import { open } from "fs/promises";
/**
 * get hash of file or directory.
 * 
 * Contains a hashing method that matches the hashing method described
 * here: [https://pypi.org/project/oshash/](https://pypi.org/project/oshash/)
 * This hashing method is particularly useful when you don't want to read
 * an entire file's bytes to generate a hash, provided you trust that any
 * changes to the file will cause byte differences in the first and last
 * bytes of the file, or a change to its file size.
 */
export async function oshash(
    path: string,
){
    const chunkSize = 4096;
    const minFileSize = chunkSize * 2;

    const fd = await open(path, "r");
    const st = await fd.stat();
    let hash = BigInt(st.size);

    if (st.size < minFileSize){
        throw new Error("File is too small to hash");
    }

    // read first and last chunk
    const firstChunk = new Uint8Array(chunkSize);
    await fd.read(firstChunk, 0, chunkSize, 0);
    const lastChunk = new Uint8Array(chunkSize);
    await fd.read(lastChunk, 0, chunkSize, st.size - chunkSize);
    // iterate over first and last chunk.
    // for each uint64_t, add it to the hash.
    const firstChunkView = new DataView(firstChunk.buffer);
    for (let i = 0; i < chunkSize; i += 8){
        hash += firstChunkView.getBigUint64(i, true);
        // prevent overflow
        hash = (hash & 0xFFFFFFFFFFFFFFFFn);
    }
    const lastChunkView = new DataView(lastChunk.buffer);
    for (let i = 0; i < chunkSize; i += 8){
        hash += lastChunkView.getBigUint64(i, true);
        // prevent overflow
        hash = (hash & 0xFFFFFFFFFFFFFFFFn);
    }
    return hash;
}