diff --git a/Cargo.toml b/Cargo.toml
index 9aab392..047ff6c 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -26,15 +26,12 @@ clap = { version = "4.5.16", features = ["derive"] }
 
 
 [[bin]]
-name = "fava"
+name = "favabean"
 path = "src/main.rs"
 
-[[bin]]
-name = "bam2bed"
-path = "src/bam2bed/main.rs"
-
-
-
+# [[bin]]
+# name = "bam2bed"
+# path = "src/bam2bed/main.rs"
 
 # anyhow = "1.0.86"
 # clap = { version = "4.5.16", features = ["derive"] }
diff --git a/README.md b/README.md
index 5e844bb..7a2b8d7 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,10 @@
-# FAVA BEAN
+# FAVA BEANS
+
+Filtering Alignments to expedite Bayesian Analysis in Base-pair-resolution Editing site Aggregation and Sifting
+
+Filtering Alignments to expedite Variant Associations in Base-pair-resolution Editing site Aggregation and Sifting
+
+
+
 
-Filtering Accessibility with Variant Alignment for Bayesian Epitranscriptomic Analysis
 
diff --git a/src/aggregate.rs b/src/aggregate.rs
new file mode 100644
index 0000000..74e51cc
--- /dev/null
+++ b/src/aggregate.rs
@@ -0,0 +1,217 @@
+use super::AggregateArgs;
+use anyhow;
+use rayon::prelude::*;
+use rust_htslib::bam::ext::BamRecordExtensions;
+use rust_htslib::bam::{self, Read};
+use std::cmp::{max, min};
+use std::sync::{Arc, Mutex};
+use std::{str, thread};
+
+use crate::util::check_bam_index;
+
+pub fn run_aggregate(args: &AggregateArgs) -> anyhow::Result<()> {
+    // Visit all the alignments and figure out
+
+    let nthread_max = thread::available_parallelism()
+        .expect("failed to figure out number of cores")
+        .get();
+
+    let nthread = match args.threads {
+        Some(x) => min(nthread_max, x),
+        None => nthread_max,
+    };
+
+    let (bam_file_bg, bam_file_fg) = (args.bg_bam.as_ref(), args.fg_bam.as_ref());
+
+    check_bam_index(bam_file_bg, args.bg_bai.as_deref())
+        .expect("check index for the background BAM");
+    check_bam_index(bam_file_fg, args.fg_bai.as_deref())
+        .expect("check index for the foreground BAM");
+
+    let block_size = match args.bsize {
+        Some(bs) => bs,
+        _ => 10_000,
+    } as i64;
+
+    let mut jobs = vec![];
+
+    let br = bam::Reader::from_path(bam_file_fg)?;
+    let hdr = br.header();
+
+    for (tid, name) in hdr.target_names().iter().enumerate() {
+        let max_size = hdr.target_len(tid as u32).unwrap() as i64;
+        let chr_name = Box::new(str::from_utf8(name).unwrap());
+        jobs.push((chr_name, make_blocks(max_size, block_size)));
+    }
+
+    // shared index reader
+    let arc_bam_bg = Arc::new(Mutex::new(bam::IndexedReader::from_path(bam_file_bg)?));
+
+    let arc_bam_fg = Arc::new(Mutex::new(bam::IndexedReader::from_path(bam_file_fg)?));
+
+    rayon::ThreadPoolBuilder::new()
+        .num_threads(nthread as usize)
+        .build_global()
+        .unwrap();
+
+    for (chr, blocks) in jobs.iter() {
+        //
+        let chr_name = *(chr.as_ref());
+
+        dbg!(chr_name);
+
+        let _ = blocks.par_iter().map(|(lb, ub)| {
+            let region = (chr_name, *lb, *ub);
+            let bg = get_dna_freq(&arc_bam_bg, region);
+            let fg = get_dna_freq(&arc_bam_fg, region);
+            match (bg, fg) {
+                (Ok(freq_bg), Ok(freq_fg)) => {
+                    freq_bg.forward;
+                    freq_bg.reverse;
+                    freq_fg.forward;
+                    freq_fg.reverse;
+                    //
+                }
+                _ => {
+                    // do nothing --> ignore errors
+                }
+            }
+        });
+
+        // blocks.par_bridge();
+    }
+
+    Ok(())
+}
+
+fn make_blocks(max_size: i64, block_size: i64) -> Vec<(i64, i64)> {
+    let mut jobs = vec![];
+    for lb in (0..max_size).step_by(block_size as usize) {
+        let ub = min(max_size, lb + block_size);
+        jobs.push((lb, ub));
+    }
+    return jobs;
+}
+
+///////////////////////////
+// DNA frequency vectors //
+///////////////////////////
+
+#[derive(Debug)]
+struct DnaFreq {
+    a: usize,   // number of A's
+    t: usize,   // number of T's
+    g: usize,   // number of G's
+    c: usize,   // number of C's
+    tot: usize, // total
+    gpos: i64,  // genomic position
+}
+
+struct DnaFreqVecs {
+    forward: Vec<DnaFreq>,
+    reverse: Vec<DnaFreq>,
+}
+
+////////////////////////////////////////////
+// Extract DNA base pair frequency tables //
+////////////////////////////////////////////
+
+fn get_dna_freq(
+    arc_bam: &Arc<Mutex<bam::IndexedReader>>,
+    region: (&str, i64, i64),
+) -> anyhow::Result<DnaFreqVecs> {
+    let (_, lb, ub) = region;
+
+    let mut bam_reader = arc_bam.lock().expect("unable to lock the reader");
+
+    bam_reader
+        .fetch(region)
+        .expect("unable to fetch the region");
+
+    if lb >= ub {
+        return Err(anyhow::anyhow!("lb >= ub"));
+    }
+
+    let nn = max(ub - lb, 0i64) as usize;
+    let mut reverse_freq = Vec::with_capacity(nn);
+    let mut forward_freq = Vec::with_capacity(nn);
+
+    for g in lb..ub {
+        forward_freq.push(DnaFreq {
+            a: 0,
+            t: 0,
+            g: 0,
+            c: 0,
+            tot: 0,
+            gpos: g,
+        });
+        reverse_freq.push(DnaFreq {
+            a: 0,
+            t: 0,
+            g: 0,
+            c: 0,
+            tot: 0,
+            gpos: g,
+        });
+    }
+
+    // Iter aligned read and reference positions on a basepair level
+    // https://docs.rs/rust-htslib/latest/src/rust_htslib/bam/ext.rs.html#135
+    // [read_pos, genome_pos]
+
+    for rr in bam_reader.rc_records() {
+        match rr {
+            Ok(rec) => {
+                if rec.is_duplicate() {
+                    continue;
+                }
+
+                // TODO: cell barcode umi
+                // extract 10x cell barcode
+                // if let Ok(cb) = rec.aux(b"CB") {
+                //     dbg!(x);
+                // }
+
+                // extract 10x UMI barcode
+                // if let Ok(umi) = rec.aux(b"UB") {
+                //     dbg!(x);
+                // }
+
+                let seq = rec.seq().as_bytes();
+
+                for [rpos, gpos] in rec.aligned_pairs() {
+                    let (r, g, v) = (rpos as usize, gpos as usize, gpos - lb);
+
+                    if g < (lb as usize) || g >= (ub as usize) || v < 0 {
+                        continue;
+                    }
+
+                    let bp = seq[r];
+
+                    let freq = match rec.is_reverse() {
+                        true => &mut reverse_freq[v as usize],
+                        _ => &mut forward_freq[v as usize],
+                    };
+
+                    debug_assert_eq!(freq.gpos, gpos);
+                    freq.tot += 1;
+                    match bp {
+                        b'A' | b'a' => freq.a += 1,
+                        b'T' | b't' => freq.t += 1,
+                        b'G' | b'g' => freq.g += 1,
+                        b'C' | b'c' => freq.c += 1,
+                        _ => (),
+                    }
+                }
+            }
+            _ => {
+                // report error message?
+            }
+        }
+    }
+
+    Ok(DnaFreqVecs {
+        forward: forward_freq,
+        reverse: reverse_freq,
+    })
+}
diff --git a/src/main.rs b/src/main.rs
index fad87ed..afcb03e 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,6 +1,79 @@
+mod aggregate;
+mod util;
+
+use aggregate::run_aggregate;
+
+use anyhow;
+// use anyhow::{self};
 use clap::{Args, Parser, Subcommand};
 
