From 46e77092326ec6a944e025afd707f656f4871458 Mon Sep 17 00:00:00 2001
From: CJ van den Berg <cj@vdbonline.com>
Date: Thu, 12 Mar 2026 21:42:13 +0100
Subject: [PATCH] feat: add slightly more robust fmtBuf API

---
 src/cbor.zig   |  6 ++++++
 test/tests.zig | 14 ++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/src/cbor.zig b/src/cbor.zig
index 65cc773..c6cbfa8 100644
--- a/src/cbor.zig
+++ b/src/cbor.zig
@@ -301,6 +301,12 @@ pub fn fmt(buf: []u8, value: anytype) []const u8 {
     return writer.buffered();
 }
 
+pub fn fmtBuf(buf: []u8, value: anytype) Io.Writer.Error![]const u8 {
+    var writer: Io.Writer = .fixed(buf);
+    try writeValue(&writer, value);
+    return writer.buffered();
+}
+
 const CborType = struct { type: u8, minor: u5, major: u3 };
 
 pub fn decodeType(iter: *[]const u8) error{TooShort}!CborType {
diff --git a/test/tests.zig b/test/tests.zig
index 6015882..538409d 100644
--- a/test/tests.zig
+++ b/test/tests.zig
@@ -9,6 +9,7 @@ const expectEqualStrings = std.testing.expectEqualStrings;
 const expectError = std.testing.expectError;
 
 const fmt = cbor_mod.fmt;
+const fmtBuf = cbor_mod.fmtBuf;
 const toJson = cbor_mod.toJson;
 const toJsonPretty = cbor_mod.toJsonPretty;
 const fromJson = cbor_mod.fromJson;
@@ -44,6 +45,19 @@ test "cbor simple" {
     );
 }
 
+test "cbor.fmtBuf success" {
+    var buf: [128]u8 = undefined;
+    try expectEqualDeep(
+        try fmtBuf(&buf, .{ "five", 5, "four", 4, .{ "three", 3 } }),
+        &[_]u8{ 0x85, 0x64, 0x66, 0x69, 0x76, 0x65, 0x05, 0x64, 0x66, 0x6f, 0x75, 0x72, 0x04, 0x82, 0x65, 0x74, 0x68, 0x72, 0x65, 0x65, 0x03 },
+    );
+}
+
+test "cbor.fmtBuf overflow" {
+    var buf: [4]u8 = undefined;
+    try expectError(error.NoSpaceLeft, fmtBuf(&buf, .{ "five", 5, "four", 4, .{ "three", 3 } }));
+}
+
 test "cbor exit message" {
     var buf: [128]u8 = undefined;
     try expectEqualDeep(