From 3ac55a64bfc3a4d43cb9496bddc57e26d542c2c7 Mon Sep 17 00:00:00 2001
From: Stephen Heumann <stephenheumann@gmail.com>
Date: Mon, 6 Mar 2023 21:54:14 -0600
Subject: [PATCH] Use an improved hash function for symbol tables.

The hash algorithm has been modified to include a rotate at each step. This should improve the quality of hashes and reduce the number of collisions. However, probably the more important change for performance is to do the modulo computation by repeated subtraction rather than by calling a slow library function.
---
 CCommon.asm | 44 +++++++++++++++++++++++++-------------------
 Header.pas  |  2 +-
 2 files changed, 26 insertions(+), 20 deletions(-)

diff --git a/CCommon.asm b/CCommon.asm
index 50d8142..d0d7d38 100644
--- a/CCommon.asm
+++ b/CCommon.asm
@@ -44,37 +44,43 @@ lb1      lda   [fromPtr],Y
 Hash     start cc
 hashSize equ   876                      # hash buckets - 1
 
-sum      equ   0                        hash
+disp     equ   0                        disp into hash table
 length   equ   2                        length of string
 
          subroutine (4:sPtr),4
 
-         stz   sum                      default to bucket 0
          lda   [sPtr]                   set the length of the string
+         tax
          and   #$00FF
          sta   length
          ldy   #1                       start with char 1
-         lda   [sPtr]                   if 1st char is '~', start with char 6
+         txa                            if 1st char is '~', start with char 6
          and   #$FF00
          cmp   #'~'*256
-         bne   lb1
+         bne   lb0
          ldy   #6
 
-lb1      lda   [sPtr],Y                 get the value to add in
-         and   #$3F3F
-         cpy   length                   if there is only 1 char left then
-         bne   lb2
-         and   #$00FF                     and out the high byte
-lb2      clc                            add it to the sum
-         adc   sum
-         sta   sum
-         iny                            next char
+lb0      lda   #0                       initial value is 0
+         bra   lb2                      while there are at least 2 chars left
+lb1      asl   a                          rotate sum left one bit
+         adc   [sPtr],Y                   add in next two bytes
+         iny                              advance two chars
          iny
-         cpy   length
-         ble   lb1
-         mod2  sum,#hashSize+1          return disp
-         asl   sum
-         asl   sum
+lb2      cpy   length
+         blt   lb1
+         bne   lb3                      if there is 1 char left then
+         asl   a                          rotate sum left one bit
+         sta   disp
+         lda   [sPtr],Y
+         and   #$00FF                     and out the high byte
+         adc   disp                       add last byte to the sum
+         sec
+lb3      sbc   #hashSize+1              disp := (sum mod (hashSize+1)) << 2
+         bcs   lb3
+         adc   #hashSize+1
+         asl   a
+         asl   a
+         sta   disp
 
-         return 2:sum
+         return 2:disp                  return disp
          end
diff --git a/Header.pas b/Header.pas
index ecc81fb..ff91a57 100644
--- a/Header.pas
+++ b/Header.pas
@@ -18,7 +18,7 @@ uses CCommon, MM, Scanner, Symbol, CGI;
 {$segment 'HEADER'}
 
 const
-   symFileVersion = 38;                 {version number of .sym file format}
+   symFileVersion = 39;                 {version number of .sym file format}
 
 var
    inhibitHeader: boolean;		{should .sym includes be blocked?}