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############################################################################
#
# File: equiv.icn
#
# Subject: Procedure to compare structures
#
# Author: Ralph E. Griswold
#
# Date: February 20, 1996
#
############################################################################
#
# This file is in the public domain.
#
############################################################################
#
# equiv(s,y) compare arbitrary structures x and y
#
############################################################################
#
# The procedure equiv() tests for the "equivalence" of two values. For types
# other than structures, it does the same thing as x1 === x2. For structures,
# the test is for "shape". For example,
#
# equiv([],[])
#
# succeeds.
#
# It handles loops, but does not recognize them as such. For example,
# given
#
# L1 := []
# L2 := []
# put(L1,L1)
# put(L2,L1)
#
# equiv(L1,L2)
#
# succeeds.
#
# The concept of equivalence for tables and sets is not quite right
# if their elements are themselves structures. The problem is that there
# is no concept of order for tables and sets, yet it is impractical to
# test for equivalence of their elements without imposing an order. Since
# structures sort by "age", there may be a mismatch between equivalent
# structures in two tables or sets.
#
# Note:
# The procedures equiv and ldag have a trailing argument that is used on
# internal recursive calls; a second argument must not be supplied
# by the user.
#
############################################################################
procedure equiv(x1,x2,done) #: compare values for equivalence
local code, i
if x1 === x2 then return x2 # Covers everything but structures.
if type(x1) ~== type(x2) then fail # Must be same type.
if type(x1) == ("procedure" | "file" | "window")
then fail # Leave only those with sizes (null
# taken care of by first two tests).
if *x1 ~= *x2 then fail # Skip a lot of possibly useless work.
# Structures (and others) remain.
/done := table() # Basic call.
(/done[x1] := set()) | # Make set of equivalences if new.
(if member(done[x1],x2) then return x2)
# Records complicate things.
image(x1) ? (code := (="record" | type(x1)))
case code of {
"list" | "record":
every i := 1 to *x1 do
if not equiv(x1[i],x2[i],done) then fail
"table": if not equiv(sort(x1,3),sort(x2,3),done) then fail
"set": if not equiv(sort(x1),sort(x2),done) then fail
default: fail # Vaues of other types are different.
}
insert(done[x1],x2) # Equivalent; add to set.
return x2
end
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