characters, you can do $(filter x$1,x$2).
streq = $(findstring x$1,$(findstring x$2,x$1))

# $(call fmt-make-assignment,foo,bar)
# Output a make assignment that stores bar in variable foo.
# The result is like `foo:=bar' but handles leading spaces, $, and
# newlines appearing in bar safely.
fmt-make-assignment = $1:=$$(empty)$(call mqas,$2)

# We use second-expansion heavily to dynamically compute prerequisites when they
# are needed.  Second-expansion lets us follow make's implicit rule search
# instead of trying to anticipate which prerequisites it will need in advance.
.SECONDEXPANSION:

# TARGET OBFUSCATION

# Mage uses two kinds of "obfuscated targets" (those that are not the simple
# names of real files):
# - An always-exists target like /.//. is used as a prerequisite of an implicit
#   rule.  Since it exists, make doesn't second-expand its own prerequisites
#   until it is actually run.  This way, a target's prerequisites can depend on
#   the results of previous command scripts.
# - An alternative-name target like /.//./proc/self/cwd/bar is used to check the
#   mtime of a file without actually building it or introducing a circular
#   dependency.

# $(newoid) allocates and returns a new obfuscation ID (oid).  Example:
#    x:=$(newoid)
# You can then refer to target $x or $x$(aname)bar (for any file bar).
# Prerequisites and commands come from $($x@opr) and $($x@ocmd).
# Target-specific variables $(oid) and $(otgt) (if alternate-name) are
# available.

opfx:=/./
# TODO If the system doesn't support /proc/self/cwd, use something else.
aname:=/proc/self/cwd/
nextoid:=/./.
newoid=$(nextoid)$(eval nextoid:=$(subst /.//////,//./,$(patsubst /.//////%,/././%,$(nextoid:.=/.))))

# High-priority implicit rule for obfuscated targets.  Works for both always-
# exists and alternate-name targets.
$(opfx)%: oid=$(word 1,$(subst $(aname), ,$@))
$(opfx)%: otgt=$(word 2,$(subst $(aname), ,$@))
$(opfx)%: $$($$(oid)@opr)
	$($(oid)@ocmd)

# MAIN BUILD LOGIC

# bar.g file format:
#     bar@cmd:=cat foo >bar.tmp
#     bar@warnings:=$(empty)yikes!
# And if dependency-logging:
# List of filename@revision, revision is x for exists and empty for doesn't
# exist.  Later perhaps x will be the mtime.
#     bar@deps:=included@x oops@
# If bar is overridden, we clear all three variables.

# $(call gload,foo.o)
# Make sure foo.o's genfile, if any, has been loaded.
define gload
$(if $($1@gloaded),,$(eval 
$1@cmd:=
$1@warnings:=
$1@deps:=
-include $1.g
$1@gloaded:=1
))
endef

# $(call mg-translate-cmd,touch $$@)
# Translates $@, $<, $^, $+, $* to Mage-ized variables if necessary.
# We won't support $%, $?, $|.
# There might be false matches, e.g., $$@ => $$(mg@) ;
# to prevent that, do $$$(empty)@ .
define mg-translate-cmd
$(subst $$@,$$(mg@),$(subst $$<,$$(mg<),$(subst $$^,$$(mg^),$(subst $$+,$$(mg+),$1))))
endef

# $(call mg-rule,target,prerequisite,cmd)
# Defines a rule.
# If cmd uses $@, quote if necessary so this function sees $@, etc.
# 
# When we generate bar from foo using a Mage rule:
# Division of work:
# - "bar.g: foo" generates bar if necessary.
# - "bar: bar.g" loads new genfile and replays warnings if bar.g was
#   already up to date.
# Cases:
# - bar is managed and up to date => replay
# - bar doesn't exist or is managed and out of date => generate
# - bar is unmanaged => warn about override 
MG-FORCE:
.PHONY: MG-FORCE
scout-oid:=$(newoid)
define mg-define-rule
$(eval 

# Define some target-specific variables.
# It might look like we could use $*, but $1 most likely isn't %.
$1.g: target = $$(@:.g=)
$1.g: cmd = $(call mg-translate-cmd,$3)
$1.g: mg@ = $$(target).tmp
$1.g: mg^ = $$(filter-out MG-% /./%,$$^)
$1.g: mg+ = $$(filter-out MG-% /./%,$$+)
$1.g: mg< = $$(firstword $$(mg^))

