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#include <cstdint>
#include "base64.h"
using namespace std;
namespace Base64{
char alphabet[65]="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
uint8_t revbet[128]={ //reverse look-up table of `alphabet` (for decoding)
#define XX (127)
XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX, //0-15
XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX, //16-31
XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,XX,62,XX,XX,XX,63, //32-47
52,53,54,55,56,57,58,59,60,61,XX,XX,XX,XX,XX,XX, //48-63
XX, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14, //64-79
15,16,17,18,19,20,21,22,23,24,25,XX,XX,XX,XX,XX, //80-95
XX,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40, //96-111
41,42,43,44,45,46,47,48,49,50,51,XX,XX,XX,XX,XX, //112-127
#undef XX
};
//This is the standard base64 algorithm; most of the code deals with correct padding, actually.
//There are a lot of magic numbers here, but they should be obvious regarding the algorithm.
string encode(const string &data){
int sz=data.size();
if(sz==0)return {};
int blocks=sz/3;
string res(4*blocks+4*(sz%3!=0),'\0');
int x;
for(int i=0;i<blocks;i++){
x=((uint8_t)data[3*i]<<16)|((uint8_t)data[3*i+1]<<8)|(uint8_t)data[3*i+2];
res[4*i+3]=alphabet[x&0x3f]; x>>=6;
res[4*i+2]=alphabet[x&0x3f]; x>>=6;
res[4*i+1]=alphabet[x&0x3f]; x>>=6;
res[4*i+0]=alphabet[x];
}
switch(sz%3){
case 1:
res[4*blocks+0]=alphabet[(uint8_t)data[3*blocks]>>2];
res[4*blocks+1]=alphabet[((uint8_t)data[3*blocks]&0x3)<<4];
res[4*blocks+2]='=';
res[4*blocks+3]='=';
break;
case 2:
res[4*blocks+0]=alphabet[(uint8_t)data[3*blocks]>>2];
res[4*blocks+1]=alphabet[(((uint8_t)data[3*blocks]&0x3)<<4)|((uint8_t)data[3*blocks+1]>>4)];
res[4*blocks+2]=alphabet[(((uint8_t)data[3*blocks+1]&0xf)<<2)];
res[4*blocks+3]='=';
break;
}
return res;
}
//The inverse of `encode`.
string decode(const string &dataS){
int szS=dataS.size();
if(szS==0)return {};
uint8_t data[szS];
int sz=0;
for(char c : dataS){ //First filter away all non-base64 characters (probably mostly newlines, if any)
if(revbet[c&0x7f]!=127)data[sz++]=revbet[c&0x7f];
}
int blocks=sz/4;
int endlen;
if(sz%4==0){ //Detect padding; tries to be nice and forgive bad padding
if(data[sz-1]=='='){
blocks--;
if(data[sz-2]=='=')endlen=1;
else endlen=2;
} else endlen=0;
} else endlen=sz%4-1; //padding not present... assume the data is OK sort-of?
string res(3*blocks+endlen,'\0');
int x;
for(int i=0;i<blocks;i++){
x=(data[4*i]<<18)|(data[4*i+1]<<12)|(data[4*i+2]<<6)|data[4*i+3];
res[3*i+2]=x&0xff; x>>=8;
res[3*i+1]=x&0xff; x>>=8;
res[3*i+0]=x;
}
switch(endlen){
case 1:
res[3*blocks+0]=(data[4*blocks]<<2)|(data[4*blocks+1]>>4);
break;
case 2:
res[3*blocks+0]=(data[4*blocks]<<2)|(data[4*blocks+1]>>4);
res[3*blocks+1]=(data[4*blocks+1]<<4)|(data[4*blocks+2]>>2);
break;
}
return res;
}
}
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