# Mage by Matt McCutchen

# 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)

mg-orig-default-goal := $(.DEFAULT_GOAL)

# bar.g file format:
# cmd-bar:=cat foo >bar.tmp
# errors-bar:=$(empty)yikes!
# exitcode-bar:=0

# $(call mg-check-file,foo.o)
# Make sure foo.o's genfile, if any, has been loaded.
define mg-check-file
$(if $(mg-checked-$1),,$(eval 
cmd-$1 :=
errors-$1 :=
exitcode-$1 :=
-include $1.g
mg-checked-$1 := done
))
endef

# $(call mg-translate-cmd,touch $$@)
# Currently translates $@, $<, $^, $+ to Mage-ized variables.
# $* needs no translation.  We won't support $%, $?, $|.
# There might be false matches, e.g., $$@ => $$(out) ;
# to prevent that, do $$$(empty)@ .
define mg-translate-cmd
$(subst $$@,$$(mg@),$(subst $$<,$$(mg<),$(subst $$^,$$(mg^),$(subst $$+,$$(mg+),$1))))
endef

# OOOH!!! .SECONDEXPANSION does let us watch the implicit rule search as it
# happens.
.SECONDEXPANSION:

# $(call mg-rule,target,prerequisite,cmd)
# Defines a rule.
# If cmd uses $@, quote if necessary so this function sees $@, etc.
# 
# When we build bar from foo using a Mage rule:
# Division of work:
# - "bar.g: foo" does the computation.
# - "bar: bar.g" replays errors and exit code.
# Cases:
# - bar is managed and up to date => replay
# - bar doesn't exist or is managed and out of date => compute
# - bar is unmanaged => warn about override 
MG-FORCE-TARGET-DNE:
MG-FORCE-CMD-CHANGED:
MG-FORCE-REPLAY:
.PHONY: MG-FORCE-TARGET-DNE MG-FORCE-CMD-CHANGED MG-FORCE-REPLAY
define mg-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^ = $$(filter-out MG-% $$(abspath $$(target)),$$^)
$1.g: mg+ = $$(filter-out MG-% $$(abspath $$(target)),$$+)
$1.g: mg< = $$(firstword $$(mg^))

# Load the target's genfile if necessary.
$1.g: $$$$(call mg-check-file,$$$$(target))

# If the target doesn't exist, we must run the rule; we'll generate it.
# If the target is unmanaged, we must run the rule; we'll see that the
# obfuscated target is in $? and complain.
$1.g: $$$$(if $$$$(wildcard $$$$(target)),$$$$(abspath $$$$(target)),$$$$(if $$(exitcode-$$$$(target)),,MG-FORCE-TARGET-DNE))

# If the command changed, we must regenerate.
$1.g: $$$$(if $$$$(call streq,$$$$(cmd),$$$$(cmd-$$$$(target))),,MG-FORCE-CMD-CHANGED)

$1.g: $2
	$(value mg-commands)

# Replay the errors and the exit code (if any of either).
# We don't have to worry about .DELETE_ON_ERROR deleting the target because
# it is only touched if we *successfully* regenerate it.
# Recheck the file in case it was regenerated.
# HMMM Maybe errors should go to stderr???
$1: $1.g MG-FORCE-REPLAY
	$$(call mg-check-file,$$@)
	$$(if $$(errors-$$@)$$(exitcode-$$@)$$(built-$$@),$$(info $$(cmd-$$@)$$(if $$(built-$$@),, [replay]))$$(if $$(errors-$$@),$$(info $$(errors-$$@)),),)
	$$(if $$(exitcode-$$@),@exit $$(exitcode-$$@),)
)
endef

# Just add additional prerequisites.
define mg-prereq
$(eval 
$1.g: $2
)
endef

# If an override:
#     1. Complain.
#     2. Clear out the errors and exit code so we don't replay them.
# Otherwise:
#     1. Store the command.
#     2. Run the command, storing errors.
#     3. Store exit code.
#     4. Update the real files as applicable.
#     5. Read the new genfile.
define mg-commands
	$(if $(filter $(abspath $(target)),$?),\
		$(info mage: warning: Manually created/modified file at $(target) overrides rule.)\
		$(eval cmd-$(target):=)$(eval errors-$(target):=)$(eval exitcode-$(target):=)\
	,\
	@$(eval mg-checked-$(target) :=)$(eval built-$(target) := yes)\
	exec 3>$@.tmp &&\
	echo $(call sq,$(call fmt-make-assignment,cmd-$(target),$(cmd))) >&3 &&\
	set -o pipefail &&\
	{ { ($(cmd)) && { [ -r $(target).tmp ] || { echo 'mage: error: Command for $(target) succeeded without creating it!'; false; }; }; } 2>&1\
	| sed -re '1s/^/errors-$(target):=$$(empty)/; 1!s/^/errors-$(target)+=$$(nl)/' >&3; xc=$$?; } &&\
	{ [ $$xc == 0 ] || echo "exitcode-$(target):=$$xc" >&3; } &&\
	if [ $$xc != 0 ] || cmp -s $(target) $(target).tmp; then mv -f $@.tmp $@ && rm -f $(target).tmp;\
	else rm -f $(target) && mv -f $@.tmp $@ && mv -f $(target).tmp $(target); fi\
	)
endef

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