refactor: shrink HPACK encode tables (#949)

Author: seanmonstar

src/hpack/huffman/decode.rs

@@ -12,7 +12,7 @@
 //!
 //! These are decoded by walking the generated `DECODE_TABLE` one input byte at a time.
 
-use crate::hpack::huffman::table::{DECODE_TABLE, ENCODE_TABLE};
+use crate::hpack::huffman::table::{DECODE_TABLE, ENCODE_CODES, ENCODE_CODE_LENGTHS};
 use crate::hpack::DecoderError;
 
 use bytes::BytesMut;
@@ -44,7 +44,8 @@ const fn build_fast_table() -> [u32; 1 << FAST_BITS] {
     // First fill in every index that starts with one whole code
     let mut a = 0;
     while a < 256 {
-        let (len1, code1) = ENCODE_TABLE[a];
+        let len1 = ENCODE_CODE_LENGTHS[a] as usize;
+        let code1 = ENCODE_CODES[a] as u64;
         if len1 <= FAST_BITS {
             let rem = FAST_BITS - len1;
             let base = (code1 as usize) << rem;
@@ -61,13 +62,15 @@ const fn build_fast_table() -> [u32; 1 << FAST_BITS] {
     // Overwrite the indices where a second whole code fit right after the first
     let mut a = 0;
     while a < 256 {
-        let (len1, code1) = ENCODE_TABLE[a];
+        let len1 = ENCODE_CODE_LENGTHS[a] as usize;
+        let code1 = ENCODE_CODES[a] as u64;
         // A second code needs at least 5 more bits (the shortest code)
         if len1 + 5 <= FAST_BITS {
             let rem = FAST_BITS - len1;
             let mut b = 0;
             while b < 256 {
-                let (len2, code2) = ENCODE_TABLE[b];
+                let len2 = ENCODE_CODE_LENGTHS[b] as usize;
+                let code2 = ENCODE_CODES[b] as u64;
                 if len2 <= rem {
                     let rem2 = rem - len2;
                     let base = ((code1 as usize) << rem) | ((code2 as usize) << rem2);
@@ -222,9 +225,10 @@ mod test {
     // out on purpose: encoded EOS is an error per RFC 7541 §5.2.
     fn reference_decode(src: &[u8]) -> Result<BytesMut, DecoderError> {
         // Each symbol's code as a string of '0' and '1', e.g. "11111000".
-        let codes: Vec<String> = ENCODE_TABLE[..256]
+        let codes: Vec<String> = ENCODE_CODES
             .iter()
-            .map(|&(len, code)| format!("{code:0len$b}"))
+            .zip(ENCODE_CODE_LENGTHS.iter())
+            .map(|(&code, &len)| format!("{code:0len$b}", len = len as usize))
             .collect();
         let bits: String = src.iter().map(|byte| format!("{byte:08b}")).collect();
 

src/hpack/huffman/encode.rs

@@ -1,4 +1,4 @@
-use crate::hpack::huffman::table::ENCODE_TABLE;
+use crate::hpack::huffman::table::{ENCODE_CODES, ENCODE_CODE_LENGTHS};
 
 use bytes::{BufMut, BytesMut};
 
@@ -7,7 +7,8 @@ pub fn encode(src: &[u8], dst: &mut BytesMut) {
     let mut bits_len = 0;
 
     for &b in src {
-        let (nbits, code) = ENCODE_TABLE[b as usize];
+        let nbits = ENCODE_CODE_LENGTHS[b as usize] as usize;
+        let code = ENCODE_CODES[b as usize] as u64;
 
         bits = (bits << nbits) | code;
         bits_len += nbits;

