base64ct 1.0.1

Pure Rust implementation of Base64 (RFC 4648) which avoids any usages of data-dependent branches/LUTs and thereby provides portable "best effort" constant-time operation and embedded-friendly no_std support
Documentation 
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//! URL-safe Base64 encoding.

use super::{Decode, Encode, Variant};

/// URL-safe Base64 encoding with `=` padding.
///
/// ```text
/// [A-Z]      [a-z]      [0-9]      -     _
/// 0x41-0x5a, 0x61-0x7a, 0x30-0x39, 0x2d, 0x5f
/// ```
pub struct Base64Url;

impl Variant for Base64Url {
    const PADDED: bool = true;
    const BASE: u8 = b'A';
    const DECODER: &'static [Decode] = DECODER;
    const ENCODER: &'static [Encode] = ENCODER;
}

/// URL-safe Base64 encoding *without* padding.
///
/// ```text
/// [A-Z]      [a-z]      [0-9]      -     _
/// 0x41-0x5a, 0x61-0x7a, 0x30-0x39, 0x2d, 0x5f
/// ```
pub struct Base64UrlUnpadded;

impl Variant for Base64UrlUnpadded {
    const PADDED: bool = false;
    const BASE: u8 = b'A';
    const DECODER: &'static [Decode] = DECODER;
    const ENCODER: &'static [Encode] = ENCODER;
}

/// URL-safe Base64 decoder
const DECODER: &[Decode] = &[
    Decode::Range(b'A'..b'Z', -64),
    Decode::Range(b'a'..b'z', -70),
    Decode::Range(b'0'..b'9', 5),
    Decode::Eq(b'-', 63),
    Decode::Eq(b'_', 64),
];

/// URL-safe Base64 encoder
const ENCODER: &[Encode] = &[
    Encode::Diff(25, 6),
    Encode::Diff(51, -75),
    Encode::Diff(61, -(b'-' as i16 - 0x20)),
    Encode::Diff(62, b'_' as i16 - b'-' as i16 - 1),
];