Fix max_satisfaction_weight calculations for taproot

* Key spend path should take into consideration the scriptSigLen,
  stackLen and itemLen (for the one and only stack item).
* Script spend path should consider `control_block` and `script` to be
  items on the stack.

I also restructured the function to improve readability.

Author: evanlinjin

examples/taproot.rs

@@ -116,11 +116,11 @@ fn main() {
 
     // Max Satisfaction Weight for compilation, corresponding to the script-path spend
     // `multi_a(2,PUBKEY_1,PUBKEY_2) at taptree depth 1, having
-    // Max Witness Size = scriptSig len + control_block size + varint(script_size) + script_size +
-    //                     varint(max satisfaction elements) + max satisfaction size
-    //                  = 4 + 65 + 1 + 70 + 1 + 132
+    // Max Witness Size = scriptSig len + witnessStack len + varint(control_block_size) +
+    //                    control_block size + varint(script_size) + script_size + max_satisfaction_size
+    //                  = 4 + 1 + 1 + 65 + 1 + 70 + 132 = 274
     let max_sat_wt = real_desc.max_satisfaction_weight().unwrap();
-    assert_eq!(max_sat_wt, 273);
+    assert_eq!(max_sat_wt, 274);
 
     // Compute the bitcoin address and check if it matches
     let network = Network::Bitcoin;

src/descriptor/tr.rs

@@ -264,27 +264,37 @@ impl<Pk: MiniscriptKey> Tr<Pk> {
     /// # Errors
     /// When the descriptor is impossible to safisfy (ex: sh(OP_FALSE)).
     pub fn max_satisfaction_weight(&self) -> Result<usize, Error> {
-        let mut max_wieght = Some(65);
-        for (depth, ms) in self.iter_scripts() {
-            let script_size = ms.script_size();
-            let max_sat_elems = match ms.max_satisfaction_witness_elements() {
-                Ok(elem) => elem,
-                Err(..) => continue,
-            };
-            let max_sat_size = match ms.max_satisfaction_size() {
-                Ok(sz) => sz,
-                Err(..) => continue,
-            };
-            let control_block_sz = control_block_len(depth);
-            let wit_size = 4 + // scriptSig len byte
-            control_block_sz + // first element control block
-            varint_len(script_size) +
-            script_size + // second element script len with prefix
-            varint_len(max_sat_elems) +
-            max_sat_size; // witness
-            max_wieght = cmp::max(max_wieght, Some(wit_size));
-        }
-        max_wieght.ok_or(Error::ImpossibleSatisfaction)
+        let tree = match self.taptree() {
+            // key spend path:
+            // scriptSigLen(4) + stackLen(1) + stack[Sig]Len(1) + stack[Sig](65)
+            None => return Ok(4 + 1 + 1 + 65),
+            // script path spend..
+            Some(tree) => tree,
+        };
+
+        tree.iter()
+            .filter_map(|(depth, ms)| {
+                let script_size = ms.script_size();
+                let max_sat_elems = ms.max_satisfaction_witness_elements().ok()?;
+                let max_sat_size = ms.max_satisfaction_size().ok()?;
+                let control_block_size = control_block_len(depth);
+                Some(
+                    // scriptSig len byte
+                    4 +
+                    // witness field stack len (+2 for control block & script)
+                    varint_len(max_sat_elems + 2) +
+                    // size of elements to satisfy script
+                    max_sat_size +
+                    // second to last element: script
+                    varint_len(script_size) +
+                    script_size +
+                    // last element: control block
+                    varint_len(control_block_size) +
+                    control_block_size,
+                )
+            })
+            .max()
+            .ok_or(Error::ImpossibleSatisfaction)
     }
 }