refactor pic_list_p
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6cc6046ea3
commit
27d4853aef
32
src/pair.c
32
src/pair.c
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@ -48,27 +48,33 @@ pic_cdr(pic_state *pic, pic_value obj)
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bool
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pic_list_p(pic_state *pic, pic_value obj)
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{
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UNUSED(pic);
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pic_value twice=obj;
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pic_value local, rapid;
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int i;
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/* This code checks whether obj is a circular list
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using Floyd's cycle-finding algorithm. */
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while ( 1 ) { /* loops until a cycle is detected or an object other than a pair is found */
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int i;
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UNUSED(pic);
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/* Floyd's cycle-finding algorithm. */
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local = rapid = obj;
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while (true) {
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/* advance rapid fast-forward; runs 2x faster than local */
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for (i = 0; i < 2; ++i) {
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if (pic_pair_p(twice)) {
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twice = pic_pair_ptr(twice)->cdr;
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}
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if (pic_pair_p(rapid)) {
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rapid = pic_pair_ptr(rapid)->cdr;
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}
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else {
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return pic_nil_p(twice);
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return pic_nil_p(rapid);
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}
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}
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obj = pic_pair_ptr(obj)->cdr; /* Since obj progresses slower than twice, it is guaranteed that obj is a pair. */
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if (pic_eq_p(obj,twice)) { /* obj is a circular list */
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/* advance local */
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local = pic_pair_ptr(local)->cdr;
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if (pic_eq_p(local, rapid)) {
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return false;
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}
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}
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/* unreachable */
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}
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pic_value
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