李显龙：会C++的新加坡总理

40年前，获得一个数学学位后，在父亲的忠告下我继续深造计算机科学。他说这里有未来。后来事实证明，他是对的。我最后的一个项目是数年前用C++写的数独，或许这也说明我过时了。目前我的孩子都在IT圈内，两个毕业于MIT。其中一个拿起一本书给我看，上面写着：“想要更上一层楼，你该学习Haskell。”我想，有一天这会是我退休时的读物。

这是摘自新加坡总理李显龙的一段演讲。

李显龙C++写的数独程序：

#include "stdio.h"

int InBlock[81], InRow[81], InCol[81];

const int BLANK = 0;

const int ONES = 0x3fe; // Binary 1111111110

int Entry[81];// Records entries 1-9 in the grid, as the corresponding bit set to 1

int Block[9], Row[9], Col[9];// Each int is a 9-bit array

int SeqPtr = 0;

int Sequence[81];

int Count = 0;

int LevelCount[81];

void SwapSeqEntries(int S1,int S2)

{

int temp = Sequence[S2];

Sequence[S2] = Sequence[S1];

Sequence[S1] = temp;

}

void InitEntry(int i, int j, int val)

{

int Square = 9 * i + j;

int valbit = 1 << val;

int SeqPtr2;

// add suitable checks for data consistency

Entry[Square] = valbit;

Block[InBlock[Square]] &= ~valbit;

Col[InCol[Square]] &= ~valbit; // Simpler Col[j] &= ~valbit;

Row[InRow[Square]] &= ~valbit; // Simpler Row[i] &= ~valbit;

SeqPtr2 = SeqPtr;

while (SeqPtr2 < 81 && Sequence[SeqPtr2] != Square)

SeqPtr2++ ;

SwapSeqEntries(SeqPtr, SeqPtr2);

SeqPtr++;

}

void PrintArray()

{

int i, j, valbit, val, Square;

char ch;

Square = 0;

for (i = 0; i < 9; i++) {

if (i % 3 == 0) putc('\n', stdout);

for (j = 0; j < 9; j++) {

if (j % 3 == 0) putc('', stdout);

valbit = Entry[Square++];

if (valbit == 0) ch = '-';

else {

for (val = 1; val <= 9; val++)

if (valbit == (1 << val)) {

ch = '0' + val;

break;

}

}

putc(ch,stdout);

}

putc ('\n', stdout);

}

}

void ConsoleInput()

{

int i, j;

char InputString[80];

for (i = 0; i < 9; i++) {

printf("Row[%d] :", i + 1);

scanf("%s", InputString);

for (j = 0; j < 9; j++) {

char ch = InputString[j];

if (ch >= '1' && ch <='9')

InitEntry(i, j, ch - '0');

}

}

PrintArray();

}

void PrintStats()

{

int i, j, S;

printf("\nLevel Counts:\n\n");

S = 0;

while (LevelCount[S] == 0) S++;

i = 0;

while (S < 81) {

int Seq = Sequence[S];

printf("(%d,%d):%4d", Seq / 9 + 1, Seq % 9 + 1, LevelCount[S]);

if (i++ > 4){

printf("\n");

i = 0;

}

S++;

}

printf("\n\nCount =%d\n", Count);

}

void Succeed()

{

PrintArray();

PrintStats();

}

int NextSeq(int S)

{

int S2, Square, Possibles, BitCount;

int T, MinBitCount = 100;

for (T = S; T < 81; T++) {

Square = Sequence[T];

Possibles = Block[InBlock[Square]] & Row[InRow[Square]] & Col[InCol[Square]];

BitCount = 0;

while (Possibles) {

Possibles &= ~(Possibles & -Possibles);

BitCount++;

}

if (BitCount < MinBitCount) {

MinBitCount = BitCount;

S2 = T;

}

}

return S2;

}

void Place(int S)

{

LevelCount[S]++;

Count++;

if (S >= 81) {

Succeed();

return;

}

int S2 = NextSeq(S);

SwapSeqEntries(S, S2);

int Square = Sequence[S];

int BlockIndex = InBlock[Square],

RowIndex = InRow[Square],

ColIndex = InCol[Square];

int Possibles = Block[BlockIndex] & Row[RowIndex] & Col[ColIndex];

while (Possibles) {

int valbit = Possibles & (-Possibles);// Lowest 1 bit in Possibles

Possibles &= ~valbit;

Entry[Square] = valbit;

Block[BlockIndex] &= ~valbit;

Row[RowIndex] &= ~valbit;

Col[ColIndex] &= ~valbit;

Place(S + 1);

Entry[Square] = BLANK; // Could be moved out of the loop

Block[BlockIndex] |= valbit;

Row[RowIndex] |= valbit;

Col[ColIndex] |= valbit;

}

SwapSeqEntries(S, S2);

}

int main(int argc, char* argv[])

{

int i, j, Square;

for (i = 0; i < 9; i++)

for (j = 0; j < 9; j++) {

Square = 9 * i + j;

InRow[Square] = i;

InCol[Square] = j;

InBlock[Square] = (i / 3) *3 + ( j / 3);

}

for (Square = 0; Square < 81; Square++) {

Sequence[Square] = Square;

Entry[Square] = BLANK;

LevelCount[Square] = 0;

}

for (i = 0; i < 9; i++)

Block[i] = Row[i] = Col[i] = ONES;

ConsoleInput();

Place(SeqPtr);

printf("\n\nTotal Count =%d\n", Count);

return 0;

}