Add dice::tree and dice::TreeBuilder

src/dice.rs

@@ -84,6 +84,9 @@ pub use fn_builder::*;
 mod index_of;
 pub use index_of::*;
 
+mod tree;
+pub use tree::*;
+
 #[cfg(any(feature = "rand_full", all(feature = "rand_core", feature = "rand")))]
 mod rand;
 #[cfg(any(feature = "rand_full", all(feature = "rand_core", feature = "rand")))]

src/dice/tree.rs

@@ -0,0 +1,225 @@
+use crate::{dice, dice::LengthRange, Die, Fate};
+
+pub trait TreeBuilder<T> {
+    fn branch_count(&self, fate: Fate, max_branch_count: usize) -> usize;
+    fn build(&mut self, fate: Fate, subtrees: impl ExactSizeIterator<Item = T>) -> T;
+}
+
+pub fn tree<T, B>(builder_die: impl Die<B>, node_count_range: impl LengthRange) -> impl Die<T>
+where
+    B: TreeBuilder<T>,
+{
+    let node_count_die = dice::length(node_count_range);
+
+    dice::from_fn(move |mut fate| {
+        let mut builder = fate.roll(&builder_die);
+        let node_count = fate.roll(&node_count_die);
+        let root_idx = 0;
+
+        // Generate tree structure
+        let nodes = {
+            let mut nodes: Vec<Vec<usize>> = Vec::with_capacity(node_count.saturating_add(1));
+            let mut nodes_to_intialize: Vec<(usize, usize)> = Vec::new();
+
+            // Add root
+            nodes.push(Vec::new());
+            nodes_to_intialize.push((root_idx, node_count));
+
+            // Initialize the nodes from top to button
+            while let Some((parent_idx, remaining_size_of_parent)) = nodes_to_intialize.pop() {
+                // Create children for parent
+                if remaining_size_of_parent > 0 {
+                    // Branch randomly
+                    let branch_count = builder.branch_count(fate.copy(), remaining_size_of_parent);
+                    assert!(
+                        branch_count >= 1,
+                        "Branch count is {}, but must be >= 1",
+                        branch_count
+                    );
+                    assert!(
+                        branch_count <= remaining_size_of_parent,
+                        "Branch count is {}, but must be <= {}",
+                        branch_count,
+                        remaining_size_of_parent
+                    );
+
+                    // Split remaining size randomly
+                    let remaining_size_of_children = fate.roll(dice::terms_of_usize(
+                        remaining_size_of_parent - branch_count,
+                        branch_count,
+                    ));
+
+                    for remaining_size_of_child in remaining_size_of_children {
+                        let child_idx = nodes.len();
+
+                        // Add child
+                        nodes.push(Vec::new());
+                        nodes_to_intialize.push((child_idx, remaining_size_of_child));
+
+                        // Register child to parant
+                        let children = &mut nodes[parent_idx];
+                        children.push(child_idx);
+                    }
+                }
+            }
+
+            nodes
+        };
+
+        // Generate actual tree
+        let tree = {
+            // Create slots for storing the subtrees temporarily
+            let mut subtree_slots: Vec<Option<T>> = Vec::with_capacity(nodes.len());
+            for _ in 0..nodes.len() {
+                subtree_slots.push(None)
+            }
+
+            // Create the subtrees from buttom to top
+            for (idx, children) in nodes.into_iter().enumerate().rev() {
+                // Collect subtrees
+                let mut subtrees = Vec::with_capacity(children.len());
+                for idx in children {
+                    subtrees.push(subtree_slots[idx].take().unwrap());
+                }
+
+                // Merge subtrees
+                let expression = builder.build(fate.copy(), subtrees.into_iter());
+                subtree_slots[idx] = Some(expression);
+            }
+
+            // Return the root
+            subtree_slots[root_idx].take().unwrap()
+        };
+
+        tree
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::prelude::*;
+
+    use super::TreeBuilder;
+
+    // Tree type for testing
+    #[derive(Debug, Clone)]
+    enum Expression {
+        Constant(u32),
+        Add(Box<Expression>, Box<Expression>),
+        Sub(Box<Expression>, Box<Expression>),
