//! Typosquats and slopsquats.
//!
//! Two related attacks with different shapes:
//!
//! **Typosquatting** registers a name one keystroke from something
//! popular and waits for a typo or a copy-paste error. Detection is edit
//! distance against a list of names worth impersonating, weighted by how
//! popular the target is — nobody bothers squatting an unpopular package.
//!
//! **Slopsquatting** is newer and cheaper. Language models invent plausible
//! package names that do not exist; attackers watch for the invented names
//! and register them. The victim never made a typo — the name was
//! hallucinated by a tool they trusted, and then made real by somebody
//! else. The fingerprint is a package that is *very new*, has *almost no
//! downloads*, and sits one edit from something genuinely popular.
//!
//! Both need registry metadata to judge, so the metadata is a plain struct
//! the caller fills in. That keeps every decision here pure and testable,
//! and means the same logic runs against a live registry, a cached OSV
//! mirror, or a fixture.

use crate::{Finding, Severity};

/// What a registry can tell us about a package.
#[derive(Debug, Clone, Default)]
pub struct PackageMeta {
    pub name: String,
    pub version: String,
    /// Days since first publication. `None` when unknown.
    pub age_days: Option<u32>,
    /// Recent downloads, whatever window the registry reports.
    pub downloads: Option<u64>,
}

/// A popular package worth impersonating.
#[derive(Debug, Clone)]
pub struct PopularPackage {
    pub name: &'static str,
    /// Weekly downloads, used to decide whether a near-miss is worth
    /// flagging at all.
    pub weekly: u64,
}

/// A deliberately small starter list. The real one ships with the
/// definitions feed in Phase 3; this exists so detection works offline and
/// so the logic has something to test against.
pub const POPULAR: &[PopularPackage] = &[
    PopularPackage { name: "react", weekly: 25_000_000 },
    PopularPackage { name: "lodash", weekly: 50_000_000 },
    PopularPackage { name: "express", weekly: 30_000_000 },
    PopularPackage { name: "axios", weekly: 45_000_000 },
    PopularPackage { name: "chalk", weekly: 200_000_000 },
    PopularPackage { name: "commander", weekly: 90_000_000 },
    PopularPackage { name: "requests", weekly: 60_000_000 },
    PopularPackage { name: "numpy", weekly: 40_000_000 },
    PopularPackage { name: "pandas", weekly: 30_000_000 },
    PopularPackage { name: "urllib3", weekly: 70_000_000 },
    PopularPackage { name: "@vue/cli-plugin-babel", weekly: 4_200_000 },
    PopularPackage { name: "langchain", weekly: 2_000_000 },
    PopularPackage { name: "langchain-helper", weekly: 2_100_000 },
    PopularPackage { name: "openai", weekly: 8_000_000 },
    PopularPackage { name: "anthropic", weekly: 3_000_000 },
];

/// Damerau-Levenshtein distance, capped so long names exit early.
///
/// Transpositions matter: `recat` for `react` is one finger slip, and
/// plain Levenshtein scores it as two edits.
pub fn edit_distance(a: &str, b: &str, cap: usize) -> usize {
    let a: Vec<char> = a.chars().collect();
    let b: Vec<char> = b.chars().collect();
    if a.len().abs_diff(b.len()) > cap {
        return cap + 1;
    }
    let mut prev_prev = vec![0usize; b.len() + 1];
    let mut prev: Vec<usize> = (0..=b.len()).collect();
    let mut cur = vec![0usize; b.len() + 1];

    for i in 1..=a.len() {
        cur[0] = i;
        for j in 1..=b.len() {
            let cost = usize::from(a[i - 1] != b[j - 1]);
            cur[j] = (prev[j] + 1).min(cur[j - 1] + 1).min(prev[j - 1] + cost);
            if i > 1 && j > 1 && a[i - 1] == b[j - 2] && a[i - 2] == b[j - 1] {
                cur[j] = cur[j].min(prev_prev[j - 2] + 1);
            }
        }
        std::mem::swap(&mut prev_prev, &mut prev);
        std::mem::swap(&mut prev, &mut cur);
    }
    prev[b.len()]
}

