根据定义计算
对文法中每一 A∈VN 计算 FOLLOW(A)
(a) 设S为文法中开始符号，把{#}加入FOLLOW(S)中(这里"#"为句子句号)。
(b) 若A→αBβ是一个产生式，则把FIRST(β)的非空元素加入FOLLOW(B)中。
如果β
(c) 反复使用(b)直到每个非终结符的FOLLOW集不再增大为止。
或：
(a)对文法开始符号S，令#∈FOLLOW(S)。
(b)若B→αAβ是一个产生式，则令FIRST(β)-{ε}属于FOLLOW(A)；
(c)若B→αA是一个产生式，或B→αAβ是一个产生式且有ε∈FOLLOW(β)，
则令FOLLOW(B)是FOLLOW(A)的子集。即把FOLLOW(B)的所有元素加入到FOLLOW(A)中。
(d)反复使用(b)直到每个非终结符的 FOLLOW集合不再增加为止。
根据以上分析求FOLLOW集要依赖于FIRST集的求解，因为上一篇已经求出FIRST集，但里面的一些函数应该可以共享给FOLLOW集的求解，所以相应的需要修改FIRST集的求法，下面为修改后的FIRST集解法和FOLLOW集解法的定义部分，其中FInfer为推导符,可以为->或→，arrTerminate为终结符集，arrNoTerminate为非终结符集：
1 TSCanner = class(TObject)
2 private
3 FInfer, FStart: string;
4 public
5 MyNoTerminate: TStringList;
6 MyToken: TToken;
7 function ScanSQL(str: string): Boolean;
8 function Pro_Process(str: string): string;
9 function DeleteBlankLines(str: string; buf: PChar): Boolean;
10 function SplitString(const Source, ch: string): TStringList;
11 function UpdateAllKey(sWords: string): string;
12 function GetAllToken(sWords: string): string;
13 function InTerminate(s: string): Boolean; //是否为终结符
14 function InNoTerminate(s: string): Boolean; //是否为非终结符
15
16 procedure GetAllNoTerminate(sContent: TStringList; var Res: TStringList);
17 function OneInOther(s, str: string): Boolean; //s字符串是否在str中
18 function DeleteOne(s, str: string):string; //在串str中删除s，避免重复
19 function GetFirst(s: TStringList): TStringList;
20 procedure GetFirstAllProduction(sNoTerminate: string; sList: TStringList; var Res: TStringList);
21 function GetFirstOne(str: string; slt: TStringList): string; //获得一个非终结符的First集
22
23 function GetFollow(s: TStringList): TStringList;
24 procedure GetFollowAllProduction(sNoTerminate: string; sList: TStringList; var Res: TStringList);
25 function GetFollowOne(str: string; slt: TStringList): string; //获得一个非终结符的Follow集
26 function CanToEmpty(str: string; slt: TStringList): Boolean; //str是否能推导出ε
27
28
29 constructor Create(sInFer: string);
30 destructor Destroy; override;
31 property Infer: string read FInfer write FInfer;
32 end;
33
34 const
35 arrTerminate: array[1..32] of string = ('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k',
36 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', ')', '(', '+', '-', '*', 'ε');
37 arrNoTerminate: array[1..26] of string = ('A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K',
38 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z');
39 SQLKey: array[1..28] of Tkey = (
40 (Content: 'SELECT'; Key: 'S01'),
41 (Content: 'WHEN'; Key: 'S02'),
42 (Content: 'DISTINCT'; Key: 'S03'),
43 (Content: 'FROM'; Key: 'S04'),
44 (Content: 'WHERE'; Key: 'S05'),
45 (Content: 'LEFT JOIN'; Key: 'S06'),
46 (Content: 'LEFT OUTER JOIN'; Key: 'S07'),
47 (Content: 'RIGHT JOIN'; Key: 'S08'),
48 (Content: 'RIGHT OUTER JOIN'; Key: 'S09'),
49 (Content: 'INNER JOIN'; Key: 'S10'),
50 (Content: 'FULL JOIN'; Key: 'S11'),
51 (Content: 'HAVING'; Key: 'S12'),