+        Sum(Vec<Expression>),
+    }
+
+    impl Expression {
+        fn into_node_count(self) -> usize {
+            let mut acc = 0;
+            let mut expressions = vec![self];
+            while let Some(expression) = expressions.pop() {
+                acc += 1;
+                match expression {
+                    Expression::Constant(_) => (),
+                    Expression::Add(left, right) => expressions.extend([*left, *right]),
+                    Expression::Sub(left, right) => expressions.extend([*left, *right]),
+                    Expression::Sum(subtrees) => expressions.extend(subtrees),
+                }
+            }
+            // Subtract 1 because the root doesn't count
+            acc - 1
+        }
+
+        fn into_max_depth(self) -> usize {
+            let mut acc = 0;
+            let mut expressions = vec![(0, self)];
+            while let Some((max_depth, expression)) = expressions.pop() {
+                let max_depth = max_depth + 1;
+                match expression {
+                    Expression::Constant(_) => {
+                        acc = acc.max(max_depth);
+                    }
+                    Expression::Add(left, right) => {
+                        expressions.extend([(max_depth, *left), (max_depth, *right)])
+                    }
+                    Expression::Sub(left, right) => {
+                        expressions.extend([(max_depth, *left), (max_depth, *right)])
+                    }
+                    Expression::Sum(subtrees) => {
+                        if subtrees.is_empty() {
+                            acc = acc.max(max_depth);
+                        } else {
+                            expressions.extend(subtrees.into_iter().map(|e| (max_depth, e)))
+                        }
+                    }
+                }
+            }
+            // Subtract 1 because the root doesn't count
+            acc - 1
+        }
+    }
+
+    #[derive(Clone)]
+    struct ExpressionBuilder;
+
+    impl TreeBuilder<Expression> for ExpressionBuilder {
+        fn branch_count(&self, mut fate: Fate, max_branch_count: usize) -> usize {
+            fate.roll(dice::weighted_one_of_die().three(
+                (16, dice::just(1)),
+                (4, dice::just(2.min(max_branch_count))),
+                (1, dice::uni_usize(1..=max_branch_count)),
+            ))
+        }
+
+        fn build(
+            &mut self,
+            mut fate: Fate,
+            mut subtrees: impl ExactSizeIterator<Item = Expression>,
+        ) -> Expression {
+            match subtrees.len() {
+                0 => {
+                    if fate.roll(dice::weighted_bool(1, 10)) {
+                        let constant = fate.roll(dice::u32(..));
+                        Expression::Constant(constant)
+                    } else {
+                        Expression::Sum(Vec::new())
+                    }
+                }
+                2 => {
+                    let right = Box::new(subtrees.next().unwrap());
+                    let left = Box::new(subtrees.next().unwrap());
+                    if fate.roll(dice::bool()) {
+                        Expression::Add(left, right)
+                    } else {
+                        Expression::Sub(left, right)
+                    }
+                }
+                _ => Expression::Sum(subtrees.collect()),
+            }
+        }
+    }
+
+    #[test]
+    fn tree_has_correct_node_count() {
+        Dicetest::repeatedly().run(|mut fate| {
+            let node_count = fate.roll(dice::length(..));
+            let tree = fate.roll(dice::tree(dice::just(ExpressionBuilder), node_count));
+            assert_eq!(node_count, tree.into_node_count());
+        });
+    }
+
+    #[test]
+    fn tree_calc_stats() {
+        Dicetest::repeatedly()
+            .passes(0)
+            .stats_enabled(true)
+            .run(|mut fate| {
+                let node_count = 100000;
+                let tree = fate.roll(dice::tree(dice::just(ExpressionBuilder), node_count));
+                let max_depth = tree.into_max_depth();
+                let resolution = 10;
+                stat!(
+                    "max depth",
+                    "~{}",
+                    (max_depth / resolution) * resolution,
+                );
+            })
+    }
+}