diff --git a/input/day10.test.txt b/input/day10.test.txt
new file mode 100644
index 0000000..7bb1248
--- /dev/null
+++ b/input/day10.test.txt
@@ -0,0 +1,8 @@
+89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732
\ No newline at end of file
diff --git a/input/day10.txt b/input/day10.txt
new file mode 100644
index 0000000..dbdbad8
--- /dev/null
+++ b/input/day10.txt
@@ -0,0 +1,55 @@
+0123450128945212434498107654876212322345432110787870123
+1214563287654302345347278923945003411276741025696983254
+2307874390901201436256361010232156500985891234899874765
+3458965481896543457101454322121267898764210348760565856
+4547889332787612898654456541010128709654321409651487943
+9656971245610502348742363456723489214545670512342396512
+8746560330541461059231072019894876523039987651229603401
+0130450101232878762108981923785955432128896560318712101
+1221321432543969456676870831276843201017698431105657892
+4349898540158950387985106740189652122012587120234598761
+0456797643267341293234203651076561043403436011231034650
+1245687653212210982178312312345878654312345190345125141
+4334567064307807891089425405658969763233438987656776032
+5321018165616906702376596534787439890120107678949889120
+6910789278965215610345687321096521763011234589030672101
+7821898347654334321256786543212310652101347654121543432
+6734787658901223456105897610105432543276978903443430563
+2105676987911010897834978923076501034789877412352321694
+3456989876854323708929870134589632385676966543261430785
+4367810105763432612210165245678745690123457898170567014
+3210121234176501523878954354776544787430342347089698923
+2345665893080787438965410167889432156561231456798789654
+1058756702191898341014321054974321047892120321887018763
+0569845212012567634323015123125410430121011010986323454
+6578954307623498765487654304034543221030345654345432125
+5434356788545349854599001216787652107845210787216701034
+0125643699237898703678104325894567856956789891209834345
+7876212765103203612363215454383898945787698900340125876
+0980101894454114503054356965212432430694543215489876965
+1098234583467023212125407870106541021583210676098920145
+2347899602898908763256910187017865652678701587187813236
+3256678711743219854567823298894976569549652490296704367
+0100345620651278343289654350765987478230743321345410198
+9251201234230341230178760541034300300121890120034326789
+8349212945145650789078921632133211212010581631128965632
+7658767876054787632107634780124504321123498745489874541
+6107323945763096549616543995435665210898589654788103450
+5236014539892124328723012876343786789867670123694012367
+4345867622101015610654322301212891270184561054543231018
+2109988913412126789961001454309750301293432163210102309
+3458776804569234697872156545678543432789430673456943212
+4567566543678985586543267836787612545676521982987856103
+0103457012987876487434586927898707654568701201276547894
+1212388967876567393325698810989898965439632320567030985
+0327890658905058212016784543298781012344543011498121076
+9456541243014149801134569650185632307655676322399876125
+8767632332123232100123678745670546998764985431087565436
+3498234501104343034598988764321457884643891056016501098
+2567107698612352125667639058901210745012342347121432167
+1989278786783961012787540147654323654321435218930345236
+0876989695894878109896039236569456788760324306543210145
+0105874504185769854385128545478998699354413457850105256
+1234763213096854763014537654360187543263509766969876567
+2303452342187943212323456963201236984102678876878103498
+3212301056789810103410567870102345676101278987989012567
diff --git a/src/day10.rs b/src/day10.rs
new file mode 100644
index 0000000..0dd7bd9
--- /dev/null
+++ b/src/day10.rs
@@ -0,0 +1,43 @@
+use crate::spatial::{Point, adjacent_cardinal, adjacent_cardinal_points, all_coords, flood};
+use std::collections::{BTreeSet, HashMap};
+
+pub fn day10_part1(input: &str) -> String {
+    let topomap = parse(input);
+    let digraph = topomap_to_digraph(&topomap);
+    let reachable = all_coords(topomap[0].len(), topomap.len())
+        .filter(|pt| topomap[pt.row][pt.col] == 0)
+        .map(|pt| flood(&digraph, pt))
