ev_lib 0.3.2

//! Framework-agnostic A/B bucketing — the port of the TypeScript
//! `@evinvest/experiments` core. Pure, I/O-free, `wasm32`-safe: variant
//! assignment is deterministic given a random source, so the same helpers drive
//! native tests and the browser (where the source is `js_sys::Math::random`).

use std::collections::BTreeMap;

/// One experiment: an ordered set of `variants` with matching `weights`.
///
/// `variants[0]` is the control — [`resolve_variant`] falls back to it when a
/// cookie is missing or invalid. `weights` are relative (they need not sum to 1);
/// [`pick_variant`] normalises by their total.
#[derive(Clone, Debug, PartialEq)]
pub struct Experiment {
	/// Variant keys, control first. Stable: these are the dashboard contract.
	pub variants: Vec<String>,
	/// Relative weights, one per variant. Normalised by their sum on assignment.
	pub weights: Vec<f64>,
}

impl Experiment {
	/// Builds an experiment from explicit variants and relative weights.
	///
	/// # Examples
	/// ```
	/// use ev_lib::experiments::Experiment;
	/// let hero = Experiment::new(["a", "b"], [0.5, 0.5]);
	/// assert_eq!(hero.variants.len(), 2);
	/// ```
	pub fn new(variants: impl IntoIterator<Item = impl Into<String>>, weights: impl IntoIterator<Item = f64>) -> Self {
		Self {
			variants: variants.into_iter().map(Into::into).collect(),
			weights: weights.into_iter().collect(),
		}
	}

	/// Builds an experiment with equal weight on every variant.
	///
	/// # Examples
	/// ```
	/// use ev_lib::experiments::Experiment;
	/// let team = Experiment::uniform(["a", "b", "c"]);
	/// assert_eq!(team.weights, vec![1.0, 1.0, 1.0]);
	/// ```
	pub fn uniform(variants: impl IntoIterator<Item = impl Into<String>>) -> Self {
		let variants: Vec<String> = variants.into_iter().map(Into::into).collect();
		let weights = vec![1.0; variants.len()];
		Self { variants, weights }
	}
}

/// The cookie name a variant is stored under: `ab_<key>` (mirrors the TS
/// `cookieName`).
///
/// # Examples
/// ```
/// use ev_lib::experiments::cookie_name;
/// assert_eq!(cookie_name("hero"), "ab_hero");
/// ```
pub fn cookie_name(key: &str) -> String {
	format!("ab_{key}")
}

/// Picks a variant by weighted random draw, mirroring the TS `pickVariant`:
/// normalise by the total weight, draw `rng() * total`, walk the cumulative
/// weights, and fall through to the last variant on rounding. `rng` must yield a
/// value in `[0, 1)`; inject a deterministic closure in tests and
/// `js_sys::Math::random` in the browser.
///
/// Returns an empty string only when the experiment has no variants.
///
/// # Examples
/// ```
/// use ev_lib::experiments::Experiment;
/// let exp = Experiment::new(["a", "b"], [0.5, 0.5]);
/// // A draw below the first weight lands on the control.
/// assert_eq!(ev_lib::experiments::pick_variant(&exp, || 0.1), "a");
/// // A draw above it lands on the second variant.
/// assert_eq!(ev_lib::experiments::pick_variant(&exp, || 0.9), "b");
/// ```
pub fn pick_variant(exp: &Experiment, mut rng: impl FnMut() -> f64) -> String {
	let total: f64 = exp.weights.iter().copied().filter(|w| *w > 0.0).sum();
	if total <= 0.0 || exp.variants.is_empty() {
		return exp.variants.first().cloned().unwrap_or_default();
	}
	let mut threshold = rng() * total;
	for (i, weight) in exp.weights.iter().enumerate() {
		threshold -= weight;
		if threshold < 0.0
			&& let Some(v) = exp.variants.get(i)
		{
			return v.clone();
		}
	}
	exp.variants.last().cloned().unwrap_or_default()
}

/// Coerces a raw cookie value to a valid variant, falling back to the control
/// (`variants[0]`) when it is missing or unknown (mirrors the TS
/// `resolveVariant`).
///
/// # Examples
/// ```
/// use ev_lib::experiments::{Experiment, resolve_variant};
/// let exp = Experiment::new(["a", "b"], [0.5, 0.5]);
/// assert_eq!(resolve_variant(&exp, Some("b")), "b");
/// assert_eq!(resolve_variant(&exp, Some("zzz")), "a");
/// assert_eq!(resolve_variant(&exp, None), "a");
/// ```
pub fn resolve_variant(exp: &Experiment, raw: Option<&str>) -> String {
	match raw {
		Some(value) if exp.variants.iter().any(|v| v == value) => value.to_string(),
		_ => exp.variants.first().cloned().unwrap_or_default(),
	}
}

