Base64 Encode Implementation in D

mybase64.d

module mybase64;

import std.stdio;
import std.conv;
import std.array;

immutable mapping = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

int main() {
    auto input = stdin.readln();
    string s;
    while ((s = stdin.readln()) !is null) { 
        input ~= s; 
    }
    base64encode(input).writeln;
    return 0;
}

string base64encode(in string input) {
    ulong len = input.length;
    char[] buf;
    buf.reserve(31);
    while(len >= 3) {
        buf ~= mapping[input[$-len] >> 2];
        buf ~= mapping[((input[$-len] & 0x3) << 4) | (input[$-len+1] & 0xf0) >> 4];
        buf ~= mapping[((input[$-len+1] & 0x0f) << 2) | input[$-len+2] >> 6];
        buf ~= mapping[input[$-len+2] & 0x3f];
        
        len -= 3;
    }
    if (len == 2) {
        buf ~= mapping[input[$-len] >> 2];
        buf ~= mapping[((input[$-len] & 0x3) << 4) | (input[$-len+1] & 0xf0) >> 4];  
        buf ~= mapping[(input[$-len+1] & 0x0f) << 2];
        buf ~= "=";
    } else if (len == 1) {
        buf ~= mapping[input[$-len] >> 2];
        buf ~= mapping[(input[$-len] & 0x3) << 4];
        buf ~= "==";
    }
    return buf.to!string;
}