Re: [PATCH dump.c] hash quality

Message-Id: <E14lUhm-0000rx-00@libra.cus.cam.ac.uk>

p4raw-id: //depot/perl@9612

diff --git a/dump.c b/dump.c
index 9010bc5..9d2595f 100644 (file)
--- a/dump.c
+++ b/dump.c
@@ -1044,15 +1044,24 @@ Perl_do_sv_dump(pTHX_ I32 level, PerlIO *file, SV *sv, I32 nest, I32 maxnest, bo
                }
             }
            PerlIO_putc(file, ')');
-           /* Now calculate quality wrt theoretical value */
+           /* The "quality" of a hash is defined as the total number of
+              comparisons needed to access every element once, relative
+              to the expected number needed for a random hash.
+
+              The total number of comparisons is equal to the sum of
+              the squares of the number of entries in each backet.
+              For a random hash of n keys into k backets, the expected
+              value is
+                               n + n(n-1)/2k
+           */
+
            for (i = max; i > 0; i--) { /* Precision: count down. */
                sum += freq[i] * i * i;
             }
            while ((keys = keys >> 1))
                pow2 = pow2 << 1;
-           /* Approximate by Poisson distribution */
            theoret = HvKEYS(sv);
-           theoret += theoret * theoret/pow2;
+           theoret += theoret * (theoret-1)/pow2;
            PerlIO_putc(file, '\n');
            Perl_dump_indent(aTHX_ level, file, "  hash quality = %.1"NVff"%%", theoret/sum*100);
        }

diff --git a/t/lib/peek.t b/t/lib/peek.t
index 7bf1793..96e24a2 100644 (file)
--- a/t/lib/peek.t
+++ b/t/lib/peek.t
@@ -171,7 +171,7 @@ do_test(12,
     IV = 1
     NV = 0
     ARRAY = $ADDR  \\(0:7, 1:1\\)
-    hash quality = 150.0%
+    hash quality = 100.0%
     KEYS = 1
     FILL = 1
     MAX = 7