// bufio — buffered I/O over [[io.stream]] (= `*io.vtable`). Subset of
// Hare's bufio:: surface (ref/hare/bufio/{scanner,stream}.ha). Project
// #94 fold-eFinal.
//
// Surface:
//
//   bufio.newscannerbuf(src: io.stream, buf: []u8) scanner
//   bufio.finish     (s: *scanner) void
//   bufio.scanbyte   (s: *scanner) (u8  | io.eof | io.error)
//   bufio.scanbytes  (s: *scanner, delim: u8)
//                    ([]u8 | io.eof | io.error | overflow)
//   bufio.scanline   (s: *scanner) (str | io.eof | io.error | overflow)
//
//   bufio.init       (src: io.stream, rbuf: []u8, wbuf: []u8) stream
//   bufio.flush      (b: *stream) (void | io.error)
//   bufio.setflush   (b: *stream, bs: []u8) void
//   bufio.unread     (b: *stream, buf: []u8) void
//   bufio.isbuffered (s: io.stream) bool
//
// VALUE-RETURN (Hare ref/hare/bufio/stream.ha:69 init, scanner.ha:92
// newscanner_buf): each constructor builds in a local `let r: T;`,
// field-assigns every slot, and `return r;`. The caller owns the
// returned struct (stack ownership, no-GC). For the buffered stream
// the caller passes `&b.vt` to the io dispatchers; the scanner is a
// plain read-ahead tokenizer (no embedded vtable) driven directly via
// scanbyte / scanbytes / scanline.
//
// SCOPE: this is the surface ww ships, NOT a port of Hare's full
// scanner. The features ww never implemented — auto-grow newscanner,
// multibyte-delim scanbytes / scanstring, scanrune / readbyte / readtok
// / readline / readrune / unreadrune — are a separate future feature
// fold (#217). The ported set: the fixed-buffer newscannerbuf,
// single-byte scanbytes, single-byte scanline, scanbyte, finish; and
// the buffered-stream init / setflush / flush / unread / isbuffered.
//
// Cast workaround per #206-payoff (ken: KEEP the explicit casts; they
// are cgen-neutral and sidestep the #214 over-acceptance surface).
// `(&fn_name): *io.<role>` at each vtable store + the isbuffered
// fn-ptr-equality comparand — the #206 cast-drop is gated on #214.
//
// Mode discrimination (drew-deferred): Hare uses three vtable
// singletons (vtable_r / vtable_w / vtable_rw); ww's vtable always
// carries all three callbacks (a zero-length rbuf/wbuf degenerates the
// matching callback in-cb). Defer-handle (MANAGED_* ownership bits)
// also deferred — caller owns rbuf/wbuf/src; bclose flushes + forwards
// close but frees nothing. Both graduate with io fold-2 (#5).

package bufio;

import io;

// flushdefault — backing storage for the default flush byte-set
// ("\n"). Hare scopes it inside `init` as `static let
// flush_default = ['\n': u8]` (ref/hare/bufio/stream.ha:75); ww
// has no function-scope statics, so it lives at module scope.
let flushdefault: [1]u8 = [10u8];

// rt_abort — terminate on a precondition violation. Used by
// [[unread]] for the "buf fits in front of rbuf" assertion that
// Hare expresses with `assert`.
@symbol("rt_abort") fn rtabort(msg: str) void;

// overflow — the scanner buffer filled before the delimiter (or
// underlying EOF) was hit. With a caller-supplied buffer we can't
// grow; bumping the budget is on the caller. Mirrors Hare bufio's
// use of errors::overflow for the same condition.
export type overflow = !void;

// ---- buffered stream ------------------------------------------------------

// stream — heap-free buffered read+write over an underlying io.stream.
// `vt` at offset 0 for the intrusive io.stream→*stream cast. `src` is
// an io.stream (the vtable-native underlying handle). Mirrors
// ref/hare/bufio/stream.ha:18.
//
// rbuf[rstart..rend] is pending read data; wbuf[0..wend] is pending
// write data. rbuf or wbuf may be zero-length: read-empty makes bread
// return io.eof immediately, write-empty makes bwrite a pass-through to
// src (no buffering).
export type stream = struct {
	vt:     io.vtable,
	src:    io.stream,
	rbuf:   []u8,
	rstart: i32,
	rend:   i32,
	wbuf:   []u8,
	wend:   i32,
	flush:  []u8,
};

