zig

commit 90ac62cc751fa85bf572e281f65064bf2935ff9c (tree)
parent 00b0beb682881da083b548423019483924736c1e
Author: Andrew Kelley <andrew@ziglang.org>
Date:   Tue,  2 Sep 2025 12:00:43 -0700

std.compress.lzma2: optimize appendLz

make the hot loop be a for loop without any failures or allocation.
change a O(N) addition into O(1)

Diffstat:
M
lib/std/compress/lzma2.zig
 | 
27
+++++++++++----------------

1 file changed, 11 insertions(+), 16 deletions(-)
diff --git a/lib/std/compress/lzma2.zig b/lib/std/compress/lzma2.zig
@@ -79,23 +79,18 @@ pub const AccumBuffer = struct {
         _ = writer;
 
         const buf_len = self.buf.items.len;
-        if (dist > buf_len) {
-            return error.CorruptInput;
-        }
+        if (dist > buf_len) return error.CorruptInput;
 
-        // ensure we have enough capacity for all new bytes.
-        // This prevents the buffer's memory from being reallocated (and freed)
-        // while we are still reading from it.
-        try self.buf.ensureTotalCapacity(allocator, buf_len + len);
-
-        var offset = buf_len - dist;
-        var i: usize = 0;
-        while (i < len) : (i += 1) {
-            const x = self.buf.items[offset];
-            // Since capacity is guaranteed, it's safe to use appendAssumeCapacity.
-            self.buf.appendAssumeCapacity(x);
-            offset += 1;
-        }
+        try self.buf.ensureUnusedCapacity(allocator, len);
+        const buffer = self.buf.allocatedSlice();
+        const src = buffer[buf_len - dist ..][0..len];
+        const dst = buffer[buf_len..][0..len];
+
+        // This is not a @memmove; it intentionally repeats patterns caused by
+        // iterating one byte at a time.
+        for (dst, src) |*d, s| d.* = s;
+
+        self.buf.items.len = buf_len + len;
         self.len += len;
     }