src/hpack/huffman/table.rs

@@ -1,266 +1,49 @@
 // !!! DO NOT EDIT !!! Generated by util/genhuff/src/main.rs
 
-// (num-bits, bits)
-pub const ENCODE_TABLE: [(usize, u64); 257] = [
-    (13, 0x1ff8),
-    (23, 0x7fffd8),
-    (28, 0xfffffe2),
-    (28, 0xfffffe3),
-    (28, 0xfffffe4),
-    (28, 0xfffffe5),
-    (28, 0xfffffe6),
-    (28, 0xfffffe7),
-    (28, 0xfffffe8),
-    (24, 0xffffea),
-    (30, 0x3ffffffc),
-    (28, 0xfffffe9),
-    (28, 0xfffffea),
-    (30, 0x3ffffffd),
-    (28, 0xfffffeb),
-    (28, 0xfffffec),
-    (28, 0xfffffed),
-    (28, 0xfffffee),
-    (28, 0xfffffef),
-    (28, 0xffffff0),
-    (28, 0xffffff1),
-    (28, 0xffffff2),
-    (30, 0x3ffffffe),
-    (28, 0xffffff3),
-    (28, 0xffffff4),
-    (28, 0xffffff5),
-    (28, 0xffffff6),
-    (28, 0xffffff7),
-    (28, 0xffffff8),
-    (28, 0xffffff9),
-    (28, 0xffffffa),
-    (28, 0xffffffb),
-    (6, 0x14),
-    (10, 0x3f8),
-    (10, 0x3f9),
-    (12, 0xffa),
-    (13, 0x1ff9),
-    (6, 0x15),
-    (8, 0xf8),
-    (11, 0x7fa),
-    (10, 0x3fa),
-    (10, 0x3fb),
-    (8, 0xf9),
-    (11, 0x7fb),
-    (8, 0xfa),
-    (6, 0x16),
-    (6, 0x17),
-    (6, 0x18),
-    (5, 0x0),
-    (5, 0x1),
-    (5, 0x2),
-    (6, 0x19),
-    (6, 0x1a),
-    (6, 0x1b),
-    (6, 0x1c),
-    (6, 0x1d),
-    (6, 0x1e),
-    (6, 0x1f),
-    (7, 0x5c),
-    (8, 0xfb),
-    (15, 0x7ffc),
-    (6, 0x20),
-    (12, 0xffb),
-    (10, 0x3fc),
-    (13, 0x1ffa),
-    (6, 0x21),
-    (7, 0x5d),
-    (7, 0x5e),
-    (7, 0x5f),
-    (7, 0x60),
-    (7, 0x61),
-    (7, 0x62),
-    (7, 0x63),
-    (7, 0x64),
-    (7, 0x65),
-    (7, 0x66),
-    (7, 0x67),
-    (7, 0x68),
-    (7, 0x69),
-    (7, 0x6a),
-    (7, 0x6b),
-    (7, 0x6c),
-    (7, 0x6d),
-    (7, 0x6e),
-    (7, 0x6f),
-    (7, 0x70),
-    (7, 0x71),
-    (7, 0x72),
-    (8, 0xfc),
-    (7, 0x73),
-    (8, 0xfd),
-    (13, 0x1ffb),
-    (19, 0x7fff0),
-    (13, 0x1ffc),
-    (14, 0x3ffc),
-    (6, 0x22),
-    (15, 0x7ffd),
-    (5, 0x3),
-    (6, 0x23),
-    (5, 0x4),
-    (6, 0x24),
-    (5, 0x5),
-    (6, 0x25),
-    (6, 0x26),
-    (6, 0x27),
-    (5, 0x6),
-    (7, 0x74),
-    (7, 0x75),
-    (6, 0x28),
-    (6, 0x29),
-    (6, 0x2a),
-    (5, 0x7),
-    (6, 0x2b),
-    (7, 0x76),
-    (6, 0x2c),
-    (5, 0x8),
-    (5, 0x9),
-    (6, 0x2d),
-    (7, 0x77),
-    (7, 0x78),
-    (7, 0x79),
-    (7, 0x7a),
-    (7, 0x7b),
-    (15, 0x7ffe),
-    (11, 0x7fc),
-    (14, 0x3ffd),
-    (13, 0x1ffd),
-    (28, 0xffffffc),
-    (20, 0xfffe6),
-    (22, 0x3fffd2),
-    (20, 0xfffe7),
-    (20, 0xfffe8),