/// Returns the variant `step` positions from `current`, wrapping around the list
/// (mirrors the TS `nextVariant`, used by the dev cycle). Unknown `current`
/// values start from the control.
///
/// # Examples
/// ```
/// use ev_lib::experiments::{Experiment, next_variant};
/// let exp = Experiment::new(["a", "b", "c"], [1.0, 1.0, 1.0]);
/// assert_eq!(next_variant(&exp, "a", 1), "b");
/// assert_eq!(next_variant(&exp, "c", 1), "a");
/// assert_eq!(next_variant(&exp, "a", -1), "c");
/// ```
pub fn next_variant(exp: &Experiment, current: &str, step: i32) -> String {
	if exp.variants.is_empty() {
		return String::new();
	}
	let len = exp.variants.len() as i32;
	let idx = exp.variants.iter().position(|v| v == current).map(|i| i as i32).unwrap_or(0);
	let next = (((idx + step) % len) + len) % len;
	exp.variants[next as usize].clone()
}

/// The exposure event name for an experiment: `<experiment>_exposed` (mirrors the
/// TS `ExperimentTracker`).
///
/// # Examples
/// ```
/// assert_eq!(ev_lib::experiments::exposed_event("hero"), "hero_exposed");
/// ```
pub fn exposed_event(experiment: &str) -> String {
	format!("{experiment}_exposed")
}

/// The scoped event name for an in-experiment action: `<experiment>_<action>`
/// (mirrors the TS `useExperimentEvent`).
///
/// # Examples
/// ```
/// assert_eq!(ev_lib::experiments::action_event("team", "cta_clicked"), "team_cta_clicked");
/// ```
pub fn action_event(experiment: &str, action: &str) -> String {
	format!("{experiment}_{action}")
}

/// An analytics event emitted by the experiment tracker, handed to the consumer's
/// injected sink. This type is owned by `experiments` so the library never
/// imports `analytics`: a consumer forwards it to whatever capture it uses.
#[derive(Clone, Debug, PartialEq)]
pub struct TrackedEvent {
	/// The event name — [`exposed_event`] or [`action_event`].
	pub name: String,
	/// The variant the user was bucketed into, merged into every event.
	pub variant: String,
	/// Extra, primitive, non-PII properties supplied at the call site.
	pub props: BTreeMap<String, String>,
}

impl TrackedEvent {
	/// Builds an exposure event (`<experiment>_exposed`) for a variant.
	pub fn exposure(experiment: &str, variant: &str) -> Self {
		Self {
			name: exposed_event(experiment),
			variant: variant.to_string(),
			props: BTreeMap::new(),
		}
	}

	/// Builds a scoped action event (`<experiment>_<action>`) for a variant.
	pub fn action(experiment: &str, variant: &str, action: &str, props: BTreeMap<String, String>) -> Self {
		Self {
			name: action_event(experiment, action),
			variant: variant.to_string(),
			props,
		}
	}
}

#[cfg(test)]
mod tests {
	use std::collections::BTreeMap;

	use super::*;

	/// A deterministic rng that yields a fixed value; injected so picks are reproducible.
	fn fixed(v: f64) -> impl FnMut() -> f64 {
		move || v
	}

	#[test]
	fn cookie_name_is_prefixed() {
		assert_eq!(cookie_name("hero"), "ab_hero");
		assert_eq!(cookie_name(""), "ab_");
	}

	#[test]
	fn experiment_new_keeps_variants_and_weights() {
		let exp = Experiment::new(["a", "b"], [0.5, 0.5]);
		assert_eq!(exp.variants, vec!["a".to_string(), "b".to_string()]);
		assert_eq!(exp.weights, vec![0.5, 0.5]);
	}

	#[test]
	fn experiment_new_empty_is_empty() {
		let exp = Experiment::new(Vec::<String>::new(), Vec::<f64>::new());
		assert!(exp.variants.is_empty());
		assert!(exp.weights.is_empty());
	}

	#[test]
	fn experiment_uniform_matches_variant_count() {
		let exp = Experiment::uniform(["a", "b", "c"]);
		assert_eq!(exp.weights, vec![1.0, 1.0, 1.0]);
		assert_eq!(exp.weights.len(), exp.variants.len());
	}

	#[test]
	fn experiment_uniform_empty_has_no_weights() {
		let exp = Experiment::uniform(Vec::<String>::new());
		assert!(exp.variants.is_empty());
		assert!(exp.weights.is_empty());
	}

