type
  TCodePage = (cpWin1251, cp866, cpKOI8R);
  PMap = ^TMap;
  TMap = array[#$80..#$FF] of Char;

function GetMap(CP: TCodePage): PMap;
{ должна возвращать указатель на таблицу перекодировки из CP в Windows1251
(nil для CP = cpWin1251) }
begin
  GetMap := nil;
end;

function DetermineRussian(Buf: PChar; Count: Integer): TCodePage;
const
  ModelBigrams: array[0..33, 0..33] of Byte = (
    {АБВГДЕЖЗИЙКЛМHОПРСТУФХЦЧШЩЪЫЬЭЮЯ_?}
    {А}(0, 20, 44, 12, 22, 23, 16, 60, 4, 9, 63, 93, 47, 110, 0, 16, 35, 61, 81,
      1, 5, 13, 24, 17, 12, 4, 0, 0, 0, 0, 14, 31, 205, 1),
    {Б}(19, 0, 0, 0, 4, 19, 0, 0, 8, 0, 2, 15, 1, 4, 41, 0, 15, 5, 0, 15, 0, 2,
      1, 0, 0, 6, 16, 37, 0, 0, 0, 4, 3, 0),
    {В}(97, 0, 1, 0, 2, 57, 0, 5, 40, 0, 4, 25, 2, 23, 78, 2, 8, 28, 4, 12, 0,
      1, 0, 0, 8, 1, 0, 40, 1, 0, 0, 5, 106, 3),
    {Г}(13, 0, 0, 0, 9, 5, 0, 0, 15, 0, 1, 17, 1, 2, 96, 0, 24, 0, 0, 7, 0, 0,
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 8, 0),
    {Д}(63, 0, 9, 1, 2, 71, 1, 0, 35, 0, 3, 16, 2, 22, 50, 2, 19, 9, 2, 25, 0,
      2, 1, 0, 1, 0, 1, 9, 4, 0, 1, 5, 17, 4),
    {Е}(4, 14, 15, 34, 56, 22, 13, 14, 2, 34, 39, 77, 73, 150, 6, 9, 101, 64,
      81, 1, 0, 15, 5, 12, 10, 6, 0, 0, 0, 0, 3, 4, 235, 1),
    {Ж}(13, 0, 0, 0, 12, 47, 0, 0, 16, 0, 1, 0, 0, 23, 0, 0, 0, 0, 0, 3, 0, 0,
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 2),
    {З}(76, 2, 11, 3, 11, 4, 1, 0, 7, 0, 2, 4, 11, 24, 17, 0, 6, 1, 0, 8, 0, 0,
      0, 0, 0, 0, 0, 16, 6, 0, 1, 4, 17, 0),
    {И}(7, 9, 32, 5, 18, 60, 4, 42, 31, 27, 28, 46, 55, 49, 12, 7, 26, 60, 53,
      0, 5, 25, 14, 28, 4, 1, 0, 0, 0, 0, 9, 56, 255, 0),
    {Й}(0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 1, 3, 0, 3, 0, 0, 0, 10, 3, 0, 0, 0, 0, 1,
      1, 0, 0, 0, 0, 0, 0, 0, 122, 0),
    {К}(92, 0, 3, 0, 0, 7, 2, 1, 39, 0, 0, 27, 0, 14, 110, 0, 18, 5, 35, 18, 0,
      0, 11, 0, 0, 0, 0, 0, 0, 0, 0, 0, 5, 5, 0),
    {Л}(85, 1, 0, 2, 1, 70, 6, 0, 85, 0, 5, 3, 0, 9, 67, 1, 0, 9, 0, 15, 0, 0,
      0, 2, 0, 0, 0, 9, 66, 0, 15, 43, 57, 4),
    {М}(44, 0, 0, 0, 0, 65, 0, 0, 47, 0, 1, 1, 10, 15, 57, 7, 0, 2, 0, 24, 0, 0,
      0, 0, 0, 0, 0, 28, 0, 0, 0, 8, 109, 3),
    {}(139, 0, 0, 1, 11, 108, 0, 4, 152, 0, 7, 0, 1, 69, 161, 0, 0, 8, 25, 24,
      5, 1, 5, 2, 0, 1, 0, 83, 10, 0, 1, 29, 38, 5),
