Rewrite matrix_solve to not contain `unwrap`

daemon/src/payout_curve/csr_tools.rs

@@ -1,5 +1,6 @@
 use crate::payout_curve::compat::{To1DArray, ToNAlgebraMatrix};
 use crate::payout_curve::Error;
+use itertools::Itertools;
 use ndarray::prelude::*;
 use std::ops::Mul;
 
@@ -49,29 +50,19 @@ impl CSR {
     // is horrible.
     pub fn matrix_solve(&self, b_arr: &Array2<f64>) -> Result<Array2<f64>, Error> {
         let a_arr = self.todense();
-        let ncols = b_arr.shape()[1];
-        let mut temp = (0..ncols)
-            .rev()
-            .map(|e| {
-                let b = b_arr.slice(s![.., e]).to_owned();
-
-                let sol = lu_solve(&a_arr, &b)?;
-                Ok(sol.to_vec())
-            })
-            .collect::<Result<Vec<_>, Error>>()?;
-
-        let nrows = temp[0].len();
-        let mut raveled = Vec::with_capacity(nrows * temp.len());
-
-        for _ in 0..nrows {
-            for vec in &mut temp {
-                raveled.push(vec.pop().unwrap());
-            }
-        }
-
-        raveled.reverse();
-
-        let result = Array2::<f64>::from_shape_vec((nrows, ncols), raveled)?.to_owned();
+        let zeros = Array2::zeros((b_arr.nrows(), 0));
+
+        let result = b_arr
+            .columns()
+            .into_iter()
+            .map(|b| lu_solve(&a_arr, &b.to_owned()))
+            .fold_ok(zeros, |mut result, column| {
+                result
+                    .push_column(column.view())
+                    .expect("shape was initialized correctly");
+
+                result
+            })?;
 
         Ok(result)
     }