+#[derive(Parser)]
+#[command(version, about, long_about=None)]
+#[command(propagate_version = true)]
+struct Cli {
+    #[command(subcommand)]
+    commands: Commands,
+}
+
+#[derive(Subcommand)]
+enum Commands {
+    /// aggregate putative editing sites
+    Aggregate(AggregateArgs),
+    /// sifting through potential sites
+    Sift,
+}
+
+#[derive(Args)]
+struct SiftArgs {
+    /// GFF file
+    #[arg(short, long)]
+    gff: Box<str>,
+}
+
+#[derive(Args)]
+struct AggregateArgs {
+    /// foreground BAM file
+    #[arg(short, long)]
+    fg_bam: Box<str>,
+
+    /// background BAM file
+    #[arg(short, long)]
+    bg_bam: Box<str>,
+
+    /// foreground BAI file (default: <FG_BAM>.bai)
+    #[arg(long)]
+    fg_bai: Option<Box<str>>,
+
+    /// background BAI file (default: <BG_BAM>.bai)
+    #[arg(long)]
+    bg_bai: Option<Box<str>>,
+
+    /// number of threads
+    #[arg(short, long)]
+    threads: Option<usize>,
+
+    /// block size (default: 10000)
+    #[arg(long)]
+    bsize: Option<usize>,
+
+    /// output file header
+    #[arg(short, long)]
+    output: Option<Box<str>>,
+}
+
 /// main CLI for FAVA
-fn main() {
-    //
+///
+fn main() -> anyhow::Result<()> {
+    let cli = Cli::parse();
+
+    match &cli.commands {
+        Commands::Aggregate(args) => {
+            //
+            run_aggregate(args)?;
+        }
+        _ => {
+            todo!("sifting");
+        }
+    }
+    Ok(())
 }
diff --git a/src/util.rs b/src/util.rs
new file mode 100644
index 0000000..3a0d904
--- /dev/null
+++ b/src/util.rs
@@ -0,0 +1,39 @@
+use rust_htslib::bam;
+use std::thread;
+use std::path::Path;
+
+pub fn check_bam_index(
+    bam_file_name: &str,
+    idx_file_name: Option<&str>,
+) -> anyhow::Result<Box<str>> {
+    // log::info!("Checking BAM index");
+
+    let idx_file = match idx_file_name {
+        Some(x) => String::from(x),
+        None => format!("{}.bai", bam_file_name),
+    };
+
+    if Path::new(&idx_file).exists() {
+        return Ok(idx_file.into_boxed_str());
+    }
+
+    let ncore = thread::available_parallelism()
+        .expect("failed to figure out number of cores")
+        .get();
+
+    // log::info!(
+    //     "Creating a new index file {} using {} cores",
+    //     &idx_file,
+    //     &ncore
+    // );
+
+    // need to build an index for this bam file
+    bam::index::build(
+        bam_file_name,
+        Some(&idx_file),
+        bam::index::Type::Bai,
+        ncore as u32,
+    )?;
+
+    Ok(idx_file.into_boxed_str())
+}