//! Shared PRNG helpers for deterministic seeding (D-176, D-174).
//!
//! Both seed.rs and currency.rs use the same FNV-1a mix + Box-Muller transform.
//! Centralised here to guarantee identical derivation chains across modules.

use std::f64::consts::PI;

use rand::Rng;
use rand_chacha::ChaCha8Rng;

/// FNV-1a 64-bit offset basis.
const FNV_OFFSET_BASIS: u64 = 0xcbf29ce484222325;

/// FNV-1a 64-bit prime.
const FNV_PRIME: u64 = 0x100000001b3;

/// Deterministic per-key seed: FNV-1a of `key` mixed with `run_seed`.
///
/// Starting from `run_seed + FNV_OFFSET_BASIS` provides per-run variation
/// while preserving the FNV avalanche properties across keys.
pub fn derive_seed(run_seed: u64, key: &str) -> u64 {
    let mut h = run_seed.wrapping_add(FNV_OFFSET_BASIS);
    for byte in key.bytes() {
        h ^= byte as u64;
        h = h.wrapping_mul(FNV_PRIME);
    }
    h
}

/// Box-Muller transform: standard normal variate from a ChaCha8 stream.
pub fn standard_normal(rng: &mut ChaCha8Rng) -> f64 {
    let u1: f64 = 1.0 - rng.random::<f64>(); // avoid ln(0)
    let u2: f64 = rng.random::<f64>();
    (-2.0 * u1.ln()).sqrt() * (2.0 * PI * u2).cos()
}