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length builtin: detect cycles and return infinity if there is any

--- a/builtins.c

+++ b/builtins.c

@@ -88,8 +88,21 @@

 		return fixnum(vector_size(a));

 	if(a == FL_NIL)

 		return fixnum(0);

-	if(iscons(a))

-		return fixnum(llength(a));

+	if(iscons(a)){

+		size_t n = 0;

+		value_t v = a, v2 = a;

+		do{

+			n++;

+			v = cdr_(v);

+			v2 = cdr_(v2);

+			if(iscons(v2))

+				v2 = cdr_(v2);

+		}while(iscons(v) && iscons(v2) && v != v2);

+		if(iscons(v2))

+			return mk_double(D_PINF);

+		n += llength(v);

+		return fixnum(n);

+	}

 	if(iscprim(a)){

 		cv = (cvalue_t*)ptr(a);

 		if(cp_class(cv) == bytetype)

--- a/test/unittest.lsp

+++ b/test/unittest.lsp

@@ -355,5 +355,9 @@

 (assert (equal? (append '(1 2)) '(1 2)))

 (assert (equal? (append '(1 2) '(3 4)) '(1 2 3 4)))

+(define a '(1))

+(set-cdr! a a)

+(assert (equal? (length a) +inf.0))

+

 (princ "all tests pass\n")

 #t

-- 

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