vtty 0.1.0

Zero-allocation, panic-free DEC VT500 terminal parser with UTF-8 decoding and no_std support.
Documentation 
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// Copyright © Fangdun Tsai <fundon@pindash.io>
// SPDX-License-Identifier: Apache-2.0 OR MIT

//! DEC VT500 escape-sequence state machine: parser states, the packed
//! `Action` table, and the `transition` lookup.
//!
//! Reference: Paul Flo Williams' VT500 parser state diagram
//! <https://vt100.net/emu/dec_ansi_parser> — the "State Definitions" and
//! "Action Definitions" sections map 1:1 to [`ParseState`] and [`Action`].
//! The grammar those states encode is ECMA-48 §5.4/§8.3
//! (<https://ecma-international.org/wp-content/uploads/ECMA-48_5th_edition_june_1991.pdf>).

/// VT500 parser states (mutually exclusive).
#[derive(Clone, Copy, Debug, PartialEq, Eq, Default)]
#[repr(u8)]
pub(crate) enum ParseState {
    /// Ground
    #[default]
    Ground = 0,
    /// Escape
    Escape = 1,
    /// `EscapeIntermediate`
    EscapeIntermediate = 2,
    /// `CsiEntry`
    CsiEntry = 3,
    /// `CsiParam`
    CsiParam = 4,
    /// `CsiIntermediate`
    CsiIntermediate = 5,
    /// `CsiIgnore`
    CsiIgnore = 6,
    /// `OscString`
    OscString = 7,
    /// `DcsEntry`
    DcsEntry = 8,
    /// `DcsParam`
    DcsParam = 9,
    /// `DcsIntermediate`
    DcsIntermediate = 10,
    /// `DcsPassthrough`
    DcsPassthrough = 11,
    /// `DcsIgnore`
    DcsIgnore = 12,
    /// `SosPmApcString`
    SosPmApcString = 13,
}

// ── Actions (bitmask — a byte can trigger several) ─────────────────────

bitflags::bitflags! {
    /// Parser actions a byte can trigger (a byte can trigger several).
    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
    pub(crate) struct Action: u16 {
        /// No action.
        const NONE = 0;
        /// print
        const PRINT = 1 << 0;
        /// execute
        const EXECUTE = 1 << 1;
        /// csi_dispatch
        const CSI_DISPATCH = 1 << 2;
        /// esc_dispatch
        const ESC_DISPATCH = 1 << 3;
        /// hook
        const HOOK = 1 << 4;
        /// put
        const PUT = 1 << 5;
        /// unhook
        const UNHOOK = 1 << 6;
        /// osc_start
        const OSC_START = 1 << 7;
        /// osc_put
        const OSC_PUT = 1 << 8;
        /// osc_end
        const OSC_END = 1 << 9;
    }
}

