perf(apis): optimize string padding function for large inputs

packages/apis/src/padding.rs

@@ -1,5 +1,36 @@
+/// Pads a string to a specified byte size with null bytes.
+///
+/// # Arguments
+///
+/// * `str` - The input string to pad.
+/// * `padded_bytes_size` - The target size in bytes after padding.
+///
+/// # Returns
+///
+/// A Vec<u8> containing the padded bytes.
+///
+/// # Performance Notes
+///
+/// This function is optimized for large strings by pre-allocating the exact
+/// capacity needed and avoiding unnecessary memory operations.
 pub fn pad_string(str: &str, padded_bytes_size: usize) -> Vec<u8> {
-    let mut padded_bytes = str.as_bytes().to_vec();
-    padded_bytes.append(&mut vec![0; padded_bytes_size - padded_bytes.len()]);
+    let str_bytes = str.as_bytes();
+    let str_len = str_bytes.len();
+
+    if padded_bytes_size <= str_len {
+        // No padding needed or string is already larger
+        return str_bytes.to_vec();
+    }
+
+    // Pre-allocate exact capacity to avoid reallocations
+    let mut padded_bytes = Vec::with_capacity(padded_bytes_size);
+
+    // Copy original string bytes
+    padded_bytes.extend_from_slice(str_bytes);
+
+    // Add padding zeros efficiently
+    let padding_size = padded_bytes_size - str_len;
+    padded_bytes.resize(padded_bytes_size, 0);
+
     padded_bytes
 }