notescraft/crates/math/tests/fuzz.rs

//! Fuzz: 20 000 adversarial/mutated inputs; parse + layout must never panic, never
//! loop forever, and only produce finite, non-negative sizes. Deterministic (xorshift).

use notescraft_math::{Style, layout, parse, testutil::FakeMetrics};

/// Deterministic xorshift64*; seeds are fixed so failures reproduce.
struct Rng(u64);

impl Rng {
    fn next(&mut self) -> u64 {
        let mut x = self.0;
        x ^= x >> 12;
        x ^= x << 25;
        x ^= x >> 27;
        self.0 = x;
        x.wrapping_mul(0x2545_F491_4F6C_DD1D)
    }

    fn below(&mut self, hi: usize) -> usize {
        let r = self.next() >> 32; // top bits: no modulo bias for the small ranges used
        let t = usize::try_from(r).unwrap_or(usize::MAX);
        t % hi
    }
}

const TOKENS: &[&str] = &[
    "x",
    "y",
    "1",
    "2",
    "+",
    "-",
    "=",
    "(",
    ")",
    "[",
    "]",
    "{",
    "}",
    "^",
    "_",
    "&",
    "\\",
    ",",
    ".",
    "!",
    "~",
    " ",
    "'",
    "é",
    "α",
    "𝔸",
    "\\frac",
    "\\sqrt",
    "\\sum",
    "\\lim",
    "\\max",
    "\\int",
    "\\iota",
    "_i",
    "^n",
    "\\left",
    "\\right",
    "\\begin{matrix}",
    "\\end{matrix}",
    "\\begin{aligned}",
    "\\end{aligned}",
    "\\begin{cases}",
    "\\end{cases}",
    "\\text{a b}",
    "\\alpha",
    "\\hat",
    "\\mathbb{R}",
    "\\mathrm{x}",
    "\\,",
    "\\quad",
    "\\frac{a}{b}",
    "\\sqrt[3]{x}",
    "\\sum_i",
    "\\lim_{x}",
    "\\cdot",
    "\\infty",
    "\\to",
    "{a}",
    "\\end",
    "\\begin",
    "\\right)",
    "&",
    "x^^\\",
    "\\frac{a}",
    "\\left(a",
    "%",
    "#",
    "\\\\",
    "\\end{x}",
];

const VALID: &[&str] = &[
    "\\frac{a}{b}+\\sqrt[3]{x^2}",
    "\\begin{pmatrix}a&b\\\\c&d\\end{pmatrix}_{i\\,j}^{n}",
    "\\sum_{k=1}^{n}\\frac{k!}{2^k} = \\lim_{n\\to\\infty}\\left(\\sum...\\right)",
    "\\left|\\frac{\\alpha}{\\beta}\\right|^2 + \\hat x \\vec v",
    "\\begin{cases} x^2 & x \\geq 0 \\\\ -x & \\text{else} \\end{cases}",
    "f(x) = \\int_{-\\infty}^{\\infty} \\mathrm{e}^{-x^2} dx",
];

fn mutate(rng: &mut Rng, src: &str) -> String {
    let chars: Vec<char> = src.chars().collect();
    let pos = rng.below(chars.len().max(1));
    let mut out = String::new();
    let pick = rng.below(3);
    for (i, ch) in chars.iter().enumerate() {
        if i == pos && pick == 0 {
            // Insert a random token.
            let tok = TOKENS[rng.below(TOKENS.len())];
            out.push_str(tok);
        }
        if i == pos && pick == 1 {
            continue; // Delete the character.
        }
        out.push(*ch);
        if i == pos && pick == 2 {
            out.push(*ch); // Duplicate it.
        }
    }
    out
}

fn random_token_soup(rng: &mut Rng, out: &mut String) {
    let n = 1 + rng.below(12);
    for _ in 0..n {
        let tok = TOKENS[rng.below(TOKENS.len())];
        out.push_str(tok);
        if rng.below(4) == 0 {
            out.push(' ');
        }
    }
}

fn check(src: &str, tag: &str) {
    match parse(src) {
        Err(_) => { /* Malformed input is always an Err, never a panic. */ }
        Ok(node) => {
            for display in [false, true] {
                let b = layout(&node, &FakeMetrics, Style { size_pt: 10.0, display });
                assert!(b.width.is_finite(), "{tag}: width");
                assert!(b.height.is_finite(), "{tag}: height");
                assert!(b.depth.is_finite(), "{tag}: depth");
                assert!(b.width >= -0.01, "{tag}: {}", b.width);
                assert!(b.height >= -0.01, "{tag}: {}", b.height);
                assert!(b.depth >= -0.01, "{tag}: {}", b.depth);
                for item in &b.items {
                    check_item(item, tag);
                }
            }
        }
    }
}

fn check_item(item: &notescraft_math::Item, tag: &str) {
    match item {
        notescraft_math::Item::Glyphs { x, baseline_y, size, text, .. } => {
            let (x, by, size) = (*x, *baseline_y, *size);
            assert!(x.is_finite() && by.is_finite() && size.is_finite(), "{tag}");
            assert!(size > 0.0, "{tag}");
            assert!(!text.is_empty(), "{tag}: empty run");
        }
        notescraft_math::Item::Rule { w, h, .. } => {
            let (w, h) = (*w, *h);
            assert!(w >= 0.0 && h > 0.0 && w.is_finite() && h.is_finite(), "{tag}");
        }
        notescraft_math::Item::Path { points, stroke } => {
            assert!(*stroke > 0.0 && stroke.is_finite(), "{tag}");
            for [x, y] in points {
                assert!((*x).is_finite() && (*y).is_finite(), "{tag}");
            }
        }
    }
}

#[test]
fn fuzz_parse_and_layout_never_panics() {
    let mut rng = Rng(0x1234_5678_9ABC_DEF0);
    for n in 0..10_000u64 {
        let mut soup = String::new();
        random_token_soup(&mut rng, &mut soup);
        check(&soup, "soup");
        if n % 1000 == 0 {
            check(&soup.repeat(400), "repeated soup");
        }
    }
    for _n in 0..10_000u64 {
        let tmpl = VALID[rng.below(VALID.len())];
        check(&mutate(&mut rng, tmpl), "mutate");
    }
}