sha1-lookup: add new "sha1_pos" function to efficiently lookup sha1

This function has been copied from the "patch_pos" function in
"patch-ids.c" but an additional parameter has been added.

The new parameter is a function pointer, that is used to access the
sha1 of an element in the table.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/sha1-lookup.c b/sha1-lookup.c
index da357479cf19aad4bebc64f874c76fdf8566712b..055dd87dc177864491905c692907ebbce059ae7e 100644 (file)
--- a/sha1-lookup.c
+++ b/sha1-lookup.c
@@ -1,6 +1,107 @@
 #include "cache.h"
 #include "sha1-lookup.h"
 
+static uint32_t take2(const unsigned char *sha1)
+{
+       return ((sha1[0] << 8) | sha1[1]);
+}
+
+/*
+ * Conventional binary search loop looks like this:
+ *
+ *      do {
+ *              int mi = (lo + hi) / 2;
+ *              int cmp = "entry pointed at by mi" minus "target";
+ *              if (!cmp)
+ *                      return (mi is the wanted one)
+ *              if (cmp > 0)
+ *                      hi = mi; "mi is larger than target"
+ *              else
+ *                      lo = mi+1; "mi is smaller than target"
+ *      } while (lo < hi);
+ *
+ * The invariants are:
+ *
+ * - When entering the loop, lo points at a slot that is never
+ *   above the target (it could be at the target), hi points at a
+ *   slot that is guaranteed to be above the target (it can never
+ *   be at the target).
+ *
+ * - We find a point 'mi' between lo and hi (mi could be the same
+ *   as lo, but never can be the same as hi), and check if it hits
+ *   the target.  There are three cases:
+ *
+ *    - if it is a hit, we are happy.
+ *
+ *    - if it is strictly higher than the target, we update hi with
+ *      it.
+ *
+ *    - if it is strictly lower than the target, we update lo to be
+ *      one slot after it, because we allow lo to be at the target.
+ *
+ * When choosing 'mi', we do not have to take the "middle" but
+ * anywhere in between lo and hi, as long as lo <= mi < hi is
+ * satisfied.  When we somehow know that the distance between the
+ * target and lo is much shorter than the target and hi, we could
+ * pick mi that is much closer to lo than the midway.
+ */
+/*
+ * The table should contain "nr" elements.
+ * The sha1 of element i (between 0 and nr - 1) should be returned
+ * by "fn(i, table)".
+ */
+int sha1_pos(const unsigned char *sha1, void *table, size_t nr,
+            sha1_access_fn fn)
+{
+       size_t hi = nr;
+       size_t lo = 0;
+       size_t mi = 0;
+
+       if (!nr)
+               return -1;
+
+       if (nr != 1) {
+               size_t lov, hiv, miv, ofs;
+
+               for (ofs = 0; ofs < 18; ofs += 2) {
+                       lov = take2(fn(0, table) + ofs);
+                       hiv = take2(fn(nr - 1, table) + ofs);
+                       miv = take2(sha1 + ofs);
+                       if (miv < lov)
+                               return -1;
+                       if (hiv < miv)
+                               return -1 - nr;
+                       if (lov != hiv) {
+                               /*
+                                * At this point miv could be equal
+                                * to hiv (but sha1 could still be higher);
+                                * the invariant of (mi < hi) should be
+                                * kept.
+                                */
+                               mi = (nr - 1) * (miv - lov) / (hiv - lov);
+                               if (lo <= mi && mi < hi)
+                                       break;
+                               die("oops");
+                       }
+               }
+               if (18 <= ofs)
+                       die("cannot happen -- lo and hi are identical");
+       }
+
+       do {
+               int cmp;
+               cmp = hashcmp(fn(mi, table), sha1);
+               if (!cmp)
+                       return mi;
+               if (cmp > 0)
+                       hi = mi;
+               else
+                       lo = mi + 1;
+               mi = (hi + lo) / 2;
+       } while (lo < hi);
+       return -lo-1;
+}
+
 /*
  * Conventional binary search loop looks like this:
  *

diff --git a/sha1-lookup.h b/sha1-lookup.h
index 3249a81b3d664afc89c98e6d9dd6b512092a82f9..20af2856818ed51b2afb1718a7e317133ee0d7bd 100644 (file)
--- a/sha1-lookup.h
+++ b/sha1-lookup.h
@@ -1,6 +1,13 @@
 #ifndef SHA1_LOOKUP_H
 #define SHA1_LOOKUP_H
 
+typedef const unsigned char *sha1_access_fn(size_t index, void *table);
+
+extern int sha1_pos(const unsigned char *sha1,
+                   void *table,
+                   size_t nr,
+                   sha1_access_fn fn);
+
 extern int sha1_entry_pos(const void *table,
                          size_t elem_size,
                          size_t key_offset,