Switch to histogram-based estimation of PFOR varbyte sizes.

This makes plocate.db ~0.1% larger, but reduces CPU usage by something
like 3%, so it's an OK tradeoff.

diff --git a/turbopfor-encode.h b/turbopfor-encode.h
index 24e007cdc070e902bb9ea4b250b9d8bbb1d33505..facd5de69ac2eb4a7d5d38ca8e42c54921050899 100644 (file)
--- a/turbopfor-encode.h
+++ b/turbopfor-encode.h
@@ -7,9 +7,7 @@
 // than encoding posting lists, so it only costs ~5-10% overall. Does not use
 // any special character sets, and generally isn't optimized at all.
 //
-// It encodes about 0.01% denser than the reference encoder (averaged over
-// a real plocate corpus), probably since it has a slower but more precise
-// method for estimating the cost of a PFOR + varbyte block.
+// It encodes roughly about as dense as the reference encoder.
 
 #include "turbopfor-common.h"
 
@@ -311,27 +309,36 @@ BlockType decide_block_type(const Docid *in, unsigned num, unsigned *bit_width,
                }
        }
 
+       // Make the histogram cumulative; histogram[n] now means the the number of
+       // elements with n bits or more.
+       for (unsigned i = max_bits; i > 0; --i) {
+               histogram[i - 1] += histogram[i];
+       }
+
        // Try PFOR with varbyte exceptions.
        bool best_is_varbyte = false;
        for (unsigned test_bit_width = 0; test_bit_width < max_bits; ++test_bit_width) {
                // 1 byte for signaling number of exceptions, plus the base values,
-               // and then we count up the varbytes and indexes. (This is precise
-               // but very slow.)
+               // and then we estimate up the varbytes and indexes using histogram
+               // indexes. This isn't exact, but it only helps ~0.1% on the total
+               // plocate.db size.
                unsigned cost = 1 + bytes_for_packed_bits(num, test_bit_width);
-               for (unsigned i = 0; i < num && cost < best_cost; ++i) {
-                       unsigned val = in[i] >> test_bit_width;
-                       if (val == 0) {
-                               // Not stored, and then also no index.
-                       } else if (val <= 176) {
-                               cost += 2;
-                       } else if (val <= 16560) {
-                               cost += 3;
-                       } else if (val <= 540848) {
-                               cost += 4;
-                       } else if (val <= 16777215) {
-                               cost += 5;
-                       } else {
-                               cost += 6;
+               if (cost >= best_cost) {
+                       break;
+               }
+               if (test_bit_width + 1 <= max_bits) {
+                       cost += 2 * histogram[test_bit_width + 1];  // > 0.
+                       if (test_bit_width + 7 <= max_bits) {
+                               cost += histogram[test_bit_width + 7];  // > 176, very roughly.
+                               if (test_bit_width + 14 <= max_bits) {
+                                       cost += histogram[test_bit_width + 14];  // > 16560, very roughly.
+                                       if (test_bit_width + 19 <= max_bits) {
+                                               cost += histogram[test_bit_width + 19];  // > 540848, very roughly.
+                                               if (test_bit_width + 24 <= max_bits) {
+                                                       cost += histogram[test_bit_width + 24];  // > 16777215, very roughly.
+                                               }
+                                       }
+                               }
                        }
                }
                if (cost < best_cost) {