From db6acff457a68e8a645594687b789c6cb13b4f49 Mon Sep 17 00:00:00 2001
From: Linus Nordberg <linus@nordu.net>
Date: Mon, 9 May 2016 13:13:29 +0200
Subject: Document keybits() and make it slightly easier to read.

Also, remove unused KEYPART_MAX.
---
 c_src/permdb.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/c_src/permdb.c b/c_src/permdb.c
index f740dd8..604899b 100644
--- a/c_src/permdb.c
+++ b/c_src/permdb.c
@@ -484,16 +484,22 @@ permdb_free(permdb_object *state)
 }
 
 
+/*
+ * Returns the two bits of KEY that are used for LEVEL, for a q=2
+ * tree.
+ */
 static unsigned char
 keybits(const unsigned char *key, unsigned int level)
 {
+    /* Finding the proper octet in key. */
     unsigned char b = key[level/4];
+    /* "Micro-level", for finding the proper two bits, below. */
+    unsigned int mlev = level % 4;
 
-    return (b >> (6-(level*2) % 8)) & 0x3;
+    /* Shift right and mask off the two lowest bits. */
+    return (b >> (6 - 2*mlev)) & 0x3;
 }
 
-#define KEYPART_MAX (keylen*4+1)
-
 static void
 keypart(const unsigned char *key, unsigned int level, ps_string *s)
 {
-- 
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