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      1 # $NetBSD: hanoi-include.mk,v 1.5 2023/10/19 18:24:33 rillig Exp $
      2 #
      3 # Implements the Towers of Hanoi puzzle, demonstrating a bunch of more or less
      4 # useful programming techniques:
      5 #
      6 #	* default assignment using the ?= assignment operator
      7 #	* including the same file recursively (rather unusual)
      8 #	* extracting the current value of a variable using the .for loop
      9 #	* using the :: dependency operator for adding commands to a target
     10 #	* on-the-fly variable assignment expressions using the ::= modifier
     11 #
     12 # usage:
     13 #	env N=3 make -r -f hanoi-include.mk
     14 #
     15 # Specifying N in the command line instead of in the environment would produce
     16 # an endless loop, since variables from the command line cannot be overridden
     17 # by global variables:
     18 #	make -r -f hanoi-include.mk N=3
     19 
     20 N?=	5			# Move this number of disks ...
     21 FROM?=	A			# ... from this stack ...
     22 VIA?=	B			# ... via this stack ...
     23 TO?=	C			# ... to this stack.
     24 
     25 # Since make has no built-in arithmetic functions, convert N to a list of
     26 # words and use the built-in word counting instead.
     27 .if ${N:[#]} == 1
     28 N:=	count ${:U:${:Urange=$N}}	# 'count' + one word for every disk
     29 .endif
     30 
     31 .if ${N:[#]} == 2
     32 .  for from to in ${FROM} ${TO}
     33 all::
     34 	@echo "Move the upper disk from stack ${from} to stack ${to}."
     35 .  endfor
     36 .else
     37 _:=	${N::=${N:[1..-2]}} ${TMP::=${VIA}} ${VIA::=${TO}} ${TO::=${TMP}}
     38 .  include "${.PARSEDIR}/${.PARSEFILE}"
     39 _:=	${N::+=D} ${TMP::=${VIA}} ${VIA::=${TO}} ${TO::=${TMP}}
     40 
     41 .  for from to in ${FROM} ${TO}
     42 all::
     43 	@echo "Move the upper disk from stack ${from} to stack ${to}."
     44 .  endfor
     45 
     46 _:=	${N::=${N:[1..-2]}} ${TMP::=${VIA}} ${VIA::=${FROM}} ${FROM::=${TMP}}
     47 .  include "${.PARSEDIR}/${.PARSEFILE}"
     48 _:=	${N::+=D} ${TMP::=${VIA}} ${VIA::=${FROM}} ${FROM::=${TMP}}
     49 .endif
     50