From 9a9fd5c936c174e13164895f85f06b4081db6fb3 Mon Sep 17 00:00:00 2001
From: Artem Vorotnikov <artem@vorotnikov.me>
Date: Mon, 21 Feb 2022 23:57:18 +0300
Subject: [PATCH] Serde support w/o impl-serde

---
 Cargo.toml         |  5 +--
 src/impls/serde.rs | 77 +++++++++++++++++++++++++++++++++++++++-------
 src/lib.rs         |  2 +-
 3 files changed, 70 insertions(+), 14 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index b6a5c06..1f6aa41 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -16,13 +16,14 @@ categories = ["cryptography::cryptocurrencies", "mathematics", "no-std"]
 members = ["bench", "fuzz", "intrinsics"]
 
 [features]
+alloc = []
 llvm-intrinsics = ["ethnum-intrinsics"]
-serde = ["impl-serde", "crate-serde"]
+serde = ["crate-serde"]
 scale = ["parity-scale-codec"]
+std = ["alloc"]
 
 [dependencies]
 ethnum-intrinsics = { version = "1", path = "intrinsics", optional = true }
-impl-serde = { version = "0.3", default-features = false, optional = true }
 rlp = { version = "0.5", optional = true }
 crate-serde = { package = "serde", version = "1", optional = true }
 parity-scale-codec = { version = "3", features = [
diff --git a/src/impls/serde.rs b/src/impls/serde.rs
index 0431858..1c6284a 100644
--- a/src/impls/serde.rs
+++ b/src/impls/serde.rs
@@ -1,6 +1,8 @@
 use crate::U256;
+use alloc::string::String;
+use core::{fmt, str::FromStr};
 use crate_serde::{
-    de::{Deserialize, Deserializer},
+    de::{self, Deserialize, Deserializer},
     ser::{Serialize, Serializer},
 };
 
@@ -9,9 +11,43 @@ impl Serialize for U256 {
     where
         S: Serializer,
     {
+        static CHARS: &[u8] = b"0123456789abcdef";
+
+        fn to_hex_raw<'a>(v: &'a mut [u8], bytes: &[u8]) -> &'a str {
+            assert!(v.len() > 1 + bytes.len() * 2);
+
+            v[0] = b'0';
+            v[1] = b'x';
+
+            let mut idx = 2;
+            let first_nibble = bytes[0] >> 4;
+            if first_nibble != 0 {
+                v[idx] = CHARS[first_nibble as usize];
+                idx += 1;
+            }
+            v[idx] = CHARS[(bytes[0] & 0xf) as usize];
+            idx += 1;
+
+            for &byte in bytes.iter().skip(1) {
+                v[idx] = CHARS[(byte >> 4) as usize];
+                v[idx + 1] = CHARS[(byte & 0xf) as usize];
+                idx += 2;
+            }
+
+            // SAFETY: all characters come either from CHARS or "0x", therefore valid UTF8
+            unsafe { core::str::from_utf8_unchecked(&v[0..idx]) }
+        }
+
         let mut slice = [0u8; 66];
         let bytes = self.to_be_bytes();
-        impl_serde::serialize::serialize_uint(&mut slice, &bytes, serializer)
+
+        let non_zero = bytes.iter().take_while(|&&b| b == 0).count();
+        let bytes = &bytes[non_zero..];
+        if bytes.is_empty() {
+            serializer.serialize_str("0x0")
+        } else {
+            serializer.serialize_str(to_hex_raw(&mut slice, bytes))
+        }
     }
 }
 
@@ -20,15 +56,30 @@ impl<'de> Deserialize<'de> for U256 {
     where
         D: Deserializer<'de>,
     {
-        let mut bytes = [0u8; 32];
-        let wrote = impl_serde::serialize::deserialize_check_len(
-            deserializer,
-            impl_serde::serialize::ExpectedLen::Between(0, &mut bytes),
-        )?;
-        let mut padded = [0u8; 32];
-        padded[32 - wrote..].copy_from_slice(&bytes[..wrote]);
-
-        Ok(U256::from_be_bytes(padded))
+        struct Visitor;
+
+        impl<'a> de::Visitor<'a> for Visitor {
+            type Value = U256;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                write!(formatter, "a (both 0x-prefixed or not) hex string",)
+            }
+
+            fn visit_str<E: de::Error>(self, v: &str) -> Result<Self::Value, E> {
+                Ok(if let Some(v) = v.strip_prefix("0x") {
+                    U256::from_str_radix(v, 16).map_err(de::Error::custom)?
+                } else {
+                    U256::from_str(v).map_err(de::Error::custom)?
+                })
+            }
+
+            #[cfg(feature = "alloc")]
+            fn visit_string<E: de::Error>(self, v: String) -> Result<Self::Value, E> {
+                self.visit_str(&v)
+            }
+        }
+
+        deserializer.deserialize_str(Visitor)
     }
 }
 
@@ -46,5 +97,9 @@ mod tests {
             serde_json::from_str::<U256>("\"0x12345\"").unwrap(),
             U256::new(0x12345),
         );
+        assert_eq!(
+            serde_json::from_str::<U256>("\"12345\"").unwrap(),
+            U256::new(12345),
+        );
     }
 }
diff --git a/src/lib.rs b/src/lib.rs
index ba6c9fd..5c37b93 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,7 +7,7 @@
 #![deny(missing_docs)]
 #![no_std]
 
-#[cfg(test)]
+#[cfg(any(test, feature = "alloc"))]
 extern crate alloc;
 
 #[macro_use]