From 164c5df92b6c14907dd4c1a82a911f509a11ae19 Mon Sep 17 00:00:00 2001
From: Hexa Dust <hxd@hexadust.net>
Date: Fri, 18 Jul 2025 21:19:30 +0100
Subject: [PATCH] display

---
 Cargo.lock |  7 +++++
 Cargo.toml |  3 ++
 src/lib.rs | 80 ++++++++++++++++++++++++++++++++++++++++++++++--------
 3 files changed, 78 insertions(+), 12 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index dd6d115..42484f5 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2,6 +2,12 @@
 # It is not intended for manual editing.
 version = 4
 
+[[package]]
+name = "arrayvec"
+version = "0.7.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7c02d123df017efcdfbd739ef81735b36c5ba83ec3c59c80a9d7ecc718f92e50"
+
 [[package]]
 name = "autocfg"
 version = "1.4.0"
@@ -12,6 +18,7 @@ checksum = "ace50bade8e6234aa140d9a2f552bbee1db4d353f69b8217bc503490fc1a9f26"
 name = "calc-math"
 version = "0.1.0"
 dependencies = [
+ "arrayvec",
  "num-traits",
 ]
 
diff --git a/Cargo.toml b/Cargo.toml
index 00e1e29..cabf607 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -5,3 +5,6 @@ edition = "2021"
 
 [dependencies]
 num-traits = { version = "0.2.19", default-features = false }
+
+[dev-dependencies]
+arrayvec = { version = "0.7.6", default-features = false }
diff --git a/src/lib.rs b/src/lib.rs
index f6364c5..c8262af 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,10 +1,26 @@
-// #![no_std]
+#![no_std]
 
 use core::{error::Error, fmt::Display};
 #[cfg(not(test))]
 use num_traits::float::FloatCore;
 use num_traits::{one, zero, PrimInt, Unsigned};
 
+#[derive(Debug)]
+pub enum DecimalError {
+    ExponentOverflow,
+    NotANumber,
+}
+
+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"),
+        }
+    }
+}
+
 #[derive(Debug, Clone, Copy)]
 pub struct Decimal<const S: usize = 5, E = u16> {
     pub minus: bool,
@@ -24,19 +40,22 @@ impl<const S: usize, E: PrimInt> Default for Decimal<S, E> {
     }
 }
 
-#[derive(Debug)]
-pub enum DecimalError {
-    ExponentOverflow,
-    NotANumber,
-}
-
-impl Error for DecimalError {}
-impl Display for DecimalError {
+impl<const S: usize, E: Display> Display for Decimal<S, E> {
     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"),
+        if self.minus {
+            write!(f, "-")?;
         }
+        for i in 0..S {
+            write!(f, "{}", self.significant[i])?;
+            if i == 0 {
+                write!(f, ".")?;
+            }
+        }
+        write!(f, "e")?;
+        if self.minus_exponent {
+            write!(f, "-")?;
+        }
+        write!(f, "{}", self.exponent)
     }
 }
 
@@ -120,6 +139,7 @@ impl<const S: usize, E: PrimInt + Unsigned> TryFrom<Decimal<S, E>> for f64 {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use arrayvec::ArrayString;
 
     #[test]
     fn float_to_decimal_zero() {
@@ -259,4 +279,40 @@ mod tests {
         let dec = Decimal::<16>::try_from(f64::MAX).unwrap();
         assert_eq!(f64::try_from(dec).unwrap(), f64::MAX);
     }
+
+    #[test]
+    fn decimal_display_zero() {
+        use core::fmt::Write;
+        let dec = Decimal::<7>::try_from(0.0).unwrap();
+        let mut str: ArrayString<100> = ArrayString::new();
+        write!(str, "{}", dec).unwrap();
+        assert_eq!("0.000000e0", str.as_str());
+    }
+
+    #[test]
+    fn decimal_display_pos_big() {
+        use core::fmt::Write;
+        let dec = Decimal::<7>::try_from(1337.42).unwrap();
+        let mut str: ArrayString<100> = ArrayString::new();
+        write!(str, "{}", dec).unwrap();
+        assert_eq!("1.337420e3", str.as_str());
+    }
+
+    #[test]
+    fn decimal_display_neg_big() {
+        use core::fmt::Write;
+        let dec = Decimal::<7>::try_from(-1337.42).unwrap();
+        let mut str: ArrayString<100> = ArrayString::new();
+        write!(str, "{}", dec).unwrap();
+        assert_eq!("-1.337420e3", str.as_str());
+    }
+
+    #[test]
+    fn decimal_display_neg_small() {
+        use core::fmt::Write;
+        let dec = Decimal::<7>::try_from(-0.0133742).unwrap();
+        let mut str: ArrayString<100> = ArrayString::new();
+        write!(str, "{}", dec).unwrap();
+        assert_eq!("-1.337420e-2", str.as_str());
+    }
 }