package main

import (
    "fmt"
    "math/rand"
    "time"
    "unsafe"
)

var (
    source   []byte
    tbl      [0xffffff]uint32
    b64alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
)

func base64b(source []byte) {
    print("Run bitwise ")
    result := make([]byte, len(source)*4/3)
    start := time.Now().UnixNano()
    j := 0
    for i := 0; i < len(source); i += 3 {
        block := (uint32(source[i]) << 16) | (uint32(source[i+1]) << 8) | uint32(source[i+2])

        result[j+0] = b64alpha[block>>18&0x3f]
        result[j+1] = b64alpha[block>>12&0x3f]
        result[j+2] = b64alpha[block>>6&0x3f]
        result[j+3] = b64alpha[block&0x3f]
        j += 4
    }
    fmt.Printf("%f s\n", float64(time.Now().UnixNano()-start)/1e9)
    fmt.Printf("Base64: %s...\n", string(result[:40]))
}

func base64t(source []byte) {
    print("Run tabular ")
    result := make([]byte, len(source)*4/3)
    start := time.Now().UnixNano()
    j := 0
    for i := 0; i < len(source); i += 3 {
        block := (*(*uint32)(unsafe.Pointer(&source[i]))) & 0xffffff
        *(*uint32)(unsafe.Pointer(&result[j])) = tbl[block]
        j += 4
    }
    fmt.Printf("%f s\n", float64(time.Now().UnixNano()-start)/1e9)
    fmt.Printf("Base64: %s...\n", string(result[:40]))
}

func main() {
    println("Prepare table")
    for i := 0; i < 0xffffff; i++ {
        block := (i & 0xff << 16) | (i & 0xff00) | (i >> 16)

        tbl[i] = uint32(b64alpha[block>>18&0x3f]) |
            uint32(b64alpha[block>>12&0x3f])<<8 |
            uint32(b64alpha[block>>6&0x3f])<<16 |
            uint32(b64alpha[block&0x3f])<<24
    }

    source = make([]byte, 3*10*1024*1024) // must be multiple of three

    rand.Read(source)
    fmt.Printf("\nRandom source data: %#v...\n", source[:10])
    base64b(source)
    base64t(source)

    alpha := "abcdefghijklmnopqrstuvwxyz0123456789 {}\":"
    for i := range source {
        source[i] = alpha[rand.Intn(len(alpha))]
    }
    fmt.Printf("\nText source data: %s...\n", source[:30])
    base64b(source)
    base64t(source)
}