//! POSIX serial port: raw mode, standard baud rates, and the DTR/RTS lines that put an Espressif
//! chip into download mode. No libc, no external tool - std.posix for termios, std.os.linux for
//! the two modem-control ioctls std does not wrap, std.Io.File for the byte traffic.
//!
//! On this board DTR drives the boot strap (GPIO35) and RTS drives CHIP_PU through a transistor
//! pair, which is why the reset sequence below needs no button press.

const std = @import("std");
const posix = std.posix;
const linux = std.os.linux;

/// Any rate the hardware can divide to, not just the historical enum values: the port is
/// configured through termios2, where the baud is a plain integer. The ESP32 ROM loader
/// auto-detects the host's rate from the 0x55 pattern in SYNC, so raising this needs no
/// CHANGE_BAUDRATE handshake.
pub const Baud = enum(u32) {
    b115200 = 115200,
    b230400 = 230400,
    b460800 = 460800,
    b921600 = 921600,
    b1500000 = 1500000,
    b2000000 = 2000000,

    pub fn rate(b: Baud) u32 {
        return @intFromEnum(b);
    }
};

/// The remedy for the one error every first run hits, in the module that owns port opening so the
/// build steps and `p4-console` cannot drift apart. Deliberately says nothing about the port's
/// path: the caller names that on its own first line, which is the only part that differs.
///
/// The last case is the one worth spelling out, because it is the one that looks like the fix did
/// not work: a shell started before `usermod` never sees the new group, since credentials are
/// captured at login and not re-read.
pub const access_denied_help =
    \\  It is owned by a group your session is not in (uucp on Arch, dialout on Debian).
    \\
    \\  If you are NOT in that group yet, join it and log in again:
    \\      sudo usermod -aG uucp $USER
    \\
    \\  If `id -nG` already lists it, this shell simply predates the change - group membership is
    \\  captured at login. Either log out and back in, or take it in this shell:
    \\      newgrp uucp
    \\
    \\  Or run one command with the group, without touching this shell at all:
    \\      sg uucp -c '...'
;

/// `struct termios2` as the Linux kernel defines it: 19 control characters, then the two integer
/// baud fields. std's `linux.termios2` uses NCCS = 32, which makes it 60 bytes instead of 44 and
/// therefore encodes TCGETS2/TCSETS2 with the wrong size field - the ioctl then fails with ENOTTY.
/// Declaring the struct here keeps the numbers right and the intent visible.
const Termios2 = extern struct {
    iflag: u32,
    oflag: u32,
    cflag: u32,
    lflag: u32,
    line: u8,
    cc: [19]u8,
    ispeed: u32,
    ospeed: u32,

    /// _IOR('T', 0x2A, struct termios2) and _IOW('T', 0x2B, struct termios2) for a 44-byte struct.
    const TCGETS2: u32 = 0x802C542A;
    const TCSETS2: u32 = 0x402C542B;
    /// _IOW('T', 0x2C, ...): TCSETS2's draining sibling. Changing the divisor while bytes are
    /// still in the kernel's output queue sends the tail of the old line at the new rate, which on
    /// a mid-session baud switch corrupts exactly the handshake line the switch was announced by.
    const TCSETSW2: u32 = 0x402C542C;

    /// CBAUD escape meaning "take the rate from ispeed/ospeed" (asm-generic/termbits.h).
    const BOTHER: u32 = 0o010000;
    const CBAUD: u32 = 0o010017;
    const CS8: u32 = 0o000060;
    const CREAD: u32 = 0o000200;
    const CLOCAL: u32 = 0o004000;
};

pub const Port = struct {
    file: std.Io.File,
    io: std.Io,
    saved: Termios2,
    /// The rate currently programmed, kept because the wire's capacity in bytes per second is
    /// `rate/10` and anything measuring this link against its ceiling needs that number. The
    /// kernel would answer a TCGETS2, but a syscall per sample to re-read a value only this file
    /// ever changes is worse than a field.
    baud: Baud,

