diff --git a/src/rsalgo/mod.rs b/src/rsalgo/mod.rs
index e80f37f2..7c5fa43f 100644
--- a/src/rsalgo/mod.rs
+++ b/src/rsalgo/mod.rs
@@ -1 +1,2 @@
 mod incremental_algorithm;
+mod stats;
diff --git a/src/rsalgo/stats.rs b/src/rsalgo/stats.rs
new file mode 100644
index 00000000..4f638c56
--- /dev/null
+++ b/src/rsalgo/stats.rs
@@ -0,0 +1,283 @@
+use crate::MutationId;
+use crate::MutationRef;
+use crate::NodeId;
+use crate::Position;
+use crate::SiteId;
+use crate::TableColumn;
+use crate::TreeSequence;
+
+pub trait SingleSiteStatistic {
+    fn update(&mut self, num_descendants: i64);
+}
+
+#[derive(Debug)]
+pub enum StatsError {
+    MissingAncestralState(SiteId),
+    MissingDerivedState(MutationId),
+    InvalidNode(NodeId),
+}
+
+impl std::fmt::Display for StatsError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "stats error")
+    }
+}
+
+impl std::error::Error for StatsError {}
+
+// Poperties of the current tree with respect to
+// a given sample set.
+// Multi-sample-set tasks will need a Vec of these.
+struct TreeData {
+    // NOTE: we manually handle all tskit conventions:
+    // * i32 for ids
+    // * -1 for "NULL" value
+    parent: Vec<i32>,
+    num_sample_descendants: Vec<i64>,
+    num_mutated_sample_descendants: Vec<i64>,
+}
+
+impl TreeData {
+    fn new(ts: &TreeSequence) -> Self {
+        Self {
+            parent: vec![-1; ts.nodes().num_rows().as_usize()],
+            num_sample_descendants: vec![0; ts.nodes().num_rows().as_usize()],
+            num_mutated_sample_descendants: vec![0; ts.mutations().num_rows().as_usize()],
+        }
+    }
+
+    fn update_ancestors(&mut self, p: i32, delta: i64) {
+        let mut p = p;
+        while p != -1 {
+            self.num_sample_descendants[p as usize] += delta;
+            debug_assert!(self.num_sample_descendants[p as usize] >= 0);
+            p = self.parent[p as usize];
+        }
+    }
+
+    fn process_output_edge(&mut self, parent: usize, child: usize) {
+        debug_assert!(
+            self.num_sample_descendants[child] <= self.num_sample_descendants[parent],
+            "{parent} ({}) -> {child} ({})",
+            self.num_sample_descendants[parent],
+            self.num_sample_descendants[child],
+        );
+        if self.num_sample_descendants[child] > 0 {
+            let delta = -self.num_sample_descendants[child];
+            self.update_ancestors(self.parent[child], delta);
+        }
+        self.parent[child] = -1;
+    }
+
+    fn process_input_edge(&mut self, parent: usize, child: usize) {
+        debug_assert!(self.num_sample_descendants[parent] >= 0);
+        debug_assert!(self.num_sample_descendants[child] >= 0);
+        self.parent[child] = parent as i32;
+        if self.num_sample_descendants[child] > 0 {
+            let delta = self.num_sample_descendants[child];
+            self.update_ancestors(self.parent[child], delta);
+        }
+    }
+
+    // This is intended as a truly private fn
+    fn get_num_sample_descendants(&mut self, mutation: &MutationRef<'_>) -> i64 {
+        let rv: i64 = self.num_sample_descendants[mutation.node().as_usize()]
+            - self.num_mutated_sample_descendants[mutation.id().as_usize()];
+        // this is a HARD error representing a serious bug.