-    (22, 0x3fffd3),
-    (22, 0x3fffd4),
-    (22, 0x3fffd5),
-    (23, 0x7fffd9),
-    (22, 0x3fffd6),
-    (23, 0x7fffda),
-    (23, 0x7fffdb),
-    (23, 0x7fffdc),
-    (23, 0x7fffdd),
-    (23, 0x7fffde),
-    (24, 0xffffeb),
-    (23, 0x7fffdf),
-    (24, 0xffffec),
-    (24, 0xffffed),
-    (22, 0x3fffd7),
-    (23, 0x7fffe0),
-    (24, 0xffffee),
-    (23, 0x7fffe1),
-    (23, 0x7fffe2),
-    (23, 0x7fffe3),
-    (23, 0x7fffe4),
-    (21, 0x1fffdc),
-    (22, 0x3fffd8),
-    (23, 0x7fffe5),
-    (22, 0x3fffd9),
-    (23, 0x7fffe6),
-    (23, 0x7fffe7),
-    (24, 0xffffef),
-    (22, 0x3fffda),
-    (21, 0x1fffdd),
-    (20, 0xfffe9),
-    (22, 0x3fffdb),
-    (22, 0x3fffdc),
-    (23, 0x7fffe8),
-    (23, 0x7fffe9),
-    (21, 0x1fffde),
-    (23, 0x7fffea),
-    (22, 0x3fffdd),
-    (22, 0x3fffde),
-    (24, 0xfffff0),
-    (21, 0x1fffdf),
-    (22, 0x3fffdf),
-    (23, 0x7fffeb),
-    (23, 0x7fffec),
-    (21, 0x1fffe0),
-    (21, 0x1fffe1),
-    (22, 0x3fffe0),
-    (21, 0x1fffe2),
-    (23, 0x7fffed),
-    (22, 0x3fffe1),
-    (23, 0x7fffee),
-    (23, 0x7fffef),
-    (20, 0xfffea),
-    (22, 0x3fffe2),
-    (22, 0x3fffe3),
-    (22, 0x3fffe4),
-    (23, 0x7ffff0),
-    (22, 0x3fffe5),
-    (22, 0x3fffe6),
-    (23, 0x7ffff1),
-    (26, 0x3ffffe0),
-    (26, 0x3ffffe1),
-    (20, 0xfffeb),
-    (19, 0x7fff1),
-    (22, 0x3fffe7),
-    (23, 0x7ffff2),
-    (22, 0x3fffe8),
-    (25, 0x1ffffec),
-    (26, 0x3ffffe2),
-    (26, 0x3ffffe3),
-    (26, 0x3ffffe4),
-    (27, 0x7ffffde),
-    (27, 0x7ffffdf),
-    (26, 0x3ffffe5),
-    (24, 0xfffff1),
-    (25, 0x1ffffed),
-    (19, 0x7fff2),
-    (21, 0x1fffe3),
-    (26, 0x3ffffe6),
-    (27, 0x7ffffe0),
-    (27, 0x7ffffe1),
-    (26, 0x3ffffe7),
-    (27, 0x7ffffe2),
-    (24, 0xfffff2),
-    (21, 0x1fffe4),
-    (21, 0x1fffe5),
-    (26, 0x3ffffe8),
-    (26, 0x3ffffe9),
-    (28, 0xffffffd),
-    (27, 0x7ffffe3),
-    (27, 0x7ffffe4),
-    (27, 0x7ffffe5),
-    (20, 0xfffec),
-    (24, 0xfffff3),
-    (20, 0xfffed),
-    (21, 0x1fffe6),
-    (22, 0x3fffe9),
-    (21, 0x1fffe7),
-    (21, 0x1fffe8),
-    (23, 0x7ffff3),
-    (22, 0x3fffea),
-    (22, 0x3fffeb),
-    (25, 0x1ffffee),
-    (25, 0x1ffffef),
-    (24, 0xfffff4),
-    (24, 0xfffff5),
-    (26, 0x3ffffea),
-    (23, 0x7ffff4),
-    (26, 0x3ffffeb),
-    (27, 0x7ffffe6),
-    (26, 0x3ffffec),
-    (26, 0x3ffffed),
-    (27, 0x7ffffe7),
-    (27, 0x7ffffe8),
-    (27, 0x7ffffe9),
-    (27, 0x7ffffea),
-    (27, 0x7ffffeb),
-    (28, 0xffffffe),
-    (27, 0x7ffffec),
-    (27, 0x7ffffed),
-    (27, 0x7ffffee),
