Refactor out sort algorithm

The `threshold` function is a candidate for refactoring because it
contains code at multiple layers of abstraction as well as duplicate
code.

Refactor out the sorting closures into three, very similar, functions.

Refactor only, no logic changes.

Author: tcharding

src/miniscript/types/extra_props.rs

@@ -809,13 +809,8 @@ impl Property for ExtData {
             );
         }
 
-        // We sort by [satisfaction cost - dissatisfaction cost] to make a worst-case (the most
-        // costy satisfaction are satisfied, the most costy dissatisfactions are dissatisfied)
-        // sum of the cost by iterating through the sorted vector *backward*.
-        stack_elem_count_sat_vec.sort_by(|a, b| {
-            a.0.map(|x| a.1.map(|y| x as isize - y as isize))
-                .cmp(&b.0.map(|x| b.1.map(|y| x as isize - y as isize)))
-        });
+        stack_elem_count_sat_vec.sort_by(sat_minus_option_dissat);
+        // Sum of the cost by iterating through the sorted vector *backward*.
         for (i, &(x, y)) in stack_elem_count_sat_vec.iter().rev().enumerate() {
             stack_elem_count_sat = if i <= k {
                 x.and_then(|x| stack_elem_count_sat.map(|count| count + x))
@@ -824,11 +819,7 @@ impl Property for ExtData {
             };
         }
 
-        // Same logic as above
-        exec_stack_elem_count_sat_vec.sort_by(|a, b| {
-            a.0.map(|x| a.1.map(|y| x as isize - y as isize))
-                .cmp(&b.0.map(|x| b.1.map(|y| x as isize - y as isize)))
-        });
+        exec_stack_elem_count_sat_vec.sort_by(sat_minus_option_dissat);
         for (i, &(x, y)) in exec_stack_elem_count_sat_vec.iter().rev().enumerate() {
             exec_stack_elem_count_sat = if i <= k {
                 opt_max(exec_stack_elem_count_sat, x)
@@ -837,14 +828,8 @@ impl Property for ExtData {
             };
         }
 
-        // Same for the size cost. A bit more intricated as we need to account for both the witness
-        // and scriptSig cost, so we end up with a tuple of Options of tuples. We use the witness
-        // cost (first element of the mentioned tuple) here.
         // FIXME: Maybe make the ExtData struct aware of Ctx and add a one_cost() method here ?
-        max_sat_size_vec.sort_by(|a, b| {
-            a.0.map(|x| a.1.map(|y| x.0 as isize - y.0 as isize))
-                .cmp(&b.0.map(|x| b.1.map(|y| x.0 as isize - y.0 as isize)))
-        });
+        max_sat_size_vec.sort_by(sat_minus_dissat_witness);
         for (i, &(x, y)) in max_sat_size_vec.iter().enumerate() {
             max_sat_size = if i <= k {
                 x.and_then(|x| max_sat_size.map(|(w, s)| (w + x.0, s + x.1)))
@@ -853,10 +838,7 @@ impl Property for ExtData {
             };
         }
 
-        ops_count_sat_vec.sort_by(|a, b| {
-            a.0.map(|x| x as isize - a.1 as isize)
-                .cmp(&b.0.map(|x| x as isize - b.1 as isize))
-        });
+        ops_count_sat_vec.sort_by(sat_minus_dissat);
         for (i, &(x, y)) in ops_count_sat_vec.iter().enumerate() {
             op_count_sat = if i <= k {
                 opt_add(op_count_sat, x)
@@ -1021,6 +1003,46 @@ impl Property for ExtData {
     }
 }
 
+// Function to pass to sort_by. Sort by (satisfaction cost - dissatisfaction cost).
+//
+// We sort by (satisfaction cost - dissatisfaction cost) to make a worst-case (the most
+// costy satisfactions are satisfied, the most costy dissatisfactions are dissatisfied).
+//
+// Args are of form: (<count_sat>, <count_dissat>)
+fn sat_minus_dissat<'r, 's>(
+    a: &'r (Option<usize>, usize),
+    b: &'s (Option<usize>, usize),
+) -> std::cmp::Ordering {
+    a.0.map(|x| x as isize - a.1 as isize)
+        .cmp(&b.0.map(|x| x as isize - b.1 as isize))
+}
+
+// Function to pass to sort_by. Sort by (satisfaction cost - dissatisfaction cost).
+//
+// We sort by (satisfaction cost - dissatisfaction cost) to make a worst-case (the most
+// costy satisfactions are satisfied, the most costy dissatisfactions are dissatisfied).
+//
+// Args are of form: (<count_sat>, <count_dissat>)
+fn sat_minus_option_dissat<'r, 's>(
+    a: &'r (Option<usize>, Option<usize>),
+    b: &'s (Option<usize>, Option<usize>),
+) -> std::cmp::Ordering {
+    a.0.map(|x| a.1.map(|y| x as isize - y as isize))
+        .cmp(&b.0.map(|x| b.1.map(|y| x as isize - y as isize)))
+}
+
+// Function to pass to sort_by. Sort by (satisfaction cost - dissatisfaction cost) of cost of witness.
+//
+// Args are of form: (<max_sat_size>, <count_dissat_size>)
+// max_[dis]sat_size of form: (<cost_of_witness>, <cost_of_sciptsig>)
+fn sat_minus_dissat_witness<'r, 's>(
+    a: &'r (Option<(usize, usize)>, Option<(usize, usize)>),
+    b: &'s (Option<(usize, usize)>, Option<(usize, usize)>),
+) -> std::cmp::Ordering {
+    a.0.map(|x| a.1.map(|y| x.0 as isize - y.0 as isize))
+        .cmp(&b.0.map(|x| b.1.map(|y| x.0 as isize - y.0 as isize)))
+}
+
 // Returns Some(max(x,y)) is both x and y are Some. Otherwise, return none
 fn opt_max<T: Ord>(a: Option<T>, b: Option<T>) -> Option<T> {
     if let (Some(x), Some(y)) = (a, b) {