From de95bf52cb560e8d7e23134257d2e3c851e38fed Mon Sep 17 00:00:00 2001
From: "Tobin C. Harding" <me@tobin.cc>
Date: Tue, 29 Aug 2023 09:01:25 +1000
Subject: [PATCH] Use checked_sub

Recently we "if" guarded subtraction manually using `> 0`, we can better
convey the meaning by using `checked_sub` and pattern match on the
option.

Refactor only, no logic changes.
---
 bitcoin/src/blockdata/witness.rs | 38 ++++++++++++++++++--------------
 1 file changed, 21 insertions(+), 17 deletions(-)

diff --git a/bitcoin/src/blockdata/witness.rs b/bitcoin/src/blockdata/witness.rs
index 889eb472..9d7102cf 100644
--- a/bitcoin/src/blockdata/witness.rs
+++ b/bitcoin/src/blockdata/witness.rs
@@ -68,26 +68,30 @@ fn fmt_debug(w: &Witness, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error>
     f.write_str("witnesses: [")?;
 
     let instructions = w.iter();
-    if instructions.len() > 0 {
-        let last_instruction = instructions.len() - 1;
-        for (i, instruction) in instructions.enumerate() {
-            let bytes = instruction.iter();
-            if bytes.len() > 0 {
-                let last_byte = bytes.len() - 1;
-                f.write_str("[")?;
-                for (j, byte) in bytes.enumerate() {
-                    write!(f, "{:#04x}", byte)?;
-                    f.write_str(comma_or_close(j, last_byte))?;
+    match instructions.len().checked_sub(1) {
+        Some(last_instruction) => {
+            for (i, instruction) in instructions.enumerate() {
+                let bytes = instruction.iter();
+                match bytes.len().checked_sub(1) {
+                    Some(last_byte) => {
+                        f.write_str("[")?;
+                        for (j, byte) in bytes.enumerate() {
+                            write!(f, "{:#04x}", byte)?;
+                            f.write_str(comma_or_close(j, last_byte))?;
+                        }
+                    }
+                    None => {
+                        // This is possible because the varint is not part of the instruction (see Iter).
+                        write!(f, "[]")?;
+                    }
                 }
-            } else {
-                // This is possible because the varint is not part of the instruction (see Iter).
-                write!(f, "[]")?;
+                f.write_str(comma_or_close(i, last_instruction))?;
             }
-            f.write_str(comma_or_close(i, last_instruction))?;
         }
-    } else {
-        // Witnesses can be empty because the 0x00 var int is not stored in content.
-        write!(f, "]")?;
+        None => {
+            // Witnesses can be empty because the 0x00 var int is not stored in content.
+            write!(f, "]")?;
+        }
     }
 
     f.write_str(" }")