Cross-reference FLOAT_E against e derived in Rust, read through e()

crates/float/src/constants.rs

@@ -0,0 +1,73 @@
+//! Cross-references the constants `LibDecimalFloat` packs against values
+//! derived here in integer arithmetic. The constants are read back through the
+//! compiled concrete's getters, so what is checked is the library's Solidity,
+//! not a copy of its digits.
+
+#[cfg(test)]
+mod tests {
+    use crate::Float;
+    use alloy::primitives::{aliases::I224, U512};
+
+    /// Decimal places the library packs the constants at.
+    const PLACES: u32 = 66;
+
+    /// Extra digits carried through the series so rounding at `PLACES` is
+    /// exact.
+    const GUARD: u32 = 12;
+
+    fn ten_pow(n: u32) -> U512 {
+        U512::from(10u64).pow(U512::from(n))
+    }
+
+    /// `e * 10^scale` by the series sum of 1/k!. Every intermediate is an
+    /// integer; the series stops once a term underflows the scale.
+    fn e_scaled(scale: u32) -> U512 {
+        let mut term = ten_pow(scale);
+        let mut sum = U512::ZERO;
+        let mut k = 1u64;
+        while !term.is_zero() {
+            sum += term;
+            term /= U512::from(k);
+            k += 1;
+        }
+        sum
+    }
+
+    /// Drops `GUARD` digits, rounding to nearest.
+    fn round_to_places(scaled: U512) -> U512 {
+        let divisor = ten_pow(GUARD);
+        let (q, r) = (scaled / divisor, scaled % divisor);
+        if r * U512::from(2u64) >= divisor {
+            q + U512::from(1u64)
+        } else {
+            q
+        }
+    }
+
+    fn coefficient(scaled: U512) -> I224 {
+        I224::from_dec_str(&round_to_places(scaled).to_string()).unwrap()
+    }
+
+    fn decimal_string(scaled: U512) -> String {
+        let digits = round_to_places(scaled).to_string();
+        let (int, frac) = digits.split_at(digits.len() - PLACES as usize);
+        format!("{int}.{frac}")
+    }
+
+    #[test]
+    fn e_is_e_rounded_to_nearest() {
+        let scaled = e_scaled(PLACES + GUARD);
+        let expected = Float::pack_lossless(coefficient(scaled), -(PLACES as i32)).unwrap();
+        let e = Float::e().unwrap();
+        assert_eq!(e.as_hex(), expected.as_hex());
+    }
+
+    #[test]
+    fn e_parses_from_its_digits() {
+        let s = decimal_string(e_scaled(PLACES + GUARD));
+        assert!(s.starts_with("2.71828182845904523536028747135266249"));
+        let parsed = Float::parse(s).unwrap();
+        let e = Float::e().unwrap();
+        assert!(e.eq(parsed).unwrap());
+    }
+}

crates/float/src/lib.rs

@@ -9,6 +9,7 @@ use wasm_bindgen_utils::prelude::*;
 #[cfg(test)]
 use alloy::primitives::aliases::I224;
 
+mod constants;
 pub mod error;
 mod evm;
 mod fuzz_ops;
@@ -500,6 +501,22 @@ impl Float {
         })
     }
 
+    /// Returns Euler's number as the library packs it, `LibDecimalFloat.FLOAT_E`,
+    /// read through the contract's `e()`.
+    ///
+    /// # Returns
+    ///
+    /// * `Ok(Float)` - The constant e.
+    /// * `Err(FloatError)` - If the EVM call fails.
+    pub fn e() -> Result<Self, FloatError> {
+        let calldata = DecimalFloat::eCall {}.abi_encode();
+
+        execute_call(Bytes::from(calldata), |output| {
+            let decoded = DecimalFloat::eCall::abi_decode_returns(output.as_ref())?;
+            Ok(Float(decoded))
+        })
+    }
+
     /// Returns the default minimum value for scientific notation formatting (1e-4).
     ///
     /// Values smaller than this (in absolute value) will be formatted in scientific notation.