    // std exposes neither these ioctl numbers nor the TIOCM bits.
    const TIOCEXCL = 0x540C;
    const TCFLSH = 0x540B;
    /// tcdrain, with a nonzero argument. Zero would transmit a break instead.
    const TCSBRK = 0x5409;
    const TCIFLUSH = 0;
    const TIOCMGET = 0x5415;
    const TIOCMSET = 0x5418;
    const DTR: u32 = 0x002;
    const RTS: u32 = 0x004;

    pub fn open(path: []const u8, baud: Baud) !Port {
        const fd = try posix.openat(posix.AT.FDCWD, path, .{
            .ACCMODE = .RDWR,
            .NOCTTY = true,
            .CLOEXEC = true,
        }, 0);
        const file: std.Io.File = .{ .handle = fd, .flags = .{ .nonblocking = false } };
        const io = std.Io.Threaded.global_single_threaded.io();
        errdefer file.close(io);

        // Configure through termios2: it is the only interface whose baud fields the kernel
        // populates. `tcsetattr` uses TCSETS, whose struct has no ispeed/ospeed, so assigning
        // those fields silently does nothing and leaves the port at whatever rate it had - which
        // is exactly the bug that made a 1.5 KB flash take 230 ms instead of 60.
        var saved: Termios2 = undefined;
        if (@as(isize, @bitCast(linux.ioctl(fd, Termios2.TCGETS2, @intFromPtr(&saved)))) < 0)
            return error.NotATerminal;

        var raw = saved;
        // A raw byte pipe: no canonical mode, no echo, no signals, no flow control, no CR/LF
        // translation, 8N1, CLOCAL so a missing carrier-detect cannot block reads, and the baud
        // taken from the integer fields.
        raw.iflag = 0;
        raw.oflag = 0;
        raw.lflag = 0;
        raw.cflag = Termios2.CS8 | Termios2.CREAD | Termios2.CLOCAL | Termios2.BOTHER;
        raw.ispeed = baud.rate();
        raw.ospeed = baud.rate();
        @memset(&raw.cc, 0);
        raw.cc[6] = 0; // VMIN: never block for a minimum count
        raw.cc[5] = 0; // VTIME: poll() owns the timeouts
        if (@as(isize, @bitCast(linux.ioctl(fd, Termios2.TCSETS2, @intFromPtr(&raw)))) < 0)
            return error.SetAttrFailed;

        // Claim the port exclusively, the way pyserial (and therefore esptool) does: otherwise a
        // `zig build monitor` left running in another terminal splits the ROM's replies between
        // two readers, and the failure looks like random SyncFailed.
        if (@as(isize, @bitCast(linux.ioctl(fd, TIOCEXCL, 0))) < 0) return error.PortBusy;
        // Drop anything the kernel captured at the previous line rate.
        _ = linux.ioctl(fd, TCFLSH, TCIFLUSH);

        return .{ .file = file, .io = io, .saved = saved, .baud = baud };
    }

    /// Re-rate an already-open port, leaving the raw-mode flags and the exclusive claim alone.
    ///
    /// This exists because the console and the flasher want different rates on the same wire. The
    /// ROM loader auto-detects the host's rate from SYNC's 0x55 pattern, so `open` can simply pick
    /// one; a running application cannot, because its UART divider was programmed by the
    /// second-stage bootloader and the only way to change it is for the firmware to reprogram its
    /// own divider and for the host to follow. That makes the switch a two-sided handshake, and
    /// `TCSETSW2` is the half that has to be ordered: the firmware announces the change on the old
    /// rate, and the announcement must have physically left the wire before either side moves.
    pub fn setBaud(p: *Port, baud: Baud) !void {
        var t: Termios2 = undefined;
        if (@as(isize, @bitCast(linux.ioctl(p.file.handle, Termios2.TCGETS2, @intFromPtr(&t)))) < 0)
            return error.NotATerminal;
        t.cflag = (t.cflag & ~Termios2.CBAUD) | Termios2.BOTHER;
        t.ispeed = baud.rate();
        t.ospeed = baud.rate();
        if (@as(isize, @bitCast(linux.ioctl(p.file.handle, Termios2.TCSETSW2, @intFromPtr(&t)))) < 0)
            return error.SetAttrFailed;
        p.baud = baud;
    }

    /// Bytes per second the wire can carry: one 8N1 byte occupies ten bit times.
    pub fn capacity(p: *const Port) u32 {
        return p.baud.rate() / 10;
    }