+        .map(|set| set.iter().filter(|pt| topomap[pt.row][pt.col] == 9).count())
+        .sum::<usize>();
+    reachable.to_string()
+}
+
+pub fn day10_part2(input: &str) -> String {
+    todo!()
+}
+
+fn parse(input: &str) -> Vec<Vec<u8>> {
+    input
+        .lines()
+        .map(|line| {
+            line.chars()
+                .map(|c| c.to_digit(10).unwrap() as u8)
+                .collect()
+        })
+        .collect()
+}
+///adjacency list in graph format
+fn topomap_to_digraph(topomap: &[Vec<u8>]) -> Vec<Vec<Vec<Point>>> {
+    let mut digraph = vec![vec![vec![]; topomap[0].len()]; topomap.len()];
+    for row in 0..topomap.len() {
+        for col in 0..topomap[0].len() {
+            let adj = adjacent_cardinal_points(topomap, Point { row, col });
+            digraph[row][col] = adj
+                .into_iter()
+                .filter(|pt| topomap[pt.row][pt.col] == 1 + topomap[row][col])
+                .collect();
+        }
+    }
+
+    digraph
+}
diff --git a/src/lib.rs b/src/lib.rs
index 9bd26be..6ff18d1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -345,11 +345,28 @@ pub mod day6;
 pub mod day7;
 pub mod day8;
 pub mod day9;
+pub mod day10;
+pub mod spatial;
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    #[test]
+    fn day10_part1_test() {
+        let test_result = day10::day10_part1(include_str!("../input/day10.test.txt"));
+        assert_eq!(test_result, "36");
+        let result = day10::day10_part1(include_str!("../input/day10.txt"));
+        assert_eq!(result, "820");
+    }
+    // #[test]
+    // fn day10_part2_test() {
+    //     let test_result = day10::day10_part2(include_str!("../input/day10.test.txt"));
+    //     assert_eq!(test_result, "2858");
+    //     // let result = day10::day10_part2(include_str!("../input/day10.txt"));
+    //     // assert_eq!(result, "6476642796832");
+    // }
+
     #[test]
     fn day9_part1_test() {
         let test_result = day9::day9_part1(include_str!("../input/day9.test.txt"));
diff --git a/src/spatial.rs b/src/spatial.rs
new file mode 100644
index 0000000..e538a86
--- /dev/null
+++ b/src/spatial.rs
@@ -0,0 +1,71 @@
+use std::collections::BTreeSet;
+
+use itertools::Itertools;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub struct Point {
+    pub row: usize,
+    pub col: usize,
+}
+pub fn all_coords(width: usize, height: usize) -> impl Iterator<Item = Point> {
+    (0..height)
+        .cartesian_product(0..width)
+        .map(|(row, col)| Point { row, col })
+}
+
+///Never Eat Shredded Wheat
+pub fn adjacent_cardinal<T: Copy>(grid: &[Vec<T>], coords: Point) -> [Option<T>; 4] {
+    let mut retval = [const { None }; 4];
+    let (row, col) = (coords.row, coords.col);
+
+    if row > 0 {
+        retval[0] = Some(grid[row - 1][col]) //N
+    }
+    if col < grid[0].len() - 1 {
+        retval[1] = Some(grid[row][col + 1]) //E
+    }
+    if row < grid.len() - 1 {
+        retval[2] = Some(grid[row + 1][col]) //S
+    }
+    if col > 0 {
+        retval[3] = Some(grid[row][col - 1]) //N
+    }
+
+    retval
+}
+
+pub fn adjacent_cardinal_points<T: Copy>(grid: &[Vec<T>], coords: Point) -> Vec<Point> {
+    let mut retval = vec![];
+    let (row, col) = (coords.row, coords.col);
+
+    if row > 0 {
+        retval.push(Point { row: row - 1, col }) //N
+    }
+    if col < grid[0].len() - 1 {
+        retval.push(Point { row, col: col + 1 }) //E
+    }
+    if row < grid.len() - 1 {
+        retval.push(Point { row: row + 1, col }) //S
+    }
+    if col > 0 {
+        retval.push(Point { row, col: col - 1 }) //W
+    }
+
+    retval
+}
+
+//2d spatial digraph
+pub fn flood(digraph: &[Vec<Vec<Point>>], start: Point) -> BTreeSet<Point> {
+    let mut reachable = BTreeSet::new();
+    let mut frontier = BTreeSet::from_iter([start]);
+
+    while let Some(cursor) = frontier.pop_first() {
+        reachable.insert(cursor);
+        let next = BTreeSet::from_iter(digraph[cursor.row][cursor.col].iter().cloned());
+        for &point in next.difference(&reachable) {
+            frontier.insert(point);
+        }
+    }
+
+    reachable
+}