1 files changed, 42 insertions, 0 deletions
diff --git a/src/hash.rs b/src/hash.rs
new file mode 100644
index 0000000..b8e5aea
--- /dev/null
+++ b/src/hash.rs
@@ -0,0 +1,42 @@
+use std::collections::hash_map;
+use std::fmt::{self, Display, Write};
+use std::hash::{Hash, Hasher};
+use syn::Ident;
+
+// 8-character symbol hash consisting of a-zA-Z0-9. We use 8 character because
+// Mach-O section specifiers are restricted to at most 16 characters (see
+// https://github.com/dtolnay/linkme/issues/35) and we leave room for a
+// linkme-specific prefix.
+pub(crate) struct Symbol(u64);
+
+pub(crate) fn hash(ident: &Ident) -> Symbol {
+    let mut hasher = hash_map::DefaultHasher::new();
+    ident.hash(&mut hasher);
+    Symbol(hasher.finish())
+}
+
+impl Display for Symbol {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        // log(62^8)/log(2) is 47.6 so we have enough bits in the 64-bit
+        // standard library hash to produce a good distribution over 8 digits
+        // from a 62-character alphabet.
+        let mut remainder = self.0;
+        for _ in 0..8 {
+            let digit = (remainder % 62) as u8;
+            remainder /= 62;
+            formatter.write_char(match digit {
+                0..=25 => b'a' + digit,
+                26..=51 => b'A' + digit - 26,
+                52..=61 => b'0' + digit - 52,
+                _ => unreachable!(),
+            } as char)?;
+        }
+        Ok(())
+    }
+}
+
+#[test]
+fn test_hash() {
+    let ident = Ident::new("EXAMPLE", proc_macro2::Span::call_site());
+    assert_eq!(hash(&ident).to_string(), "0GPSzIoo");
+}