Decode numbers from a reader whose buffer is smaller than them

CHANGELOG.md

@@ -16,6 +16,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Encoding writes straight into the writer's buffer when there is room, and keeps the drain path out of line, which is about 13% cheaper on a typical struct and 37% on an array of integers
 - Encoding a struct is about 25% cheaper again, by telling the compiler that writing into the buffer cannot change the writer itself
 
+### Fixed
+- Decoding numbers from a streaming reader whose buffer is smaller than the number (under 8 bytes) works, instead of failing with `error.ReaderBufferTooSmall`; that error is now only for strings that have to be buffered whole, such as struct field names
+
 ## [0.9.0] - 2026-09-05
 
 ### Changed

src/msgpack.zig

@@ -278,7 +278,8 @@ pub const SliceDecodeError = error{
 pub const DecodeError = SliceDecodeError || error{
     /// The reader failed, and its own error says why.
     ReadFailed,
-    /// A value had to be buffered whole, and the reader's buffer is smaller.
+    /// A string had to be buffered whole, such as a struct field name, and
+    /// the reader's buffer is smaller.
     ReaderBufferTooSmall,
 };
 
@@ -453,21 +454,28 @@ test "decode from a streaming reader whose buffer holds the largest value" {
     }
 }
 
-test "decode from a streaming reader with too small a buffer errors, never panics" {
-    const Numeric = struct { id: u64, seq: i32, ratio: f64, ok: bool };
+test "decode numbers from a streaming reader whose buffer is smaller than them" {
+    const Numeric = struct {
+        id: u64,
+        seq: i32,
+        ratio: f64,
+        ok: bool,
+
+        pub fn msgpackFormat() StructFormat {
+            return .{ .as_array = .{} };
+        }
+    };
     const value = Numeric{ .id = 0xdead_beef_cafe_1234, .seq = -4242, .ratio = 3.14159, .ok = true };
 
     var encoded: [64]u8 = undefined;
     const bytes = try encodeToBuffer(value, &encoded);
 
-    // The u64 payload needs 8 buffered bytes; anything smaller cannot rebase.
+    // The u64 and f64 payloads are 8 bytes, more than any of these buffers.
     for ([_]usize{ 1, 2, 4, 7 }) |buffer_len| {
         var buffer: [8]u8 = undefined;
         var trickle = TrickleReader.init(buffer[0..buffer_len], bytes);
-        try std.testing.expectError(
-            error.ReaderBufferTooSmall,
-            decodeLeaky(Numeric, std.testing.allocator, &trickle.reader),
-        );
+        const decoded = try decodeLeaky(Numeric, std.testing.allocator, &trickle.reader);
+        try std.testing.expectEqualDeep(value, decoded);
     }
 }
 

src/utils.zig

@@ -62,15 +62,30 @@ pub noinline fn writeAllSlow(writer: *std.Io.Writer, bytes: []const u8) std.Io.W
     return writer.writeAll(bytes);
 }
 
-/// Reads a big-endian `T` from the reader. `Reader.takeInt` rebases, which
-/// asserts the reader's buffer can hold `@sizeOf(T)`; report that as an error
-/// rather than letting the assert fire.
+/// Reads a big-endian `T` from the reader. When the bytes are already
+/// buffered, which is always the case when decoding from a slice, this is one
+/// compare and a load. `Reader.takeInt` would check the same thing, but it
+/// rebases on a miss, which asserts the reader's buffer can hold `@sizeOf(T)`,
+/// so it needed a check of the buffer's capacity in front of it as well.
 pub inline fn takeInt(reader: *std.Io.Reader, comptime T: type) !T {
-    if (reader.buffer.len < @divExact(@bitSizeOf(T), 8)) {
-        @branchHint(.unlikely);
-        return error.ReaderBufferTooSmall;
+    const n = @divExact(@bitSizeOf(T), 8);
+    const buffered = reader.buffered();
+    if (buffered.len >= n) {
+        @branchHint(.likely);
+        reader.toss(n);
+        return std.mem.readInt(T, buffered[0..n], .big);
     }
-    return reader.takeInt(T, .big);
+    return takeIntSlow(reader, T);
+}
+
+/// The fallback of `takeInt`, for when the bytes are not all buffered. Copying
+/// them out with `readSliceAll` works across refills, so a reader whose buffer
+/// is smaller than `T` still works.
+noinline fn takeIntSlow(reader: *std.Io.Reader, comptime T: type) !T {
+    @branchHint(.cold);
+    var bytes: [@divExact(@bitSizeOf(T), 8)]u8 = undefined;
+    try reader.readSliceAll(&bytes);
+    return std.mem.readInt(T, &bytes, .big);
 }
 
 /// Copies `dest.len` bytes out of the reader. When they are already buffered,