Remaining builtin

crates/uplc/src/machine/runtime.rs

@@ -1702,7 +1702,42 @@ impl DefaultFunction {
 
                 Ok(Value::integer(hamming::weight(bytes).into()))
             }
-            DefaultFunction::FindFirstSetBit => todo!(),
+            DefaultFunction::FindFirstSetBit => {
+                let bytes = args[0].unwrap_byte_string()?;
+
+                let first_bit =
+                    bytes
+                        .into_iter()
+                        .rev()
+                        .enumerate()
+                        .find_map(|(byte_index, value)| {
+                            let value = value.reverse_bits();
+
+                            let first_bit: Option<usize> = if value > 128 {
+                                Some(0)
+                            } else if value > 64 {
+                                Some(1)
+                            } else if value > 32 {
+                                Some(2)
+                            } else if value > 16 {
+                                Some(3)
+                            } else if value > 8 {
+                                Some(4)
+                            } else if value > 4 {
+                                Some(5)
+                            } else if value > 2 {
+                                Some(6)
+                            } else if value > 1 {
+                                Some(7)
+                            } else {
+                                None
+                            };
+
+                            first_bit.map(|bit| isize::try_from(bit + byte_index * 8).unwrap())
+                        });
+
+                Ok(Value::integer(first_bit.unwrap_or(-1).into()))
+            }
             DefaultFunction::Ripemd_160 => {
                 use cryptoxide::{digest::Digest, ripemd160::Ripemd160};