perf(binary): unpack packed values through a byte-pair table

wacore/binary/benches/binary_benchmark.rs

@@ -22,6 +22,44 @@ fn create_small_node() -> Node {
         .build()
 }
 
+/// The shape the wire is mostly made of: a short ack whose `to` is a JID and
+/// whose `id` is hex, so decoding it goes through `read_jid_pair` and
+/// `read_packed`. `create_small_node` has neither, which left the two paths
+/// that dominate a small-stanza profile uncovered.
+fn create_ack_node() -> Node {
+    NodeBuilder::new("ack")
+        .attr("to", "5511999990000@s.whatsapp.net")
+        .attr("id", "3EB0A1B2C3D4E5F60718")
+        .attr("class", "message")
+        .build()
+}
+
+/// A device fanout, where the same two paths repeat per child.
+fn create_fanout_node() -> Node {
+    let devices: Vec<Node> = (0..8)
+        .map(|i| {
+            NodeBuilder::new("to")
+                .attr("jid", format!("5511999990000:{i}@s.whatsapp.net"))
+                .children(vec![
+                    NodeBuilder::new("enc")
+                        .attr("v", "2")
+                        .attr("type", "msg")
+                        .bytes(vec![0xAB; 128])
+                        .build(),
+                ])
+                .build()
+        })
+        .collect();
+    NodeBuilder::new("message")
+        .attr("to", "5511999990000@g.us")
+        .attr("id", "3EB0A1B2C3D4E5F60718")
+        .attr("type", "text")
+        .children(vec![
+            NodeBuilder::new("participants").children(devices).build(),
+        ])
+        .build()
+}
+
 fn create_large_node() -> Node {
     NodeBuilder::new("iq")
         .attr("to", "server@s.whatsapp.net")
@@ -243,6 +281,32 @@ fn bench_unmarshal_small(bencher: divan::Bencher) {
         });
 }
 
+fn setup_ack_marshaled() -> Vec<u8> {
+    marshal(&create_ack_node()).unwrap()
+}
+
+fn setup_fanout_marshaled() -> Vec<u8> {
+    marshal(&create_fanout_node()).unwrap()
+}
+
+#[divan::bench]
+fn bench_unmarshal_ack(bencher: divan::Bencher) {
+    bencher
+        .with_inputs(setup_ack_marshaled)
+        .bench_refs(|marshaled| {
+            black_box(unmarshal_ref(black_box(&marshaled[1..])).unwrap());
+        });
+}
+
+#[divan::bench]
+fn bench_unmarshal_fanout(bencher: divan::Bencher) {
+    bencher
+        .with_inputs(setup_fanout_marshaled)
+        .bench_refs(|marshaled| {
+            black_box(unmarshal_ref(black_box(&marshaled[1..])).unwrap());
+        });
+}
+
 #[divan::bench]
 fn bench_unmarshal_large(bencher: divan::Bencher) {
     bencher

wacore/binary/src/decoder.rs

@@ -15,6 +15,44 @@ fn jid_ref_to_compact(j: &JidRef<'_>) -> CompactString {
     s
 }
 
+/// Each byte's two output characters, so unpacking is one load and one 2-byte
+/// store per input byte instead of two shifts, two lookups and two bounds
+/// checks. Packed values on the wire (a 13-digit phone number, a 20-character
+/// id) are shorter than the SIMD chunk above, so this is the path that runs.
+static HEX_PAIRS: [[u8; 2]; 256] = {
+    const HEX: [u8; 16] = *b"0123456789ABCDEF";
+    let mut table = [[0u8; 2]; 256];
+    let mut i = 0;
+    while i < 256 {
+        table[i] = [HEX[i >> 4], HEX[i & 0x0F]];
+        i += 1;
+    }
+    table
+};
+
+/// Nibble values 12, 13 and 14 encode nothing, so they are marked and the byte
+/// carrying one falls back to the scalar path, which reports which half was
+/// bad. `NIBBLE_INVALID` cannot collide with an output character.
+const NIBBLE_INVALID: u8 = 0xFF;
+static NIBBLE_PAIRS: [[u8; 2]; 256] = {
+    const fn glyph(nibble: usize) -> u8 {
+        match nibble {
+            0..=9 => b'0' + nibble as u8,
+            10 => b'-',
+            11 => b'.',
+            15 => 0,
+            _ => NIBBLE_INVALID,
+        }
+    }
+    let mut table = [[0u8; 2]; 256];
+    let mut i = 0;
+    while i < 256 {
+        table[i] = [glyph(i >> 4), glyph(i & 0x0F)];
+        i += 1;
+    }
+    table
+};
+
 /// Node-nesting cap rejecting deep-`LIST` frames that would overflow the stack via
 /// unbounded `read_node_ref` recursion (real WA trees are well under 20 levels).
 const MAX_NODE_DEPTH: usize = 128;
@@ -357,14 +395,14 @@ impl<'a> Decoder<'a> {
             remainder
         };
 
