// From https://npmjs.com/package/@exodus/bytes

// Copyright Exodus Movement. Licensed under MIT License.

'use strict';

const {

Uint8Array,

} = primordials;

/**

* Get a number of last bytes in an Uint8Array `data` ending at `len` that don't

* form a codepoint yet, but can be a part of a single codepoint on more data.

* @param {Uint8Array} data Uint8Array of potentially UTF-8 bytes

* @param {number} len Position to look behind from

* @returns {number} Number of unfinished potentially valid UTF-8 bytes ending at position `len`

*/

function unfinishedBytesUtf8(data, len) {

// 0-3

let pos = 0;

while (pos < 2 && pos < len && (data[len - pos - 1] & 0xc0) === 0x80) pos++; // Go back 0-2 trailing bytes

if (pos === len) return 0; // no space for lead

const lead = data[len - pos - 1];

if (lead < 0xc2 || lead > 0xf4) return 0; // not a lead

if (pos === 0) return 1; // Nothing to recheck, we have only lead, return it. 2-byte must return here

if (lead < 0xe0 || (lead < 0xf0 && pos >= 2)) return 0; // 2-byte, or 3-byte or less and we already have 2 trailing

const lower = lead === 0xf0 ? 0x90 : lead === 0xe0 ? 0xa0 : 0x80;

const upper = lead === 0xf4 ? 0x8f : lead === 0xed ? 0x9f : 0xbf;

const next = data[len - pos];

return next >= lower && next <= upper ? pos + 1 : 0;

}

/**

* Merge prefix `chunk` with `data` and return new combined prefix.

* For data.length < 3, fully consumes data and can return unfinished data,

* otherwise returns a prefix with no unfinished bytes

* @param {Uint8Array} data Uint8Array of potentially UTF-8 bytes

* @param {Uint8Array} chunk Prefix to prepend before `data`

* @returns {Uint8Array} If data.length >= 3: an Uint8Array containing `chunk` and a slice of `data`

* so that the result has no unfinished UTF-8 codepoints. If data.length < 3: concat(chunk, data).

*/

function mergePrefixUtf8(data, chunk) {

if (data.length === 0) return chunk;

if (data.length < 3) {

// No reason to bruteforce offsets, also it's possible this doesn't yet end the sequence

const res = new Uint8Array(data.length + chunk.length);

res.set(chunk);

res.set(data, chunk.length);

return res;

}

// Slice off a small portion of data into prefix chunk so we can decode them separately without extending array size

const temp = new Uint8Array(chunk.length + 3); // We have 1-3 bytes and need 1-3 more bytes

temp.set(chunk);

temp.set(data.subarray(0, 3), chunk.length);

// Stop at the first offset where unfinished bytes reaches 0 or fits into data

// If that doesn't happen (data too short), just concat chunk and data completely (above)

for (let i = 1; i <= 3; i++) {

const unfinished = unfinishedBytesUtf8(temp, chunk.length + i); // 0-3

if (unfinished <= i) {

// Always reachable at 3, but we still need 'unfinished' value for it

const add = i - unfinished; // 0-3

return add > 0 ? temp.subarray(0, chunk.length + add) : chunk;

}

}

// Unreachable

return null;

}

module.exports = { unfinishedBytesUtf8, mergePrefixUtf8 };