feat: span pad end zeroes (#277)

* feat: span pad end zeroes

* rename fn

* docs: add cairodoc

crates/utils/src/helpers.cairo

@@ -206,6 +206,44 @@ impl ArrayExtension of ArrayExtensionTrait {
     }
 }
 
+#[generate_trait]
+impl SpanExtension of SpanExtensionTrait {
+    /// Pads a span of bytes with zeroes on the right.
+    ///
+    /// It creates a new `Array<u8>` instance and clones each element of the input span to it,
+    /// and then adds the required amount of zeroes.
+    ///
+    /// # Arguments
+    ///
+    /// * `self` - The `Span<u8>` instance to pad with zeroes.
+    /// * `n_zeroes` - The number of zeroes to add to the right of the span.
+    ///
+    /// # Returns
+    ///
+    /// A new `Span<u8>` instance which has a length equal to the length of the input
+    /// span plus the number of zeroes specified.
+    fn pad_right(self: Span<u8>, n_zeroes: usize) -> Span<u8> {
+        let mut res: Array<u8> = array![];
+        let mut i = 0;
+        loop {
+            if i == self.len() {
+                break;
+            }
+            res.append(*self[i]);
+            i += 1;
+        };
+        let mut i = 0;
+        loop {
+            if i == n_zeroes {
+                break ();
+            }
+            res.append(0);
+            i += 1;
+        };
+        res.span()
+    }
+}
+
 // Raise a number to a power.
 fn pow(base: felt252, exp: felt252) -> felt252 {
     if exp == 0 {

crates/utils/src/tests/test_helpers.cairo

@@ -1,4 +1,5 @@
 use utils::helpers;
+use utils::helpers::{SpanExtension, SpanExtensionTrait};
 use array::{ArrayTrait, SpanTrait};
 use debug::PrintTrait;
 
@@ -155,3 +156,10 @@ fn test_split_word() {
     };
 }
 
+#[test]
+#[available_gas(2000000000)]
+fn test_clone_pad_zeroes() {
+    let mut original: Span<u8> = array![1, 2, 3, 4].span();
+    let res = original.pad_right(3);
+    assert(res == array![1, 2, 3, 4, 0, 0, 0].span(), 'padding mismatch');
+}