// init — wire a buffered stream over an underlying io.stream with
// caller-supplied read/write buffers. Both rbuf and wbuf may be empty;
// bread / bwrite degenerate (see [[stream]]). The flush byte-set
// defaults to "\n" (line-buffered writes); [[setflush]] swaps it.
//
// Returns the stream BY VALUE; the caller passes `&b.vt` to the io
// dispatchers. Mirrors ref/hare/bufio/stream.ha:69.
export fn init(src: io.stream, rbuf: []u8, wbuf: []u8) stream = {
	let r: stream;
	r.vt.reader = (&bread):  *io.reader;
	r.vt.writer = (&bwrite): *io.writer;
	r.vt.closer = (&bclose): *io.closer;
	r.src    = src;
	// rstart=rbuf.len, rend=rbuf.len: pre-read unread budget = rbuf.len
	// (Hare bufio/stream.ha:101).
	r.rstart = rbuf.len;
	r.rend   = rbuf.len;
	r.rbuf   = rbuf;
	r.wbuf   = wbuf;
	r.wend   = 0;
	r.flush  = flushdefault[0:1];
	return r;
};

// setflush — install a new flush byte-set. Any byte from `bs` appearing
// in a write payload triggers an automatic flush after the write copies
// into wbuf. Mirrors ref/hare/bufio/stream.ha:128.
export fn setflush(b: *stream, bs: []u8) void = {
	b.flush = bs;
};

// flush — drain any pending wbuf data to src. Public API AND the
// internal drain called by bwrite / bclose. A 0-byte non-error write
// means the sink made no progress (memio.fixed full) — surfaced as a
// nomem-carried io.error rather than spinning, matching what io.writeall
// would do in Hare. Mirrors ref/hare/bufio/stream.ha.
export fn flush(b: *stream) (void | io.error) = {
	if (b.wend == 0) { return; };
	let off: i32 = 0;
	for (off < b.wend) {
		let r: (size | io.error) = io.write(b.src, b.wbuf[off:b.wend]);
		match (r) {
		case let n: size => {
			if (n == 0: size) {
				let nm: nomem;
				let e: io.error = nm;
				return e;
			};
			off += n: i32;
		};
		case let e: io.error => return e;
		};
	};
	b.wend = 0;
	return;
};

// unread — push `buf` back into the read buffer so the next bread
// returns it first. The bytes must fit in front of the pending region
// (rstart >= buf.len); Hare aborts on overflow, ww does the same via
// rtabort. Mirrors ref/hare/bufio/stream.ha:164.
export fn unread(b: *stream, buf: []u8) void = {
	if (b.rstart < buf.len) {
		rtabort("bufio.unread: more data than rbuf has room for");
	};
	let i: i32 = 0;
	for (i < buf.len) {
		b.rbuf[b.rstart - buf.len + i] = buf[i];
		i += 1;
	};
	b.rstart -= buf.len;
};

// isbuffered — true when `s` was returned by [[init]]. Hare's
// discriminator is callback identity (ref/hare/bufio/stream.ha:179).
// Either reader or writer matching is sufficient.
export fn isbuffered(s: io.stream) bool = {
	match (s.reader) {
	case let r: *io.reader => {
		if (r == (&bread): *io.reader) { return true; };
	};
	case void => { };
	};
	match (s.writer) {
	case let w: *io.writer => {
		if (w == (&bwrite): *io.writer) { return true; };
	};
	case void => { };
	};
	return false;
};