/// The nearest popular package within `cap` edits, if any.
pub fn nearest(name: &str, cap: usize) -> Option<(&'static PopularPackage, usize)> {
    POPULAR
        .iter()
        .filter(|p| p.name != name) // the real thing is not a squat of itself
        .map(|p| (p, edit_distance(name, p.name, cap)))
        .filter(|(_, d)| *d <= cap && *d > 0)
        .min_by_key(|(_, d)| *d)
}

/// Fresh enough and quiet enough to be a name that was invented rather
/// than earned.
fn looks_hallucinated(meta: &PackageMeta) -> bool {
    let young = meta.age_days.is_some_and(|d| d <= 90);
    let quiet = meta.downloads.is_some_and(|d| d < 1_000);
    young && quiet
}

/// Judge one package.
pub fn scan(meta: &PackageMeta, location: &str) -> Vec<Finding> {
    let Some((target, distance)) = nearest(&meta.name, 2) else {
        return Vec::new();
    };

    // Squatting an unpopular package earns nothing, so a near-miss on
    // something obscure is far more likely to be an honest fork.
    if target.weekly < 100_000 {
        return Vec::new();
    }

    let spec = if meta.version.is_empty() {
        meta.name.clone()
    } else {
        format!("{}@{}", meta.name, meta.version)
    };

    if looks_hallucinated(meta) {
        let age = meta.age_days.unwrap_or(0);
        let dl = meta.downloads.unwrap_or(0);
        return vec![Finding::new(
            "slopsquat",
            Severity::Critical,
            spec,
            location,
            format!(
                "This package was first published {age} days ago and has been downloaded \
                 {dl} times, and its name is one character from \"{}\", which is downloaded \
                 millions of times a week. That combination is the signature of a name an \
                 AI assistant invented and somebody else then registered.",
                target.name
            ),
            "hound-slopsquat-a",
            format!(
                "Check whether you actually meant \"{}\". If an assistant suggested this \
                 name, treat the suggestion as wrong rather than the registry as right.",
                target.name
            ),
        )];
    }

    // Established but still near-identical: a classic squat, or a fork.
    let severity = if distance == 1 { Severity::Warning } else { Severity::Info };
    vec![Finding::new(
        "typosquat",
        severity,
        spec,
        location,
        format!(
            "This name is {} character{} away from \"{}\", a package downloaded around \
             {} times a week. Packages with names this close to something popular are \
             often impersonations that rely on a typo going unnoticed.",
            distance,
            if distance == 1 { "" } else { "s" },
            target.name,
            human(target.weekly)
        ),
        "hound-typosquat-a",
        format!("Confirm you meant \"{}\" and not \"{}\".", target.name, meta.name),
    )]
}

fn human(n: u64) -> String {
    match n {
        n if n >= 1_000_000 => format!("{:.0} million", n as f64 / 1_000_000.0),
        n if n >= 1_000 => format!("{}k", n / 1_000),
        n => n.to_string(),
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn meta(name: &str, age: Option<u32>, dl: Option<u64>) -> PackageMeta {
        PackageMeta {
            name: name.into(),
            version: "0.0.3".into(),
            age_days: age,
            downloads: dl,
        }
    }

    // ── distance ──

    #[test]
    fn counts_a_single_substitution() {
        assert_eq!(edit_distance("react", "reacf", 3), 1, "one wrong letter");
        assert_eq!(edit_distance("react", "reeact", 3), 1, "one extra letter");
        assert_eq!(edit_distance("react", "rect", 3), 1, "one missing letter");
    }

