#***********************************************************************
#
# Rules for generating the compilation DB.
#
# This is used by the clang tooling. It contains a list of translation units
# and the commands used to generate them (also the working directory, but that
# is uninteresting for our build system).
#
# See the clang docs for more information:
# http://clang.llvm.org/docs/JSONCompilationDatabase.html
#
# To generate the compilation DB we must be building with clang and have
# CMD_CHECK set. We must also undo the effects of SKIPDEP *not* being set.
#
# The clang compiler is required so the flags used are consistent with what the
# clang tooling expects. If we are using some other compiler then we just error
# out.
#
# CMD_CHECK is required as without it there is no dependency from the object
# file to the command file. That dependency is required so the target-specific
# variables used to build the object file are present when writing the command
# file. To address this if it is missing we just reinvoke make with it.
#
# Copyright (c) 2016 Process Systems Enterprise Ltd
# ALL RIGHTS RESERVED
#
#***********************************************************************

.PHONY: compilation_db

# The JSON compilation database file.
COMPILATION_DB := $(GEN_DIR)/compile_commands.json

# Note we also need an (order-only) dependency on messaging headers otherwise
# we will get a failure when building from clean. Note also that ordering
# matters: this must be declared before the other dependencies below.
$(COMPILATION_DB): | GenerateMessagingHeaders

# Adds a dependency on the object file and command file to the compilation
# database and adds the command file to the list to be processed.
#
# NOTE: Adding the object file as a dependency is bad and wrong but necessary
#       to make this all work. Unfortunately it seems that make (at least as of
#       4.2.1) determines target-specific variables based on whether
#       dependencies that set them are rebuilt or not. I.e. the fact that the
#       object file has a dependency on the command file does not necessarily
#       mean the command file rules will run with the target-specific variables
#       defined for the object files. That will happen if and only if the
#       object files are also being rebuilt.
#
#       This means that in order to generate a correct command file we need to
#       also be building the object file at the same time. If we've previously
#       built the object file and command file together then everything should
#       be fine. However if not (e.g. because we are building from clean or
#       CMD_CHECK is not set) then to get it right we must rebuild the object
#       file.
#
# TODO: It would be really nice if we could avoid this!
CMD_FILE_SRCS :=
define DefineCommandFile
$(COMPILATION_DB): $(addprefix $(call GenObjBaseName,$(1)),.o .ccmd)
CMD_FILE_SRCS += $(1)
endef

# Text to generate for each entry in the compilation database.
define CompilationDBEntry
{ "file": "$(1)",
  "directory": "$(abspath $(TOP))",
  "command": "$(shell cat $(call GenObjBaseName,$(1)).ccmd)" },
endef

CommandFileSuffixes := %.c %.cpp %.cxx
CommandFileSources := $(filter $(CommandFileSuffixes),$(SOURCES))
$(foreach src,$(CommandFileSources),$(eval $(call DefineCommandFile,$(src))))

# Building with clang and CMD_CHECK: all good.
define CompilationDBReady

# Pseudo-target to generate the compilation database.
compilation_db: $(COMPILATION_DB)

# The compilation database's build rule.
#
# We add a bogus empty entry last so we don't need to worry about not adding a
# trailing comma to the last entry (stupid JSON!)
#
# NOTE: Dependencies on all command files have been added above (see
#       DefineCommandFile)
$(COMPILATION_DB):
	$$(file >$$@,[)
	$$(foreach cmdfile,$(CMD_FILE_SRCS),$$(file >>$$@,$$(call CompilationDBEntry,$$(cmdfile))))
	$$(file >>$$@,{"file": "", "command": "", "directory": ""}])
	@echo "Generated compilation database: $$@"

# Keep the compilation database up-to-date when doing a regular build with
# appropriate settings
TARGETS += $(COMPILATION_DB)
endef

# Building with clang but not CMD_CHECK: reinvoke with it.
define CompilationDBReinvoke
compilation_db::
	@$$(MAKE) --no-print-directory CC=clang CMD_CHECK=1 compilation_db
endef

# Not building with clang: fail.
define CompilationDBNeedsClang
compilation_db::
	@echo "You must build with clang (CC=clang) in order to generate the compilation database."
	@/bin/false
endef

# Select the appropriate rule for the pseudo-target depending on whether we
# have already been invoked with the correct flags.
CompilationDBSelectedRule = $(if $(findstring clang,$(CC)),$(if $(CMD_CHECK),$(CompilationDBReady),$(CompilationDBReinvoke)),$(CompilationDBNeedsClang))
$(eval $(CompilationDBSelectedRule))