// memio — in-memory io stream. Project #94 fold-eFinal.
//
// Hare's memio:: surface, drop underscores. Two flavours behind a
// single [[io.stream]] (= `*io.vtable`):
//
//   fixed       caller owns the buffer, writes stop when full.
//   dynamic     memio owns the buffer, writes grow it; close frees.
//
// Hare's memio::fixed/dynamic/dynamic_from return a `stream` whose
// FIRST field IS the `io::stream` (= `*vtable`). ww mirrors that
// intrusively: `stream`'s first field is `vt: io.vtable` (the vtable
// embedded INLINE) so a stack `stream` is castable to
// `io.stream = *vtable` via `&s.vt` — and the callbacks recover the
// outer `stream` by casting the dispatch arg back to `*stream`. Same
// intrusive shape as lib/bufio + lib/log over their embedded vtables.
//
//   let s: memio.stream = memio.fixed(buf);
//   io.write(&s.vt, bytes);
//   let view: str = memio.string(&s);
//
// Constructors return the `stream` BY VALUE (Hare shape,
// ref/hare/memio/stream.ha:46,58,64): build the struct in a local,
// field-assign every slot, and `return r;` (the proven sret round-trip
// pinned by test/wcc/925 + 776). NO heap, NO `nomem`: the alloc that
// forced an earlier heap return is gone, so the constructor cannot
// fail. Caller owns the returned `stream` (stack ownership, no-GC) and
// passes `&s.vt` to the io dispatchers — exactly Hare's `&s` into
// io::write.
//
// Subset of Hare's surface: io's variants are {eof, error}, so memio
// drops Hare's NONBLOCK flag (would need an `again` variant in lib/io).
// string()'s utf8-validating constructor is omitted per CLAUDE.md
// rule 9 carve-out — see [[string]]. Hare's seek / copy callbacks are
// likewise absent: io's stream vtable has only read/write/close slots,
// so memio can't wire a seeker or copier yet (io fold-2, #5).
//
// Cast workaround per #206-payoff (ken: KEEP the explicit casts; they
// are cgen-neutral and sidestep the #214 over-acceptance surface). The
// `(&fn_name): *io.<role>` cast at each store site is the Hare-faithful
// minimum-touch route — the #206 cast-drop is a separate deferred
// payoff gated on #214.
//
// ptr/len/cap kept flat (no `buf: []u8`) per a historical cgen note:
// chained-dot writes through a state pointer into a slice subfield
// miscompile silently (#195 family); the flat shape sidesteps it.
// dynamicfrom uses the slice's `cap` (NOT `len`) to track the
// allocated-capacity-to-free on close — passing a half-filled append
// slice with len < cap and using only `buf.len` would under-free.

package memio;

import io;
import os;
import rt;

// stream — Hare's memio::stream (ref/hare/memio/stream.ha:18). `vt` at
// offset 0 for the intrusive stream→io.stream cast (`&s.vt`) and the
// callbacks' reverse `s: *stream` cast. Unified across fixed/dynamic
// (Hare keeps a single `stream` over per-mode vtable singletons; ww
// wires the per-mode callbacks post-construction instead). ptr/len/cap
// flat per the header note.
export type stream = struct {
	vt:  io.vtable,
	ptr: *u8,
	len: i32,
	cap: i32,
	pos: i32,
};

// fixed — wire a stream over a caller-supplied buffer. Writes never
// grow; they return 0 once `pos` reaches the end of the buffer (Hare
// returns `nomem` here; ww surfaces 0 — graduating to Hare's `nomem`
// return needs the widen-from-bare-nomem path that #173-family work
// gates).
//
// Mirrors ref/hare/memio/stream.ha:46.
export fn fixed(buf: []u8) stream = {
	let r: stream;
	r.vt.reader = (&readfn):     *io.reader;
	r.vt.writer = (&fixedwrite): *io.writer;
	r.ptr = buf.ptr;
	r.len = buf.len;
	r.cap = buf.len;
	r.pos = 0;
	return r;
};

// dynamic — wire a stream with no initial buffer. Writes grow the
// backing allocation; [[io.close]] frees it.
//
// Mirrors ref/hare/memio/stream.ha:58.
export fn dynamic() stream = {
	let r: stream;
	r.vt.reader = (&readfn):       *io.reader;
	r.vt.writer = (&dynamicwrite): *io.writer;
	r.vt.closer = (&dynamicclose): *io.closer;
	r.ptr = nil;
	r.len = 0;
	r.cap = 0;
	r.pos = 0;
	return r;
};

// dynamicfrom — like [[dynamic]] but seeded with an existing slice.
// Ownership transfers; close frees `cap` bytes from the slice's
// allocated capacity (NOT logical length).
//
// Mirrors ref/hare/memio/stream.ha:64.
export fn dynamicfrom(buf: []u8) stream = {
	let r: stream;
	r.vt.reader = (&readfn):       *io.reader;
	r.vt.writer = (&dynamicwrite): *io.writer;
	r.vt.closer = (&dynamicclose): *io.closer;
	r.ptr = buf.ptr;
	r.len = buf.len;
	r.cap = buf.cap;
	r.pos = 0;
	return r;
};