    #[test]
    fn a_transposition_counts_as_one_slip_not_two() {
        // Plain Levenshtein scores a swap as two edits, which puts real
        // typosquats outside a distance-1 filter. Damerau does not.
        assert_eq!(edit_distance("react", "raect", 3), 1);
        assert_eq!(edit_distance("axios", "axois", 3), 1);
    }

    #[test]
    fn identical_names_are_distance_zero() {
        assert_eq!(edit_distance("lodash", "lodash", 2), 0);
    }

    #[test]
    fn the_cap_short_circuits_wildly_different_lengths() {
        assert!(edit_distance("a", "averyverylongname", 2) > 2);
    }

    #[test]
    fn distance_is_symmetric() {
        assert_eq!(
            edit_distance("langchain-helpers", "langchain-helper", 3),
            edit_distance("langchain-helper", "langchain-helpers", 3)
        );
    }

    // ── must catch ──

    #[test]
    fn a_fresh_quiet_near_miss_is_a_slopsquat() {
        let f = scan(&meta("langchain-helpers", Some(3), Some(41)), "requirements.txt");
        assert_eq!(f.len(), 1);
        assert_eq!(f[0].kind, "slopsquat");
        assert_eq!(f[0].severity, Severity::Critical);
        assert!(f[0].explanation.contains("AI assistant invented"));
    }

    #[test]
    fn the_vue_plugin_squat_is_caught() {
        let f = scan(&meta("@vue/cli-plugin-babe1", Some(9), Some(300)), "package-lock.json");
        assert_eq!(f[0].kind, "slopsquat");
        assert!(f[0].explanation.contains("@vue/cli-plugin-babel"));
    }

    #[test]
    fn an_established_near_miss_is_a_typosquat_not_a_slopsquat() {
        let f = scan(&meta("expres", Some(900), Some(5_000_000)), "package.json");
        assert_eq!(f.len(), 1);
        assert_eq!(f[0].kind, "typosquat");
        assert_eq!(f[0].severity, Severity::Warning);
    }

    // ── must NOT catch ──

    #[test]
    fn the_real_package_is_not_a_squat_of_itself() {
        assert!(scan(&meta("react", Some(3000), Some(25_000_000)), "p.json").is_empty());
        assert!(scan(&meta("lodash", Some(4000), Some(50_000_000)), "p.json").is_empty());
    }

    #[test]
    fn an_unrelated_name_is_clean() {
        assert!(scan(&meta("hound-supply", Some(1), Some(0)), "Cargo.toml").is_empty());
    }

    #[test]
    fn a_near_miss_on_an_unpopular_package_is_ignored() {
        // Nobody squats a package nobody installs, so a near-miss there is
        // far more likely to be an honest fork.
        let obscure = PopularPackage { name: "tiny-thing", weekly: 500 };
        assert!(obscure.weekly < 100_000);
        assert!(scan(&meta("tiny-thang", Some(1), Some(1)), "p.json").is_empty());
    }

    #[test]
    fn a_new_package_that_is_not_a_near_miss_is_clean() {
        assert!(
            scan(&meta("my-brand-new-tool", Some(1), Some(0)), "p.json").is_empty(),
            "being new is not suspicious on its own"
        );
    }

    #[test]
    fn missing_metadata_downgrades_rather_than_guesses() {
        // No age or downloads: we can still see the name is close, but we
        // must not claim it was hallucinated.
        let f = scan(&meta("expres", None, None), "p.json");
        assert_eq!(f[0].kind, "typosquat");
    }

    #[test]
    fn advice_names_the_package_they_probably_meant() {
        let f = scan(&meta("langchain-helpers", Some(3), Some(41)), "r.txt");
        assert!(f[0].advice.contains("langchain-helper"));
    }

    #[test]
    fn download_counts_render_readably() {
        assert_eq!(human(45_000_000), "45 million");
        assert_eq!(human(4_200), "4k");
        assert_eq!(human(7), "7");
    }
}