Feature: Add opcodes::All::decode_pushnum and Script::get_sigop_count

Co-authored-by: Tobin C. Harding <me@tobin.cc>

bitcoin/src/blockdata/opcodes.rs

@@ -411,6 +411,30 @@ impl All {
     /// Encodes [`All`] as a byte.
     #[inline]
     pub const fn to_u8(self) -> u8 { self.code }
+
+    /// Encodes PUSHNUM [`All`] as a `u8` representing its number (1-16).
+    ///
+    /// Does not convert `OP_FALSE` to 0. Only `1` to `OP_PUSHNUM_16` are covered.
+    ///
+    /// # Returns
+    ///
+    /// Returns `None` if `self` is not a PUSHNUM.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bitcoin::opcodes::all::*;
+    /// assert_eq!(OP_PUSHNUM_5.decode_pushnum().expect("pushnum"), 5)
+    /// ```
+    #[inline]
+    pub const fn decode_pushnum(self) -> Option<u8> {
+        const START: u8 = OP_PUSHNUM_1.code;
+        const END: u8 = OP_PUSHNUM_16.code;
+        match self.code {
+            START..=END => Some(self.code - START + 1),
+            _ => None,
+        }
+    }
 }
 
 impl From<u8> for All {

bitcoin/src/blockdata/script/borrowed.rs

@@ -318,6 +318,77 @@ impl Script {
         crate::Amount::from_sat(sats)
     }
 
+    /// Get the sigop count for this Script using accurate counting.
+    ///
+    /// In Bitcoin Core, there are two ways to count sigops, "accurate" and "legacy".
+    /// This method uses "accurate" counting. This means that OP_CHECKMULTISIG and its
+    /// verify variant count for N sigops where N is the number of pubkeys used in the
+    /// multisig. However, it will count for 20 sigops if CHECKMULTISIG is not preceeded by an
+    /// OP_PUSHNUM from 1 - 16 (this would be an invalid script)
+    ///
+    /// Bitcoin Core uses accurate counting for sigops contained within redeemScripts (P2SH)
+    /// and witnessScripts (P2WSH) only. It uses legacy for sigops in scriptSigs and scriptPubkeys.
+    ///
+    /// (Note: taproot scripts don't count toward the sigop count of the block,
+    /// nor do they have CHECKMULTISIG operations. This function does not count OP_CHECKSIGADD,
+    /// so do not use this to try and estimate if a taproot script goes over the sigop budget.)
+    pub fn get_sigop_count(&self) -> Result<usize, super::Error> {
+        self.get_sigop_count_internal(true)
+    }
+
+    /// Get the sigop count for this Script using legacy counting.
+    ///
+    /// In Bitcoin Core, there are two ways to count sigops, "accurate" and "legacy".
+    /// This method uses "legacy" counting. This means that OP_CHECKMULTISIG and its
+    /// verify variant count for 20 sigops.
+    ///
+    /// Bitcoin Core uses legacy counting for sigops contained within scriptSigs and
+    /// scriptPubkeys. It uses accurate for redeemScripts (P2SH) and witnessScripts (P2WSH).
+    ///
+    /// (Note: taproot scripts don't count toward the sigop count of the block,
+    /// nor do they have CHECKMULTISIG operations. This function does not count OP_CHECKSIGADD,
+    /// so do not use this to try and estimate if a taproot script goes over the sigop budget.)
+    pub fn get_sigop_count_legacy(&self) -> Result<usize, super::Error> {
+        self.get_sigop_count_internal(false)
+    }
+
+    fn get_sigop_count_internal(&self, accurate: bool) -> Result<usize, super::Error> {
+        let mut n = 0;
+        let mut pushnum_cache = None;
+        for inst in self.instructions() {
+            match inst? {
+                Instruction::Op(opcode) => {
+                    match opcode {
+                        OP_CHECKSIG | OP_CHECKSIGVERIFY => {
+                            n += 1;
+                        }
+                        OP_CHECKMULTISIG | OP_CHECKMULTISIGVERIFY => {
+                            match (accurate, pushnum_cache) {
+                                (true, Some(pushnum)) => {
+                                    // Add the number of pubkeys in the multisig as sigop count
+                                    n += pushnum as usize;
+                                }
+                                _ => {
+                                    // MAX_PUBKEYS_PER_MULTISIG from Bitcoin Core
+                                    // https://github.com/bitcoin/bitcoin/blob/v25.0/src/script/script.h#L29-L30
+                                    n += 20;
+                                }
+                            }
+                        }
+                        _ => {
+                            pushnum_cache = opcode.decode_pushnum();
+                        }
+                    }
+                }
+                Instruction::PushBytes(_) => {
+                    pushnum_cache = None;
+                }
+            }
+        }
+
+        Ok(n)
+    }
+
     /// Iterates over the script instructions.
     ///
     /// Each returned item is a nested enum covering opcodes, datapushes and errors.