// ---- vtable callbacks ----------------------------------------------------

// readfn — recover the stream from the io.stream's `*vtable` via the
// intrusive offset-0 cast. Single fn over the common header (Hare's
// single `read` at ref/hare/memio/stream.ha:103); fixed and dynamic
// share it because the read path is buffer-flavour-agnostic.
fn readfn(s: io.stream, buf: []u8) (size | io.eof | io.error) = {
	let m: *stream = s: *stream;
	if (m.pos >= m.len) {
		let e: io.eof;
		return e;
	};
	let avail: i32 = m.len - m.pos;
	let n: i32 = buf.len;
	if (avail < n) { n = avail; };
	let i: i32 = 0;
	for (i < n) {
		buf[i] = m.ptr[m.pos + i];
		i += 1;
	};
	m.pos += n;
	return n: size;
};

fn fixedwrite(s: io.stream, buf: []u8) (size | io.error) = {
	let m: *stream = s: *stream;
	if (m.pos >= m.len) { return 0: size; };
	let space: i32 = m.len - m.pos;
	let n: i32 = buf.len;
	if (space < n) { n = space; };
	let i: i32 = 0;
	for (i < n) {
		m.ptr[m.pos + i] = buf[i];
		i += 1;
	};
	m.pos += n;
	return n: size;
};

fn dynamicwrite(s: io.stream, buf: []u8) (size | io.error) = {
	let m: *stream = s: *stream;
	let need: i32 = m.pos + buf.len;
	if (need > m.cap) { dynamicgrow(m, need); };
	let i: i32 = 0;
	for (i < buf.len) {
		m.ptr[m.pos + i] = buf[i];
		i += 1;
	};
	m.pos += buf.len;
	if (m.pos > m.len) { m.len = m.pos; };
	return buf.len: size;
};

fn dynamicclose(s: io.stream) (void | io.error) = {
	let m: *stream = s: *stream;
	if (m.cap > 0) { os.free(m.ptr: *void, m.cap: u64); };
	m.ptr = nil;
	m.len = 0;
	m.cap = 0;
	m.pos = 0;
	return;
};

// Double-and-copy growth with floor at 8. `dynamicgrow`, not Hare's
// bare `grow`: cstage bundles all imported modules into a flat TU and
// resolves private fns by unqualified name (task #9), so the
// module-prefixed name keeps the symmetry with dynamicwrite/dynamicclose.
fn dynamicgrow(d: *stream, need: i32) void = {
	let newcap: i32 = d.cap;
	if (newcap < 8) { newcap = 8; };
	for (newcap < need) { newcap *= 2; };
	let nbuf: *u8 = rt.malloc(newcap: u64): *u8;
	let i: i32 = 0;
	for (i < d.len) {
		nbuf[i] = d.ptr[i];
		i += 1;
	};
	if (d.cap > 0) { os.free(d.ptr: *void, d.cap: u64); };
	d.ptr = nbuf;
	d.cap = newcap;
};

// ---- accessors over the common `stream` header -----------------------
//
// Single fn each: Hare's string/reset/buffer/borrowedread all take
// `*stream` and read the flat header.

// string — bytes written so far, as a str view (buf[0..pos]).
//
// Mirrors ref/hare/memio/stream.ha:81 string(in: *stream). Hare returns
// (str | utf8::invalid) — the validating constructor. ww returns a bare
// `str` per the CLAUDE.md rule-9 frombytes carve-out: utf8.validate at
// the IO source is opt-in, never wrapped per-construction; the honest
// name reserves a future validating helper.
export fn string(s: *stream) str = {
	let r: str;
	r.ptr = s.ptr;
	r.len = s.pos;
	return r;
};

// buffer — borrowed []u8 view of bytes written so far (buf[0..pos]).
//
// Mirrors ref/hare/memio/stream.ha:74 buffer(in: *stream).
export fn buffer(s: *stream) []u8 = {
	let r: []u8;
	r.ptr = s.ptr;
	r.len = s.pos;
	return r;
};

// reset — rewind the cursor and truncate the logical content to 0.
// Backing storage is preserved; subsequent writes (dynamic) re-fill
// from the start without reallocation.
//
// Mirrors ref/hare/memio/stream.ha:87 reset(in: *stream).
export fn reset(s: *stream) void = {
	s.pos = 0;
	s.len = 0;
};

// borrowedread — return an `amt`-byte view starting at `pos` without
// copying, advancing the cursor. eof if fewer bytes are available.
//
// Mirrors ref/hare/memio/stream.ha:94 borrowedread(st: *stream, amt).
// `amt: i32` (not Hare's `size`) per the i32-index convention.
export fn borrowedread(s: *stream, amt: i32) ([]u8 | io.eof) = {
	if (s.len - s.pos < amt) {
		let e: io.eof;
		return e;
	};
	let r: []u8;
	r.ptr = s.ptr + (s.pos: u64);
	r.len = amt;
	s.pos += amt;
	return r;
};