/**
 * https://github.com/kawanet/git-cat-file
 *
 * @see https://github.com/git/git/blob/master/Documentation/technical/pack-format.txt
 */

import {promises as fs} from "fs"

export type PackIndex = {[oid: string]: number}

const toHex = (buf: Buffer) => buf.toString("hex").replace(/(\w.)(?=\w)/g, "$1 ")

export async function readPackIndex(path: string): Promise<PackIndex> {
    path = path.replace(/\.pack$/, ".idx")
    if (!/\.idx$/.test(path)) throw TypeError(`Invalid pack index file: ${path}`)
    const data = await fs.readFile(path)
    return parsePackIndex(data)
}

async function parsePackIndex(data: Buffer): Promise<PackIndex> {
    // validate header
    const headByte = data.slice(0, 4)
    const head = headByte.toString("latin1")
    if (head !== "\xFFtOc") throw TypeError(`Invalid header: ${toHex(headByte)}`)

    // validate version number
    const version = data.readUInt32BE(4)
    if (version !== 2) throw TypeError(`Invalid version number: ${version}`)

    let oidIdx = 8 + 4 * 256

    // total objects count
    const total = data.readUInt32BE(oidIdx - 4)
    // console.warn(`found: ${total} packed objects`);

    const oidLast = oidIdx + 20 * total
    let table4Idx = oidLast + 4 * total
    let table8Idx = table4Idx + 4 * total

    const packIndex: PackIndex = {}
    while (oidIdx < oidLast) {
        const end = oidIdx + 20
        const oid = data.slice(oidIdx, end).toString("hex")
        let pos = data.readUInt32BE(table4Idx)
        table4Idx += 4
        if (pos & 0x80000000) {
            // console.warn(pos.toString(16));
            const high = data.readUInt32BE(table8Idx)
            table8Idx += 4
            const low = data.readUInt32BE(table8Idx)
            table8Idx += 4
            pos = high * 0x100000000 + low
        }
        packIndex[oid] = pos

        oidIdx = end
    }

    return packIndex
}