1 files changed, 158 insertions, 0 deletions

diff --git a/reversible.rs b/reversible.rs
new file mode 100644
index 0000000..19e6f98
--- /dev/null
+++ b/reversible.rs
@@ -0,0 +1,158 @@
+use std::marker::PhantomData;
+
+pub trait Rev {
+    type Input;
+    type Output;
+
+    fn forward(&self, i: Self::Input) -> Self::Output;
+    fn reverse(&self, o: Self::Output) -> Self::Input;
+}
+
+#[derive(Clone)]
+pub struct Addition<A> {
+    amount: A
+}
+impl<A> Rev for Addition<A>
+where
+    A: std::ops::Add<Output = A> + std::ops::Sub<Output = A> + Copy
+{
+    type Input = A;
+    type Output = A;
+
+    fn forward(&self, a: A) -> A {
+        a + self.amount
+    }
+
+    fn reverse(&self, b: A) -> A {
+        b - self.amount
+    }
+}
+pub fn plus<A>(amount: A) -> Addition<A> {
+    Addition { amount }
+}
+
+// more blocks
+
+#[derive(Clone)]
+pub struct Negate<A>(PhantomData<A>);
+impl<A: std::ops::Neg<Output = A>> Rev for Negate<A> {
+    type Input = A;
+    type Output = A;
+
+    fn forward(&self, a: Self::Input) -> Self::Output {
+        -a
+    }
+
+    fn reverse(&self, b: Self::Output) -> Self::Input {
+        -b
+    }
+}
+pub fn neg<A>() -> Negate<A> {
+    Negate(PhantomData)
+}
+
+#[derive(Clone)]
+pub struct Multiplication<A> {
+    amount: A
+}
+impl<A> Rev for Multiplication<A>
+where
+    A: std::ops::Mul<Output = A> + std::ops::Div<Output = A> + Copy,
+{
+    type Input = A;
+    type Output = A;
+
+    fn forward(&self, a: A) -> A {
+        a * self.amount
+    }
+
+    fn reverse(&self, b: A) -> A {
+        b / self.amount
+    }
+}
+pub fn times<A>(amount: A) -> Multiplication<A> {
+    Multiplication { amount }
+}
+
+#[derive(Clone)]
+pub struct DiscreteMap<A, B>(Vec<(A, B)>);
+impl<A: PartialEq + Clone, B: PartialEq + Clone> Rev for DiscreteMap<A, B> {
+    type Input = A;
+    type Output = B;
+    
+    fn forward(&self, a: A) -> B {
+        for i in self.0.iter() {
+            if i.0 == a {
+                return i.1.clone()
+            }
+        }
+        panic!()
+    }
+
+    fn reverse(&self, b: B) -> A {
+        for i in self.0.iter() {
+            if i.1 == b {
+                return i.0.clone()
+            }
+        }
+        panic!()
+    }
+}
+pub fn discrete<A, B>(vec: Vec<(A, B)>) -> DiscreteMap<A, B> {
+    DiscreteMap(vec)
+}
+
+// combinators
+
+#[derive(Clone)]
+pub struct Pipe<X, Y>(X, Y);
+impl<X, Y> Rev for Pipe<X, Y>
+where
+    X: Rev,
+    Y: Rev<Input = X::Output>
+{
+    type Input = X::Input;
+    type Output = Y::Output;
+
+    fn forward(&self, a: Self::Input) -> Self::Output {
+        self.1.forward(self.0.forward(a))
+    }
+
+    fn reverse(&self, c: Self::Output) -> Self::Input {
+        self.0.reverse(self.1.reverse(c))
+    }
+}
+
+#[derive(Clone)]
+pub struct Inverse<X>(X);
+impl<X: Rev> Rev for Inverse<X> {
+    type Input = X::Output;
+    type Output = X::Input;
+
+    fn forward(&self, a: Self::Input) -> Self::Output {
+        self.0.reverse(a)
+    }
+
+    fn reverse(&self, b: Self::Output) -> Self::Input {
+        self.0.forward(b)
+    }
+}
+
+pub trait RevExt: Rev + Sized {
+    fn then<N: Rev<Input = Self::Output>>(self, other: N) -> Pipe<Self, N> {
+        Pipe(self, other)
+    }
+
+    fn inverse(self) -> Inverse<Self> {
+        Inverse(self)
+    }
+}
+impl<T: Rev> RevExt for T {}
+
+pub fn under<Op, Val>(op: Op, val: Val) -> impl Rev<Input = Op::Input, Output = Op::Input>
+where
+    Op: Rev + Clone,
+    Val: Rev<Input = Op::Output, Output = Op::Output>
+{
+    op.clone().then(val).then(op.inverse())
+}
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