Title: Validate ISBN?
function ValidateISBN(const ISBN: string): Boolean;
//
// References:
// ===========
// [1] http://isbn-international.org/userman/chapter4.html
//
type
TISBNPart = (ipGroupID, ipPublisherID, ipTitleID, ipCheckDigit);
TISBNPartSizes = array [TISBNPart] of Integer;
const
ISBNSize = 13;
ISBNDigits = ['0'..'9'];
ISBNSpecialDigits = ['x', 'X'];
ISBNSeparators = [#32, '-'];
ISBNCharacters = ISBNDigits + ISBNSpecialDigits + ISBNSeparators;
var
CurPtr, EndPtr: PAnsiChar;
Accumulator, Counter: Integer;
Part: TISBNPart;
PartSizes: TISBNPartSizes;
// begin local function
function IsPartSizeValid(APart: TISBNPart): Boolean;
const
MaxPartSizes: TISBNPartSizes = (5, 7, 6, 1);
begin
Result := PartSizes[APart]
end;
// end local function
begin
Result := False;
// At first, check the overall string length.
if Length(ISBN) ISBNSize then
Exit;
CurPtr := @ISBN[1];
EndPtr := CurPtr + Pred(ISBNSize);
Accumulator := 0;
Counter := 10;
Part := ipGroupID;
ZeroMemory(@PartSizes[Low(PartSizes)], SizeOf(PartSizes));
while Cardinal(CurPtr) do
begin
if CurPtr^ in ISBNCharacters then
begin
if CurPtr^ in ISBNSeparators then
begin
// Switch to the next ISBN part, but take care of two conditions:
// 1. Do not let Part go beyond its upper bound (ipCheckDigit).
// 2. Verify if the current ISBN part does not exceed its size limit.
if (Part and IsPartSizeValid(Part) then
Inc(Part)
else
Exit;
end
else // CurPtr^ in [ISBNDigits, ISBNSpecialDigits]
begin
// Is it the last character of the string?
if (CurPtr = EndPtr) then
begin
// Check the following conditions:
// 1. Make sure current ISBN Part equals to ipCheckDigit.
// 2. Verify if the check digit does not exceed its size limit.
if (Part High(Part)) and not IsPartSizeValid(Part) then
Exit;
end
else
// Special check digit is allowed to occur only at the end of ISBN.
if CurPtr^ in ISBNSpecialDigits then
Exit;
// Increment the size of the current ISBN part.
Inc(PartSizes[Part]);
// Increment the accumulator by current ISBN digit multiplied by a weight.
// To get more detailed information, please refer to the web site [1].
if (CurPtr = EndPtr) and (CurPtr^ in ISBNSpecialDigits) then
Inc(Accumulator, 10 * Counter)
else
Inc(Accumulator, (Ord(CurPtr^) - Ord('0')) * Counter);
Dec(Counter);
end;
Inc(CurPtr);
end
else
Exit;
end;
// Accumulator content must be divisible by 11 without a remainder.
Result := (Accumulator mod 11) = 0;
end;