1 files changed, 166 insertions, 0 deletions
diff --git a/tvix/nix-compat/src/nar/wire/tag.rs b/tvix/nix-compat/src/nar/wire/tag.rs
new file mode 100644
index 0000000000..4982a0d707
--- /dev/null
+++ b/tvix/nix-compat/src/nar/wire/tag.rs
@@ -0,0 +1,166 @@
+/// A type implementing Tag represents a static hash set of byte strings,
+/// with a very simple perfect hash function: every element has a unique
+/// discriminant at a common byte offset. The values of the type represent
+/// the members by this single discriminant byte; they are indices into the
+/// hash set.
+pub trait Tag: Sized {
+    /// Discriminant offset
+    const OFF: usize;
+    /// Minimum variant length
+    const MIN: usize;
+
+    /// Minimal suitably sized buffer for reading the wire representation
+    ///
+    /// HACK: This is a workaround for const generics limitations.
+    type Buf: AsMut<[u8]> + Send;
+
+    /// Make an instance of [Self::Buf]
+    fn make_buf() -> Self::Buf;
+
+    /// Convert a discriminant into the corresponding variant
+    fn from_u8(x: u8) -> Option<Self>;
+
+    /// Convert a variant back into the wire representation
+    fn as_bytes(&self) -> &'static [u8];
+}
+
+/// Generate an enum implementing [Tag], enforcing at compile time that
+/// the discriminant values are distinct.
+macro_rules! make {
+    (
+        $(
+            $(#[doc = $doc:expr])*
+            $vis:vis enum $Enum:ident[$off:expr] {
+                $(
+                    $(#[doc = $var_doc:expr])*
+                    $Var:ident = $TOK:ident,
+                )+
+            }
+        )*
+    ) => {
+        $(
+            $(#[doc = $doc])*
+            #[derive(Debug, PartialEq, Eq)]
+            #[repr(u8)]
+            $vis enum $Enum {
+                $(
+                    $(#[doc = $var_doc])*
+                    $Var = $TOK[$Enum::OFF]
+                ),+
+            }
+
+            impl Tag for $Enum {
+                /// Discriminant offset
+                const OFF: usize = $off;
+                /// Minimum variant length
+                const MIN: usize = tag::min_of(&[$($TOK.len()),+]);
+
+                /// Minimal suitably sized buffer for reading the wire representation
+                type Buf = [u8; tag::buf_of(&[$($TOK.len()),+])];
+
+                /// Make an instance of [Self::Buf]
+                #[inline(always)]
+                fn make_buf() -> Self::Buf {
+                    [0u8; tag::buf_of(&[$($TOK.len()),+])]
+                }
+
+                /// Convert a discriminant into the corresponding variant
+                #[inline(always)]
+                fn from_u8(x: u8) -> Option<Self> {
+                    #[allow(non_upper_case_globals)]
+                    mod __variant {
+                        $(
+                            pub const $Var: u8 = super::$Enum::$Var as u8;
+                        )+
+                    }
+
+                    match x {
+                        $(__variant::$Var => Some(Self::$Var),)+
+                        _ => None
+                    }
+                }
+
+                /// Convert a variant back into the wire representation
+                #[inline(always)]
+                fn as_bytes(&self) -> &'static [u8] {
+                    match self {
+                        $(Self::$Var => &$TOK,)+
+                    }
+                }
+            }
+        )*
+    };
+}
+
+// The following functions are written somewhat unusually,
+// since they're const functions that cannot use iterators.
+
+/// Maximum element of a slice
+const fn max_of(mut xs: &[usize]) -> usize {
+    let mut y = usize::MIN;
+    while let &[x, ref tail @ ..] = xs {
+        y = if x > y { x } else { y };
+        xs = tail;
+    }
+    y
+}
+
+/// Minimum element of a slice
+pub const fn min_of(mut xs: &[usize]) -> usize {
+    let mut y = usize::MAX;
+    while let &[x, ref tail @ ..] = xs {
+        y = if x < y { x } else { y };
+        xs = tail;
+    }
+    y
+}
+
+/// Minimum buffer size to contain either of `0..Tag::MIN` and `Tag::MIN..`
+/// at a particular time, for all possible tag wire representations, given
+/// the sizes of all wire representations.
+///
+/// # Example
+///
+/// ```plain
+/// OFF = 16
+/// MIN = 24
+/// MAX = 64
+///
+/// BUF = max(MIN, MAX-MIN)
+///     = max(24, 64-24)
+///     = max(24, 40)
+///     = 40
+/// ```
+pub const fn buf_of(xs: &[usize]) -> usize {
+    max_of(&[min_of(xs), max_of(xs) - min_of(xs)])
+}
+
+pub(crate) use make;
+
+#[cfg(test)]
+mod test {
+    use super::super::tag::{self, Tag};
+
+    const TOK_A: [u8; 3] = [0xed, 0xef, 0x1c];
+    const TOK_B: [u8; 3] = [0xed, 0xf0, 0x1c];
+
+    const OFFSET: usize = 1;
+
+    make! {
+        enum Token[OFFSET] {
+            A = TOK_A,
+            B = TOK_B,
+        }
+    }
+
+    #[test]
+    fn example() {
+        assert_eq!(Token::from_u8(0xed), None);
+
+        let tag = Token::from_u8(0xef).unwrap();
+        assert_eq!(tag.as_bytes(), &TOK_A[..]);
+
+        let tag = Token::from_u8(0xf0).unwrap();
+        assert_eq!(tag.as_bytes(), &TOK_B[..]);
+    }
+}