# $NetBSD: hanoi-include.mk,v 1.1 2020/10/03 17:30:54 rillig Exp $
#
# Implements the Towers of Hanoi puzzle, thereby demonstrating a bunch of
# useful programming techniques:
#
# * default assignment using the ?= assignment operator
# * including the same file recursively
# * extracting the current value of a variable using the .for loop
# * using shell commands for calculations since make is a text processor
# * using the :: dependency operator for adding commands to a target
# * on-the-fly variable assignment expressions using the ::= modifier
#
# usage:
#	env N=3 make -f hanoi-include.mk
# endless loop:
#	make -f hanoi-include.mk N=3

N?=	5			# Move this number of disks ...
FROM?=	A			# ... from this stack ...
VIA?=	B			# ... via this stack ...
TO?=	C			# ... to this stack.

.if $N == 1
.  for from to in ${FROM} ${TO}
all::
	@echo "Move the upper disk from stack ${from} to stack ${to}."
.  endfor
.else
_:=	${N::!=expr $N - 1} ${TMP::=${VIA}} ${VIA::=${TO}} ${TO::=${TMP}}
.  include "${.PARSEDIR}/${.PARSEFILE}"
_:=	${N::!=expr $N + 1} ${TMP::=${VIA}} ${VIA::=${TO}} ${TO::=${TMP}}

.  for from to in ${FROM} ${TO}
all::
	@echo "Move the upper disk from stack ${from} to stack ${to}."
.  endfor

_:=	${N::!=expr $N - 1} ${TMP::=${VIA}} ${VIA::=${FROM}} ${FROM::=${TMP}}
.  include "${.PARSEDIR}/${.PARSEFILE}"
_:=	${N::!=expr $N + 1} ${TMP::=${VIA}} ${VIA::=${FROM}} ${FROM::=${TMP}}
.endif