// ---- buffered-stream vtable callbacks ------------------------------------

// bread — buffered read. Recover stream via the intrusive cast; refill
// rbuf from src via io.read on empty pending region. Mirrors
// ref/hare/bufio/stream.ha (stream_read).
fn bread(s: io.stream, buf: []u8) (size | io.eof | io.error) = {
	let b: *stream = s: *stream;
	if (b.rbuf.len == 0) {
		let e: io.eof; return e;
	};
	if (b.rstart >= b.rend) {
		b.rstart = 0;
		b.rend = 0;
		let r: (size | io.eof | io.error) = io.read(b.src, b.rbuf);
		match (r) {
		case let n: size => { b.rend = n: i32; };
		case io.eof => { let e: io.eof; return e; };
		case let e: io.error => return e;
		};
	};
	let avail: i32 = b.rend - b.rstart;
	let n: i32 = buf.len;
	if (avail < n) { n = avail; };
	let i: i32 = 0;
	for (i < n) {
		buf[i] = b.rbuf[b.rstart + i];
		i += 1;
	};
	b.rstart += n;
	return n: size;
};

// bwrite — buffered write. wbuf-empty passes through to src; otherwise
// default-flush scan + per-batch copy + post-write conditional flush.
// Labeled-break inlined (ww has no labeled break). Mirrors
// ref/hare/bufio/stream.ha (stream_write).
fn bwrite(s: io.stream, buf: []u8) (size | io.error) = {
	let b: *stream = s: *stream;
	if (b.wbuf.len == 0) {
		return io.write(b.src, buf);
	};
	let doflush: bool = false;
	if (b.flush.len != 0) {
		let i: i32 = 0;
		for (i < buf.len) {
			let j: i32 = 0;
			for (j < b.flush.len) {
				if (buf[i] == b.flush[j]) {
					doflush = true;
					i = buf.len;
					j = b.flush.len;
				};
				j += 1;
			};
			i += 1;
		};
	};
	let z: i32 = 0;
	for (z < buf.len) {
		let avail: i32 = b.wbuf.len - b.wend;
		if (avail == 0) {
			let fr: (void | io.error) = flush(b);
			match (fr) {
			case void => { };
			case let e: io.error => return e;
			};
			avail = b.wbuf.len;
		};
		let n: i32 = buf.len - z;
		if (avail < n) { n = avail; };
		let i: i32 = 0;
		for (i < n) {
			b.wbuf[b.wend + i] = buf[z + i];
			i += 1;
		};
		b.wend += n;
		z += n;
	};
	if (doflush) {
		let fr: (void | io.error) = flush(b);
		match (fr) {
		case void => { };
		case let e: io.error => return e;
		};
	};
	return buf.len: size;
};

// bclose — flush pending wbuf, forward close to src via io.close.
// Caller-owned buffers/src are not freed (drew defer-handle deferral).
fn bclose(s: io.stream) (void | io.error) = {
	let b: *stream = s: *stream;
	let fr: (void | io.error) = flush(b);
	match (fr) {
	case void => { };
	case let e: io.error => return e;
	};
	let cr: (void | io.error) = io.close(b.src);
	match (cr) {
	case void => return void;
	case let e: io.error => return e;
	};
};

// ---- scanner (read-ahead tokenizer over an io.stream src) ----------------
//
// Plain tokenizer — no embedded vtable; driven directly via scanbyte /
// scanbytes / scanline, not through io dispatch. Its `src` is an
// io.stream and refills go through io.read. Value-return constructor
// (Hare newscanner_buf, scanner.ha:92).
//
// ptr/cap kept flat (no `buf: []u8`); the scanner predates struct-held
// slice support.

export type scanner = struct {
	src:   io.stream,
	ptr:   *u8,
	cap:   i32,
	start: i32,	// index where the pending region starts in ptr
	avail: i32,	// pending byte count; pending = ptr[start..start+avail]
};

// newscannerbuf — wire a scanner to read through `src` using `buf` as
// the fixed read-ahead window. Returns the scanner BY VALUE. This is
// Hare's newscanner_buf (ref/hare/bufio/scanner.ha:92); the auto-grow
// newscanner is a separate feature (#217).
export fn newscannerbuf(src: io.stream, buf: []u8) scanner = {
	let r: scanner;
	r.src = src;
	r.ptr = buf.ptr;
	r.cap = buf.len;
	r.start = 0;
	r.avail = 0;
	return r;
};