+        assert!(rv >= 0);
+        rv
+    }
+
+    #[must_use]
+    fn process_mutation<M>(&mut self, mutation: &MutationRef<'_>, mutation_parent: &M) -> i64
+    where
+        M: TableColumn<MutationId, MutationId>,
+    {
+        let nd = self.get_num_sample_descendants(mutation);
+        if nd > 0 {
+            // Propagate number of nodes inheriting this mutation up the tree
+            let delta = self.num_sample_descendants[mutation.node().as_usize()]
+                - self.num_mutated_sample_descendants[mutation.id().as_usize()];
+            assert!(!delta.is_negative());
+            let mut current_mut_parent = mutation_parent[mutation.id()];
+            while !current_mut_parent.is_null() {
+                self.num_mutated_sample_descendants[current_mut_parent.as_usize()] += delta;
+                current_mut_parent = mutation_parent[current_mut_parent];
+            }
+        }
+        nd
+    }
+}
+
+trait SampleSets<'s> {
+    fn process_input_edge(&mut self, parent: usize, child: usize);
+    fn process_output_edge(&mut self, parent: usize, child: usize);
+    fn initialize_site<'ts, 'a>(
+        &'a mut self,
+        ts: &'ts TreeSequence,
+        site: SiteId,
+    ) -> Result<(), StatsError>
+    where
+        'ts: 's,
+        's: 'a;
+    fn process_mutation<'ts, 'a, M>(
+        &'a mut self,
+        ts: &'ts TreeSequence,
+        mutation_parent: &'a M,
+        mutation: MutationRef<'a>,
+    ) -> Result<(), StatsError>
+    where
+        'ts: 's,
+        's: 'a,
+        M: TableColumn<MutationId, MutationId>;
+    fn update_allele_counts(&mut self) -> Result<(), StatsError>;
+}
+
+struct SingleSampleSet<'ts> {
+    tree_data: TreeData,
+    num_sampled_genomes: i64,
+    alleles_at_site: Vec<&'ts [u8]>,
+    allele_counts: Vec<i64>,
+}
+
+impl<'s> SampleSets<'s> for SingleSampleSet<'s> {
+    fn process_input_edge(&mut self, parent: usize, child: usize) {
+        self.tree_data.process_input_edge(parent, child);
+    }
+    fn process_output_edge(&mut self, parent: usize, child: usize) {
+        self.tree_data.process_output_edge(parent, child);
+    }
+
+    fn process_mutation<'ts, 'a, M>(
+        &'a mut self,
+        ts: &'ts TreeSequence,
+        mutation_parent: &'a M,
+        mutation: MutationRef<'a>,
+    ) -> Result<(), StatsError>
+    where
+        'ts: 's,
+        's: 'a,
+        M: TableColumn<MutationId, MutationId>,
+    {
+        let num_samples_inheriting_derived_state =
+            self.tree_data.process_mutation(&mutation, mutation_parent);
+        if num_samples_inheriting_derived_state > 0
+            && num_samples_inheriting_derived_state < self.num_sampled_genomes
+        {
+            let derived_state = *ts
+                .mutations()
+                .derived_state(mutation.id())
+                .as_ref()
+                .ok_or(StatsError::MissingDerivedState(mutation.id()))?;
+            match self
+                .alleles_at_site
+                .iter()
+                .position(|&x| x == derived_state)
+            {
+                Some(index) => {
+                    if index > 0 {
+                        self.allele_counts[index] += num_samples_inheriting_derived_state
+                    }
+                }
+                None => {
+                    self.alleles_at_site.push(derived_state);
+                    self.allele_counts
+                        .push(num_samples_inheriting_derived_state);
+                }
+            }
+        }
+        Ok(())
+    }
+
+    fn update_allele_counts(&mut self) -> Result<(), StatsError> {
+        self.allele_counts[0] =
+        // TODO: we should simply sum the desired quantity as we go along,
+        // eliminating the need for an iteration here.
+            (self.num_sampled_genomes) - self.allele_counts.iter().skip(1).sum::<i64>();
+        assert!(self.allele_counts[0] >= 0);
+        if self
+            .allele_counts
+            .iter()
+            .filter(|&&i| i > 0 && i < self.num_sampled_genomes)
+            .count()
+            > 1
+        {
+            todo!()
+        }
+        Ok(())
+    }
+
+    fn initialize_site<'ts, 'a>(
+        &'a mut self,
+        ts: &'ts TreeSequence,
+        site: SiteId,
+    ) -> Result<(), StatsError>
+    where
+        'ts: 's,
+        's: 'a,
+    {
+        setup_alleles_at_site(ts, site, &mut self.alleles_at_site)?;
+        self.allele_counts.resize(1, 0);
+        Ok(())
+    }
+}
+
+fn setup_alleles_at_site<'ts, 'a>(
+    ts: &'ts TreeSequence,
+    site: SiteId,
+    alleles_at_site: &mut Vec<&'a [u8]>,
+) -> Result<(), StatsError>
+where
+    'ts: 'a,
+{
+    alleles_at_site.clear();
+    // NOTE: trying to store the derived state
+    // from the current_site as a slice runs
+    // into lifetime issues because current_site
+    // goes away. So what we do instead is get a slice
+    // for the same row whose lifetime depends on
+    // the tree sequence!
+    alleles_at_site.push(
+        *ts.sites()
+            .ancestral_state(site)
+            .as_ref()
+            .ok_or(StatsError::MissingAncestralState(site))?,
+    );
+    Ok(())
+}
+
+fn setup_samples_from_node_ids<I>(
+    num_nodes: usize,
+    iter: I,
+    td: &mut TreeData,
+) -> Result<i32, StatsError>
+where
+    I: Iterator<Item = NodeId>,
+{
+    let mut num_sampled_genomes = 0;
+    for node_id in iter {
+        // Should be an Err condition!
+        if node_id == NodeId::NULL {
+            return Err(StatsError::InvalidNode(node_id));
+        }
+        if node_id.as_usize() >= num_nodes {
+            return Err(StatsError::InvalidNode(node_id));
+        }
+        if let Some(value) = td.num_sample_descendants.get_mut(node_id.as_usize()) {
+            *value += 1;
+        } else {
+            return Err(StatsError::InvalidNode(node_id));
+        }
+        num_sampled_genomes += 1;
+    }
+    Ok(num_sampled_genomes)
+}
+
+pub fn single_site_statistic<N: Iterator<Item = NodeId>, S: SingleSiteStatistic>(
+    samples: N,
+    statistic: S,
+    ts: &TreeSequence,
+) {
+    let mutation_parent = ts.tables().mutations().parent_column();
+    let num_edges = ts.edges().num_rows().as_usize();
+}