simplicity: add 'simplicity pset create' command

This should be compatible with any non-pegin non-issuance calls to elements-cli
createpsbt.

Co-authored-by: aider (anthropic/claude-sonnet-4-20250514) <[email protected]>

Author: apoelstra

src/bin/hal-simplicity/cmd/simplicity/pset/create.rs

@@ -0,0 +1,172 @@
+// Copyright 2025 Andrew Poelstra
+// SPDX-License-Identifier: CC0-1.0
+
+use super::super::{Error, ErrorExt as _};
+use super::UpdatedPset;
+use crate::cmd;
+
+use elements::confidential;
+use elements::pset::PartiallySignedTransaction;
+use elements::{Address, AssetId, OutPoint, Transaction, TxIn, TxOut, Txid};
+use serde::Deserialize;
+
+use std::collections::HashMap;
+
+#[derive(Deserialize)]
+struct InputSpec {
+	txid: Txid,
+	vout: u32,
+	#[serde(default)]
+	sequence: Option<u32>,
+}
+
+#[derive(Deserialize)]
+struct FlattenedOutputSpec {
+	address: String,
+	asset: AssetId,
+	#[serde(with = "elements::bitcoin::amount::serde::as_btc")]
+	amount: elements::bitcoin::Amount,
+}
+
+#[derive(Deserialize)]
+#[serde(untagged)]
+enum OutputSpec {
+	Explicit {
+		address: String,
+		asset: AssetId,
+		#[serde(with = "elements::bitcoin::amount::serde::as_btc")]
+		amount: elements::bitcoin::Amount,
+	},
+	Map(HashMap<String, f64>),
+}
+
+impl OutputSpec {
+	fn flatten(self) -> Box<dyn Iterator<Item = Result<FlattenedOutputSpec, Error>>> {
+		match self {
+			Self::Map(map) => Box::new(map.into_iter().map(|(address, amount)| {
+				// Use liquid bitcoin asset as default for map format
+				let default_asset = AssetId::from_slice(&[
+					0x49, 0x9a, 0x81, 0x85, 0x45, 0xf6, 0xba, 0xe3, 0x9f, 0xc0, 0x3b, 0x63, 0x7f,
+					0x2a, 0x4e, 0x1e, 0x64, 0xe5, 0x90, 0xca, 0xc1, 0xbc, 0x3a, 0x6f, 0x6d, 0x71,
+					0xaa, 0x44, 0x43, 0x65, 0x4c, 0x14,
+				])
+				.expect("valid asset id");
+
+				Ok(FlattenedOutputSpec {
+					address,
+					asset: default_asset,
+					amount: elements::bitcoin::Amount::from_btc(amount)
+						.result_context("parsing amount")?,
+				})
+			})),
+			Self::Explicit {
+				address,
+				asset,
+				amount,
+			} => Box::new(
+				Some(Ok(FlattenedOutputSpec {
+					address,
+					asset,
+					amount,
+				}))
+				.into_iter(),
+			),
+		}
+	}
+}
+
+pub fn cmd<'a>() -> clap::App<'a, 'a> {
+	cmd::subcommand("create", "create an empty PSET").args(&cmd::opts_networks()).args(&[
+		cmd::arg(
+			"inputs",
+			"input outpoints (JSON array of objects containing txid, vout, sequence)",
+		)
+		.takes_value(true)
+		.required(true),
+		cmd::arg("outputs", "outputs (JSON array of objects containing address, asset, amount)")
+			.takes_value(true)
+			.required(true),
+	])
+}
+
+pub fn exec<'a>(matches: &clap::ArgMatches<'a>) {
+	let inputs_json = matches.value_of("inputs").expect("inputs mandatory");
+	let outputs_json = matches.value_of("outputs").expect("inputs mandatory");
+
+	match exec_inner(inputs_json, outputs_json) {
+		Ok(info) => cmd::print_output(matches, &info),
+		Err(e) => cmd::print_output(matches, &e),
+	}
+}
+
+fn exec_inner(inputs_json: &str, outputs_json: &str) -> Result<UpdatedPset, Error> {
+	// Parse inputs JSON
+	let input_specs: Vec<InputSpec> =
+		serde_json::from_str(inputs_json).result_context("parsing inputs JSON")?;
+
+	// Parse outputs JSON - support both array and map formats
+	let output_specs: Vec<OutputSpec> =
+		serde_json::from_str(outputs_json).result_context("parsing outputs JSON")?;
+
+	// Create transaction inputs
+	let mut inputs = Vec::new();
+	for input_spec in &input_specs {
+		let outpoint = OutPoint::new(input_spec.txid, input_spec.vout);
+		let sequence = elements::Sequence(input_spec.sequence.unwrap_or(0xffffffff));
+
+		inputs.push(TxIn {
+			previous_output: outpoint,
+			script_sig: elements::Script::new(),
+			sequence,
+			asset_issuance: Default::default(),
+			witness: Default::default(),
+			is_pegin: false,
+		});
+	}
+
+	// Create transaction outputs
+	let mut outputs = Vec::new();
+	for output_spec in output_specs.into_iter().flat_map(OutputSpec::flatten) {
+		let output_spec = output_spec?; // serde has crappy error messages so we defer parsing and then have to unwrap errors
+
+		let script_pubkey = match output_spec.address.as_str() {
+			"fee" => elements::Script::new(),
+			x => {
+				let addr = x.parse::<Address>().result_context("parsing address")?;
+				if addr.is_blinded() {
+					return Err("confidential addresses are not yet supported")
+						.result_context("output address");
+				}
+				addr.script_pubkey()
+			}
+		};
+
+		outputs.push(TxOut {
+			asset: confidential::Asset::Explicit(output_spec.asset),
+			value: confidential::Value::Explicit(output_spec.amount.to_sat()),
+			nonce: elements::confidential::Nonce::Null,
+			script_pubkey,
+			witness: elements::TxOutWitness::empty(),
+		});
+	}
+
+	// Create the transaction
+	let tx = Transaction {
+		version: 2,
+		lock_time: elements::LockTime::ZERO,
+		input: inputs,
+		output: outputs,
+	};
+
+	// Create PSET from transaction
+	let pset = PartiallySignedTransaction::from_tx(tx);
+
+	Ok(UpdatedPset {
+		pset: pset.to_string(),
+		updated_values: vec![
+			// FIXME we technically update a whole slew of fields; see the implementation
+			// of PartiallySignedTransaction::from_tx. Should we attempt to exhaustively
+			// list them here? Or list none? Or what?
+		],
+	})
+}

src/bin/hal-simplicity/cmd/simplicity/pset/mod.rs

@@ -1,6 +1,7 @@
 // Copyright 2025 Andrew Poelstra
 // SPDX-License-Identifier: CC0-1.0
 
+mod create;
 mod extract;
 mod finalize;
 mod update_input;
@@ -16,13 +17,15 @@ struct UpdatedPset {
 
 pub fn cmd<'a>() -> clap::App<'a, 'a> {
 	cmd::subcommand_group("pset", "manipulate PSETs for spending from Simplicity programs")
+		.subcommand(self::create::cmd())
 		.subcommand(self::extract::cmd())
 		.subcommand(self::finalize::cmd())
 		.subcommand(self::update_input::cmd())
 }
 
 pub fn exec<'a>(matches: &clap::ArgMatches<'a>) {
 	match matches.subcommand() {
+		("create", Some(m)) => self::create::exec(m),
 		("extract", Some(m)) => self::extract::exec(m),
 		("finalize", Some(m)) => self::finalize::exec(m),
 		("update-input", Some(m)) => self::update_input::exec(m),