From 2d3b51665e2190879616318828ba6244db6033ad Mon Sep 17 00:00:00 2001
From: Max Kellermann <max@musicpd.org>
Date: Wed, 5 Feb 2020 19:51:46 +0100
Subject: [PATCH] util/BitReverse: generate table with constexpr function

Get rid of the macro hell.
---
 src/util/BitReverse.cxx | 18 +++++++++---------
 src/util/BitReverse.hxx | 21 +++++++++++++++++++--
 2 files changed, 28 insertions(+), 11 deletions(-)

diff --git a/src/util/BitReverse.cxx b/src/util/BitReverse.cxx
index 29ffb49eb..0564c2c51 100644
--- a/src/util/BitReverse.cxx
+++ b/src/util/BitReverse.cxx
@@ -19,13 +19,13 @@
 
 #include "BitReverse.hxx"
 
-/**
- * @see http://graphics.stanford.edu/~seander/bithacks.html#BitReverseTable
- */
-const uint8_t bit_reverse_table[256] =
+static constexpr BitReverseTable
+GenerateBitReverseTable() noexcept
 {
-#define R2(n)     n,     n + 2*64,     n + 1*64,     n + 3*64
-#define R4(n) R2(n), R2(n + 2*16), R2(n + 1*16), R2(n + 3*16)
-#define R6(n) R4(n), R4(n + 2*4 ), R4(n + 1*4 ), R4(n + 3*4 )
-    R6(0), R6(2), R6(1), R6(3)
-};
+	BitReverseTable table{};
+	for (unsigned i = 0; i < 256; ++i)
+		table.data[i] = BitReverseMultiplyModulus(i);
+	return table;
+}
+
+const BitReverseTable bit_reverse_table = GenerateBitReverseTable();
diff --git a/src/util/BitReverse.hxx b/src/util/BitReverse.hxx
index c67d8a68e..f8e9cf63b 100644
--- a/src/util/BitReverse.hxx
+++ b/src/util/BitReverse.hxx
@@ -24,13 +24,30 @@
 
 #include <stdint.h>
 
-extern const uint8_t bit_reverse_table[256];
+/**
+ * @see http://graphics.stanford.edu/~seander/bithacks.html#ReverseByteWith64BitsDiv
+ */
+constexpr uint8_t
+BitReverseMultiplyModulus(uint8_t _in) noexcept
+{
+	uint64_t in = _in;
+	return uint8_t((in * 0x0202020202ULL & 0x010884422010ULL) % 1023);
+}
+
+/* in order to avoid including <array> in this header, this `struct`
+   is a workaround for GenerateBitReverseTable() being able to return
+   the plain array */
+struct BitReverseTable {
+	uint8_t data[256];
+};
+
+extern const BitReverseTable bit_reverse_table;
 
 gcc_const
 static inline uint8_t
 bit_reverse(uint8_t x) noexcept
 {
-	return bit_reverse_table[x];
+	return bit_reverse_table.data[x];
 }
 
 #endif