diff --git a/flatgfa/src/cli/cmds.rs b/flatgfa/src/cli/cmds.rs
index 3bf22cd4..9b55fd9c 100644
--- a/flatgfa/src/cli/cmds.rs
+++ b/flatgfa/src/cli/cmds.rs
@@ -196,10 +196,14 @@ pub fn extract(
 /// compute node depth, the number of times paths cross a node
 #[derive(FromArgs, PartialEq, Debug)]
 #[argh(subcommand, name = "depth")]
-pub struct Depth {}
+pub struct Depth {
+    /// enable usage of StepsBySegIndex
+    #[argh(switch, short = 'n')]
+    index: bool,
+}
 
-pub fn depth(gfa: &flatgfa::FlatGFA) {
-    let (depths, uniq_paths) = ops::depth::depth(gfa);
+pub fn depth(gfa: &flatgfa::FlatGFA, args: Depth) {
+    let (depths, uniq_paths) = ops::depth::depth(gfa, args.index);
 
     println!("#node.id\tdepth\tdepth.uniq");
     for (id, seg) in gfa.segs.items() {
diff --git a/flatgfa/src/cli/main.rs b/flatgfa/src/cli/main.rs
index 03ee8357..f24226ba 100644
--- a/flatgfa/src/cli/main.rs
+++ b/flatgfa/src/cli/main.rs
@@ -127,8 +127,8 @@ fn main() -> Result<(), &'static str> {
             let store = cmds::extract(&gfa, sub_args)?;
             dump(&store.as_ref(), &args.output);
         }
-        Some(Command::Depth(_)) => {
-            cmds::depth(&gfa);
+        Some(Command::Depth(sub_args)) => {
+            cmds::depth(&gfa, sub_args);
         }
         Some(Command::Chop(sub_args)) => {
             let store = cmds::chop(&gfa, sub_args)?;
diff --git a/flatgfa/src/index.rs b/flatgfa/src/index.rs
new file mode 100644
index 00000000..90c42591
--- /dev/null
+++ b/flatgfa/src/index.rs
@@ -0,0 +1,124 @@
+use std::ops::Range;
+
+use crate::flatgfa::{FlatGFA, Path, Segment};
+use crate::pool::{Id, Span};
+
+/// A reference to a specific Step within a Path
+pub struct StepRef {
+    /// The ID of the Path
+    pub path: Id<Path>,
+
+    /// The offset of the Step within that Path's Span of Steps
+    pub offset: u32,
+}
+
+/// An in memory index that provides all Steps over a given Segment
+pub struct StepsBySegIndex {
+    /// All Steps that are accessible by the index.
+    /// StepRefs that are cross over the same segment are contiguous
+    pub steps: Vec<StepRef>,
+
+    /// Spans into the steps vector. All Steps in the Span cross over one Segment.
+    /// A Span's index maps to a Segment's index in a corresponding FlatGFA
+    pub segment_steps: Vec<Span<StepRef>>,
+}
+
+// TODO: make this return an Id<Segment> instead of a usize
+/// Extract the segment index from a StepRef in a given FlatGFA
+fn segment_of_step(fgfa: &FlatGFA, step: &StepRef) -> Id<Segment> {
+    let path = &fgfa.paths[step.path];
+    let step_span = path.steps;
+    let step_slice = &fgfa.steps[step_span];
+    step_slice[step.offset as usize].segment()
+}
+
+impl StepsBySegIndex {
+    pub fn new(fgfa: &FlatGFA) -> Self {
+        // will be our `steps` vector that contains all steprefs
+        let mut all_steps = Vec::new();
+
+        // build the vector of all the steprefs, by iterating over each path, so we can construct using a path id and offset
+        // package the Id<Segment> with the StepRef's into tuples so sorting by it is easier
+        for (path_id, path) in fgfa.paths.items() {
+            for (offset, _) in fgfa.get_path_steps(path).enumerate() {
+                // the constructed StepRef based on the path and step offset
+                let step = StepRef {
+                    path: path_id,
+                    offset: offset as u32,
+                };
+
+                // the segment that this step passes over in the form of a Id<Segment>
+                let seg = segment_of_step(fgfa, &step);
+
+                // push the tuple of the Id<Segment> and StepRef into the steps vector
+                all_steps.push((seg, step));
+            }
+        }
+
+        // sort the steprefs by the index of the segment in the segment pool
+        // by extracting the actual numeric index from the Id<Segment>
+        all_steps.sort_by_key(|a| a.0.index());
+
+        // once sorted, steps that cross the same segment will be contiguous in all_steps
+        // this allows us to generate spans over steps that cross over the same segment,
+        // storing them in segment_steps
+
+        // the working segment's index
+        let mut seg_ind: &Id<Segment> = &all_steps[0].0;
+
+        // the working start of the span of StepRefs
+        let mut span_start: usize = 0;
+
+        // the vector of spans of step refs that will act as our index
+        let mut segment_steps: Vec<Span<StepRef>> = Vec::new();
+
+        // fill the segment_steps with empty spans
+        // TODO: we definitely don't need to do another iteration to fill this with empty spans
+        // It's likely more efficient to push empty spans as needed
+        for _ in 0..fgfa.segs.len() {
+            segment_steps.push(Span::new_empty());
+        }
+
+        // enumerate over each step ref in all_steps to build our spans
+        for (i, (curr_seg_ind, _)) in all_steps.iter().enumerate() {
+            // if we have moved onto a new segment add to the segment_steps vec
+            if curr_seg_ind.index() > seg_ind.index() {
+                // create and set the new span of step refs for this segment
+                // it will be left inclusive right exclusive
+                let new_span: Span<StepRef> = Span::new(Id::new(span_start), Id::new(i));
+
+                // assign the span to the index in segment_steps that maps to the index of the segment in the FlatGFA segment pool
+                segment_steps[seg_ind.index()] = new_span;
+
+                // update the working seg_ind and range_start variables to reflect the new segment/range
+                seg_ind = curr_seg_ind;
+                span_start = i;
+            }
+        }
+
+        // because we've only set a new span whenever we move onto a new segment
+        // we will need to set the final span after iterating
+        let new_span: Span<_> = Span::new(Id::new(span_start), Id::new(all_steps.len()));
+        segment_steps[seg_ind.index()] = new_span;
+
+        // unpackage all_steps into a vector with just spans
+        let steps: Vec<StepRef> = all_steps.into_iter().map(|x| x.1).collect();
+
+        Self {
+            steps,
+            segment_steps,
+        }
+    }
+
+    /// Returns a slice of StepRefs that cross over the given segment
+    pub fn get_steps_slice(&self, segment: Id<Segment>) -> &[StepRef] {
+        let range: Range<usize> = Range::from(&self.segment_steps[segment.index()]);
+
+        &(self.steps[range])
+    }
+
+    /// Returns the number of steps that cross over this segment
+    pub fn get_num_steps(&self, segment: Id<Segment>) -> usize {
+        self.segment_steps[segment.index()].len()
+    }
+}
diff --git a/flatgfa/src/lib.rs b/flatgfa/src/lib.rs
index 7b5eeafe..c132ddd9 100644
--- a/flatgfa/src/lib.rs
+++ b/flatgfa/src/lib.rs
@@ -3,6 +3,7 @@ pub mod file;
 pub mod flatbed;
 pub mod flatgfa;
 pub mod gfaline;
+pub mod index;
 pub mod memfile;
 pub mod namemap;
 pub mod ops;
diff --git a/flatgfa/src/ops/depth.rs b/flatgfa/src/ops/depth.rs
index b46070d5..1467e16e 100644
--- a/flatgfa/src/ops/depth.rs
+++ b/flatgfa/src/ops/depth.rs
@@ -1,4 +1,5 @@
 use crate::flatgfa;
+use crate::index;
 use bit_vec::BitVec;
 
