hex.js

// Hex JavaScript decoder
// Copyright (c) 2008-2024 Lapo Luchini <lapo@lapo.it>

// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

const
    haveU8 = (typeof Uint8Array == 'function');

let decoder; // populated on first usage

export class Hex {

    /**
     * Decodes an hexadecimal value.
     * @param {string|Array|Uint8Array} a - a string representing hexadecimal data, or an array representation of its charcodes
     */
    static decode(a) {
        let isString = (typeof a == 'string');
        let i;
        if (decoder === undefined) {
            let hex = '0123456789ABCDEF',
                ignore = ' \f\n\r\t\u00A0\u2028\u2029';
            decoder = [];
            for (i = 0; i < 16; ++i)
                decoder[hex.charCodeAt(i)] = i;
            hex = hex.toLowerCase();
            for (i = 10; i < 16; ++i)
                decoder[hex.charCodeAt(i)] = i;
            for (i = 0; i < ignore.length; ++i)
                decoder[ignore.charCodeAt(i)] = -1;
        }
        let out = haveU8 ? new Uint8Array(a.length >> 1) : [],
            bits = 0,
            char_count = 0,
            len = 0;
        for (i = 0; i < a.length; ++i) {
            let c = isString ? a.charCodeAt(i) : a[i];
            c = decoder[c];
            if (c == -1)
                continue;
            if (c === undefined)
                throw 'Illegal character at offset ' + i;
            bits |= c;
            if (++char_count >= 2) {
                out[len++] = bits;
                bits = 0;
                char_count = 0;
            } else {
                bits <<= 4;
            }
        }
        if (char_count)
            throw 'Hex encoding incomplete: 4 bits missing';
        if (haveU8 && out.length > len) // in case it was originally longer because of ignored characters
            out = out.subarray(0, len);
        return out;
    }

}