/**
 * SHA-256 hash function reference implementation.
 *
 * This is an annotated direct implementation of FIPS 180-4, without any optimisations.
 * it is intended to aid understanding of the algorithm rather than for production use!!!
 *
 * While it could be used where performance is not critical, I would recommend using the ‘Web
 * Cryptography API’ (developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/digest) for the browser,
 * or the ‘crypto’ library (nodejs.org/api/crypto.html#crypto_class_hash) in Node.js.
 *
 * See csrc.nist.gov/groups/ST/toolkit/secure_hashing.html
 *     csrc.nist.gov/groups/ST/toolkit/examples.html
 */
'use strict';
class Sha256 {

    /**
     * Generates SHA-256 hash of string.
     *
     * @param   {string} msg - (Unicode) string to be hashed.
     * @param   {Object} [options]
     * @param   {string} [options.msgFormat=string] - Message format: 'string' for JavaScript string
     *   (gets converted to UTF-8 for hashing); 'hex-bytes' for string of hex bytes ('616263' ≡ 'abc') .
     * @param   {string} [options.outFormat=hex] - Output format: 'hex' for string of contiguous
     *   hex bytes; 'hex-w' for grouping hex bytes into groups of (4 byte / 8 character) words.
     * @returns {string} Hash of msg as hex character string.
     */
    static hash(msg, options) {
        const defaults = { msgFormat: 'string', outFormat: 'hex' };
        const opt = Object.assign(defaults, options);

        // note use throughout this routine of 'n >>> 0' to coerce Number 'n' to unsigned 32-bit integer

        switch (opt.msgFormat) {
            default: // default is to convert string to UTF-8, as SHA only deals with byte-streams
            case 'string':   msg = utf8Encode(msg);       break;
            case 'hex-bytes':msg = hexBytesToString(msg); break; // mostly for running tests
        }

        // constants
        const K = [
            0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
            0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
            0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
            0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
            0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
            0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
            0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
            0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2 ];

        // initial hash value
        const H = [
            0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19 ];

        // PREPROCESSING

        msg += String.fromCharCode(0x80);  // add trailing '1' bit (+ 0's padding) to string

        // convert string msg into 512-bit blocks (array of 16 32-bit integers)
        const l = msg.length/4 + 2; // length (in 32-bit integers) of msg + ‘1’ + appended length
        const N = Math.ceil(l/16);  // number of 16-integer (512-bit) blocks required to hold 'l' ints
        const M = new Array(N);     // message M is N×16 array of 32-bit integers

        for (let i=0; i<N; i++) {
            M[i] = new Array(16);
            for (let j=0; j<16; j++) { // encode 4 chars per integer (64 per block), big-endian encoding
                M[i][j] = (msg.charCodeAt(i*64+j*4+0)<<24) | (msg.charCodeAt(i*64+j*4+1)<<16)
                        | (msg.charCodeAt(i*64+j*4+2)<< 8) | (msg.charCodeAt(i*64+j*4+3)<< 0);
            } // note running off the end of msg is ok 'cos bitwise ops on NaN return 0
        }
        // add length (in bits) into final pair of 32-bit integers (big-endian)
        // note: most significant word would be (len-1)*8 >>> 32, but since JS converts
        // bitwise-op args to 32 bits, we need to simulate this by arithmetic operators
        const lenHi = ((msg.length-1)*8) / Math.pow(2, 32);
        const lenLo = ((msg.length-1)*8) >>> 0;
        M[N-1][14] = Math.floor(lenHi);
        M[N-1][15] = lenLo;


        // HASH COMPUTATION

        for (let i=0; i<N; i++) {
            const W = new Array(64);

            // 1 - prepare message schedule 'W'
            for (let t=0;  t<16; t++) W[t] = M[i][t];
            for (let t=16; t<64; t++) {
                W[t] = (Sha256.σ1(W[t-2]) + W[t-7] + Sha256.σ0(W[t-15]) + W[t-16]) >>> 0;
            }

            // 2 - initialise working variables a, b, c, d, e, f, g, h with previous hash value
            let a = H[0], b = H[1], c = H[2], d = H[3], e = H[4], f = H[5], g = H[6], h = H[7];

            // 3 - main loop (note '>>> 0' for 'addition modulo 2^32')
            for (let t=0; t<64; t++) {
                const T1 = h + Sha256.Σ1(e) + Sha256.Ch(e, f, g) + K[t] + W[t];
                const T2 =     Sha256.Σ0(a) + Sha256.Maj(a, b, c);
                h = g;
                g = f;
                f = e;
                e = (d + T1) >>> 0;
                d = c;
                c = b;
                b = a;
                a = (T1 + T2) >>> 0;
            }

            // 4 - compute the new intermediate hash value (note '>>> 0' for 'addition modulo 2^32')
            H[0] = (H[0]+a) >>> 0;
            H[1] = (H[1]+b) >>> 0;
            H[2] = (H[2]+c) >>> 0;
            H[3] = (H[3]+d) >>> 0;
            H[4] = (H[4]+e) >>> 0;
            H[5] = (H[5]+f) >>> 0;
            H[6] = (H[6]+g) >>> 0;
            H[7] = (H[7]+h) >>> 0;
        }

        // convert H0..H7 to hex strings (with leading zeros)
        for (let h=0; h<H.length; h++) H[h] = ('00000000'+H[h].toString(16)).slice(-8);

        // concatenate H0..H7, with separator if required
        const separator = opt.outFormat=='hex-w' ? ' ' : '';

        return H.join(separator);

        /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */

        function utf8Encode(str) {
            try {
                return new TextEncoder().encode(str, 'utf-8').reduce((prev, curr) => prev + String.fromCharCode(curr), '');
            } catch (e) { // no TextEncoder available?
                return unescape(encodeURIComponent(str)); // monsur.hossa.in/2012/07/20/utf-8-in-javascript.html
            }
        }

        function hexBytesToString(hexStr) { // convert string of hex numbers to a string of chars (eg '616263' -> 'abc').
            const str = hexStr.replace(' ', ''); // allow space-separated groups
            return str=='' ? '' : str.match(/.{2}/g).map(byte => String.fromCharCode(parseInt(byte, 16))).join('');
        }
    }



    /**
     * Rotates right (circular right shift) value x by n positions [§3.2.4].
     * @private
     */
    static ROTR(n, x) {
        return (x >>> n) | (x << (32-n));
    }


    /**
     * Logical functions.
     * @private
     */
    static Σ0(x) { return Sha256.ROTR(2,  x) ^ Sha256.ROTR(13, x) ^ Sha256.ROTR(22, x); }
    static Σ1(x) { return Sha256.ROTR(6,  x) ^ Sha256.ROTR(11, x) ^ Sha256.ROTR(25, x); }
    static σ0(x) { return Sha256.ROTR(7,  x) ^ Sha256.ROTR(18, x) ^ (x>>>3);  }
    static σ1(x) { return Sha256.ROTR(17, x) ^ Sha256.ROTR(19, x) ^ (x>>>10); }
    static Ch(x, y, z)  { return (x & y) ^ (~x & z); }          // 'choice'
    static Maj(x, y, z) { return (x & y) ^ (x & z) ^ (y & z); } // 'majority'

}


/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */

if (typeof module != 'undefined' && module.exports) module.exports = Sha256; // ≡ export default Sha256