-        for &byte in packed_data {
-            let high = (byte & 0xF0) >> 4;
-            let low = byte & 0x0F;
-            out[*pos] = Self::unpack_hex(high);
-            *pos += 1;
-            out[*pos] = Self::unpack_hex(low);
-            *pos += 1;
+        let written = packed_data.len() * 2;
+        for (slot, &byte) in out[*pos..*pos + written]
+            .chunks_exact_mut(2)
+            .zip(packed_data)
+        {
+            slot.copy_from_slice(&HEX_PAIRS[byte as usize]);
         }
+        *pos += written;
     }
 
     #[inline]
@@ -415,14 +453,20 @@ impl<'a> Decoder<'a> {
             remainder
         };
 
-        for &byte in packed_data {
-            let high = (byte & 0xF0) >> 4;
-            let low = byte & 0x0F;
-            out[*pos] = Self::unpack_nibble(high)?;
-            *pos += 1;
-            out[*pos] = Self::unpack_nibble(low)?;
-            *pos += 1;
+        let written = packed_data.len() * 2;
+        for (slot, &byte) in out[*pos..*pos + written]
+            .chunks_exact_mut(2)
+            .zip(packed_data)
+        {
+            let pair = NIBBLE_PAIRS[byte as usize];
+            if pair[0] == NIBBLE_INVALID || pair[1] == NIBBLE_INVALID {
+                // Report the bad half in the order the byte carries it.
+                Self::unpack_nibble((byte & 0xF0) >> 4)?;
+                Self::unpack_nibble(byte & 0x0F)?;
+            }
+            slot.copy_from_slice(&pair);
         }
+        *pos += written;
 
         Ok(())
     }
@@ -438,15 +482,6 @@ impl<'a> Decoder<'a> {
         }
     }
 
-    #[inline(always)]
-    fn unpack_hex(value: u8) -> u8 {
-        match value {
-            0..=9 => b'0' + value,
-            10..=15 => b'A' + value - 10,
-            _ => unreachable!("hex nibble validated by 4-bit mask"),
-        }
-    }
-
     fn read_attributes(&mut self, size: usize) -> Result<AttrsRef<'a>> {
         if size == 0 {
             return Ok(AttrsRef::Empty);

wacore/binary/tests/packed_equivalence.rs

@@ -0,0 +1,44 @@
+//! The packed-value tables must decode exactly what the scalar nibble/hex
+//! walk did, including where an invalid nibble reports the failure.
+use wacore_binary::builder::NodeBuilder;
+use wacore_binary::marshal::{marshal, unmarshal_ref};
+
+fn roundtrip_attr(value: &str) -> String {
+    let node = NodeBuilder::new("t").attr("v", value).build();
+    let bytes = marshal(&node).unwrap();
+    let decoded = unmarshal_ref(&bytes[1..]).unwrap();
+    match decoded.get_attr("v").unwrap() {
+        wacore_binary::node::ValueRef::String(s) => s.to_string(),
+        other => panic!("unexpected value {other:?}"),
+    }
+}
+
+#[test]
+fn packed_values_round_trip_across_the_simd_boundary() {
+    // Hex and nibble, at lengths below, at, and above the 16-byte chunk the
+    // SIMD path handles, plus odd lengths that exercise the half-byte trim.
+    for len in [1, 2, 3, 15, 16, 17, 31, 32, 33, 63, 100, 127] {
+        let hex: String = (0..len)
+            .map(|i| b"0123456789ABCDEF"[i % 16] as char)
+            .collect();
+        assert_eq!(roundtrip_attr(&hex), hex, "hex len {len}");
+
+        let nibble: String = (0..len).map(|i| b"0123456789-."[i % 12] as char).collect();
+        assert_eq!(roundtrip_attr(&nibble), nibble, "nibble len {len}");
+    }
+}
+
+#[test]
+fn an_invalid_nibble_is_still_rejected() {
+    // Nibble 12, 13 and 14 encode nothing. Build the frame by hand, since the
+    // encoder would never emit one.
+    for bad in [0x0Cu8, 0x0D, 0x0E] {
+        // NIBBLE_8 tag, length 1, one byte whose low half is invalid.
+        let frame = [248u8, 2, 1, 255, 1, 0xF0 | bad];
+        let err = unmarshal_ref(&frame).unwrap_err();
+        assert!(
+            format!("{err}").contains(&bad.to_string()),
+            "error must name the bad nibble {bad}: {err}"
+        );
+    }
+}