/**
 * Just enough of the ELF format to describe a native file the packager is about to copy (ADR 0049):
 * which machine it was built for, which shared libraries and C library symbol versions it needs, and
 * which Node.js addon interface it exports. Read from the file's own bytes, never by running it or
 * shelling out to a system tool.
 *
 * Only 64-bit little-endian ELF is understood, because that is every target this packager produces.
 * Anything else that starts with the ELF magic is reported as unsupported rather than guessed at.
 */

export class ElfFormatError extends Error {
  constructor() {
    super("The file is not a 64-bit little-endian ELF object this packager can read.");
    this.name = "ElfFormatError";
  }
}

/** `EM_X86_64` in the ELF machine field. */
export const ELF_MACHINE_X86_64 = 62;

export interface ElfDescription {
  readonly machine: number;
  /** `DT_NEEDED` entries: the shared libraries the file links against. */
  readonly needed: readonly string[];
  /** Symbol version names the file requires, for example `GLIBC_2.28`. */
  readonly requiredVersions: readonly string[];
  /** Names of the dynamic symbols the file itself defines. */
  readonly exported: ReadonlySet<string>;
  /** The value a Node-API addon's `node_api_module_get_api_version_v1` returns, when it exports
   * that function and its body is the plain constant return every compiler emits for it.
   * `undefined` when the function is absent; `"unrecognized"` when its body is something else. */
  readonly nodeApiModuleVersion: number | "unrecognized" | undefined;
}

const SHT_NOBITS = 8;
const SHT_DYNSYM = 11;
const SHT_DYNAMIC = 6;
const SHT_GNU_VERNEED = 0x6ffffffe;
const DT_NULL = 0;
const DT_NEEDED = 1;

export function isElf(data: Uint8Array): boolean {
  return (
    data.length >= 4 && data[0] === 0x7f && data[1] === 0x45 && data[2] === 0x4c && data[3] === 0x46
  );
}

interface Section {
  readonly type: number;
  readonly address: number;
  readonly offset: number;
  readonly size: number;
  readonly link: number;
  readonly entrySize: number;
}

export function describeElf(input: Uint8Array): ElfDescription {
  const data = Buffer.from(input.buffer, input.byteOffset, input.byteLength);
  if (!isElf(data) || data.length < 64 || data[4] !== 2 || data[5] !== 1) {
    throw new ElfFormatError();
  }
  const u16 = (offset: number): number => {
    if (offset < 0 || offset + 2 > data.length) throw new ElfFormatError();
    return data.readUInt16LE(offset);
  };
  const u32 = (offset: number): number => {
    if (offset < 0 || offset + 4 > data.length) throw new ElfFormatError();
    return data.readUInt32LE(offset);
  };
  const u64 = (offset: number): number => {
    if (offset < 0 || offset + 8 > data.length) throw new ElfFormatError();
    const value = data.readBigUInt64LE(offset);
    if (value > BigInt(Number.MAX_SAFE_INTEGER)) throw new ElfFormatError();
    return Number(value);
  };
  const string = (tableOffset: number, tableSize: number, index: number): string => {
    if (index >= tableSize) throw new ElfFormatError();
    const start = tableOffset + index;
    const end = data.indexOf(0, start);
    if (end === -1 || end > tableOffset + tableSize) throw new ElfFormatError();
    return data.toString("latin1", start, end);
  };

  const machine = u16(18);
  const sectionOffset = u64(40);
  const sectionEntrySize = u16(58);
  const sectionCount = u16(60);
  if (sectionEntrySize < 64 || sectionCount === 0) {
    throw new ElfFormatError();
  }
  const sections: Section[] = [];
  for (let index = 0; index < sectionCount; index += 1) {
    const base = sectionOffset + index * sectionEntrySize;
    sections.push({
      address: u64(base + 16),
      entrySize: u64(base + 56),
      link: u32(base + 40),
      offset: u64(base + 24),
      size: u64(base + 32),
      type: u32(base + 4),
    });
  }
  const linkedStrings = (section: Section): { offset: number; size: number } => {
    const table = sections[section.link];
    if (table === undefined || table.offset + table.size > data.length) throw new ElfFormatError();
    return { offset: table.offset, size: table.size };
  };

  const needed: string[] = [];
  const requiredVersions: string[] = [];
  const exported = new Set<string>();
  let apiVersionAddress: number | undefined;

  for (const section of sections) {
    if (section.type === SHT_DYNAMIC) {
      const strings = linkedStrings(section);
      for (let entry = section.offset; entry + 16 <= section.offset + section.size; entry += 16) {
        const tag = u64(entry);
        if (tag === DT_NULL) break;
        if (tag === DT_NEEDED) needed.push(string(strings.offset, strings.size, u64(entry + 8)));
      }
    } else if (section.type === SHT_DYNSYM) {
      const strings = linkedStrings(section);
      const size = section.entrySize === 0 ? 24 : section.entrySize;
      for (let entry = section.offset; entry + 24 <= section.offset + section.size; entry += size) {
        const nameIndex = u32(entry);
        const sectionIndex = u16(entry + 6);
        if (nameIndex === 0 || sectionIndex === 0) continue;
        const name = string(strings.offset, strings.size, nameIndex);
        exported.add(name);
        if (name === "node_api_module_get_api_version_v1") {
          apiVersionAddress = u64(entry + 8);
        }
      }
    } else if (section.type === SHT_GNU_VERNEED) {
      const strings = linkedStrings(section);
      let need = section.offset;
      for (let guard = 0; guard < 4096; guard += 1) {
        const count = u16(need + 2);
        let aux = need + u32(need + 8);
        for (let index = 0; index < count; index += 1) {
          requiredVersions.push(string(strings.offset, strings.size, u32(aux + 8)));
          const next = u32(aux + 12);
          if (next === 0) break;
          aux += next;
        }
        const next = u32(need + 12);
        if (next === 0) break;
        need += next;
      }
    }
  }

  let nodeApiModuleVersion: number | "unrecognized" | undefined;
  if (apiVersionAddress !== undefined) {
    const text = sections.find(
      (section) =>
        section.type !== SHT_NOBITS &&
        section.address <= apiVersionAddress &&
        apiVersionAddress < section.address + section.size,
    );
    nodeApiModuleVersion = "unrecognized";
    if (text !== undefined) {
      let offset = text.offset + (apiVersionAddress - text.address);
      // An optional `endbr64` (Intel CET), then `mov eax, imm32; ret`: what every compiler emits for
      // a function that returns the NAPI_VERSION constant. Anything else is not guessed at.
      if (
        data[offset] === 0xf3 &&
        data[offset + 1] === 0x0f &&
        data[offset + 2] === 0x1e &&
        data[offset + 3] === 0xfa
      ) {
        offset += 4;
      }
      if (data[offset] === 0xb8 && data[offset + 5] === 0xc3) {
        nodeApiModuleVersion = u32(offset + 1);
      }
    }
  }

  return Object.freeze({
    exported,
    machine,
    needed: Object.freeze(needed),
    nodeApiModuleVersion,
    requiredVersions: Object.freeze(requiredVersions),
  });
}