52 (Content: 'ASC'; Key: 'S13'),
53 (Content: 'DESC'; Key: 'S14'),
54 (Content: 'CASE'; Key: 'S15'),
55 (Content: 'END'; Key: 'S16'),
56 (Content: 'ELSE'; Key: 'S17'),
57 (Content: 'ON'; Key: 'S18'),
58 (Content: 'UNION'; Key: 'S19'),
59 (Content: 'UNION ALL'; Key: 'S20'),
60 (Content: 'AS'; Key: 'S21'),
61 (Content: 'ORDER BY'; Key: 'S22'),
62 (Content: 'GOURP BY'; Key: 'S23'),
63 (Content: 'IN'; Key: 'S24'),
64 (Content: 'OR'; Key: 'S25'),
65 (Content: 'AND'; Key: 'S26'),
66 (Content: 'THEN'; Key: 'S27'),
67 (Content: 'LIKE'; Key: 'S28')
68 );
实现部分
function TSCanner.CanToEmpty(str: string; slt: TStringList): Boolean;
var
i, j: Integer;
sTemp, s1, s2: string;
begin
Result := False;
for i := 0 to slt.Count - 1 do
begin
sTemp := Trim(slt[i]);
j := Pos(FInfer, sTemp);
if (j > 0) then
begin
s1 := Copy(sTemp, j + Length(FInfer), 1);
s2 := Copy(sTemp, j + Length(FInfer), 2);
if s2 = 'ε' then
begin
Result := True;
Break;
end
else if InNoTerminate(s1) then
begin
Result := CanToEmpty(s1, slt);
end
else if InTerminate(s1) then
Result :=False;
end;
end;
end;
constructor TSCanner.Create(sInFer: string);
begin
MyToken := TToken.Create;
FInfer := sInFer;
end;
function TSCanner.DeleteBlankLines(str: string; buf: PChar): Boolean;
var
sTemp: string;
i, j, iLen, kFlag: Integer;
old_c, cur_c: Char;
bComment: Boolean;
begin
Result := False;
bComment := False;
sTemp := Pro_Process(str);
iLen := Length(sTemp);
j := 1;
i := 0;
kFlag := 0;
cur_c := sTemp[j];
while j <= iLen do
begin
case bComment of
false:
begin
if ((old_c = '/') and (cur_c = '*')) then
begin
i := i - 1;
bComment := True;
end
else if ((old_c = '/') and (cur_c = '/')) then
begin
i := i - 1;
kFlag := 1; //状态1为"//"注视状态
end
else
begin
if kFlag = 1 then
begin
j := j + 1;
old_c := cur_c;
cur_c := str[j];
Continue;
end
else if (kFlag = 2) then
begin
kFlag := 0;
buf[i] := ' ';
buf[i + 1] := cur_c;
i := i + 2;
end
else
begin
buf[i] := cur_c;
i := i + 1;
end;
end;
//ShowMessage(buf);
end;
True:
begin
if (old_c = '*') and (cur_c = '/') then
bComment := False
else if cur_c = #10 then
begin
if kFlag = 1 then
begin
kFlag := 0;
bComment := False;
end;
end;
end;
end;
j := j + 1;
old_c := cur_c;
cur_c := sTemp[j];
end;
//i := i + 1;
buf[i] := '#';
Result := True;
end;
function TSCanner.DeleteOne(s, str: string): string;
var
sliTemp: TStringList;
begin
sliTemp := TStringList.Create;
sliTemp.Delimiter := ',';
sliTemp.CommaText := str;
if sliTemp.IndexOf(s) >= 0 then
begin
sliTemp.Delete(sliTemp.IndexOf(s));
end;
Result :=sliTemp.CommaText;
sliTemp.Free;
end;
destructor TSCanner.destroy;
begin
inherited;
MyToken.Free;
end;
procedure TSCanner.GetAllNoTerminate(sContent: TStringList; var Res: TStringList);
var
iCou, Cou, i, j: Integer;
sTemp: string;
begin
Cou := sContent.Count;
Res.Clear;
for iCou := 0 to Cou - 1 do
begin
i := Pos(FInfer, sContent[iCou]);
if (i = 0) then Exit;
if i > 0 then
