############################################################################
#
#	Name:	lmap.icn
#
#	Title:	Map list elements
#
#	Author:	Ralph E. Griswold
#
#	Date:	June 10, 1988
#
############################################################################
#  
#  The procedure lmap(L1,L2,L3) maps elements of L1 according to L2
#  and L3.  This procedure is the analog for lists of the built-in
#  string-mapping function map(s1,s2,s3). Elements in L1 that are
#  the same as elements in L2 are mapped into the corresponding ele-
#  ments of L3. For example, given the lists
#  
#     L1 := [1,2,3,4]
#     L2 := [4,3,2,1]
#     L3 := ["a","b","c","d"]
#  
#  then
#  
#     lmap(L1,L2,L3)
#  
#  produces a new list
#  
#     ["d","c","b","a"]
#  
#     Lists that are mapped can have any kinds of elements. The
#  operation
#  
#     x === y
#  
#  is used to determine if elements x and y are equivalent.
#  
#     All cases in lmap are handled as they are in map, except that
#  no defaults are provided for omitted arguments. As with map, lmap
#  can be used for transposition as well as substitution.
#  
#  Warning:
#
#     If lmap is called with the same lists L2 and L3 as in
#  the immediately preceding call, the same mapping is performed,
#  even if the values in L2 and L3 have been changed. This improves
#  performance, but it may cause unexpected effects.
#  
#     This ``caching'' of the mapping table based on L2 and L3
#  can be easily removed to avoid this potential problem.
#  
############################################################################

procedure lmap(L1,L2,L3)
   static lmem2, lmem3, lmaptbl, tdefault
   local i, a

   initial tdefault := []

   if type(a := L1 | L2 | L3) ~== "list" then runerr(108,a)
   if *L2 ~= *L3 then runerr(208,L2)

   L1 := copy(L1)

   if not(lmem2 === L2 & lmem3 === L3) then {	# if an argument is new, rebuild
      lmem2 := L2				# save for future reference
      lmem3 := L3
      lmaptbl := table(tdefault)		# new mapping table
      every i := 1 to *L2 do			# build the map
         lmaptbl[L2[i]] := L3[i]
      }
   every i := 1 to *L1 do			# map the values
      L1[i] := (tdefault ~=== lmaptbl[L1[i]])
   return L1
end