    /// Block until every byte written has physically left the wire - `tcdrain`, spelled as the
    /// ioctl because std exposes neither.
    ///
    /// Distinct from `drain` above in both direction and meaning, which is worth stating because
    /// getting them the wrong way round silently invalidates a measurement: `drain` discards what
    /// has ARRIVED, this waits for what is LEAVING. A `write` returns once the kernel has accepted
    /// the bytes, so timing a transfer to the write measures a memcpy into a tty buffer - at 115200
    /// that reported 202% of the wire's capacity, which is how the confusion was noticed.
    pub fn flushOutput(p: *Port) void {
        // TCSBRK with a nonzero argument is tcdrain on Linux; with zero it would send a break.
        _ = linux.ioctl(p.file.handle, TCSBRK, 1);
    }

    pub fn close(p: *Port) void {
        _ = linux.ioctl(p.file.handle, Termios2.TCSETS2, @intFromPtr(&p.saved));
        p.file.close(p.io);
    }

    pub fn write(p: *Port, bytes: []const u8) !void {
        try p.file.writeStreamingAll(p.io, bytes);
    }

    /// Read whatever is available, waiting at most `timeout_ms`. Returns 0 on timeout.
    pub fn read(p: *Port, buf: []u8) !usize {
        var pfd = [_]posix.pollfd{.{ .fd = p.file.handle, .events = posix.POLL.IN, .revents = 0 }};
        const ready = try posix.poll(&pfd, 0);
        if (ready == 0) return 0;
        return p.file.readStreaming(p.io, &.{buf}) catch |err| switch (err) {
            error.EndOfStream => 0,
            else => err,
        };
    }

    pub fn readTimeout(p: *Port, buf: []u8, timeout_ms: i32) !usize {
        var pfd = [_]posix.pollfd{.{ .fd = p.file.handle, .events = posix.POLL.IN, .revents = 0 }};
        const ready = try posix.poll(&pfd, timeout_ms);
        if (ready == 0) return 0;
        return p.file.readStreaming(p.io, &.{buf}) catch |err| switch (err) {
            error.EndOfStream => 0,
            else => err,
        };
    }

    pub fn drain(p: *Port) void {
        var scratch: [512]u8 = undefined;
        while (true) {
            const n = p.read(&scratch) catch return;
            if (n == 0) return;
        }
    }

    fn setLines(p: *Port, dtr: bool, rts: bool) !void {
        var flags: u32 = 0;
        if (linux.ioctl(p.file.handle, TIOCMGET, @intFromPtr(&flags)) != 0) return error.IoctlFailed;
        flags = if (dtr) flags | DTR else flags & ~DTR;
        flags = if (rts) flags | RTS else flags & ~RTS;
        if (linux.ioctl(p.file.handle, TIOCMSET, @intFromPtr(&flags)) != 0) return error.IoctlFailed;
    }

    /// How long to hold each phase of the reset sequence. esptool uses 100/50/50 ms; the shorter
    /// numbers below were measured on this board over repeated runs. Tunable because a different
    /// carrier's RC network may need longer.
    pub const ResetTiming = struct {
        hold_reset_ms: i64 = 40,
        strap_settle_ms: i64 = 25,
        release_ms: i64 = 15,
    };

    /// Classic reset into the ROM download loader: hold the boot strap asserted across a reset
    /// pulse. Both lines are inverted by the board's transistor pair, so an asserted RS-232 line
    /// pulls its pin low.
    pub fn resetToDownload(p: *Port, timing: ResetTiming) !void {
        // Never leave the board held in reset because a modem-line ioctl failed halfway through.
        errdefer p.setLines(false, false) catch {};
        try p.setLines(false, true); // strap released, EN low: held in reset
        p.sleepMs(timing.hold_reset_ms);
        try p.setLines(true, false); // strap asserted, EN high: enters the ROM loader
        p.sleepMs(timing.strap_settle_ms);
        try p.setLines(false, false);
        p.sleepMs(timing.release_ms);
        p.drain();
    }

    /// Reset and let the flashed application run.
    pub fn resetToRun(p: *Port, timing: ResetTiming) !void {
        errdefer p.setLines(false, false) catch {};
        try p.setLines(false, true);
        p.sleepMs(timing.hold_reset_ms);
        try p.setLines(false, false);
        p.sleepMs(timing.release_ms);
    }

    fn sleepMs(p: *Port, ms: i64) void {
        std.Io.sleep(p.io, .fromMilliseconds(ms), .boot) catch {};
    }

    /// Milliseconds on a monotonic clock, for deadlines.
    pub fn nowMs(p: *Port) i64 {
        return std.Io.Timestamp.now(p.io, .boot).toMilliseconds();
    }
};