begin
sTemp := Copy(Trim(sContent[icou]), 1, i - (Length(FInfer)) + 1);
if Res.IndexOf(sTemp) < 0 then
Res.Add(sTemp);
end;
end;
end;
function TSCanner.GetAllToken(sWords: string): string;
var
i, j: Integer;
slTemp: TStringList;
sTemp: string;
bTemp: Boolean;
begin
slTemp := TStringList.Create;
sTemp := sWords;
bTemp := False;
for j := Low(SQLKey) to High(SQLKEY) do
sTemp := StringReplace(sTemp, SQLKey[j].Content, SQLKey[j].Key, [rfReplaceAll]);
slTemp.CommaText := sTemp;
sTemp := '';
for i := 0 to slTemp.Count - 1 do
begin
bTemp := False;
for j := Low(SQLKey) to High(SQLKEY) do
begin
if SQLKey[j].Key = slTemp[i] then
begin
bTemp := True;
MyToken.Add(SQLKey[j].Key + '=' + SQLKey[j].Content);
sTemp := sTemp + SQLKey[j].Key;
Break;
end;
end;
if not bTemp then
sTemp := sTemp + slTemp[i];
end;
slTemp.Free;
Result := sTemp;
end;
//获得FIRST集
function TSCanner.GetFirst(s: TStringList): TStringList;
var
i, j, k: Integer;
sTemp, FFirst: string;
Data: PFirst;
s1: PChar;
sList, slTemp1: TStringList;
begin
sList := TStringList.Create;
FStart := 'E';
Result := TStringList.Create;
try
GetAllNoTerminate(s, sList);
for i := 0 to sList.Count - 1 do
begin
//获取所有sList[i]推导的产生式 如A->a或A->BCa
FFirst := '';
FFirst := GetFirstOne(sList[i], s);
if Trim(FFirst) <> '' then
Delete(FFirst, Length(FFirst), 1);
New(Data);
Data^.Name := sList[i];
Data^.Value := FFirst;
Result.AddObject(Data^.Name, TObject(Data));
end;
finally
sList.Free;
//slTemp1.Free;
end;
end;
//获得FOLLOW集
procedure TSCanner.GetFirstAllProduction(sNoTerminate: string;
sList: TStringList; var Res: TStringList);
var
i, j: Integer;
s1: string;
begin
for i := 0 to sList.Count - 1 do
begin
j := Pos(FInfer, sList[i]);
if j > 0 then
begin
if Trim(MidStr(sList[i], 1, j - 1)) = Trim(sNoTerminate) then
Res.Add(sList[i]);
end;
end;
end;
function TSCanner.GetFirstOne(str: string; slt: TStringList): string;
var
i, j: integer;
s1, s2, sTemp,sFirst,sFollow: string;
slTemp1: TStringList;
begin
slTemp1 := TStringList.Create;
Result := '';
GetFirstAllProduction(str, slt, slTemp1);
for i := 0 to slTemp1.Count - 1 do
begin
sTemp := Trim(slTemp1[i]);
j := Pos(FInfer, sTemp);
if (j > 0) then
begin
s1 := Copy(sTemp, j + Length(FInfer), 1);
s2 := Copy(sTemp, j + Length(FInfer), 2);
if (InTerminate(s1)) then
begin
if not OneInOther(s1, sTemp) then
Result := Result + s1 + ',';
end
else if (s2 = 'ε') then
begin
if not OneInOther(s2, sTemp) then
Result := Result + s2 + ','
end
else if InNoTerminate(s1) then
begin
Result := GetFirstOne(s1, slt);
end;
end;
end;
slTemp1.Free;
end;
function TSCanner.GetFollow(s: TStringList): TStringList;
var
i, j, k: Integer;
sTemp, FFollow: string;
Data: PFirst;
s1: PChar;
sList, slTemp1: TStringList;
begin
sList := TStringList.Create;
FStart := 'E';
Result := TStringList.Create;
try
GetAllNoTerminate(s, sList);
for i := 0 to sList.Count - 1 do
begin
