1 files changed, 54 insertions, 0 deletions

diff --git a/src/crash.rs b/src/crash.rs
index 9d6e98c..40b8743 100644
--- a/src/crash.rs
+++ b/src/crash.rs
@@ -6,6 +6,7 @@
 //! clock, never from anything the client sends.
 
 use serde::{Deserialize, Serialize};
+use sha2::{Digest, Sha256};
 use std::time::Duration;
 
 /// `exp(GROWTH_RATE * t)` doubles the multiplier every five seconds.
@@ -47,6 +48,53 @@ pub fn crash_point_from_uniform(u: f64) -> f64 {
     ((1.0 - HOUSE_EDGE) / (1.0 - u)).clamp(1.0, MAX_MULTIPLIER)
 }
 
+/// How many bytes a round seed contains. The first eight bytes choose the
+/// crash point; the rest are revealed and hashed with it.
+pub const SEED_LEN: usize = 32;
+
+/// Hex-encoded SHA-256 of a round seed, sent before the round starts.
+pub fn commit_seed(seed: &[u8]) -> String {
+    let digest = Sha256::digest(seed);
+    format!("{digest:x}")
+}
+
+/// Uniform draw encoded by a seed: the first eight bytes, big-endian.
+pub fn seed_to_uniform(seed: &[u8]) -> f64 {
+    let mut bytes = [0u8; 8];
+    let take = seed.len().min(8);
+    bytes[..take].copy_from_slice(&seed[..take]);
+    u64::from_be_bytes(bytes) as f64 / 18_446_744_073_709_551_616.0
+}
+
+/// The crash point a seed commits to. A client recomputes this after the
+/// reveal and checks it against the published commit.
+pub fn crash_point_from_seed(seed: &[u8]) -> f64 {
+    crash_point_from_uniform(seed_to_uniform(seed))
+}
+
+/// Decode a lowercase or uppercase hex seed. Returns `None` for odd lengths,
+/// bad digits or a seed that is not exactly [`SEED_LEN`] bytes.
+pub fn seed_from_hex(raw: &str) -> Option<Vec<u8>> {
+    if raw.len() != SEED_LEN * 2 {
+        return None;
+    }
+    let mut seed = Vec::with_capacity(SEED_LEN);
+    for pair in raw.as_bytes().chunks_exact(2) {
+        let high = (pair[0] as char).to_digit(16)?;
+        let low = (pair[1] as char).to_digit(16)?;
+        seed.push((high * 16 + low) as u8);
+    }
+    Some(seed)
+}
+
+/// Check a revealed seed against the commit published before the round.
+pub fn verify_seed(seed_hex: &str, commit: &str) -> bool {
+    let Some(seed) = seed_from_hex(seed_hex) else {
+        return false;
+    };
+    commit_seed(&seed).eq_ignore_ascii_case(commit)
+}
+
 /// Whole-Kr cashout value. Balances are integers, so the payout is floored.
 pub fn payout(bet: u64, multiplier: f64) -> u64 {
     if bet == 0 || !multiplier.is_finite() || multiplier <= 0.0 {
@@ -72,10 +120,16 @@ pub struct CrashState {
     pub multiplier: f64,
     /// `floor(bet * multiplier)` for the value above.
     pub potential: u64,
+    /// Server Unix time when the round started, milliseconds.
+    pub started_at: u64,
     /// Hidden until the round ends.
     pub crash_point: Option<f64>,
     pub payout: u64,
     pub result: String,
+    /// `sha256(seed)`, present for the whole round.
+    pub commit: Option<String>,
+    /// The seed itself, present only after the round ends.
+    pub seed: Option<String>,
 }
 
 impl CrashState {