// finish — release scanner-owned resources. No-op (buffer is
// caller-owned, src isn't closed); kept on the surface so callers won't
// churn. Mirrors ref/hare/bufio/scanner.ha:110.
export fn finish(s: *scanner) void = { };

// readahead — make room and read once from src into the back of the
// pending region. Returns bytes newly buffered (>=0), or io.eof/io.error
// from src. The size from io.read narrows to i32 (buffer-length type).
fn readahead(s: *scanner) (i32 | io.eof | io.error) = {
	if (s.start + s.avail == s.cap && s.start > 0) {
		let i: i32 = 0;
		for (i < s.avail) {
			s.ptr[i] = s.ptr[s.start + i];
			i += 1;
		};
		s.start = 0;
	};
	let off: i32 = s.start + s.avail;
	let v: []u8;
	v.ptr = s.ptr + (off: u64);
	v.len = s.cap - off;
	let r: (size | io.eof | io.error) = io.read(s.src, v);
	match (r) {
	case let n: size => {
		s.avail += n: i32;
		return n: i32;
	};
	case io.eof => { let e: io.eof; return e; };
	case let e: io.error => return e;
	};
};

// scanbyte — pop one byte, refilling from src on demand. Mirrors
// ref/hare/bufio/scanner.ha:204.
export fn scanbyte(s: *scanner) (u8 | io.eof | io.error) = {
	for (s.avail == 0) {
		let r: (i32 | io.eof | io.error) = readahead(s);
		match (r) {
		case let n: i32 => { };
		case io.eof => { let e: io.eof; return e; };
		case let e: io.error => return e;
		};
	};
	let b: u8 = s.ptr[s.start];
	s.start += 1;
	s.avail -= 1;
	return b;
};

// scanbytes — read up to (and not including) the next byte equal to
// `delim`. The delim is consumed but not returned. The returned slice
// borrows from the scanner buffer and is invalidated by the next scan.
// EOF without delim discards the trailing fragment and returns io.eof
// (Hare EOF_DISCARD default); buffer-full without delim returns
// overflow. Single-byte delim only — Hare's `(u8 | []u8)` multibyte
// form is #217. Mirrors ref/hare/bufio/scanner.ha:220 (narrowed).
export fn scanbytes(s: *scanner, delim: u8) ([]u8 | io.eof | io.error | overflow) = {
	let i: i32 = 0;
	for (true) {
		for (i < s.avail) {
			if (s.ptr[s.start + i] == delim) {
				let v: []u8;
				v.ptr = s.ptr + (s.start: u64);
				v.len = i;
				s.start += i + 1;
				s.avail -= i + 1;
				return v;
			};
			i += 1;
		};
		if (s.start + s.avail == s.cap && s.start == 0) {
			let e: overflow; return e;
		};
		let r: (i32 | io.eof | io.error) = readahead(s);
		match (r) {
		case let n: i32 => { };
		case io.eof => { let e: io.eof; return e; };
		case let e: io.error => return e;
		};
	};
	let e: io.eof; return e;
};

// scanline — read up to (and not including) the next '\n'. The newline
// is consumed; the returned str view borrows from the scanner buffer.
// Single-byte route (Hare's scan_line = scan_string(s, "\n"); the
// arbitrary multibyte-delim scan_string is #217). Mirrors
// ref/hare/bufio/scanner.ha:307.
export fn scanline(s: *scanner) (str | io.eof | io.error | overflow) = {
	let r: ([]u8 | io.eof | io.error | overflow) = scanbytes(s, 10u8);
	match (r) {
	case let bs: []u8 => {
		let v: str;
		v.ptr = bs.ptr;
		v.len = bs.len;
		return v;
	};
	case io.eof => { let e: io.eof; return e; };
	case let e: io.error => return e;
	case overflow => { let e: overflow; return e; };
	};
};