From 8bff8dda04eb060f1748768e4949a782004407ed Mon Sep 17 00:00:00 2001
From: Hexa Dust <hxd@hexadust.net>
Date: Fri, 18 Jul 2025 21:19:30 +0100
Subject: [PATCH] strict stack op

---
 src/calc.rs | 104 ++++++++++++++++++++++++++++++++++++----------------
 src/lib.rs  |   4 +-
 2 files changed, 75 insertions(+), 33 deletions(-)

diff --git a/src/calc.rs b/src/calc.rs
index 0d49217..510e91d 100644
--- a/src/calc.rs
+++ b/src/calc.rs
@@ -1,46 +1,83 @@
+use core::{convert::Infallible, error::Error, fmt::Display};
+
 use num_traits::{PrimInt, Unsigned};
 
 use crate::{Decimal, DecimalError};
 
+#[derive(Debug)]
+pub enum StackCalcError {
+    StackOverflow,
+    StackUnderflow,
+    DecimalError(DecimalError),
+}
+
+impl Error for StackCalcError {}
+impl Display for StackCalcError {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            StackCalcError::StackOverflow => f.write_str("stack overflow"),
+            StackCalcError::StackUnderflow => f.write_str("stack underflow"),
+            StackCalcError::DecimalError(de) => de.fmt(f),
+        }
+    }
+}
+
 #[derive(Debug, Clone)]
-pub struct RPNCalc<const SS: usize = 2, const DS: usize = 5, E = u16> {
+pub struct StackCalc<const SS: usize = 2, const DS: usize = 5, E = u16> {
     stack: [Decimal<DS, E>; SS],
-    cur: usize,
+    index: usize,
 }
 
-impl<const SS: usize, const DS: usize, E: PrimInt> Default for RPNCalc<SS, DS, E> {
+impl<const SS: usize, const DS: usize, E: PrimInt> Default for StackCalc<SS, DS, E> {
     fn default() -> Self {
-        RPNCalc {
+        StackCalc {
             stack: [Decimal::default(); SS],
-            cur: 0,
+            index: 0,
         }
     }
 }
 
-impl<const SS: usize, const DS: usize, E: PrimInt + Unsigned> RPNCalc<SS, DS, E> {
-    pub fn enter(&mut self, val: Decimal<DS, E>) {
-        self.stack[self.cur] = val;
-        if self.cur == SS - 1 {
-            self.cur = 0;
-        } else {
-            self.cur += 1;
+impl From<DecimalError> for StackCalcError {
+    fn from(val: DecimalError) -> Self {
+        StackCalcError::DecimalError(val)
+    }
+}
+
+impl From<Infallible> for StackCalcError {
+    fn from(_: Infallible) -> Self {
+        unreachable!()
+    }
+}
+
+impl<const SS: usize, const DS: usize, E: PrimInt + Unsigned> StackCalc<SS, DS, E> {
+    pub fn push<V>(&mut self, val: V) -> Result<(), StackCalcError>
+    where
+        V: TryInto<Decimal<DS, E>>,
+        StackCalcError: From<<V as TryInto<Decimal<DS, E>>>::Error>,
+    {
+        if self.index == SS {
+            return Err(StackCalcError::StackOverflow);
         }
+        self.stack[self.index] = val.try_into()?;
+        self.index += 1;
+        Ok(())
     }
 
-    pub fn pop(&mut self) -> Decimal<DS, E> {
-        self.cur = if self.cur == 0 { SS - 1 } else { self.cur - 1 };
-        let ret = self.stack[self.cur];
-        self.stack[self.cur] = Decimal::default();
-        ret
+    pub fn pop(&mut self) -> Result<Decimal<DS, E>, StackCalcError> {
+        if self.index == 0 {
+            return Err(StackCalcError::StackUnderflow);
+        }
+        self.index -= 1;
+        Ok(self.stack[self.index])
     }
 
-    pub fn add(&mut self) -> Result<Decimal<DS, E>, DecimalError> {
-        let a = self.pop();
-        let b = self.pop();
+    pub fn add(&mut self) -> Result<Decimal<DS, E>, StackCalcError> {
+        let a = self.pop()?;
+        let b = self.pop()?;
         let a = f64::try_from(a)?;
         let b = f64::try_from(b)?;
         let res = Decimal::try_from(a + b)?;
-        self.enter(res);
+        self.push(res)?;
         Ok(res)
     }
 }
@@ -49,20 +86,25 @@ impl<const SS: usize, const DS: usize, E: PrimInt + Unsigned> RPNCalc<SS, DS, E>
 mod tests {
     use super::*;
     #[test]
-    fn test_add() -> Result<(), DecimalError> {
-        let mut calc = RPNCalc::<3, 5>::default();
-        calc.enter(Decimal::try_from(1337.42)?);
+    fn test_add() -> Result<(), StackCalcError> {
+        let mut calc = StackCalc::<3, 7>::default();
+        calc.push(Decimal::default())?;
+        calc.push(1337.42)?;
         assert_eq!(calc.add()?, Decimal::try_from(1337.42)?);
-        calc.enter(Decimal::try_from(1337.42)?);
+        calc.push(1337.42)?;
         assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0)?);
-        calc.enter(Decimal::try_from(2.0)?);
-        calc.enter(Decimal::try_from(3.0)?);
+        calc.push(2.0)?;
+        calc.push(3.0)?;
+        assert!(matches!(
+            calc.push(3.0).unwrap_err(),
+            StackCalcError::StackOverflow
+        ));
         assert_eq!(calc.add()?, Decimal::try_from(5.0)?);
         assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0 + 5.0)?);
-        assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0 + 5.0)?);
-        assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0 + 5.0)?);
-        assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0 + 5.0)?);
-        assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0 + 5.0)?);
+        assert!(matches!(
+            calc.add().unwrap_err(),
+            StackCalcError::StackUnderflow
+        ));
         Ok(())
     }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 1e3e90c..bf64f0a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -17,8 +17,8 @@ impl Error for DecimalError {}
 impl Display for DecimalError {
     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         match self {
-            DecimalError::ExponentOverflow => f.write_str("Exponent overflow"),
-            DecimalError::NotANumber => f.write_str("Not a number"),
+            DecimalError::ExponentOverflow => f.write_str("exponent overflow"),
+            DecimalError::NotANumber => f.write_str("not a number"),
         }
     }
 }