From 34b6ad594e80196674506d21809126b230c11a0d Mon Sep 17 00:00:00 2001
From: "dgrowley@gmail.com"
Date: Fri, 6 Apr 2018 22:17:58 +1200
Subject: [PATCH v22 2/4] Add bms_prev_member function
This works very much like the existing bms_last_member function, only it
traverses through the Bitmapset in the opposite direction from the most
significant bit down to the least significant bit. A special prevbit value of
-1 may be used to have the function determine the most significant bit. This
is useful for starting a loop. When there are no members less than prevbit,
the function returns -2 to indicate there are no more members.
---
src/backend/nodes/bitmapset.c | 95 +++++++++++++++++++++++++++++++++++++++++++
src/include/nodes/bitmapset.h | 1 +
2 files changed, 96 insertions(+)
diff --git a/src/backend/nodes/bitmapset.c b/src/backend/nodes/bitmapset.c
index edcd19a4fd..9341bf579e 100644
--- a/src/backend/nodes/bitmapset.c
+++ b/src/backend/nodes/bitmapset.c
@@ -58,6 +58,9 @@
* rightmost_one_pos[x] gives the bit number (0-7) of the rightmost one bit
* in a nonzero byte value x. The entry for x=0 is never used.
*
+ * leftmost_one_pos[x] gives the bit number (0-7) of the leftmost one bit in a
+ * nonzero byte value x. The entry for x=0 is never used.
+ *
* number_of_ones[x] gives the number of one-bits (0-8) in a byte value x.
*
* We could make these tables larger and reduce the number of iterations
@@ -84,6 +87,25 @@ static const uint8 rightmost_one_pos[256] = {
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
};
+static const uint8 leftmost_one_pos[256] = {
+ 0, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3,
+ 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
+ 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
+ 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
+ 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+ 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+ 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+ 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7
+};
+
static const uint8 number_of_ones[256] = {
0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
@@ -1089,6 +1111,79 @@ bms_next_member(const Bitmapset *a, int prevbit)
}
/*
+ * bms_prev_member - find prev member of a set
+ *
+ * Returns largest member less than "prevbit", or -2 if there is none.
+ * "prevbit" must NOT be more than one above the highest possible bit that can
+ * be set at the Bitmapset at its current size.
+ *
+ * To ease finding the highest set bit for the initial loop, the special
+ * prevbit value of -1 can be passed to have the function find the highest
+ * valued member in the set.
+ *
+ * This is intended as support for iterating through the members of a set in
+ * reverse. The typical pattern is
+ *
+ * x = -1;
+ * while ((x = bms_prev_member(inputset, x)) >= 0)
+ * process member x;
+ *
+ * Notice that when there are no more members, we return -2, not -1 as you
+ * might expect. The rationale for that is to allow distinguishing the
+ * loop-not-started state (x == -1) from the loop-completed state (x == -2).
+ * It makes no difference in simple loop usage, but complex iteration logic
+ * might need such an ability.
+ */
+
+int
+bms_prev_member(const Bitmapset *a, int prevbit)
+{
+ int wordnum;
+ int ushiftbits;
+ bitmapword mask;
+
+ /*
+ * If set is NULL or if there are no more bits to the right then we've
+ * nothing to do.
+ */
+ if (a == NULL || prevbit == 0)
+ return -2;
+
+ /* transform -1 to the highest possible bit we could have set */
+ if (prevbit == -1)
+ prevbit = a->nwords * BITS_PER_BITMAPWORD - 1;
+ else
+ prevbit--;
+
+ ushiftbits = BITS_PER_BITMAPWORD - (BITNUM(prevbit) + 1);
+ mask = (~(bitmapword) 0) >> ushiftbits;
+ for (wordnum = WORDNUM(prevbit); wordnum >= 0; wordnum--)
+ {
+ bitmapword w = a->words[wordnum];
+
+ /* mask out bits left of prevbit */
+ w &= mask;
+
+ if (w != 0)
+ {
+ int result;
+ int shift = 24;
+ result = wordnum * BITS_PER_BITMAPWORD;
+
+ while ((w >> shift) == 0)
+ shift -= 8;
+
+ result += shift + leftmost_one_pos[(w >> shift) & 255];
+ return result;
+ }
+
+ /* in subsequent words, consider all bits */
+ mask = (~(bitmapword) 0);
+ }
+ return -2;
+}
+
+/*
* bms_hash_value - compute a hash key for a Bitmapset
*
* Note: we must ensure that any two bitmapsets that are bms_equal() will
diff --git a/src/include/nodes/bitmapset.h b/src/include/nodes/bitmapset.h
index 67e8920f65..b6f1a9e6e5 100644
--- a/src/include/nodes/bitmapset.h
+++ b/src/include/nodes/bitmapset.h
@@ -99,6 +99,7 @@ extern Bitmapset *bms_join(Bitmapset *a, Bitmapset *b);
/* support for iterating through the integer elements of a set: */
extern int bms_first_member(Bitmapset *a);
extern int bms_next_member(const Bitmapset *a, int prevbit);
+extern int bms_prev_member(const Bitmapset *a, int prevbit);
/* support for hashtables using Bitmapsets as keys: */
extern uint32 bms_hash_value(const Bitmapset *a);
--
2.11.0