From 373c512397dde9c767ac65453780931a84704ce4 Mon Sep 17 00:00:00 2001
From: Ben Bridle <ben@derelict.engineering>
Date: Fri, 18 Apr 2025 20:34:39 +1200
Subject: Implement <len> operator for expressions

The <len> operator returns the width of an integer in bits, using the
same calculation as for packing an integer into a bit field.
---
 src/stages/bytecode.rs        | 6 +-----
 src/types/expression_stack.rs | 9 +++++++++
 src/types/operator.rs         | 3 +++
 3 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/src/stages/bytecode.rs b/src/stages/bytecode.rs
index e4464a1..1d27b28 100644
--- a/src/stages/bytecode.rs
+++ b/src/stages/bytecode.rs
@@ -162,11 +162,7 @@ impl BytecodeParser {
                     *value
                 }
             };
-            let value_width = match field_value.cmp(&0) {
-                std::cmp::Ordering::Less => (-field_value).ilog2() + 2,
-                std::cmp::Ordering::Equal => 0,
-                std::cmp::Ordering::Greater => field_value.ilog2() + 1,
-            };
+            let value_width = width(field_value);
             if field.width < value_width {
                 let error = BytecodeError::ValueTooWide(field.width, value_width);
                 self.errors.push(Tracked::from(error, field_source.clone()));
diff --git a/src/types/expression_stack.rs b/src/types/expression_stack.rs
index 4d26eb2..d14d808 100644
--- a/src/types/expression_stack.rs
+++ b/src/types/expression_stack.rs
@@ -61,11 +61,20 @@ impl ExpressionStack {
             Operator::BitOr            => { pop!(b); pop!(a); push!(a | b) },
             Operator::BitXor           => { pop!(b); pop!(a); push!(a ^ b) },
             Operator::BitNot           => {          pop!(a); push!(!a) },
+            Operator::Length           => {          pop!(a); push!(width(a) as isize) },
         }
         return Ok(());
     }
 }
 
+/// Find the number of bits required to hold an integer.
+pub fn width(value: isize) -> u32 {
+    match value.cmp(&0) {
+        std::cmp::Ordering::Less => (-value).ilog2() + 2,
+        std::cmp::Ordering::Equal => 0,
+        std::cmp::Ordering::Greater => value.ilog2() + 1,
+    }
+}
 
 pub enum StackError {
     Underflow,
diff --git a/src/types/operator.rs b/src/types/operator.rs
index a7e7b9b..af607e3 100644
--- a/src/types/operator.rs
+++ b/src/types/operator.rs
@@ -18,6 +18,7 @@ pub enum Operator {
     BitOr,
     BitXor,
     BitNot,
+    Length,
 }
 
 impl Operator {
@@ -55,6 +56,7 @@ impl Operator {
             "<or>"  => Some(Operator::BitOr),
             "<xor>" => Some(Operator::BitXor),
             "<not>" => Some(Operator::BitNot),
+            "<len>" => Some(Operator::Length),
             _      => None,
         }
     }
@@ -81,6 +83,7 @@ impl std::fmt::Display for Operator {
             Operator::BitOr            => "<or>",
             Operator::BitXor           => "<xor>",
             Operator::BitNot           => "<not>",
+            Operator::Length           => "<len>",
         };
         write!(f, "{string}")
     }
-- 
cgit v1.2.3-70-g09d2