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# $NetBSD: hanoi-include.mk,v 1.5 2023/10/19 18:24:33 rillig Exp $
#
# Implements the Towers of Hanoi puzzle, demonstrating a bunch of more or less
# useful programming techniques:
#
# * default assignment using the ?= assignment operator
# * including the same file recursively (rather unusual)
# * extracting the current value of a variable using the .for loop
# * using the :: dependency operator for adding commands to a target
# * on-the-fly variable assignment expressions using the ::= modifier
#
# usage:
# env N=3 make -r -f hanoi-include.mk
#
# Specifying N in the command line instead of in the environment would produce
# an endless loop, since variables from the command line cannot be overridden
# by global variables:
# make -r -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.
# Since make has no built-in arithmetic functions, convert N to a list of
# words and use the built-in word counting instead.
.if ${N:[#]} == 1
N:= count ${:U:${:Urange=$N}} # 'count' + one word for every disk
.endif
.if ${N:[#]} == 2
. for from to in ${FROM} ${TO}
all::
@echo "Move the upper disk from stack ${from} to stack ${to}."
. endfor
.else
_:= ${N::=${N:[1..-2]}} ${TMP::=${VIA}} ${VIA::=${TO}} ${TO::=${TMP}}
. include "${.PARSEDIR}/${.PARSEFILE}"
_:= ${N::+=D} ${TMP::=${VIA}} ${VIA::=${TO}} ${TO::=${TMP}}
. for from to in ${FROM} ${TO}
all::
@echo "Move the upper disk from stack ${from} to stack ${to}."
. endfor
_:= ${N::=${N:[1..-2]}} ${TMP::=${VIA}} ${VIA::=${FROM}} ${FROM::=${TMP}}
. include "${.PARSEDIR}/${.PARSEFILE}"
_:= ${N::+=D} ${TMP::=${VIA}} ${VIA::=${FROM}} ${FROM::=${TMP}}
.endif
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