/// The DEC VT500 transition table for the non-ground states (each `ParseState`
/// arm maps to a "State Definitions" entry on
/// <https://vt100.net/emu/dec_ansi_parser>). This is the compile-time
/// evaluator, expanded into the flat [`TABLE`]; runtime [`transition`] is one
/// indexed load.
const fn transition_impl(state: ParseState, b: u8) -> (Action, ParseState) {
    match state {
        ParseState::Escape => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x1f => (Action::EXECUTE, ParseState::Escape),
            0x18 | 0x1a => (Action::EXECUTE, ParseState::Ground),
            0x1b => (Action::NONE, ParseState::Escape),
            0x20..=0x2f => (Action::NONE, ParseState::EscapeIntermediate),
            0x30..=0x4f | 0x51..=0x57 | 0x59 | 0x5a | 0x5c | 0x60..=0x7e => {
                (Action::ESC_DISPATCH, ParseState::Ground)
            }
            0x50 => (Action::NONE, ParseState::DcsEntry), // 'P' → DCS
            0x58 | 0x5e | 0x5f => (Action::NONE, ParseState::SosPmApcString), // 'X' '^' '_' → SOS/PM/APC
            0x5b => (Action::NONE, ParseState::CsiEntry),                     // '[' → CSI
            0x5d => (Action::OSC_START, ParseState::OscString),               // ']' → OSC
            _ => (Action::NONE, ParseState::Escape),
        },
        ParseState::EscapeIntermediate => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x1f => (Action::EXECUTE, ParseState::EscapeIntermediate),
            0x18 | 0x1a => (Action::EXECUTE, ParseState::Ground),
            0x1b => (Action::NONE, ParseState::Escape),
            0x20..=0x2f => (Action::NONE, ParseState::EscapeIntermediate),
            0x30..=0x7e => (Action::ESC_DISPATCH, ParseState::Ground),
            _ => (Action::NONE, ParseState::EscapeIntermediate),
        },
        ParseState::CsiEntry => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x1f => (Action::EXECUTE, ParseState::CsiEntry),
            0x18 | 0x1a => (Action::EXECUTE, ParseState::Ground),
            0x1b => (Action::NONE, ParseState::Escape),
            0x20..=0x2f => (Action::NONE, ParseState::CsiIntermediate),
            0x30..=0x39 => (Action::NONE, ParseState::CsiParam),
            0x3a => (Action::NONE, ParseState::CsiParam),
            0x3b => (Action::NONE, ParseState::CsiParam),
            0x3c..=0x3f => (Action::NONE, ParseState::CsiParam),
            0x40..=0x7e => (Action::CSI_DISPATCH, ParseState::Ground),
            _ => (Action::NONE, ParseState::CsiEntry),
        },
        ParseState::CsiParam => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x1f => (Action::EXECUTE, ParseState::CsiParam),
            0x18 | 0x1a => (Action::EXECUTE, ParseState::Ground),
            0x1b => (Action::NONE, ParseState::Escape),
            0x20..=0x2f => (Action::NONE, ParseState::CsiIntermediate),
            0x30..=0x39 => (Action::NONE, ParseState::CsiParam),
            0x3a | 0x3b => (Action::NONE, ParseState::CsiParam),
            0x3c..=0x3f => (Action::NONE, ParseState::CsiIgnore),
            0x40..=0x7e => (Action::CSI_DISPATCH, ParseState::Ground),
            _ => (Action::NONE, ParseState::CsiParam),
        },
        ParseState::CsiIntermediate => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x1f => (Action::EXECUTE, ParseState::CsiIntermediate),
            0x18 | 0x1a => (Action::EXECUTE, ParseState::Ground),
            0x1b => (Action::NONE, ParseState::Escape),
            0x20..=0x2f => (Action::NONE, ParseState::CsiIntermediate),
            0x30..=0x3f => (Action::NONE, ParseState::CsiIgnore),
            0x40..=0x7e => (Action::CSI_DISPATCH, ParseState::Ground),
            _ => (Action::NONE, ParseState::CsiIntermediate),
        },
        ParseState::CsiIgnore => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x1f => (Action::EXECUTE, ParseState::CsiIgnore),
            0x18 | 0x1a => (Action::EXECUTE, ParseState::Ground),
            0x1b => (Action::NONE, ParseState::Escape),
            0x20..=0x3f => (Action::NONE, ParseState::CsiIgnore),
            0x40..=0x7e => (Action::NONE, ParseState::Ground),
            _ => (Action::NONE, ParseState::CsiIgnore),
        },
        ParseState::OscString => match b {
            0x00..=0x06 | 0x08..=0x17 | 0x19 | 0x1c..=0x1f => (Action::NONE, ParseState::OscString),
            0x07 => (Action::OSC_END, ParseState::Ground),
            0x18 | 0x1a => (
                Action::from_bits_retain(Action::OSC_END.bits() | Action::EXECUTE.bits()),
                ParseState::Ground,
            ),
            0x1b => (Action::OSC_END, ParseState::Escape),
            0x9c => (Action::OSC_END, ParseState::Ground),
            _ => (Action::OSC_PUT, ParseState::OscString),
        },
        ParseState::DcsEntry => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x1f => (Action::NONE, ParseState::DcsEntry),
            0x18 | 0x1a => (Action::EXECUTE, ParseState::Ground),
            0x1b => (Action::NONE, ParseState::Escape),
            0x20..=0x2f => (Action::NONE, ParseState::DcsIntermediate),
            0x30..=0x39 => (Action::NONE, ParseState::DcsParam),
            0x3a => (Action::NONE, ParseState::DcsParam),
            0x3b => (Action::NONE, ParseState::DcsParam),
            0x3c..=0x3f => (Action::NONE, ParseState::DcsParam),
            0x40..=0x7e => (Action::HOOK, ParseState::DcsPassthrough),
            _ => (Action::NONE, ParseState::DcsEntry),
        },
        ParseState::DcsParam => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x1f => (Action::NONE, ParseState::DcsParam),
            0x18 | 0x1a => (Action::EXECUTE, ParseState::Ground),
            0x1b => (Action::NONE, ParseState::Escape),
            0x20..=0x2f => (Action::NONE, ParseState::DcsIntermediate),
            0x30..=0x39 => (Action::NONE, ParseState::DcsParam),
            0x3a | 0x3b => (Action::NONE, ParseState::DcsParam),
            0x3c..=0x3f => (Action::NONE, ParseState::DcsIgnore),
            0x40..=0x7e => (Action::HOOK, ParseState::DcsPassthrough),
            _ => (Action::NONE, ParseState::DcsParam),
        },
        ParseState::DcsIntermediate => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x1f => (Action::NONE, ParseState::DcsIntermediate),
            0x18 | 0x1a => (Action::EXECUTE, ParseState::Ground),
            0x1b => (Action::NONE, ParseState::Escape),
            0x20..=0x2f => (Action::NONE, ParseState::DcsIntermediate),
            0x30..=0x3f => (Action::NONE, ParseState::DcsIgnore),
            0x40..=0x7e => (Action::HOOK, ParseState::DcsPassthrough),
            _ => (Action::NONE, ParseState::DcsIntermediate),
        },
        ParseState::DcsPassthrough => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x7e => (Action::PUT, ParseState::DcsPassthrough),
            0x18 | 0x1a => (
                Action::from_bits_retain(Action::UNHOOK.bits() | Action::EXECUTE.bits()),
                ParseState::Ground,
            ),
            0x1b => (Action::UNHOOK, ParseState::Escape),
            0x9c => (Action::UNHOOK, ParseState::Ground),
            _ => (Action::NONE, ParseState::DcsPassthrough),
        },
        ParseState::DcsIgnore => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x1f => (Action::NONE, ParseState::DcsIgnore),
            0x18 | 0x1a => (Action::EXECUTE, ParseState::Ground),
            0x1b => (Action::NONE, ParseState::Escape),
            0x20..=0x7f => (Action::NONE, ParseState::DcsIgnore),
            0x9c => (Action::NONE, ParseState::Ground),
            _ => (Action::NONE, ParseState::DcsIgnore),
        },
        ParseState::SosPmApcString => match b {
            0x00..=0x17 | 0x19 | 0x1c..=0x1f => (Action::NONE, ParseState::SosPmApcString),
            0x18 | 0x1a => (Action::EXECUTE, ParseState::Ground),
            0x1b => (Action::NONE, ParseState::Escape),
            // SOS/PM/APC: the first content byte is the "action" (e.g. `G`
            // for kitty graphics APC `_G`); dispatch it as a hook and reuse
            // the DCS string state for put/unhook. This is more useful than
            // vte's blanket drop (vte predates kitty graphics).
            0x20..=0x7e => (Action::HOOK, ParseState::DcsPassthrough),
            0x9c => (Action::NONE, ParseState::Ground),
            _ => (Action::NONE, ParseState::SosPmApcString),
        },
        ParseState::Ground => (Action::NONE, ParseState::Ground),
    }
}

