Add mempool fees histogram estimation

src/bin/tracker.rs

@@ -25,6 +25,7 @@ fn main() {
             "update took {:.3} ms",
             (dt.as_secs() as f64 + 1e-9f64 * dt.subsec_nanos() as f64) * 1e3
         );
+        info!("histogram: {:?}", tracker.fee_histogram());
         thread::sleep(Duration::from_secs(1));
     }
 }

src/daemon.rs

@@ -44,6 +44,14 @@ impl MempoolEntry {
     pub fn fee_per_vbyte(&self) -> f32 {
         self.fee as f32 / self.vsize as f32
     }
+
+    pub fn fee(&self) -> u64 {
+        self.fee
+    }
+
+    pub fn vsize(&self) -> u32 {
+        self.vsize
+    }
 }
 
 impl Daemon {

src/mempool.rs

@@ -5,11 +5,10 @@ use std::collections::hash_map::Entry;
 use std::collections::{HashMap, HashSet};
 use std::iter::FromIterator;
 
-use query::{FundingOutput, SpendingInput};
-use types::FullHash;
-
 error_chain!{}
 
+const VSIZE_BIN_WIDTH: u32 = 100_000; // in vbytes
+
 pub struct Stats {
     tx: Transaction,
     entry: MempoolEntry,
@@ -28,6 +27,26 @@ impl<'a> Tracker<'a> {
         }
     }
 
+    /// Returns vector of (fee_rate, vsize) pairs, where fee_{n-1} > fee_n and vsize_n is the
+    /// cumulative virtual size of mempool transaction with fee in the interval [fee_{n-1}, fee_n].
+    /// Note: fee_0 is implied to be infinity.
+    pub fn fee_histogram(&self) -> Vec<(f32, u32)> {
+        let mut entries: Vec<&MempoolEntry> = self.txids.values().map(|stat| &stat.entry).collect();
+        entries.sort_unstable_by(|e1, e2| {
+            e2.fee_per_vbyte().partial_cmp(&e1.fee_per_vbyte()).unwrap()
+        });
+        let mut histogram = Vec::new();
+        let mut cumulative_vsize = 0;
+        for e in entries {
+            cumulative_vsize += e.vsize();
+            if cumulative_vsize > VSIZE_BIN_WIDTH {
+                histogram.push((e.fee_per_vbyte(), cumulative_vsize));
+                cumulative_vsize = 0;
+            }
+        }
+        histogram
+    }
+
     pub fn update_from_daemon(&mut self) -> Result<()> {
         let new_txids = HashSet::<Sha256dHash>::from_iter(self.daemon
             .getmempooltxids()