--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/resolv_addr.rs	Fri Nov 27 15:05:19 2020 +0100
@@ -0,0 +1,75 @@
+use thiserror::Error;
+use std::fmt;
+
+#[derive(Debug, PartialEq)]
+pub(crate) struct ResolvAddr(String);
+
+impl ResolvAddr {
+    pub(crate) fn as_str(&self) -> &str {
+        &self.0
+    }
+
+    pub(crate) fn try_from_generic<T: std::ops::Deref<Target=str> + Into<String>>(string: T) -> Result<Self, ResolvAddrError> {
+        // can't use a combinator due to borrowing
+        let colon = match string.rfind(':') {
+            Some(colon) => colon,
+            None => return Err(ResolvAddrError::MissingPort(string.into())),
+        };
+
+        let (hostname, port) = string.split_at(colon);
+
+        if let Err(error) = port[1..].parse::<u16>() {
+            return Err(ResolvAddrError::InvalidPort { string: string.into(), error, });
+        }
+
+        let len = hostname.len();
+        if len > 253 {
+            return Err(ResolvAddrError::TooLong { string: string.into(), len, } )
+        }
+
+        let mut label_start = 0usize;
+
+        for (i, c) in hostname.chars().enumerate() {
+            match c {
+                '.' => {
+                    if i - label_start == 0 {
+                        return Err(ResolvAddrError::EmptyLabel { string: string.into(), label_start, });
+                    }
+
+                    label_start = i + 1;
+                },
+                'a'..='z' | 'A'..='Z' | '0'..='9' | '-' => (),
+                _ => return Err(ResolvAddrError::InvalidCharacter { string: string.into(), c, pos: i, }),
+            }
+
+            if i - label_start > 63 {
+                return Err(ResolvAddrError::LongLabel { string: string.into(), label_start, label_end: i, });
+            }
+        }
+
+        Ok(ResolvAddr(string.into()))
+    }
+}
+
+impl fmt::Display for ResolvAddr {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Display::fmt(&self.0, f)
+    }
+}
+
+
+#[derive(Debug, Error)]
+pub(crate) enum ResolvAddrError {
+    #[error("hostname {string} has {len} character which exceeds the limit of 253")]
+    TooLong { string: String, len: usize },
+    #[error("invalid character {c} in hostname {string} at position {pos}")]
+    InvalidCharacter { string: String, pos: usize, c: char, },
+    #[error("hostname {string} contains a label {} at position {label_start} which is {} characters long - more than the limit 63", &string[(*label_start)..(*label_end)], label_end - label_start)]
+    LongLabel { string: String, label_start: usize, label_end: usize, },
+    #[error("hostname {string} contains an empty label at position {label_start}")]
+    EmptyLabel { string: String, label_start: usize, },
+    #[error("the address {0} is missing a port")]
+    MissingPort(String),
+    #[error("failed to parse port numer in the address {string}")]
+    InvalidPort { string: String, error: std::num::ParseIntError, },
+}