/// A packed transition entry: bits 0..=15 hold [`Action`], bits 16..=23 hold
/// the next [`ParseState`].
type Entry = u32;

const fn pack(action: Action, state: ParseState) -> Entry {
    (action.bits() as u32) | ((state as u8 as u32) << 16)
}

const fn parse_state(v: u8) -> ParseState {
    match v {
        0 => ParseState::Ground,
        1 => ParseState::Escape,
        2 => ParseState::EscapeIntermediate,
        3 => ParseState::CsiEntry,
        4 => ParseState::CsiParam,
        5 => ParseState::CsiIntermediate,
        6 => ParseState::CsiIgnore,
        7 => ParseState::OscString,
        8 => ParseState::DcsEntry,
        9 => ParseState::DcsParam,
        10 => ParseState::DcsIntermediate,
        11 => ParseState::DcsPassthrough,
        12 => ParseState::DcsIgnore,
        13 => ParseState::SosPmApcString,
        _ => ParseState::Ground,
    }
}

/// The DEC VT500 transition table expanded to a flat `[state][byte]` lookup
/// table at compile time. 14 states × 256 bytes = 3584 × 4 bytes ≈ 14 KB,
/// resident in `.rodata`; the runtime lookup is a single branchless load.
static TABLE: [Entry; 14 * 256] = build_table();

const fn build_table() -> [Entry; 14 * 256] {
    let mut t = [0_u32; 14 * 256];
    let mut s = 0_usize;
    while s < 14 {
        let mut b = 0_usize;
        while b < 256 {
            let (action, next) = transition_impl(parse_state(s as u8), b as u8);
            t[s * 256 + b] = pack(action, next);
            b += 1;
        }
        s += 1;
    }
    t
}

/// Runtime transition: one indexed load into [`TABLE`], no per-byte match.
#[inline(always)]
pub(crate) fn transition(state: ParseState, b: u8) -> (Action, ParseState) {
    let e = TABLE[(state as usize) << 8 | b as usize];
    (
        Action::from_bits_retain(e as u16),
        parse_state((e >> 16) as u8),
    )
}