Simple string hashing function

Question

I\'m trying to hash a string into an integer for placing it in an array. However I do not know all too much about hashing functions, and that\'s why my current method is jus

Answer 1

See http://www.strchr.com/hash_functions for a very good panel of hashing functions.

In Delphi implementation, here are several versions:

The first coming to mind is the one used in TStringHash.HashOf method from official IniFiles.pas unit. Including a faster asm version:

function HashOf(P: PByteArray; Len: integer): cardinal;
// algorithm from IniFiles.TStringHash.HashOf
{$ifdef PUREPASCAL}
var I: Integer;
begin
  Result := 0;
  for I := 1 to Len do
    Result := ((Result shl 2) or (Result shr (SizeOf(Result)*8-2))) xor P[I];
end;
{$else}
asm // faster asm version by Synopse
    or edx,edx
    jz @z
    push ebx
    mov ebx,edx     // ebx = length(Key)
    mov edx,eax     // edx = Text
    xor eax,eax     // eax = Result
    xor ecx,ecx     // ecx = Result shl 2 = 0
@1: shr eax,$1e     // eax = Result shr (SizeOf(Result) * 8 - 2))
    or ecx,eax      // ecx = ((Result shl 2) or (Result shr (SizeOf(Result)*8-2)))
    movzx eax,byte ptr [edx] // eax = ord(Key[i])
    inc edx
    xor eax,ecx     // eax = () xor ord(Key[i])
    dec ebx
    lea ecx,[eax*4] // ecx = Result shl 2
    jnz @1
    pop ebx
@z:
end;
{$endif}

The classic Kernighan & Ritchie hash from "The C programming Language", 3rd edition - not the best, but simple and efficient code.

function kr32(crc: cardinal; buf: PAnsiChar; len: cardinal): cardinal;
var i: integer;
begin
  for i := 0 to len-1 do
    crc := ord(buf[i])+crc*31;
  result := crc;
end;

The fast "Adler" CRC as implemented in zlib - optimized asm version here:

function Adler32Pas(Adler: cardinal; p: pointer; Count: Integer): cardinal;
var s1, s2: cardinal;
    i, n: integer;
begin
  s1 := LongRec(Adler).Lo;
  s2 := LongRec(Adler).Hi;
  while Count>0 do begin
    if Count<5552 then
      n := Count else
      n := 5552;
    for i := 1 to n do begin
      inc(s1,pByte(p)^);
      inc(cardinal(p));
      inc(s2,s1);
    end;
    s1 := s1 mod 65521;
    s2 := s2 mod 65521;
    dec(Count,n);
  end;
  result := word(s1)+cardinal(word(s2)) shl 16;
end;

My own faster variant - not re-entrant, but faster since it will read by DWORDs - and an even faster asm version here:

function Hash32(Data: pointer; Len: integer): cardinal;
function SubHash(P: PCardinalArray; L: integer): cardinal;
{$ifdef HASINLINE}inline;{$endif}
var s1,s2: cardinal;
    i: PtrInt;
const Mask: array[0..3] of cardinal = (0,$ff,$ffff,$ffffff);
begin
  if P<>nil then begin
    s1 := 0;
    s2 := 0;
    for i := 1 to L shr 4 do begin // 16 bytes (4 DWORD) by loop - aligned read
      inc(s1,P^[0]);
      inc(s2,s1);
      inc(s1,P^[1]);
      inc(s2,s1);
      inc(s1,P^[2]);
      inc(s2,s1);
      inc(s1,P^[3]);
      inc(s2,s1);
      inc(PtrUInt(P),16);
    end;
    for i := 1 to (L shr 2)and 3 do begin // 4 bytes (DWORD) by loop
      inc(s1,P^[0]);
      inc(s2,s1);
      inc(PtrUInt(P),4);
    end;
    inc(s1,P^[0] and Mask[L and 3]);      // remaining 0..3 bytes
    inc(s2,s1);
    result := s1 xor (s2 shl 16);
  end else
    result := 0;
end;
begin // use a sub function for better code generation under Delphi
  result := SubHash(Data,Len);
end;

The classic CRC32 version - you can find a very optimized asm version (using 8 tables) here:

function UpdateCrc32(aCRC32: cardinal; inBuf: pointer; inLen: integer) : cardinal;
var i: integer;
begin
  result := aCRC32;
  // if we used a dynamic table, we assume we want shorter code size
  for i := 1 to inLen do begin
    result := crc32Tab[byte(result xor pByte(inBuf)^)] xor (result shr 8);
    inc(cardinal(inBuf));
  end;
end;