package helpers

const b64encode = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"

// Base64Encode encodes bytes to standard base64. The output size is known
// from the input size, so it is presized and written through a cursor: push
// does not grow, and the padding concat returned a full slice that the next
// push could not extend.
func Base64Encode(data []byte) (s string) {
	n := len(data)
	out := []byte{:(n + 2) / 3 * 4}
	j := 0
	for i := 0; i < n; i += 3 {
		var b0, b1, b2 byte
		b0 = data[i]
		if i+1 < n {
			b1 = data[i+1]
		}
		if i+2 < n {
			b2 = data[i+2]
		}
		out[j] = b64encode[(b0>>2)&0x3f]
		j++
		out[j] = b64encode[((b0<<4)|(b1>>4))&0x3f]
		j++
		if i+1 < n {
			out[j] = b64encode[((b1<<2)|(b2>>6))&0x3f]
		} else {
			out[j] = '='
		}
		j++
		if i+2 < n {
			out[j] = b64encode[b2&0x3f]
		} else {
			out[j] = '='
		}
		j++
	}
	return string(out)
}

// Base64Decode decodes standard base64. Returns nil on invalid input.
func Base64Decode(s string) (buf []byte) {
	// Strip padding.
	n := len(s)
	pad := 0
	for n > 0 && s[n-1] == '=' {
		pad++
		n--
	}
	out := []byte{:n * 3 / 4}
	j := 0
	var acc uint32
	var bits int32
	for i := 0; i < n; i++ {
		v := b64val(s[i])
		if v < 0 {
			return nil
		}
		acc = (acc << 6) | uint32(v)
		bits += 6
		if bits >= 8 {
			bits -= 8
			out[j] = byte(acc >> uint32(bits))
			j++
		}
	}
	_ = pad
	return out[:j]
}

func b64val(c byte) (n int32) {
	switch {
	case c >= 'A' && c <= 'Z':
		return int32(c - 'A')
	case c >= 'a' && c <= 'z':
		return int32(c-'a') + 26
	case c >= '0' && c <= '9':
		return int32(c-'0') + 52
	case c == '+':
		return 62
	case c == '/':
		return 63
	default:
		return -1
	}
}