countdown loops

compiler/src/statement/iteration/iteration.rs

@@ -53,7 +53,13 @@ impl<'a, F: PrimeField, G: GroupType<F>> ConstrainedProgram<'a, F, G> {
             .to_usize()
             .ok_or_else(|| StatementError::loop_index_const(&span))?;
 
-        for i in from..to {
+        let iter: Box<dyn Iterator<Item = usize>> = if from < to {
+            Box::new(from..to)
+        } else {
+            Box::new((to..from).rev())
+        };
+
+        for i in iter {
             // Store index in current function scope.
             // For loop scope is not implemented.
             let variable = statement.variable.borrow();

tests/compiler/statements/reverse_loop.leo

@@ -0,0 +1,19 @@
+/*
+namespace: Compile
+expectation: Pass
+input_file: inputs/dummy.in
+*/
+
+function main(k: bool) -> bool {
+    let reverse: u32 = 0;
+    for i in 10..0 {
+        reverse += i;
+    }
+
+    let forward: u32 = 0;
+    for x in 0..10 {
+        forward += x;
+    }
+
+    return reverse == forward && k;
+}

tests/expectations/compiler/compiler/statements/reverse_loop.leo.out

@@ -0,0 +1,21 @@
+---
+namespace: Compile
+expectation: Pass
+outputs:
+  - circuit:
+      num_public_variables: 0
+      num_private_variables: 1
+      num_constraints: 1
+      at: 042610d0fd1fe6d6ac112138f8755752f44c7d2a00f1b5960574d6da5cda393f
+      bt: e97756698880ab7555a959a5fb5c6b4e15bd64612aa677adbfe2d0bd91f0a83c
+      ct: cf1cbb66a638b4860a516671fb74850e6ccf787fe6c4c8d29e9c04efe880bd05
+    output:
+      - input_file: inputs/dummy.in
+        output:
+          registers:
+            r0:
+              type: bool
+              value: "true"
+    initial_ast: 2de2209f674d98d4c079ab820b1e154ab47cf45a0413a8b0643b322afb836518
+    canonicalized_ast: 780fd70bae5c39651481e3966332f3759fd4b80a03cabc812682aa0e5aebb0b7
+    type_inferenced_ast: 4256bce9f7fa10cd6265340993d77abc18eff6f71cb6eb13a0e064c310b75658