add const implementation of from_as_i32

Author: corwinkuiper

agb-fixnum/Cargo.toml

@@ -16,6 +16,7 @@ num-traits = { version = "0.2", default-features = false }
 serde = { version = "1", features = [
     "derive",
 ], default-features = false, optional = true }
+typewit = "1.14.2"
 
 [build-dependencies]
 quote = "1"

agb-fixnum/src/integer.rs

@@ -4,6 +4,7 @@ use core::{
 };
 
 use num_traits::Signed;
+use typewit::HasTypeWitness;
 
 /// A trait for integers that don't implement unary negation
 pub trait FixedWidthUnsignedInteger:
@@ -18,6 +19,7 @@ pub trait FixedWidthUnsignedInteger:
     + num_traits::Num
     + Not<Output = Self>
     + num_traits::AsPrimitive<usize>
+    + HasTypeWitness<IntegerWitness<Self>>
 {
     /// Converts an i32 to it's own representation, panics on failure
     fn from_as_i32(v: i32) -> Self;
@@ -81,3 +83,27 @@ fixed_width_unsigned_integer_impl!(u16, u32);
 
 fixed_width_unsigned_integer_impl!(i32, optimised_64_bit);
 fixed_width_unsigned_integer_impl!(u32, optimised_64_bit);
+
+#[doc(hidden)]
+/// A const implemnetation of the trait method `from_as_i32`.
+pub const fn from_as_i32<I: FixedWidthUnsignedInteger>(n: i32) -> I {
+    match HasTypeWitness::WITNESS {
+        IntegerWitness::U32(x) => x.to_left(n as u32),
+        IntegerWitness::I16(x) => x.to_left(n as i16),
+        IntegerWitness::I32(x) => x.to_left(n as i32),
+        IntegerWitness::U16(x) => x.to_left(n as u16),
+        IntegerWitness::I8(x) => x.to_left(n as i8),
+        IntegerWitness::U8(x) => x.to_left(n as u8),
+    }
+}
+
+typewit::simple_type_witness! {
+    enum IntegerWitness {
+        I32 = i32,
+        U32 = u32,
+        I16 = i16,
+        U16 = u16,
+        I8 = i8,
+        U8 = u8,
+    }
+}