From ba314c1508ae0e7a29af55d9ddf1c636b1d0ec3a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=A9r=C3=B4me=20Rx?=
 <jerome.git@eventuallyconsultant.com>
Date: Tue, 10 Dec 2024 16:25:27 +0100
Subject: [PATCH] less allocations in parse_svcb_ip (#41)

---
 src/decoders/dns.rs | 62 ++++++++++++++++++++++-----------------------
 1 file changed, 31 insertions(+), 31 deletions(-)

diff --git a/src/decoders/dns.rs b/src/decoders/dns.rs
index a6b0334..7a02083 100644
--- a/src/decoders/dns.rs
+++ b/src/decoders/dns.rs
@@ -14,8 +14,8 @@ use byteorder::{BigEndian, WriteBytesExt};
 use log::{error, warn};
 use nom::{
     bytes::complete::take,
-    combinator::{fail, map, map_parser, verify},
-    multi::{fold_many0, many0},
+    combinator::{fail, iterator, map, map_parser, verify},
+    multi::fold_many0,
     number::complete::{be_u128, be_u16, be_u32, be_u8, le_u32},
     sequence::tuple,
     IResult,
@@ -227,46 +227,46 @@ fn parse_svcb_alpn(mut input: &[u8]) -> nom::IResult<&[u8], String> {
 }
 
 /// Parse the IPs
-fn parse_svcb_ip(data: &[u8]) -> nom::IResult<&[u8], String> {
+fn parse_svcb_ip(mut input: &[u8]) -> nom::IResult<&[u8], String> {
     const IPV4: u16 = 4;
     const IPV6: u16 = 6;
 
-    let mut dns_data = data;
+    let ipv4_parser = || map(be_u32, Ipv4Addr::from);
+    let ipv6_parser = || map(be_u128, Ipv6Addr::from);
 
-    let mut ipv4_addrs = vec![];
-    let mut ipv6_addrs = vec![];
+    let mut ipv4s = String::with_capacity(2 * 16); // let's reserve max space for 2 IPV4 addresses
+    let mut ipv6s = String::with_capacity(2 * 40); // let's reserve max space for 2 IPV6 addresses
 
     // IPs can either be IPv4 or/and IPv6
-    while !dns_data.is_empty() {
-        let (remaining_dns_data, (ip_version, ip_size)) =
-            tuple((verify(be_u16, |val| *val == IPV4 || *val == IPV6), be_u16))(dns_data)?;
-
-        let (remaining_dns_data, ip_data) = take(ip_size)(remaining_dns_data)?;
-
-        dns_data = remaining_dns_data;
+    while !input.is_empty() {
+        let (i, ip_version) = verify(be_u16, |val| *val == IPV4 || *val == IPV6)(input)?;
+        let (i, ip_size) = be_u16(i)?;
+        let (i, ip_data) = take(ip_size)(i)?;
+        input = i;
 
-        // There can be multiple IPs
-        match ip_version {
-            IPV4 => {
-                let (_, mut addrs) =
-                    many0(map(be_u32, |raw| Ipv4Addr::from(raw).to_string()))(ip_data)?;
-                ipv4_addrs.append(&mut addrs);
+        if ip_version == IPV4 {
+            let mut iter = iterator(ip_data, ipv4_parser());
+            for ip in iter.into_iter() {
+                if !ipv4s.is_empty() {
+                    ipv4s.push(',');
+                }
+                write!(ipv4s, "{}", ip).ok(); // ignore errors on write in String
             }
-            IPV6 => {
-                let (_, mut addrs) =
-                    many0(map(be_u128, |raw| Ipv6Addr::from(raw).to_string()))(ip_data)?;
-                ipv6_addrs.append(&mut addrs);
+            iter.finish()?;
+        } else if ip_version == IPV6 {
+            let mut iter = iterator(ip_data, ipv6_parser());
+            for ip in iter.into_iter() {
+                if !ipv6s.is_empty() {
+                    ipv6s.push(',');
+                }
+                write!(ipv6s, "{}", ip).ok(); // ignore errors on write in String
             }
-            _ => {}
-        };
+            iter.finish()?;
+        }
     }
 
-    let message = format!(
-        "ipv4 hint:{}, ipv6 hint:{}",
-        ipv4_addrs.join(","),
-        ipv6_addrs.join(",")
-    );
-    Ok((dns_data, message))
+    let message = format!("ipv4 hint:{ipv4s}, ipv6 hint:{ipv6s}");
+    Ok((input, message))
 }
 
 /// Get the MAC Address from the log data