Add BIP174 pseudo code for signing checks

src/psbt.js

@@ -8,6 +8,27 @@ class Psbt extends bip174_1.Psbt {
   signInput(inputIndex, keyPair) {
     // TODO: Implement BIP174 pre-sign checks:
     // https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki#signer
+    //
+    // if non_witness_utxo.exists:
+    //     assert(sha256d(non_witness_utxo) == psbt.tx.innput[i].prevout.hash)
+    //     if redeemScript.exists:
+    //         assert(non_witness_utxo.vout[psbt.tx.input[i].prevout.n].scriptPubKey == P2SH(redeemScript))
+    //         sign_non_witness(redeemScript)
+    //     else:
+    //         sign_non_witness(non_witness_utxo.vout[psbt.tx.input[i].prevout.n].scriptPubKey)
+    // else if witness_utxo.exists:
+    //     if redeemScript.exists:
+    //         assert(witness_utxo.scriptPubKey == P2SH(redeemScript))
+    //         script = redeemScript
+    //     else:
+    //         script = witness_utxo.scriptPubKey
+    //     if IsP2WPKH(script):
+    //         sign_witness(P2PKH(script[2:22]))
+    //     else if IsP2WSH(script):
+    //         assert(script == P2WSH(witnessScript))
+    //         sign_witness(witnessScript)
+    // else:
+    //     assert False
     // TODO: Get hash to sign
     const hash = Buffer.alloc(32);
     const partialSig = {

ts_src/psbt.ts

@@ -9,6 +9,27 @@ export class Psbt extends PsbtBase {
   signInput(inputIndex: number, keyPair: Signer): Psbt {
     // TODO: Implement BIP174 pre-sign checks:
     // https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki#signer
+    //
+    // if non_witness_utxo.exists:
+    //     assert(sha256d(non_witness_utxo) == psbt.tx.innput[i].prevout.hash)
+    //     if redeemScript.exists:
+    //         assert(non_witness_utxo.vout[psbt.tx.input[i].prevout.n].scriptPubKey == P2SH(redeemScript))
+    //         sign_non_witness(redeemScript)
+    //     else:
+    //         sign_non_witness(non_witness_utxo.vout[psbt.tx.input[i].prevout.n].scriptPubKey)
+    // else if witness_utxo.exists:
+    //     if redeemScript.exists:
+    //         assert(witness_utxo.scriptPubKey == P2SH(redeemScript))
+    //         script = redeemScript
+    //     else:
+    //         script = witness_utxo.scriptPubKey
+    //     if IsP2WPKH(script):
+    //         sign_witness(P2PKH(script[2:22]))
+    //     else if IsP2WSH(script):
+    //         assert(script == P2WSH(witnessScript))
+    //         sign_witness(witnessScript)
+    // else:
+    //     assert False
 
     // TODO: Get hash to sign
     const hash = Buffer.alloc(32);