// rt/ensure.ww — slice growth helper, archived into libwwrt.a.
//
// Companion to the `append(s, v)` builtin. The compiler lowers
// `append(s, v)` to:
//
//   ; push v
//   ; s.len += 1
//   ; CALL rt_ensure(&s, sizeof(elem))
//   ; ; ensure may have realloc'd, so re-read s.ptr
//   ; pop v
//   ; *(s.ptr + (s.len - 1) * elem_size) = v
//
// One helper handles every element width via the membsz parameter —
// no per-type wrapper functions (appendu8 / appendi64) needed.
//
// User code never `use`s this — the symbol is resolved at link time
// from libwwrt.a, like rt_malloc and rt_streq. rt/ensure.ww is compiled
// standalone via `w6c rt/ensure.ww` (not through the driver). Under
// strict-package (#24a) it declares `package rt;` (matching its
// directory) like every primary section; the link-resolved name is
// pinned bare via @symbol on the export (the clause would otherwise
// mangle it to rt.rt_ensure).
package rt;

@symbol("rt_malloc") fn malloc(n: u64) *void;

// Mirrors ww's []T header layout: 24 bytes with 8-byte slots.
// ww's source uses i32 for len/cap but the compiler stores them in
// 8-byte slots; declaring as i64 here keeps the field offsets right
// for this polymorphic alias.
type slice = struct {
	ptr: *u8,
	len: i64,
	cap: i64,
};

@symbol("rt_ensure") export fn rt_ensure(s: *slice, membsz: u64) void = {
	if (s.cap >= s.len) { return; };
	let nc: i64 = s.cap * 2i64;
	if (nc < 8i64) { nc = 8i64; };
	for (nc < s.len) { nc *= 2i64; };
	// Task #30 (commit 3) upgrades to nullable *void + null-check.
	// For now, rt_malloc returns plain *void; OOM faults on deref.
	let np: *u8 = malloc((nc: u64) * membsz): *u8;
	let n: u64 = (s.cap: u64) * membsz;
	let i: u64 = 0u64;
	for (i < n) {
		np[i] = s.ptr[i];
		i += 1u64;
	};
	// No free: the bump allocator (rt/malloc.ww) can't reclaim a
	// mid-chunk region; the old buffer leaks until process exit. (#8)
	s.ptr = np;
	s.cap = nc;
};