 /// Compute the *depth* of each segment in the variation graph.
@@ -8,7 +9,7 @@ use bit_vec::BitVec;
 /// which only counts each path that tarverses a given segment once.
 ///
 /// Both outputs are depth values indexed by segment ID.
-pub fn depth(gfa: &flatgfa::FlatGFA) -> (Vec<usize>, Vec<usize>) {
+pub fn depth(gfa: &flatgfa::FlatGFA, use_index: bool) -> (Vec<usize>, Vec<usize>) {
     // Our output vectors: the ordinary and unique depths of each segment.
     let mut depths = vec![0; gfa.segs.len()];
     let mut uniq_depths = vec![0; gfa.segs.len()];
@@ -18,15 +19,53 @@ pub fn depth(gfa: &flatgfa::FlatGFA) -> (Vec<usize>, Vec<usize>) {
     // subsequent traversals (for the purpose of counting unique depth).
     let mut seen = BitVec::from_elem(gfa.segs.len(), false);
 
-    for path in gfa.paths.all().iter() {
-        seen.clear(); // All segments are unseen.
-        for step in &gfa.steps[path.steps] {
-            let seg_id = step.segment().index();
-            depths[seg_id] += 1;
-            if !seen[seg_id] {
-                // The first traversal of this path over this segment.
-                uniq_depths[seg_id] += 1;
-                seen.set(seg_id, true);
+    if use_index {
+        // Build the index
+        let step_seg_index = index::StepsBySegIndex::new(gfa);
+
+        // This bit vector keeps track of whether the current *path* has already been seen
+        // by the current working *segment*. A reverse approach to the non-indexed option
+        let mut path_seen: BitVec = BitVec::from_elem(gfa.paths.len(), false);
+
+        // iterate over each segment and populate the output vectors
+        for (seg_id, _) in gfa.segs.items() {
+            // clear the path_seen vector, since all paths should be unseen
+            path_seen.clear();
+
+            // get the actual offset of the id of thesegment
+            let ind = seg_id.index();
+
+            // get the span of StepRefs for this segment
+            let span = step_seg_index.get_steps_slice(seg_id);
+
+            // use that offset to directly modify the depths vector
+            depths[ind] = span.len();
+
+            // iterate over the span to populate the uniq_depths vector
+            for stepref in span.iter() {
+                // extract the path index
+                let path_id = stepref.path.index();
+
+                // if the path has not been seen, increment the unique depth
+                if !path_seen[path_id] {
+                    uniq_depths[ind] += 1;
+
+                    // set the path to seen in the path_seen bitvec
+                    path_seen.set(path_id, true);
+                }
+            }
+        }
+    } else {
+        for path in gfa.paths.all().iter() {
+            seen.clear(); // All segments are unseen.
+            for step in &gfa.steps[path.steps] {
+                let seg_id = step.segment().index();
+                depths[seg_id] += 1;
+                if !seen[seg_id] {
+                    // The first traversal of this path over this segment.
+                    uniq_depths[seg_id] += 1;
+                    seen.set(seg_id, true);
+                }
             }
         }
     }