Implement simple random mixin selection which passes sanity

coins/monero/src/transaction/mixins.rs

@@ -1,15 +1,113 @@
-// TODO
-pub(crate) fn select(o: u64) -> (u8, Vec<u64>) {
-  let mut mixins: Vec<u64> = (o .. o + 11).into_iter().collect();
-  mixins.sort();
-  (0, mixins)
+use std::collections::HashSet;
+
+use rand_core::{RngCore, CryptoRng};
+
+use curve25519_dalek::edwards::EdwardsPoint;
+
+use monero::VarInt;
+
+use crate::{transaction::SpendableOutput, rpc::{RpcError, Rpc}};
+
+const MIXINS: usize = 11;
+
+async fn select_single<R: RngCore + CryptoRng>(
+  rng: &mut R,
+  rpc: &Rpc,
+  height: usize,
+  high: u64,
+  used: &mut HashSet<u64>
+) -> Result<(u64, [EdwardsPoint; 2]), RpcError> {
+  let mut o;
+  let mut output = None;
+  while {
+    o = rng.next_u64() % u64::try_from(high).unwrap();
+    used.contains(&o) || {
+      output = rpc.get_outputs(&[o], height).await?[0];
+      output.is_none()
+    }
+  } {}
+  used.insert(o);
+  Ok((o, output.unwrap()))
 }
 
-pub(crate) fn offset(mixins: &[u64]) -> Vec<u64> {
-  let mut res = vec![mixins[0]];
-  res.resize(11, 0);
+// Uses VarInt as this is solely used for key_offsets which is serialized by monero-rs
+fn offset(mixins: &[u64]) -> Vec<VarInt> {
+  let mut res = vec![VarInt(mixins[0])];
+  res.resize(mixins.len(), VarInt(0));
   for m in (1 .. mixins.len()).rev() {
-    res[m] = mixins[m] - mixins[m - 1];
+    res[m] = VarInt(mixins[m] - mixins[m - 1]);
   }
   res
 }
+
+pub(crate) async fn select<R: RngCore + CryptoRng>(
+  rng: &mut R,
+  rpc: &Rpc,
+  height: usize,
+  inputs: &[SpendableOutput]
+) -> Result<Vec<(Vec<VarInt>, u8, Vec<[EdwardsPoint; 2]>)>, RpcError> {
+  // Convert the inputs in question to the raw output data
+  let mut outputs = Vec::with_capacity(inputs.len());
+  for input in inputs {
+    outputs.push((
+      rpc.get_o_indexes(input.tx).await?[input.o],
+      [input.key, input.commitment.calculate()]
+    ));
+  }
+
+  let high = rpc.get_high_output(height - 1).await?;
+  let high_f = high as f64;
+  if (high_f as u64) != high {
+    panic!("Transaction output index exceeds f64");
+  }
+
+  let mut used = HashSet::<u64>::new();
+  for o in &outputs {
+    used.insert(o.0);
+  }
+
+  let mut res = Vec::with_capacity(inputs.len());
+  for (i, o) in outputs.iter().enumerate() {
+    let mut mixins = Vec::with_capacity(MIXINS);
+    for _ in 0 .. MIXINS {
+      mixins.push(select_single(rng, rpc, height, high, &mut used).await?);
+    }
+    mixins.sort_by(|a, b| a.0.cmp(&b.0));
+
+    // Make sure the TX passes the sanity check that the median output is within the last 40%
+    // This actually checks the median is within the last third, a slightly more aggressive boundary,
+    // as the height used in this calculation will be slightly under the height this is sanity
+    // checked against
+    while mixins[MIXINS / 2].0 < (high * 2 / 3) {
+      // If it's not, update the bottom half with new values to ensure the median only moves up
+      for m in 0 .. MIXINS / 2 {
+        // We could not remove this, saving CPU time and removing low values as possibilities, yet
+        // it'd increase the amount of mixins required to create this transaction and some banned
+        // outputs may be the best options
+        used.remove(&mixins[m].0);
+        mixins[m] = select_single(rng, rpc, height, high, &mut used).await?;
+      }
+      mixins.sort_by(|a, b| a.0.cmp(&b.0));
+    }
+
+    // Replace the closest selected decoy with the actual
+    let mut replace = 0;
+    let mut distance = u64::MAX;
+    for m in 0 .. mixins.len() {
+      let diff = mixins[m].0.abs_diff(o.0);
+      if diff < distance {
+        replace = m;
+        distance = diff;
+      }
+    }
+
+    mixins[replace] = outputs[i];
+    res.push((
+      offset(&mixins.iter().map(|output| output.0).collect::<Vec<_>>()),
+      u8::try_from(replace).unwrap(),
+      mixins.iter().map(|output| output.1).collect()
+    ));
+  }
+
+  Ok(res)
+}