    {О}(0, 72, 139, 76, 74, 32, 32, 19, 12, 52, 21, 93, 68, 72, 7, 34, 93, 102,
      98, 1, 2, 6, 6, 19, 15, 2, 0, 0, 0, 1, 4, 9, 252, 2),
    {П}(17, 0, 0, 0, 0, 43, 0, 0, 14, 0, 1, 9, 0, 1, 125, 3, 120, 1, 2, 8, 0, 0,
      0, 0, 0, 0, 0, 3, 6, 0, 0, 3, 2, 2),
    {Р}(151, 1, 6, 4, 3, 103, 7, 0, 76, 0, 4, 0, 11, 10, 117, 1, 0, 5, 9, 39, 2,
      5, 0, 1, 3, 0, 0, 24, 7, 0, 1, 10, 22, 5),
    {С}(24, 1, 21, 0, 3, 39, 0, 0, 33, 0, 56, 41, 11, 15, 58, 30, 5, 30, 183,
      16, 0, 4, 1, 4, 1, 0, 0, 8, 25, 0, 1, 50, 41, 2),
    {Т}(83, 0, 43, 0, 3, 87, 0, 0, 71, 0, 9, 3, 2, 26, 180, 0, 55, 33, 1, 23, 1,
      0, 1, 4, 0, 0, 0, 20, 78, 0, 0, 5, 82, 4),
    {У}(3, 6, 7, 14, 19, 8, 13, 6, 0, 1, 13, 15, 10, 7, 0, 12, 17, 16, 19, 0, 1,
      3, 0, 12, 5, 8, 0, 0, 0, 0, 22, 1, 65, 0),
    {Ф}(4, 0, 0, 0, 0, 4, 0, 0, 11, 0, 0, 1, 0, 0, 9, 0, 3, 0, 0, 4, 1, 0, 0, 0,
      0, 0, 0, 0, 0, 0, 0, 0, 2, 0),
    {Х}(9, 0, 2, 0, 0, 2, 0, 0, 5, 0, 0, 1, 0, 5, 26, 0, 4, 1, 0, 1, 0, 0, 0, 0,
      0, 0, 0, 0, 0, 0, 0, 0, 76, 0),
    {Ц}(5, 0, 0, 0, 0, 16, 0, 0, 48, 0, 1, 0, 0, 0, 4, 0, 0, 0, 0, 3, 0, 0, 0,
      0, 0, 0, 0, 2, 0, 0, 0, 0, 3, 0),
    {Ч}(30, 0, 0, 0, 0, 52, 0, 0, 23, 0, 3, 1, 0, 14, 1, 0, 0, 0, 36, 5, 0, 0,
      0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 2, 2),
    {Ш}(13, 0, 0, 0, 0, 28, 0, 0, 17, 0, 4, 4, 0, 4, 3, 0, 0, 0, 1, 3, 0, 0, 0,
      0, 0, 0, 0, 0, 3, 0, 0, 0, 1, 1),
    {Щ}(6, 0, 0, 0, 0, 23, 0, 0, 16, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0,
      0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1),
    {Ъ}(0, 0, 0, 0, 0, 16, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
      0, 0, 0, 0, 0, 0, 1, 1, 0, 0),
    {Ы}(0, 5, 14, 1, 3, 28, 0, 2, 0, 22, 6, 19, 21, 2, 0, 5, 4, 7, 10, 0, 0, 37,
      0, 3, 4, 0, 0, 0, 0, 0, 0, 1, 84, 0),
    {Ь}(0, 1, 0, 0, 0, 9, 0, 10, 1, 0, 13, 0, 2, 26, 0, 0, 0, 10, 3, 0, 0, 0, 1,
      0, 6, 0, 0, 0, 0, 0, 6, 4, 117, 0),
    {Э}(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 3, 3, 0, 0, 0, 0, 0, 0, 31, 0, 1, 0, 0, 0,
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0),
    {Ю}(0, 5, 0, 0, 3, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 15, 0, 0, 0, 1, 4,
      1, 15, 0, 0, 0, 0, 0, 0, 38, 0),
    {Я}(0, 0, 9, 2, 7, 10, 3, 19, 0, 0, 1, 6, 7, 8, 0, 0, 2, 6, 19, 0, 0, 3, 5,
      1, 0, 3, 0, 0, 0, 0, 5, 2, 177, 0),
