Move txin base weight calculation

The base weight calculation was previously provided by rust-bitcoin but
has since been removed.  Adding the base weight calculation to this
local crate instead.

Cargo.toml

@@ -15,7 +15,7 @@ keywords = ["crypto", "bitcoin"]
 readme = "README.md"
 
 [dependencies]
-bitcoin = { git="https://github.com/rust-bitcoin/rust-bitcoin", branch="master" }
+bitcoin = { git="https://github.com/yancyribbens/rust-bitcoin.git", branch="add-from-vb-const" }
 rand = {version = "0.8.5", default-features = false, optional = true}
 
 [dev-dependencies]

src/lib.rs

@@ -41,6 +41,9 @@ pub trait Utxo: Clone {
 // https://github.com/bitcoin/bitcoin/blob/f722a9bd132222d9d5cd503b5af25c905b205cdb/src/wallet/coinselection.h#L20
 const CHANGE_LOWER: Amount = Amount::from_sat(50_000);
 
+/// The weight of a `TxIn` excluding the `script_sig` and `witness`.
+pub const BASE_WEIGHT: Weight = Weight::from_vb_const(32 + 4 + 4);
+
 /// This struct contains the weight of all params needed to satisfy the UTXO.
 ///
 /// The idea of using a WeightUtxo type was inspired by the BDK implementation:

src/single_random_draw.rs

@@ -4,9 +4,9 @@
 use crate::errors::Error;
 use crate::WeightedUtxo;
 use crate::CHANGE_LOWER;
+use crate::BASE_WEIGHT;
 use bitcoin::Amount;
 use bitcoin::FeeRate;
-use bitcoin::TxIn;
 use rand::seq::SliceRandom;
 
 /// Calculates the effective_value of an input.
@@ -24,8 +24,8 @@ fn get_effective_value(
     let satisfaction_weight = weighted_utxo.satisfaction_weight;
 
     let weight = satisfaction_weight
-        .checked_add(TxIn::BASE_WEIGHT)
-        .ok_or(Error::AdditionOverflow(satisfaction_weight, TxIn::BASE_WEIGHT))?;
+        .checked_add(BASE_WEIGHT)
+        .ok_or(Error::AdditionOverflow(satisfaction_weight, BASE_WEIGHT))?;
 
     let input_fee = fee_rate
         .checked_mul_by_weight(weight)
@@ -239,6 +239,6 @@ mod tests {
         let result: Error = select_coins_srd(target, FEE_RATE, &mut weighted_utxos, &mut get_rng())
             .expect_err("expected error");
 
-        assert_eq!(result.to_string(), "18446744073709551615 + 40 exceeds u64 Max");
+        assert_eq!(result.to_string(), "18446744073709551615 + 160 exceeds u64 Max");
     }
 }