-    (27, 0x7ffffef),
-    (27, 0x7fffff0),
-    (26, 0x3ffffee),
-    (30, 0x3fffffff),
+pub const ENCODE_CODES: [u32; 256] = [
+    0x1ff8, 0x7fffd8, 0xfffffe2, 0xfffffe3, 0xfffffe4, 0xfffffe5, 0xfffffe6, 0xfffffe7, 0xfffffe8,
+    0xffffea, 0x3ffffffc, 0xfffffe9, 0xfffffea, 0x3ffffffd, 0xfffffeb, 0xfffffec, 0xfffffed,
+    0xfffffee, 0xfffffef, 0xffffff0, 0xffffff1, 0xffffff2, 0x3ffffffe, 0xffffff3, 0xffffff4,
+    0xffffff5, 0xffffff6, 0xffffff7, 0xffffff8, 0xffffff9, 0xffffffa, 0xffffffb, 0x14, 0x3f8,
+    0x3f9, 0xffa, 0x1ff9, 0x15, 0xf8, 0x7fa, 0x3fa, 0x3fb, 0xf9, 0x7fb, 0xfa, 0x16, 0x17, 0x18,
+    0x0, 0x1, 0x2, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, 0x5c, 0xfb, 0x7ffc, 0x20, 0xffb,
+    0x3fc, 0x1ffa, 0x21, 0x5d, 0x5e, 0x5f, 0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68,
+    0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f, 0x70, 0x71, 0x72, 0xfc, 0x73, 0xfd, 0x1ffb, 0x7fff0,
+    0x1ffc, 0x3ffc, 0x22, 0x7ffd, 0x3, 0x23, 0x4, 0x24, 0x5, 0x25, 0x26, 0x27, 0x6, 0x74, 0x75,
+    0x28, 0x29, 0x2a, 0x7, 0x2b, 0x76, 0x2c, 0x8, 0x9, 0x2d, 0x77, 0x78, 0x79, 0x7a, 0x7b, 0x7ffe,
+    0x7fc, 0x3ffd, 0x1ffd, 0xffffffc, 0xfffe6, 0x3fffd2, 0xfffe7, 0xfffe8, 0x3fffd3, 0x3fffd4,
+    0x3fffd5, 0x7fffd9, 0x3fffd6, 0x7fffda, 0x7fffdb, 0x7fffdc, 0x7fffdd, 0x7fffde, 0xffffeb,
+    0x7fffdf, 0xffffec, 0xffffed, 0x3fffd7, 0x7fffe0, 0xffffee, 0x7fffe1, 0x7fffe2, 0x7fffe3,
+    0x7fffe4, 0x1fffdc, 0x3fffd8, 0x7fffe5, 0x3fffd9, 0x7fffe6, 0x7fffe7, 0xffffef, 0x3fffda,
+    0x1fffdd, 0xfffe9, 0x3fffdb, 0x3fffdc, 0x7fffe8, 0x7fffe9, 0x1fffde, 0x7fffea, 0x3fffdd,
+    0x3fffde, 0xfffff0, 0x1fffdf, 0x3fffdf, 0x7fffeb, 0x7fffec, 0x1fffe0, 0x1fffe1, 0x3fffe0,
+    0x1fffe2, 0x7fffed, 0x3fffe1, 0x7fffee, 0x7fffef, 0xfffea, 0x3fffe2, 0x3fffe3, 0x3fffe4,
+    0x7ffff0, 0x3fffe5, 0x3fffe6, 0x7ffff1, 0x3ffffe0, 0x3ffffe1, 0xfffeb, 0x7fff1, 0x3fffe7,
+    0x7ffff2, 0x3fffe8, 0x1ffffec, 0x3ffffe2, 0x3ffffe3, 0x3ffffe4, 0x7ffffde, 0x7ffffdf,
+    0x3ffffe5, 0xfffff1, 0x1ffffed, 0x7fff2, 0x1fffe3, 0x3ffffe6, 0x7ffffe0, 0x7ffffe1, 0x3ffffe7,
+    0x7ffffe2, 0xfffff2, 0x1fffe4, 0x1fffe5, 0x3ffffe8, 0x3ffffe9, 0xffffffd, 0x7ffffe3, 0x7ffffe4,
+    0x7ffffe5, 0xfffec, 0xfffff3, 0xfffed, 0x1fffe6, 0x3fffe9, 0x1fffe7, 0x1fffe8, 0x7ffff3,
+    0x3fffea, 0x3fffeb, 0x1ffffee, 0x1ffffef, 0xfffff4, 0xfffff5, 0x3ffffea, 0x7ffff4, 0x3ffffeb,
+    0x7ffffe6, 0x3ffffec, 0x3ffffed, 0x7ffffe7, 0x7ffffe8, 0x7ffffe9, 0x7ffffea, 0x7ffffeb,
+    0xffffffe, 0x7ffffec, 0x7ffffed, 0x7ffffee, 0x7ffffef, 0x7fffff0, 0x3ffffee,
 ];
 
