Start working on factor graph.

src/factor_graph.rs

@@ -0,0 +1,54 @@
+use std::cmp;
+
+use gaussian::Gaussian;
+
+pub struct Variable {
+    gaussian: Gaussian,
+}
+
+impl Variable {
+    pub fn new() -> Variable {
+        Variable {
+            gaussian: Gaussian::new(0.0, 0.0),
+        }
+    }
+
+    fn delta(&self, other: &Variable) -> f32 {
+        let pi_delta = self.gaussian.pi - other.gaussian.pi;
+
+        if pi_delta.is_infinite() {
+            0.0
+        } else {
+            let tau_delta = (self.gaussian.tau - other.gaussian.tau).abs();
+
+            if pi_delta > tau_delta {
+               pi_delta
+            } else {
+                tau_delta
+            }
+        }
+    }
+}
+
+pub trait Factor {
+    fn down(&self) -> f32 {
+        0.0
+    }
+}
+
+pub struct PriorFactor {
+    variable: Variable,
+    dynamic: f32
+}
+
+impl PriorFactor {
+    pub fn new(variable: Variable, dynamic: f32) -> PriorFactor {
+        PriorFactor { variable, dynamic }
+    }
+}
+
+impl Factor for PriorFactor {
+    fn down(&self) -> f32 {
+        0.0
+    }
+}