// Transaction building for P2PKH spends on Bitcoin Cash: serialization, the
// BIP143-style replay-protected sighash (SIGHASH_ALL | FORKID), coin
// selection and fee estimation. Signing itself is delegated to WalletKeys so
// private keys stay in one module.
module.exports = function makeTx({ sha256 }) {
  const SIGHASH_ALL_FORKID = 0x41;
  const DUST = 546;
  const P2PKH_INPUT_SIZE = 149;   // outpoint 36 + len 1 + sig push 74 + pubkey push 34 + sequence 4
  const P2PKH_OUTPUT_SIZE = 34;
  const OVERHEAD = 10;            // version 4 + in/out counts 2 + locktime 4

  const dsha = (b) => sha256(sha256(b));
  const toHex = (b) => Array.from(b, (x) => x.toString(16).padStart(2, "0")).join("");
  const fromHex = (h) => Uint8Array.from(h.match(/../g) || [], (x) => parseInt(x, 16));
  const concat = (...parts) => {
    const n = parts.reduce((a, p) => a + p.length, 0);
    const out = new Uint8Array(n); let o = 0;
    for (const p of parts) { out.set(p, o); o += p.length; }
    return out;
  };
  const u32le = (n) => Uint8Array.from([n & 0xff, (n >>> 8) & 0xff, (n >>> 16) & 0xff, (n >>> 24) & 0xff]);
  const u64le = (n) => {
    let v = BigInt(n); const out = new Uint8Array(8);
    for (let i = 0; i < 8; i++) { out[i] = Number(v & 0xffn); v >>= 8n; }
    return out;
  };
  const varint = (n) => {
    if (n < 0xfd) return Uint8Array.from([n]);
    if (n <= 0xffff) return Uint8Array.from([0xfd, n & 0xff, n >> 8]);
    return concat(Uint8Array.from([0xfe]), u32le(n));
  };
  const varbytes = (b) => concat(varint(b.length), b);
  const pushdata = (b) => {
    if (b.length < 0x4c) return concat(Uint8Array.from([b.length]), b);
    return concat(Uint8Array.from([0x4c, b.length]), b);
  };
  const outpoint = (inp) => concat(fromHex(inp.txid).reverse(), u32le(inp.vout));

  const estimateSize = (nIn, nOut) => OVERHEAD + nIn * P2PKH_INPUT_SIZE + nOut * P2PKH_OUTPUT_SIZE;
  const feeFor = (nIn, nOut, satPerByte) => Math.ceil(estimateSize(nIn, nOut) * satPerByte);

  // inputs: [{ txid, vout, value, script(Uint8Array), sig?(Uint8Array) }]
  // outputs: [{ value, script(Uint8Array) }]
  function serialize(tx) {
    return concat(
      u32le(tx.version ?? 2),
      varint(tx.inputs.length),
      ...tx.inputs.map((i) => concat(outpoint(i), varbytes(i.unlocking || new Uint8Array(0)), u32le(i.sequence ?? 0xffffffff))),
      varint(tx.outputs.length),
      ...tx.outputs.map((o) => concat(u64le(o.value), varbytes(o.script))),
      u32le(tx.locktime ?? 0),
    );
  }

  function sighash(tx, index) {
    const inp = tx.inputs[index];
    const hashPrevouts = dsha(concat(...tx.inputs.map(outpoint)));
    const hashSequence = dsha(concat(...tx.inputs.map((i) => u32le(i.sequence ?? 0xffffffff))));
    const hashOutputs = dsha(concat(...tx.outputs.map((o) => concat(u64le(o.value), varbytes(o.script)))));
    const preimage = concat(
      u32le(tx.version ?? 2),
      hashPrevouts, hashSequence,
      outpoint(inp),
      varbytes(inp.script),
      u64le(inp.value),
      u32le(inp.sequence ?? 0xffffffff),
      hashOutputs,
      u32le(tx.locktime ?? 0),
      u32le(SIGHASH_ALL_FORKID),
    );
    return dsha(preimage);
  }

  // signer(input, index, digest) -> { sig: DER bytes, publicKey }
  function sign(tx, signer) {
    tx.inputs.forEach((inp, i) => {
      const { sig, publicKey } = signer(inp, i, sighash(tx, i));
      inp.unlocking = concat(pushdata(concat(sig, Uint8Array.from([SIGHASH_ALL_FORKID]))), pushdata(publicKey));
    });
    const raw = serialize(tx);
    return { raw, hex: toHex(raw), txid: toHex(dsha(raw).reverse()) };
  }

  // Largest-first accumulation. `targets` = [{ value, script }]; returns
  // { inputs, outputs, fee, change } or throws when funds don't cover it.
  // sendMax: spend every UTXO into targets[0] and no change.
  function select(utxos, targets, satPerByte, changeScript, { sendMax = false } = {}) {
    const sorted = utxos.slice().sort((a, b) => b.value - a.value);
    const total = sorted.reduce((a, u) => a + u.value, 0);
    if (sendMax) {
      if (targets.length !== 1) throw new Error("send max needs exactly one recipient");
      const fee = feeFor(sorted.length, 1, satPerByte);
      const value = total - fee;
      if (!sorted.length || value < DUST) throw new Error("balance too small to send");
      return { inputs: sorted, outputs: [{ value, script: targets[0].script }], fee, change: 0 };
    }
    const want = targets.reduce((a, t) => a + t.value, 0);
    for (const t of targets) if (t.value < DUST) throw new Error(`amount below dust limit (${DUST} sat)`);
    const chosen = []; let sum = 0;
    for (const u of sorted) {
      chosen.push(u); sum += u.value;
      const feeWithChange = feeFor(chosen.length, targets.length + 1, satPerByte);
      if (sum >= want + feeWithChange) {
        const change = sum - want - feeWithChange;
        if (change >= DUST) {
          return { inputs: chosen, outputs: [...targets, { value: change, script: changeScript }], fee: feeWithChange, change };
        }
        // Change would be dust: fold it into the fee, one output fewer.
        const fee = sum - want;
        return { inputs: chosen, outputs: targets.slice(), fee, change: 0 };
      }
      const feeNoChange = feeFor(chosen.length, targets.length, satPerByte);
      if (sum >= want + feeNoChange && sum - want - feeNoChange < DUST) {
        return { inputs: chosen, outputs: targets.slice(), fee: sum - want, change: 0 };
      }
    }
    const short = want + feeFor(Math.max(1, sorted.length), targets.length + 1, satPerByte) - total;
    throw new Error(`insufficient funds: need about ${short} more sat`);
  }

  return { SIGHASH_ALL_FORKID, DUST, serialize, sighash, sign, select, feeFor, estimateSize, toHex, fromHex, dsha };
};