/* eslint-disable no-bitwise */

import assert from 'node:assert';

export default class Salsa20 {
    private readonly fixed: Uint32Array;

    private readonly key: Uint32Array;

    private readonly nonce: Uint32Array;

    private counter = new Uint32Array([0, 0]);

    private state = new Uint32Array(16);

    private block = new Uint8Array(64);

    private position = 0;

    constructor(key: Uint8Array, nonce: Uint8Array) {
        assert(key.length === 32 || key.length === 16, 'Salsa20 requires 128-bit or 256-bit key');
        assert(nonce.length === 8, 'Salsa20 requires 64-bit nonce');

        this.key = new Uint32Array(8);
        const keyView = new DataView(key.buffer);
        if (key.length === 32) {
            for (let i = 0; i < 8; i += 1) {
                this.key[i] = keyView.getUint32(i * 4, true);
            }
            this.fixed = new Uint32Array([
                0x61707865,
                0x3320646e,
                0x79622d32,
                0x6b206574,
            ]);
        } else {
            for (let i = 0; i < 4; i += 1) {
                const word = keyView.getUint32(i * 4, true);
                this.key[i] = word;
                this.key[i + 4] = word;
            }
            this.fixed = new Uint32Array([
                0x61707865,
                0x3120646e,
                0x79622d36,
                0x6b206574,
            ]);
        }

        this.nonce = new Uint32Array(2);
        const nonceView = new DataView(nonce.buffer);
        for (let i = 0; i < 2; i += 1) {
            this.nonce[i] = nonceView.getUint32(i * 4, true);
        }

        this.generateBlock();
    }

    // eslint-disable-next-line @typescript-eslint/naming-convention
    private QR(a: number, b: number, c: number, d: number) {
        let t: number;

        t = (this.state[a] + this.state[d]) & 0xffffffff;
        this.state[b] ^= (t << 7) | (t >>> 25);

        t = (this.state[b] + this.state[a]) & 0xffffffff;
        this.state[c] ^= (t << 9) | (t >>> 23);

        t = (this.state[c] + this.state[b]) & 0xffffffff;
        this.state[d] ^= (t << 13) | (t >>> 19);

        t = (this.state[d] + this.state[c]) & 0xffffffff;
        this.state[a] ^= (t << 18) | (t >>> 14);
    }

    private generateBlock() {
        const init = new Uint32Array([
            this.fixed[0],
            this.key[0],
            this.key[1],
            this.key[2],

            this.key[3],
            this.fixed[1],
            this.nonce[0],
            this.nonce[1],

            this.counter[0],
            this.counter[1],
            this.fixed[2],
            this.key[4],

            this.key[5],
            this.key[6],
            this.key[7],
            this.fixed[3],
        ]);
        this.state = new Uint32Array(init);

        for (let i = 0; i < 20; i += 2) {
            // Odd round
            this.QR(0, 4, 8, 12);
            this.QR(5, 9, 13, 1);
            this.QR(10, 14, 2, 6);
            this.QR(15, 3, 7, 11);
            // Even round
            this.QR(0, 1, 2, 3);
            this.QR(5, 6, 7, 4);
            this.QR(10, 11, 8, 9);
            this.QR(15, 12, 13, 14);
        }

        for (let i = 0; i < 16; i += 1) {
            const word = (this.state[i] + init[i]) & 0xffffffff;
            this.block[i * 4] = word & 0xff;
            this.block[i * 4 + 1] = (word >>> 8) & 0xff;
            this.block[i * 4 + 2] = (word >>> 16) & 0xff;
            this.block[i * 4 + 3] = (word >>> 24) & 0xff;
        }

        this.counter[0] = (this.counter[0] + 1) & 0xffffffff;
        if (this.counter[0] === 0) {
            this.counter[1] = (this.counter[1] + 1) & 0xffffffff;
        }
    }

    process(input: Uint8Array): Uint8Array {
        const { length } = input;
        const result = new Uint8Array(length);

        for (let i = 0; i < length; i += 1) {
            if (this.position === 64) {
                this.generateBlock();
                this.position = 0;
            }

            result[i] = input[i] ^ this.block[this.position];
            this.position += 1;
        }

        return result;
    }
}