diff --git a/src/arraymap.rs b/src/arraymap.rs
new file mode 100644
index 0000000..514ce85
--- /dev/null
+++ b/src/arraymap.rs
@@ -0,0 +1,28 @@
+pub struct ArrayMap<T> {
+    offset: usize,
+    elements: Vec<Option<T>>
+}
+
+impl <T: std::clone::Clone> ArrayMap<T> {
+    pub fn new(start_key: usize, end_key: usize) -> ArrayMap<T> {
+        ArrayMap {
+            offset: start_key,
+            elements: vec![None; end_key - start_key]
+        }
+    }
+
+    pub fn with_default(start_key: usize, end_key: usize, default_value: T) -> ArrayMap<T> {
+        ArrayMap {
+            offset: start_key,
+            elements: vec![Some(default_value); end_key - start_key]
+        }
+    }
+
+    pub fn insert(&mut self, key: usize, value: T) {
+        self.elements[key - self.offset] = Some(value);
+    }
+
+    pub fn get(&self, key: usize) -> T {
+        self.elements[key - self.offset].clone().unwrap()
+    }
+}
\ No newline at end of file
diff --git a/src/base.rs b/src/base.rs
index 578fd33..584729a 100644
--- a/src/base.rs
+++ b/src/base.rs
@@ -251,7 +251,7 @@ impl IntermediateSymbolDecoder {
                 let mut chosen_hdpc = None;
                 let mut chosen_non_hdpc = None;
                 for row in self.i..self.L {
-                    let non_zero = non_zero_counts[row - self.i];
+                    let non_zero = non_zero_counts.get(row);
                     if non_zero == r as u32 {
                         if hdpc_rows[row] {
                             chosen_hdpc = Some(row);
diff --git a/src/bin_matrix_sparsity.rs b/src/bin_matrix_sparsity.rs
index 5eb7c37..4b6ea88 100644
--- a/src/bin_matrix_sparsity.rs
+++ b/src/bin_matrix_sparsity.rs
@@ -2,6 +2,7 @@ extern crate petgraph;
 extern crate primal;
 extern crate raptorq;
 
+mod arraymap;
 mod octets;
 #[allow(dead_code)]
 mod systematic_constants;
diff --git a/src/lib.rs b/src/lib.rs
index 3f0edc8..c88525e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,6 +1,7 @@
 extern crate petgraph;
 extern crate primal;
 
+mod arraymap;
 mod systematic_constants;
 mod rng;
 mod octet;
diff --git a/src/matrix.rs b/src/matrix.rs
index e77a417..6cab1fd 100644
--- a/src/matrix.rs
+++ b/src/matrix.rs
@@ -1,4 +1,5 @@
 use std::ops::Mul;
+use arraymap::ArrayMap;
 use octet::Octet;
 use octets::fused_addassign_mul_scalar;
 use octets::add_assign;
@@ -75,11 +76,11 @@ impl OctetMatrix {
     // Helper method for decoder phase 1
     // selects from [start_row, end_row) reading [start_col, end_col)
     // Returns (rows with two 1s, a row with two values > 1,
-    // mapping from row number (offset by start_row) to number of non-zero values, "r" minimum positive number of non-zero values a row has)
-    pub fn first_phase_selection(&self, start_row: usize, end_row: usize, start_col: usize, end_col: usize) -> (Vec<usize>, Option<usize>, Vec<u32>, Option<u32>) {
+    // mapping from row number to number of non-zero values, "r" minimum positive number of non-zero values a row has)
+    pub fn first_phase_selection(&self, start_row: usize, end_row: usize, start_col: usize, end_col: usize) -> (Vec<usize>, Option<usize>, ArrayMap<u32>, Option<u32>) {
         let mut rows_with_two_ones = vec![];
         let mut row_with_two_greater_than_one = None;
-        let mut non_zeros = vec![0; end_row - start_row];
+        let mut non_zeros = ArrayMap::with_default(start_row, end_row, 0);
         let mut r = std::u32::MAX;
         for row in start_row..end_row {
             let mut non_zero = 0;
@@ -93,7 +94,7 @@ impl OctetMatrix {
                 }
             }
             if non_zero > 0 {
-                non_zeros[row - start_row] = non_zero;
+                non_zeros.insert(row, non_zero);
                 if non_zero < r {
                     r = non_zero;
                 }