# Rule for the genfile.  Evidently all the prerequisites we want second-expanded
# have to go on the same rule.
$1.g: $2 $$$$(mg-scout-target) MG-FORCE
	$$(mg-generate)

$(mg-file-from-genfile)
)
endef

define mg-file-from-genfile
# If the file was regenerated, load the new genfile.
# If not, replay any warnings.
# HMMM Maybe errors should go to stderr???
$1: MG-FORCE | $1.g
	$$(call gload,$$@)
	$$(if $$($$@@warnings),$$(if $$($$@@built),,$$(info $$($$@@cmd) # warning replay)$$(info $$($$@@warnings))),)
endef

# If the target is unmanaged, we must run the rule; we'll see that the
# obfuscated target is in $? and complain.
mg-scout-target=$(if $(wildcard $(target)),$(scout-oid)$(aname)$(target),)

# If the command changed, we must regenerate.
mg-check-cmd=$(if $(call streq,$(cmd),$($(target)@cmd)),,x)

# Just add additional prerequisites.
# I don't think this can add patterns to implicit rules, but it should be able
# to add specific prerequisites to uses of implicit rules.
define mg-define-prereq
$(eval 
$1.g: $2
)
endef

# Procedure to generate bar.  Remember, $@ is bar.g.
# If an override:
#     1. Complain.  (Should we stop "`bar' is up to date" using @:; ?)
#     2. Clear out the warnings so we don't replay them.
# Otherwise, check prereqs, command, and nonexistence to decide whether bar
# needs to be regenerated.  If so:
#     1. Set a flag so Mage knows to reread the genfile when make runs target
#        "bar".
#     2. Echo the command being run.
#     On error, skip to 8:
#        3. Open the new genfile bar.g.tmp for writing.
#        4. Store the command in bar.g.tmp.
#        5. Run the command to bar.tmp, storing warnings in bar.g.tmp.
#        6. If bar.tmp differs from bar:
#           a. Clear and touch bar.g (so bar doesn't become an override if we're
#              killed between steps 6b and 7).
#           b. Move in the new bar.
#        7. Now that bar is known to be up to date, move in the new bar.g.
#     8. Defensively remove both temporary files and exit with the exit code
#        from before the removal. (trap EXIT)
define mg-generate
	$(call gload,$(target))\
	$(if $(filter $(scout-oid)$(aname)$(target),$?),\
		$(info mage: warning: Manually created/modified file at $(target) overrides rule.)\
		$(eval $(target)@cmd:=)$(eval $(target)@warnings:=)$(eval $(target)@deps:=)\
	,\
	$(if $?$(if $(wildcard $(target)),,x)$(mg-check-cmd),\
	$(eval $(target)@gloaded:=)$(eval $(target)@built:=1)$(info $(cmd))\
	@trap 'rm -f $(target).tmp $@.tmp' EXIT &&\
	exec 3>$@.tmp && $(mg-assign-cmd) >&3 &&\
	set -o pipefail && { $(mg-run-cmd) | tee /dev/fd/4 | $(mg-wrap-warnings) >&3; } 4>&1 &&\
	$(mg-maybe-move-target) && mv -f $@.tmp $@\
	))
endef

# Pieces of mg-generate that I factored out to make mg-generate more readable.
mg-assign-cmd=echo $(call sq,$(call fmt-make-assignment,$(target)@cmd,$(cmd)))
mg-run-cmd={ ($(cmd)) 2>&1 && { [ -r $(target).tmp ] || { echo 'mage: error: Command for $(target) succeeded without creating it!'; false; }; }; }
mg-wrap-warnings=sed -re '1s/^/$(target)@warnings:=$$(empty)/; 1!s/^/$(target)@warnings+=$$(nl)/'
mg-maybe-move-target={ cmp -s $(target) $(target).tmp || echo >$@ && mv -f $(target).tmp $(target); }

# END

# We don't want anything we defined to become the default goal.
.DEFAULT_GOAL := $(mg-orig-default-goal)