// IMG archive reader.
//
// San Andreas ships VER2 archives: the 'VER2' tag, a u32 entry count, then one
// 32 byte directory entry per file -
//   u32 offset, in 2048 byte sectors
//   u16 streaming size, u16 size in archive (zero on retail, so prefer the former)
//   char name[24], null padded
// Bodies follow the directory, each starting on a sector boundary.

const fs = require('fs');

const SECTOR = 2048;

class Img {
  constructor(file) {
    this.fd = fs.openSync(file, 'r');
    this.path = file;

    const header = Buffer.alloc(8);
    fs.readSync(this.fd, header, 0, 8, 0);
    if (header.toString('ascii', 0, 4) !== 'VER2') {
      throw new Error(`${file} is not a VER2 img archive`);
    }
    const count = header.readUInt32LE(4);

    const dir = Buffer.alloc(count * 32);
    fs.readSync(this.fd, dir, 0, dir.length, 8);

    // Lower cased, because the ide names and the archive names disagree on case.
    this.entries = new Map();
    for (let i = 0; i < count; i++) {
      const at = i * 32;
      const name = dir.toString('ascii', at + 8, at + 32).split('\0')[0];
      const sectors = dir.readUInt16LE(at + 4) || dir.readUInt16LE(at + 6);
      this.entries.set(name.toLowerCase(), {
        name,
        offset: dir.readUInt32LE(at) * SECTOR,
        size: sectors * SECTOR,
      });
    }
  }

  has(name) {
    return this.entries.has(name.toLowerCase());
  }

  // Entry bodies are padded out to the sector size, and every reader downstream
  // walks chunk headers rather than trusting the length, so the padding is
  // harmless and gets written as-is.
  read(name) {
    const entry = this.entries.get(name.toLowerCase());
    if (!entry) return null;
    const buffer = Buffer.alloc(entry.size);
    fs.readSync(this.fd, buffer, 0, entry.size, entry.offset);
    return buffer;
  }

  close() {
    fs.closeSync(this.fd);
  }
}

module.exports = { Img };