task 14

Author: WeetHet

tasks/human_eval_014.rs

@@ -9,7 +9,54 @@ use vstd::prelude::*;
 
 verus! {
 
-// TODO: Put your solution (the specification, implementation, and proof) to the task here
+#[verifier::loop_isolation(false)]
+fn all_prefixes(s: &Vec<u8>) -> (prefixes: Vec<Vec<u8>>)
+    ensures
+        prefixes.len() == s.len(),
+        forall|i: int| #![auto] 0 <= i < s.len() ==> prefixes[i]@ == s@.subrange(0, i + 1),
+{
+    let mut prefixes: Vec<Vec<u8>> = vec![];
+    let mut prefix = vec![];
+    assert(forall|i: int|
+        #![auto]
+        0 <= i < prefix.len() ==> prefix@.index(i) == prefix@.subrange(
+            0,
+            prefix.len() as int,
+        ).index(i));
+
+    assert(prefix@ == prefix@.subrange(0, 0));
+    assert(forall|i: int|
+        #![auto]
+        0 <= i < prefix.len() ==> prefix@.index(i) == s@.subrange(0, prefix.len() as int).index(i));
+    assert(prefix@ == s@.subrange(0, 0));
+    for i in 0..s.len()
+        invariant
+            prefixes.len() == i,
+            prefix.len() == i,
+            forall|j: int| #![auto] 0 <= j < i ==> prefixes[j]@ == s@.subrange(0, j + 1),
+            prefix@ == s@.subrange(0, i as int),
+            prefix@ == prefix@.subrange(0, i as int),
+    {
+        let ghost pre_prefix = prefix;
+        prefix.push(s[i]);
+        assert(pre_prefix@.subrange(0, i as int) == pre_prefix@ && prefix@.subrange(0, i as int)
+            == pre_prefix@.subrange(0, i as int));
+        assert(prefix@.subrange(0, i as int) == s@.subrange(0, i as int));
+        assert(prefix[i as int] == s@.subrange(0, i + 1).index(i as int));
+
+        assert(forall|j: int|
+            #![auto]
+            0 <= j < i + 1 ==> prefix@.index(j) == prefix@.subrange(0, (i + 1) as int).index(j));
+        assert(prefix@ == prefix@.subrange(0, (i + 1) as int));
+        assert(forall|j: int|
+            #![auto]
+            0 <= j < i + 1 ==> prefix@.index(j) == s@.subrange(0, (i + 1) as int).index(j));
+        assert(prefix@ == s@.subrange(0, (i + 1) as int));
+
+        prefixes.push(prefix.clone());
+    }
+    return prefixes;
+}
 
 } // verus!
 fn main() {}