	#[test]
	fn pick_variant_respects_weight_boundaries() {
		let exp = Experiment::new(["a", "b"], [0.3, 0.7]);
		// draw just under 0.3 of the total → control
		assert_eq!(pick_variant(&exp, fixed(0.0)), "a");
		assert_eq!(pick_variant(&exp, fixed(0.29)), "a");
		// draw above the first weight → second variant
		assert_eq!(pick_variant(&exp, fixed(0.31)), "b");
		assert_eq!(pick_variant(&exp, fixed(0.999)), "b");
	}

	#[test]
	fn pick_variant_at_exact_cumulative_boundary_lands_on_next() {
		// total 1.0, control weight 0.3. r = 0.3 exactly: 0.3 - 0.3 = 0.0, which is
		// NOT < 0, so the boundary belongs to the second variant.
		let exp = Experiment::new(["a", "b"], [0.3, 0.7]);
		assert_eq!(pick_variant(&exp, fixed(0.3)), "b");
		// Just below the boundary stays on the control.
		assert_eq!(pick_variant(&exp, fixed(0.2999)), "a");
	}

	#[test]
	fn pick_variant_rng_one_falls_through_to_last() {
		// rng() -> 1.0 makes threshold == total, so the cumulative walk never trips
		// `< 0`; the fall-through must return the last variant, never "".
		let exp = Experiment::new(["a", "b", "c"], [1.0, 1.0, 1.0]);
		assert_eq!(pick_variant(&exp, fixed(1.0)), "c");
	}

	#[test]
	fn pick_variant_normalises_unnormalised_weights() {
		let exp = Experiment::new(["a", "b", "c"], [1.0, 1.0, 2.0]);
		// total 4: [0,0.25)→a, [0.25,0.5)→b, [0.5,1)→c
		assert_eq!(pick_variant(&exp, fixed(0.1)), "a");
		assert_eq!(pick_variant(&exp, fixed(0.3)), "b");
		assert_eq!(pick_variant(&exp, fixed(0.6)), "c");
	}

	#[test]
	fn pick_variant_maps_each_subinterval_to_its_variant() {
		// Property-style: a handful of rng points across [0,1) land on the variant
		// whose cumulative sub-interval contains them. total 4 → a [0,.5) b [.5,.75) c [.75,1).
		let exp = Experiment::new(["a", "b", "c"], [2.0, 1.0, 1.0]);
		for (r, expected) in [(0.0, "a"), (0.49, "a"), (0.5, "b"), (0.74, "b"), (0.75, "c"), (0.99, "c")] {
			assert_eq!(pick_variant(&exp, fixed(r)), expected, "rng={r}");
		}
	}

	#[test]
	fn pick_variant_single_variant_always_picks_it() {
		let exp = Experiment::new(["only"], [1.0]);
		assert_eq!(pick_variant(&exp, fixed(0.0)), "only");
		assert_eq!(pick_variant(&exp, fixed(0.5)), "only");
		assert_eq!(pick_variant(&exp, fixed(1.0)), "only");
	}

	#[test]
	fn pick_variant_empty_experiment_returns_empty_string() {
		let exp = Experiment::new(Vec::<String>::new(), Vec::<f64>::new());
		assert_eq!(pick_variant(&exp, fixed(0.5)), "");
	}

	#[test]
	fn pick_variant_falls_back_when_no_weight() {
		let exp = Experiment::new(["a", "b"], [0.0, 0.0]);
		assert_eq!(pick_variant(&exp, fixed(0.5)), "a");
	}

	#[test]
	fn pick_variant_zero_total_falls_back_to_control() {
		// Zero total weight must fall back to the control (variants[0]), regardless of rng.
		let exp = Experiment::new(["control", "b", "c"], [0.0, 0.0, 0.0]);
		assert_eq!(pick_variant(&exp, fixed(0.0)), "control");
		assert_eq!(pick_variant(&exp, fixed(0.99)), "control");
	}

	#[test]
	fn pick_variant_negative_weights_are_ignored_in_total() {
		// Negative weights are filtered out of the total; the positive variant wins.
		let exp = Experiment::new(["a", "b"], [-1.0, 1.0]);
		assert_eq!(pick_variant(&exp, fixed(0.5)), "b");
	}

	#[test]
	fn pick_variant_more_variants_than_weights_stays_valid() {
		// Missing weights are treated as 0; must never panic and must return a real variant.
		let exp = Experiment::new(["a", "b", "c"], [1.0]);
		// Only "a" carries weight, so every draw lands on it.
		assert_eq!(pick_variant(&exp, fixed(0.0)), "a");
		assert_eq!(pick_variant(&exp, fixed(0.99)), "a");
		assert!(exp.variants.contains(&pick_variant(&exp, fixed(0.5))));
	}

