From 4b53479153806e3a1e7a4944313f77dc262117b2 Mon Sep 17 00:00:00 2001
From: Ben Bridle <ben@derelict.engineering>
Date: Sat, 26 Apr 2025 09:09:59 +1200
Subject: Implement <tal> operator for expressions

The <tal> operator returns the number of set bits in the binary
representation of an integer. For negative numbers, only a single sign
bit is counted in the result.
---
 src/types/expression_stack.rs | 8 ++++++++
 src/types/operator.rs         | 3 +++
 2 files changed, 11 insertions(+)

diff --git a/src/types/expression_stack.rs b/src/types/expression_stack.rs
index d14d808..8e1c87d 100644
--- a/src/types/expression_stack.rs
+++ b/src/types/expression_stack.rs
@@ -62,6 +62,7 @@ impl ExpressionStack {
             Operator::BitXor           => { pop!(b); pop!(a); push!(a ^ b) },
             Operator::BitNot           => {          pop!(a); push!(!a) },
             Operator::Length           => {          pop!(a); push!(width(a) as isize) },
+            Operator::Tally            => {          pop!(a); push!(tally(a) as isize) },
         }
         return Ok(());
     }
@@ -76,6 +77,13 @@ pub fn width(value: isize) -> u32 {
     }
 }
 
+pub fn tally(value: isize) -> u32 {
+    let width = width(value);
+    let mask = 2i32.pow(width) -1;
+    let value = (value as usize) & (mask as usize);
+    return value.count_ones();
+}
+
 pub enum StackError {
     Underflow,
     MultipleReturnValues,
diff --git a/src/types/operator.rs b/src/types/operator.rs
index af607e3..d5b5e94 100644
--- a/src/types/operator.rs
+++ b/src/types/operator.rs
@@ -19,6 +19,7 @@ pub enum Operator {
     BitXor,
     BitNot,
     Length,
+    Tally,
 }
 
 impl Operator {
@@ -57,6 +58,7 @@ impl Operator {
             "<xor>" => Some(Operator::BitXor),
             "<not>" => Some(Operator::BitNot),
             "<len>" => Some(Operator::Length),
+            "<tal>" => Some(Operator::Tally),
             _      => None,
         }
     }
@@ -84,6 +86,7 @@ impl std::fmt::Display for Operator {
             Operator::BitXor           => "<xor>",
             Operator::BitNot           => "<not>",
             Operator::Length           => "<len>",
+            Operator::Tally            => "<tal>",
         };
         write!(f, "{string}")
     }
-- 
cgit v1.2.3-70-g09d2