+pub const ENCODE_CODE_LENGTHS: [u8; 256] = [
+    13, 23, 28, 28, 28, 28, 28, 28, 28, 24, 30, 28, 28, 30, 28, 28, 28, 28, 28, 28, 28, 28, 30, 28,
+    28, 28, 28, 28, 28, 28, 28, 28, 6, 10, 10, 12, 13, 6, 8, 11, 10, 10, 8, 11, 8, 6, 6, 6, 5, 5,
+    5, 6, 6, 6, 6, 6, 6, 6, 7, 8, 15, 6, 12, 10, 13, 6, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+    7, 7, 7, 7, 7, 7, 7, 7, 8, 7, 8, 13, 19, 13, 14, 6, 15, 5, 6, 5, 6, 5, 6, 6, 6, 5, 7, 7, 6, 6,
+    6, 5, 6, 7, 6, 5, 5, 6, 7, 7, 7, 7, 7, 15, 11, 14, 13, 28, 20, 22, 20, 20, 22, 22, 22, 23, 22,
+    23, 23, 23, 23, 23, 24, 23, 24, 24, 22, 23, 24, 23, 23, 23, 23, 21, 22, 23, 22, 23, 23, 24, 22,
+    21, 20, 22, 22, 23, 23, 21, 23, 22, 22, 24, 21, 22, 23, 23, 21, 21, 22, 21, 23, 22, 23, 23, 20,
+    22, 22, 22, 23, 22, 22, 23, 26, 26, 20, 19, 22, 23, 22, 25, 26, 26, 26, 27, 27, 26, 24, 25, 19,
+    21, 26, 27, 27, 26, 27, 24, 21, 21, 26, 26, 28, 27, 27, 27, 20, 24, 20, 21, 22, 21, 21, 23, 22,
+    22, 25, 25, 24, 24, 26, 23, 26, 27, 26, 26, 27, 27, 27, 27, 27, 28, 27, 27, 27, 27, 27, 26,
+];
+
+const _: [(); 1024] = [(); std::mem::size_of::<[u32; 256]>()];
+const _: [(); 256] = [(); std::mem::size_of::<[u8; 256]>()];
+
 // A branch has bit 15 set and its table index in bits 8..=14.
 // A leaf stores the number of consumed bits in its high byte and the decoded byte in its low byte.
 pub const DECODE_TABLE: [u16; 15 * 256] = [