### Setup
source_d := source
target_d := target
template := ./template

# Check that our directories are set
ifndef source_d
  $(error 'source_d' not defined)
endif
ifndef target_d
  $(error 'target_d' not defined)
endif
# Check that our directories exist
source_is_dir = $(shell test -d $(source_d) && echo true || echo false)
ifeq ($(source_is_dir),false)
  $(error directory '$(source_d)' does not exist)
endif
target_is_dir = $(shell test -d $(target_d) && echo true || echo false)
ifeq ($(target_is_dir),false)
  $(error directory '$(target_d)' does not exist)
endif
# Check that $(template) is set
ifndef template
  $(error 'template' not defined)
endif
# Check that $(template) exists
template_exists = $(shell test -f $(template) && echo true || echo false)
ifeq ($(template_exists),false)
  $(error template file '$(template)' does not exist)
endif
# Check that $(template) is executable
template_is_exec = $(shell test -x $(template) && echo true || echo false)
ifeq ($(template_is_exec),false)
  $(error template file '$(template)' is not executable)
endif

# Generate Target Names
source_dirs  := $(shell find -L $(source_d) -mindepth 1 -type d)
source_files := $(shell find -L $(source_d) -type f | rev | sort | rev)
# '| rev | sort | rev' sorts filenames by their extenstion
# this isn't neccessary, it just makes for cleaner output

target_dirs  := $(source_dirs:$(source_d)%=$(target_d)%)
target_files := $(source_files:$(source_d)%=$(target_d)%)
target_files := $(target_files:%.md=%.html)
target_files := $(target_files:%.mmd=%.html)
target_files := $(target_files:%.scss=%.css)

# Check that we actually have targets to build
# if $(target_*) is empty $(old_files) will delete everything
ifndef target_files
  $(error no files found in 'target_files')
endif
ifndef target_dirs
  $(error no directories found in 'target_dirs')
endif

# files in $(target_d) that are no longer targets
old_files := $(filter-out $(target_files) $(target_dirs), $(shell find $(target_d) -mindepth 1 -depth -type f,d))
# because $(old_files) conists of files that already exist
# the $(old_files) rule will not run if it is not .PHONY
.PHONY : $(old_files)


### Rules

default : all
all : $(target_dirs) $(target_files) $(old_files)

$(target_dirs) :
	mkdir -p "$(@)"

$(target_d)/%.html : $(source_d)/%.md template
	markdown "$(<)" | $(template) > "$(@)"

$(target_d)/%.css : $(source_d)/%.scss
	sassc -t expanded "$(<)" > "$(@)"

$(target_d)/% : $(source_d)/%
	cp "$(<)" "$(@)"

$(old_files) :
	@if [[ "$(@)" == "/" ]]; then \
		echo error: attempting to rm "$(@)"; \
	else \
		echo rm -rf "$(@)"; \
		     rm -rf "$(@)"; \
	fi
# this method is slightly inefficient. Because $(old_files) contains the
# names of the directories to be removed and their contents, this rule will
# redundantly run rm on both the contents of a directory and
# each of it's contents

clean :
	rm -r $(target_d)/*