1 files changed, 51 insertions, 0 deletions
diff --git a/ext/B/ramblings/flip-flop b/ext/B/ramblings/flip-flop
new file mode 100644
index 0000000000..183d541b98
--- /dev/null
+++ b/ext/B/ramblings/flip-flop
@@ -0,0 +1,51 @@
+PP(pp_range)
+{
+    if (GIMME == G_ARRAY)
+        return cCONDOP->op_true;
+    return SvTRUEx(PAD_SV(op->op_targ)) ? cCONDOP->op_false : cCONDOP->op_true;
+}
+
+pp_range is a CONDOP.
+In array context, it just returns op_true.
+In scalar context it checks the truth of targ and returns
+op_false if true, op_true if false.
+
+flip is an UNOP.
+It "looks after" its child which is always a pp_range CONDOP.
+In array context, it just returns the child's op_false.
+In scalar context, there are three possible outcomes:
+  (1) set child's targ to 1, our targ to 1 and return op_next.
+  (2) set child's targ to 1, our targ to 0, sp-- and return child's op_false.
+  (3) Blank targ and  TOPs and return op_next.
+Case 1 happens for a "..." with a matching lineno... or true TOPs.
+Case 2 happens for a ".." with a matching lineno... or true TOPs.
+Case 3 happens for a non-matching lineno or false TOPs.
+
+               $a = lhs..rhs;
+
+        ,------->  range
+        ^         /     \
+        |    true/       \false
+        |       /         \
+   first|     lhs        rhs
+        |      \   first   /
+        ^--- flip <----- flop
+                 \       /
+                  \     /
+                  sassign
+
+
+/* range */
+if (SvTRUE(curpad[op->op_targ]))
+    goto label(op_false);
+/* op_true */
+...
+/* flip */
+/* For "..." returns op_next. For ".." returns op_next or op_first->op_false */
+/* end of basic block */
+goto out;
+label(range op_false):
+...
+/* flop */
+out:
+...