bitcoin/src/blockdata/script/tests.rs

@@ -607,6 +607,82 @@ fn defult_dust_value_tests() {
     assert_eq!(script_p2pkh.dust_value(), crate::Amount::from_sat(546));
 }
 
+#[test]
+fn test_script_get_sigop_count() {
+    assert_eq!(
+        Builder::new()
+            .push_opcode(OP_DUP)
+            .push_opcode(OP_HASH160)
+            .push_slice([42; 20])
+            .push_opcode(OP_EQUAL)
+            .into_script()
+            .get_sigop_count(),
+        Ok(0)
+    );
+    assert_eq!(
+        Builder::new()
+            .push_opcode(OP_DUP)
+            .push_opcode(OP_HASH160)
+            .push_slice([42; 20])
+            .push_opcode(OP_EQUALVERIFY)
+            .push_opcode(OP_CHECKSIG)
+            .into_script()
+            .get_sigop_count(),
+        Ok(1)
+    );
+    assert_eq!(
+        Builder::new()
+            .push_opcode(OP_DUP)
+            .push_opcode(OP_HASH160)
+            .push_slice([42; 20])
+            .push_opcode(OP_EQUALVERIFY)
+            .push_opcode(OP_CHECKSIGVERIFY)
+            .push_opcode(OP_PUSHNUM_1)
+            .into_script()
+            .get_sigop_count(),
+        Ok(1)
+    );
+    let multi = Builder::new()
+        .push_opcode(OP_PUSHNUM_1)
+        .push_slice([3; 33])
+        .push_slice([3; 33])
+        .push_slice([3; 33])
+        .push_opcode(OP_PUSHNUM_3)
+        .push_opcode(OP_CHECKMULTISIG)
+        .into_script();
+    assert_eq!(multi.get_sigop_count(), Ok(3));
+    assert_eq!(multi.get_sigop_count_legacy(), Ok(20));
+    let multi_verify = Builder::new()
+        .push_opcode(OP_PUSHNUM_1)
+        .push_slice([3; 33])
+        .push_slice([3; 33])
+        .push_slice([3; 33])
+        .push_opcode(OP_PUSHNUM_3)
+        .push_opcode(OP_CHECKMULTISIGVERIFY)
+        .push_opcode(OP_PUSHNUM_1)
+        .into_script();
+    assert_eq!(multi_verify.get_sigop_count(), Ok(3));
+    assert_eq!(multi_verify.get_sigop_count_legacy(), Ok(20));
+    let multi_nopushnum_pushdata = Builder::new()
+        .push_opcode(OP_PUSHNUM_1)
+        .push_slice([3; 33])
+        .push_slice([3; 33])
+        .push_slice([3; 33])
+        .push_opcode(OP_CHECKMULTISIG)
+        .into_script();
+    assert_eq!(multi_nopushnum_pushdata.get_sigop_count(), Ok(20));
+    assert_eq!(multi_nopushnum_pushdata.get_sigop_count_legacy(), Ok(20));
+    let multi_nopushnum_op = Builder::new()
+        .push_opcode(OP_PUSHNUM_1)
+        .push_slice([3; 33])
+        .push_slice([3; 33])
+        .push_opcode(OP_DROP)
+        .push_opcode(OP_CHECKMULTISIG)
+        .into_script();
+    assert_eq!(multi_nopushnum_op.get_sigop_count(), Ok(20));
+    assert_eq!(multi_nopushnum_op.get_sigop_count_legacy(), Ok(20));
+}
+
 #[test]
 #[cfg(feature = "serde")]
 fn test_script_serde_human_and_not() {