first steps

.gitignore

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+/target

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "f8"
+version = "0.1.0"
+dependencies = [
+ "static_assertions",
+]
+
+[[package]]
+name = "static_assertions"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a2eb9349b6444b326872e140eb1cf5e7c522154d69e7a0ffb0fb81c06b37543f"

Cargo.toml

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+[package]
+name = "f8"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+static_assertions = "1.1.0"

src/lib.rs

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+pub const M_BITS: u8 = 5;
+pub const E_BITS: u8 = 3;
+const E_MAX: u8 = 4;
+
+static_assertions::const_assert_eq!(M_BITS + E_BITS, 8);
+
+const M_STORAGE_MAX: u8 = (1 << M_BITS) - 1;
+const E_STORAGE_MAX: u8 = (1 << E_BITS) - 1;
+const M_BIAS: u8 = 1 << M_BITS;
+const E_BIAS: u8 = E_STORAGE_MAX - E_MAX;
+const M_MASK: u8 = M_STORAGE_MAX << E_BITS;
+const E_MASK: u8 = E_STORAGE_MAX;
+
+#[repr(transparent)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct F8(u8);
+
+impl F8 {
+	/// Split self into the mantissa and exponent, as stored.
+	fn split(self) -> (u8, u8) {
+		(self.0 >> E_BITS, self.0 & E_MASK)
+	}
+
+	/// Split self into integers (m, e) such that `self == m * 2.pow(e)`.
+	fn split_unbias(self) -> (u8, i8) {
+		let (m, e) = self.split();
+		(m | M_BIAS, e as i8 - (E_BIAS + M_BITS) as i8)
+	}
+
+	fn merge(m: u8, e: u8) -> Self {
+		assert!(m <= M_STORAGE_MAX);
+		assert!(e <= E_STORAGE_MAX);
+		Self((m << E_BITS) | e)
+	}
+}
+
+impl From<u8> for F8 {
+	fn from(value: u8) -> Self {
+		todo!()
+	}
+}
+
+impl From<F8> for u8 {
+	fn from(value: F8) -> Self {
+		if value.0 == 0 {
+			return 0;
+		}
+		let (m, e) = value.split_unbias();
+		if e >= 0 { m << e } else { m >> -e }
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn test_int_conv() {
+		assert_eq!(u8::from(F8(0)), 0);
+		for e in 0..E_MAX {
+			assert_eq!(u8::from(F8(e + E_BIAS)), 1 << e, "e={e}");
+		}
+	}
+}