// memio — in-memory io stream.
//
// Hare's memio:: surface, drop underscores. Two flavours behind a
// single [[io.stream]]:
//
//   fixed       caller owns the buffer, writes stop when full.
//   dynamic     memio owns the buffer, writes grow it; close frees.
//
// Call shape divergence from Hare: the caller supplies both the
// memio `state` and the `io.stream` slot, by pointer. ww cgen does
// not yet implement &x.field or 32B-struct return-by-value, so the
// Hare `let s = memio::fixed(buf)` shape isn't reachable; collapse
// to a single returned struct when those land (lib/CLAUDE.md
// "graduate in one go").
//
//   let mem: memio.state;
//   let s: io.stream;
//   memio.fixed(&mem, &s, buf);
//   io.write(&s, bytes);
//
// Subset of Hare's surface: io.stream's variants are {eof, closed},
// 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. Hare's seek / copy callbacks are
// likewise absent: lib/io's stream vtable has only read/write/close
// slots, so memio can't wire a seeker or copier even if we wanted
// to. All three come back when their dependencies do.

package memio;

import io;
import os;
import rt;

// state — memio's per-stream bookkeeping. The caller owns the slot
// and passes its address into a constructor. `ptr/len/cap` are the
// slice fields kept flat to dodge a chained-dot write through the
// state pointer (cgen doesn't store into `m.buf.ptr` reliably).
export type state = struct {
	ptr: *u8,
	len: i32,
	cap: i32,
	pos: i32,
};

// fixed — wire `s` over a caller-supplied buffer. Writes never grow;
// they return 0 once `pos` reaches the end of the buffer.
export fn fixed(m: *state, s: *io.stream, buf: []u8) void = {
	m.ptr = buf.ptr;
	m.len = buf.len;
	m.cap = buf.len;
	m.pos = 0;
	s.ctx = m: *void;
	s.read = readfn;
	s.write = fixedwrite;
	s.close = closenoop;
};

// dynamic — wire `s` with no initial buffer. Writes grow the backing
// allocation; [[io.close]] frees it.
export fn dynamic(m: *state, s: *io.stream) void = {
	m.ptr = nil;
	m.len = 0;
	m.cap = 0;
	m.pos = 0;
	s.ctx = m: *void;
	s.read = readfn;
	s.write = dynamicwrite;
	s.close = dynamicclose;
};

// dynamicfrom — like [[dynamic]] but seeded with an existing slice.
// Ownership of the slice transfers to the stream; [[io.close]] frees
// it. The slice must come from the runtime allocator: close calls
// [[os.free]] with `m.cap` bytes, which is taken from `buf.cap` (the
// slice's allocated capacity), not its logical length. Passing a
// half-filled append slice (len < cap) and using only `buf.len` here
// would under-free on close.
export fn dynamicfrom(m: *state, s: *io.stream, buf: []u8) void = {
	m.ptr = buf.ptr;
	m.len = buf.len;
	m.cap = buf.cap;
	m.pos = 0;
	s.ctx = m: *void;
	s.read = readfn;
	s.write = dynamicwrite;
	s.close = dynamicclose;
};

// buffer — borrowed view of bytes written so far (buf[..pos]).
// Seek to the end before calling if the full buffer is wanted.
export fn buffer(m: *state) []u8 = {
	let r: []u8;
	r.ptr = m.ptr;
	r.len = m.pos;
	return r;
};

// string — bytes written so far, as a str view. Hare returns
// (str | utf8::invalid); ww doesn't ship utf8 validation yet, so
// this returns the unchecked view.
export fn string(m: *state) str = {
	let r: str;
	r.ptr = m.ptr;
	r.len = m.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.
export fn reset(m: *state) void = {
	m.pos = 0;
	m.len = 0;
};

// borrowedread — return an `amt`-byte view starting at `pos` without
// copying, advancing the cursor. eof if fewer bytes are available.
export fn borrowedread(m: *state, amt: i32) ([]u8 | io.eof) = {
	if (m.len - m.pos < amt) {
		let e: io.eof;
		return e;
	};
	let r: []u8;
	r.ptr = m.ptr + (m.pos: u64);
	r.len = amt;
	m.pos += amt;
	return r;
};

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

fn readfn(s: *io.stream, buf: []u8) (i32 | io.eof | io.closed) = {
	let m: *state = s.ctx: *state;
	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;
};

fn fixedwrite(s: *io.stream, buf: []u8) (i32 | io.closed) = {
	let m: *state = s.ctx: *state;
	if (m.pos >= m.len) { return 0; };
	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;
};

fn dynamicwrite(s: *io.stream, buf: []u8) (i32 | io.closed) = {
	let m: *state = s.ctx: *state;
	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;
};

fn dynamicclose(s: *io.stream) (void | io.closed) = {
	let m: *state = s.ctx: *state;
	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;
};

fn closenoop(s: *io.stream) (void | io.closed) = {
	return;
};

// Double-and-copy growth. Initial bump from 0 lands at 8 to amortise
// small write bursts without a tail of reallocs.
//
// `dynamicgrow`, not Hare's bare `grow`: cstage bundles all imported
// modules into a flat TU and resolves private fns by unqualified
// name, so two `fn grow` decls (here + selfhost/cmd/wcc/mem.ww's
// arena `grow`) collide. Module-prefixed name keeps the symmetry
// with `dynamicwrite`/`dynamicclose` until task #9 (module-aware
// private-fn scoping in cstage) lands.
fn dynamicgrow(m: *state, need: i32) void = {
	let newcap: i32 = m.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 < m.len) {
		nbuf[i] = m.ptr[i];
		i += 1;
	};
	if (m.cap > 0) { os.free(m.ptr: *void, m.cap: u64); };
	m.ptr = nbuf;
	m.cap = newcap;
};