    {_}(42, 80, 193, 43, 109, 41, 18, 53, 159, 0, 144, 27, 83, 176, 187, 229,
      70, 231, 99, 47, 15, 13, 6, 58, 7, 0, 0, 0, 0, 38, 0, 22, 0, 2),
    {?}(0, 0, 0, 0, 3, 0, 0, 0, 0, 0, 2, 4, 4, 8, 0, 0, 5, 3, 4, 0, 0, 0, 0, 0,
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0));
  { " рейтинг"  буквы ? условно принимается равным 1/20 от " рейтинга"  буквы E,
  если сочетание с участием ? корректно, иначе - 0 }
type
  TVariation = array[0..33, 0..33] of Integer;
var
  I, J, iC, iPredC, Max: Integer;
  C: Char;
  CP: TCodePage;
  D, MinD, Factor: Double;
  AMap: PMap;
  PV: ^TVariation;
  Vars: array[TCodePage] of TVariation;
begin
  DetermineRussian := cpWin1251; { по yмолчанию }
  { вычисление распределений биграмм }
  FillChar(Vars, SizeOf(Vars), 0);
  for CP := Low(Vars) to High(Vars) do
  begin
    AMap := GetMap(CP);
    PV := @Vars[CP];
    iPredC := 32;
    for I := 0 to Count - 1 do
    begin
      C := Buf[I];
      iC := 32;
      if C > = #128 then
      begin
        if AMap < > nil then
          C := AMap^[C];
        if not (C in ['?', '?']) then
        begin
          C := Chr(Ord(C) and not 32); { 'a'..'я' ->  'А'..'Я' }
          if C in ['А'..'Я'] then
            iC := Ord(C) - Ord('А');
        end
        else
          iC := 33;
      end;
      Inc(PV^[iPredC, iC]);
      iPredC := iC;
    end;
  end;
  { вычисление метрики и определение наиболее правдоподобной кодировки }
  MinD := 0;
  for CP := Low(Vars) to High(Vars) do
  begin
    PV := @Vars[CP];
    PV^[32, 32] := 0;
    Max := 1;
    for I := 0 to 33 do
      for J := 0 to 33 do
        if PV^[I, J] > Max then
          Max := PV^[I, J];
    Factor := 255 / Max; { ноpмализация }
    D := 0;
    for I := 0 to 33 do
      for J := 0 to 33 do
        D := D + Abs(PV^[I, J] * Factor - ModelBigrams[I, J]);
    if (MinD = 0) or (D < MinD) then
    begin
      MinD := D;
      DetermineRussian := CP;
    end;
  end;
end;

begin
  { тест: слово 'Пример' в разных кодировках (веpоятность ошибок на таких
  коpотких текстах высока - в данном слyчае пpосто повезло!) }
  writeln(DetermineRussian(#$CF#$F0#$E8#$EC#$E5#$F0, 6) = cpWin1251);
  writeln(DetermineRussian(#$8F#$E0#$A8#$AC#$A5#$E0, 6) = cp866);
  writeln(DetermineRussian(#$F0#$D2#$C9#$CD#$C5#$D2, 6) = cpKOI8R);
  readln;
end.