	#[test]
	fn pick_variant_more_weights_than_variants_stays_valid() {
		// Extra weights have no variant to map to; the walk must not panic and must
		// fall through to a real variant rather than "".
		let exp = Experiment::new(["a"], [1.0, 1.0]);
		assert_eq!(pick_variant(&exp, fixed(0.0)), "a");
		assert_eq!(pick_variant(&exp, fixed(0.9)), "a");
		let exp2 = Experiment::new(["a", "b"], [1.0, 1.0, 1.0]);
		// total 3 → a [0,1/3) b [1/3,2/3); the surplus third weight maps to no variant
		// and falls through to the last real variant.
		assert_eq!(pick_variant(&exp2, fixed(0.1)), "a");
		assert_eq!(pick_variant(&exp2, fixed(0.5)), "b");
		assert_eq!(pick_variant(&exp2, fixed(0.9)), "b");
	}

	#[test]
	fn resolve_variant_valid_unknown_none_and_empty() {
		let exp = Experiment::new(["a", "b"], [0.5, 0.5]);
		assert_eq!(resolve_variant(&exp, Some("b")), "b");
		assert_eq!(resolve_variant(&exp, Some("garbage")), "a");
		assert_eq!(resolve_variant(&exp, Some("")), "a");
		assert_eq!(resolve_variant(&exp, None), "a");

		let empty = Experiment::new(Vec::<String>::new(), Vec::<f64>::new());
		assert_eq!(resolve_variant(&empty, Some("anything")), "");
		assert_eq!(resolve_variant(&empty, None), "");
	}

	#[test]
	fn next_variant_wraps_both_directions() {
		let exp = Experiment::new(["a", "b", "c"], [1.0, 1.0, 1.0]);
		assert_eq!(next_variant(&exp, "a", 1), "b");
		assert_eq!(next_variant(&exp, "c", 1), "a");
		assert_eq!(next_variant(&exp, "a", -1), "c");
		assert_eq!(next_variant(&exp, "unknown", 1), "b");
	}

	#[test]
	fn next_variant_step_zero_is_identity() {
		let exp = Experiment::new(["a", "b", "c"], [1.0, 1.0, 1.0]);
		assert_eq!(next_variant(&exp, "b", 0), "b");
		// Unknown current with step 0 falls to the control (index 0).
		assert_eq!(next_variant(&exp, "unknown", 0), "a");
	}

	#[test]
	fn next_variant_large_steps_wrap_via_modulo() {
		let exp = Experiment::new(["a", "b", "c"], [1.0, 1.0, 1.0]);
		// |step| > len: 5 % 3 == 2, so from "a" we land two ahead → "c".
		assert_eq!(next_variant(&exp, "a", 5), "c");
		// Negative big step: from "a" stepping back 5 (== back 2) → "b".
		assert_eq!(next_variant(&exp, "a", -5), "b");
		// Full multiple of len is identity.
		assert_eq!(next_variant(&exp, "b", 6), "b");
		assert_eq!(next_variant(&exp, "b", -6), "b");
	}

	#[test]
	fn next_variant_single_variant_is_always_itself() {
		let exp = Experiment::new(["only"], [1.0]);
		assert_eq!(next_variant(&exp, "only", 1), "only");
		assert_eq!(next_variant(&exp, "only", -1), "only");
		assert_eq!(next_variant(&exp, "unknown", 3), "only");
	}

	#[test]
	fn next_variant_empty_experiment_returns_empty_string() {
		let exp = Experiment::new(Vec::<String>::new(), Vec::<f64>::new());
		assert_eq!(next_variant(&exp, "a", 1), "");
		assert_eq!(next_variant(&exp, "", 0), "");
	}

	#[test]
	fn event_names_match_taxonomy() {
		assert_eq!(exposed_event("hero"), "hero_exposed");
		assert_eq!(action_event("team", "cta_clicked"), "team_cta_clicked");
	}

	#[test]
	fn tracked_event_exposure_carries_name_and_variant() {
		let ev = TrackedEvent::exposure("hero", "a");
		assert_eq!(ev.name, "hero_exposed");
		assert_eq!(ev.variant, "a");
		assert!(ev.props.is_empty());
	}

	#[test]
	fn tracked_event_action_carries_scoped_name_variant_and_props() {
		let mut props = BTreeMap::new();
		props.insert("cta".to_string(), "careers".to_string());
		let ev = TrackedEvent::action("team", "b", "cta_clicked", props.clone());
		assert_eq!(ev.name, "team_cta_clicked");
		assert_eq!(ev.variant, "b");
		assert_eq!(ev.props, props);
	}
}