Added option to output directed graphs of the group heirarchy before and after compression (#43)

It outputs nodes and edges information before and after the
compressor has run - these can be visualised in a tool like Gephi

A good way to visualise what the compressor is actually doing!

src/graphing.rs

@@ -0,0 +1,71 @@
+use std::collections::BTreeMap;
+use std::{fs::File, io::Write};
+
+use super::StateGroupEntry;
+
+type Graph = BTreeMap<i64, StateGroupEntry>;
+
+/// Outputs information from a state group graph into an edges file and a node file
+///
+/// These can be loaded into something like Gephi to visualise the graphs
+///
+/// # Arguments
+///
+/// * `groups`          - A map from state group ids to StateGroupEntries
+/// * `edges_output`    - The file to output the predecessor link information to
+/// * `nodes_output`    - The file to output the state group information to
+fn output_csv(groups: &Graph, edges_output: &mut File, nodes_output: &mut File) {
+    // The line A;B in the edges file means:
+    //      That state group A has predecessor B
+    writeln!(edges_output, "Source;Target",).unwrap();
+
+    // The line A;B;C;"B" in the nodes file means:
+    //      The state group id is A
+    //      This state group has B rows in the state_groups_state table
+    //      If C is true then A has no predecessor
+    writeln!(nodes_output, "Id;Rows;Root;Label",).unwrap();
+
+    for (source, entry) in groups {
+        // If the group has a predecessor then write an edge in the edges file
+        if let Some(target) = entry.prev_state_group {
+            writeln!(edges_output, "{};{}", source, target,).unwrap();
+        }
+
+        // Write the state group's information to the nodes file
+        writeln!(
+            nodes_output,
+            "{};{};{};\"{}\"",
+            source,
+            entry.state_map.len(),
+            entry.prev_state_group.is_none(),
+            entry.state_map.len(),
+        )
+        .unwrap();
+    }
+}
+
+/// Outputs information from two state group graph into files
+///
+/// These can be loaded into something like Gephi to visualise the graphs
+/// before and after the compressor is run
+///
+/// # Arguments
+///
+/// * `before`      - A map from state group ids to StateGroupEntries
+///                   the information from this map goes into before_edges.csv
+///                   and before_nodes.csv
+/// * `after`       - A map from state group ids to StateGroupEntries
+///                   the information from this map goes into after_edges.csv
+///                   and after_nodes.csv
+pub fn make_graphs(before: Graph, after: Graph) {
+    // Open all the files to output to
+    let mut before_edges_file = File::create("before_edges.csv").unwrap();
+    let mut before_nodes_file = File::create("before_nodes.csv").unwrap();
+    let mut after_edges_file = File::create("after_edges.csv").unwrap();
+    let mut after_nodes_file = File::create("after_nodes.csv").unwrap();
+
+    // Write before's information to before_edges and before_nodes
+    output_csv(&before, &mut before_edges_file, &mut before_nodes_file);
+    // Write afters's information to after_edges and after_nodes
+    output_csv(&after, &mut after_edges_file, &mut after_nodes_file);
+}