//获取所有sList[i]推导的产生式 如A->a或A->BCa
FFollow := '';
FFollow := FFollow + GetFollowOne(sList[i], s);
if Trim(FFollow) <> '' then
Delete(FFollow, Length(FFollow), 1);
New(Data);
Data^.Name := sList[i];
Data^.Value := FFollow;
Result.AddObject(Data^.Name, TObject(Data));
end;
finally
sList.Free;
//slTemp1.Free;
end;
end;
procedure TSCanner.GetFollowAllProduction(sNoTerminate: string;
sList: TStringList; var Res: TStringList);
var
i, j: Integer;
s1: string;
s2, s3: string;
begin
for i := 0 to sList.Count - 1 do
begin
j := Pos(FInfer, sList[i]);
if j > 0 then
begin
s2 := Trim(MidStr(sList[i], j + 1, Length(sList[i]) - j));
s3 := Trim(sNoTerminate);
if Pos(s3, s2) > 0 then
begin
Res.Add(sList[i]);
end;
end;
end;
end;
function TSCanner.GetFollowOne(str: string; slt: TStringList): string;
var
i, j, k,l: integer;
s1, s2, s3, sTemp,sFirst,sFollow: string;
slTemp1,slTemp2: TStringList;
begin
slTemp1 := TStringList.Create;
Result := '';
if str = FStart then
Result := '#,';
GetFollowAllProduction(str, slt, slTemp1);
for i := 0 to slTemp1.Count - 1 do
begin
sTemp := Trim(slTemp1[i]);
j := Pos(FInfer, sTemp);
if (j > 0) then
begin
s1 := Copy(sTemp, 1, 1);
//s2 := Copy(sTemp, j + Length(FInfer), 2);
k := Pos(str, sTemp);
if k > 0 then
begin
s3 := Copy(sTemp, k + 1, Length(sTemp) - k);
if (InTerminate(s3)) then
begin
if not OneInOther(s3, Result) then
begin
Result := Result + s3 + ',';
end;
end
else if s3 = '' then
begin
Result := Result + GetFollowOne(s1, slt);
end
else if InNoTerminate(s3) then
begin
if not OneInOther(s3,Result) then
begin
sFirst :=GetFirstOne(s3, slt);
slTemp2 :=TStringList.Create;
slTemp2.CommaText :=sFirst;
for l:=0 to slTemp2.Count -1 do
begin
if not OneInOther(slTemp2[l],Result) then
Result := Result + slTemp2[l]+',';
end;
slTemp2.Free;
end;
if Pos('ε',Result)>0 then
begin
sFollow := GetFollowOne(s3, slt);
slTemp2 :=TStringList.Create;
slTemp2.CommaText :=sFollow;
for l:=0 to slTemp2.Count -1 do
begin
if not OneInOther(slTemp2[l],Result) then
Result := Result + slTemp2[l]+',';
end;
slTemp2.Free;
end;
end;
end;
end;
end;
if OneInOther('ε',Result) then
begin
Result :=DeleteOne('ε',Result);
end;
slTemp1.Free;
end;
function TSCanner.InNoTerminate(s: string): Boolean;
var
i: integer;
begin
Result := False;
for i := Low(arrNoTerminate) to High(arrNoTerminate) do
begin
if s = arrNoTerminate[i] then
begin
Result := True;
Break;
end;
end;
end;
function TSCanner.InTerminate(s: string): Boolean;
var
i: Integer;
begin
Result := False;
for i := Low(arrTerminate) to High(arrTerminate) do
begin
if s = arrTerminate[i] then
begin
Result := True;
Break;
end;
end;
end;
function TSCanner.OneInOther(s, str: string): Boolean;
var
sliTemp: TStringList;
begin
Result := False;
sliTemp := TStringList.Create;
sliTemp.Delimiter := ',';
sliTemp.CommaText := str;
if sliTemp.IndexOf(s) >= 0 then
Result := True;
sliTemp.Free;
end;
实际的测试界面如下：
