Speed up StorePath::to_string()

1.81% -> 0.56%

nix-rust/src/c.rs

@@ -1,6 +1,7 @@
 use super::{
     error,
     foreign::{self, CBox},
+    store::path,
     store::StorePath,
     util,
 };
@@ -53,8 +54,12 @@ pub unsafe extern "C" fn ffi_StorePath_drop(self_: *mut StorePath) {
 }
 
 #[no_mangle]
-pub extern "C" fn ffi_StorePath_to_string(self_: &StorePath) -> String {
-    format!("{}", self_)
+pub extern "C" fn ffi_StorePath_to_string(self_: &StorePath) -> Vec<u8> {
+    let mut buf = vec![0; path::STORE_PATH_HASH_CHARS + 1 + self_.name.name().len()];
+    util::base32::encode_into(self_.hash.hash(), &mut buf[0..path::STORE_PATH_HASH_CHARS]);
+    buf[path::STORE_PATH_HASH_CHARS] = b'-';
+    buf[path::STORE_PATH_HASH_CHARS + 1..].clone_from_slice(self_.name.name().as_bytes());
+    buf
 }
 
 #[no_mangle]

nix-rust/src/store/path.rs

@@ -70,7 +70,9 @@ impl StorePathHash {
 
 impl fmt::Display for StorePathHash {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.write_str(&base32::encode(&self.0))
+        let mut buf = vec![0; STORE_PATH_HASH_CHARS];
+        base32::encode_into(&self.0, &mut buf);
+        f.write_str(std::str::from_utf8(&buf).unwrap())
     }
 }
 

nix-rust/src/util/base32.rs

@@ -25,27 +25,36 @@ lazy_static! {
 }
 
 pub fn encode(input: &[u8]) -> String {
-    let mut res = String::new();
-    res.reserve(encoded_len(input.len()));
+    let mut buf = vec![0; encoded_len(input.len())];
+    encode_into(input, &mut buf);
+    std::str::from_utf8(&buf).unwrap().to_string()
+}
+
+pub fn encode_into(input: &[u8], output: &mut [u8]) {
+    let len = encoded_len(input.len());
+    assert_eq!(len, output.len());
 
     let mut nr_bits_left: usize = 0;
     let mut bits_left: u16 = 0;
+    let mut pos = len;
 
     for b in input {
         bits_left |= (*b as u16) << nr_bits_left;
         nr_bits_left += 8;
         while nr_bits_left > 5 {
-            res.push(BASE32_CHARS[(bits_left & 0x1f) as usize] as char);
+            output[pos - 1] = BASE32_CHARS[(bits_left & 0x1f) as usize];
+            pos -= 1;
             bits_left >>= 5;
             nr_bits_left -= 5;
         }
     }
 
     if nr_bits_left > 0 {
-        res.push(BASE32_CHARS[(bits_left & 0x1f) as usize] as char);
+        output[pos - 1] = BASE32_CHARS[(bits_left & 0x1f) as usize];
+        pos -= 1;
     }
 
-    res.chars().rev().collect()
+    assert_eq!(pos, 0);
 }
 
 pub fn decode(input: &str) -> Result<Vec<u8>, crate::Error> {