Update ffi/write to append to a buffer by default.

CHANGELOG.md

@@ -1,6 +1,9 @@
 # Changelog
 All notable changes to this project will be documented in this file.
 
+## Unreleased - ???
+- Make `ffi/write` append to a buffer instead of insert at 0 by default.
+
 ## 1.38.0 - 2025-03-18
 - Add `bundle/replace`
 - Add CLI flags for the `bundle/` module to install and manage bundles.

src/core/ffi.c

@@ -1686,7 +1686,7 @@ JANET_CORE_FN(cfun_ffi_buffer_write,
     JanetFFIType type = decode_ffi_type(argv[0]);
     uint32_t el_size = (uint32_t) type_size(type);
     JanetBuffer *buffer = janet_optbuffer(argv, argc, 2, el_size);
-    int32_t index = janet_optnat(argv, argc, 3, 0);
+    int32_t index = janet_optnat(argv, argc, 3, buffer->count);
     int32_t old_count = buffer->count;
     if (index > old_count) janet_panic("index out of bounds");
     buffer->count = index;

test/suite-ffi.janet

@@ -55,4 +55,11 @@
 (compwhen has-ffi
   (assert-error "bad struct issue #1512" (ffi/struct :void)))
 
+(compwhen has-ffi
+  (def buf @"")
+  (ffi/write :u8 10 buf)
+  (assert (= 1 (length buf)))
+  (ffi/write :u8 10 buf)
+  (assert (= 2 (length buf))))
+
 (end-suite)